Belete Highway II-1

WOLLO UNIVERSITY
INSTITUTE of TECHNOLOGY
CIVIL ENGINEERING DEPRTIMENT
Highway Engineering II – CENG 4302
Samuel
Kifle
COURSE OUTLINE
Course Number – CENG 4402
Course Title – Highway Engineering II
Contact Hours (per week) – 3
Course Objectives
– The student will incorporate and utilize technology
in pavement analysis and design.
COURSE OUTLINE
Course Description/Course Content
Chapter One
• Overview of pavement structures
– Types of pavement structures
– Basic design data
Chapter Two
• Stresses in pavement structures.
Chapter Three
• Traffic volume and loading
– Fixed traffic procedures
– Fixed vehicle procedures
– Axle load survey
– Comulative equivalent standard axles
Chapter Four
• Subgrade soils
– Special soil tests for pavement design
– Soil classification for highway purposes
COURSE OUTLINE
Chaper Five
• Unbound pavement materials
– Sources and properties of aggregates
– Aggregates for surfacing, base and sub-base courses
– Materials for gravel surfacing
– Capping layers
Chapter Six
• Stabilized pavement materials.
Chapter Seven
• Bituminous materials
– Sources and properties of binders
– Types of asphalt mixtures
– Marshall method of mix design
– Surface treatments
COURSE OUTLINE
Chapter Eight
– Structural design of flexible pavements
– AASHTO method of flexible pavement design
– Design of flexible pavement structures ERA and
AACRA design procedures
Chapter Nine
– Design of gravel surfaced road
COURSE OUTLINE
Pre-requisites –CENG 3302 and CENG 3301
Status of Course – Compursory
Teaching and Learning Methods – lectures, tutorials and project
Evaluation – written exam 70% and continuous assessment 30%
Attendance Requirements – 75% minimum class attendance
Reference
1. Huang, Y.H. (2003), Pavement Analysis & Design, 2nd edition,
Prentice-Hall.
2. Ritter L. J., Paquette, R.J. and Wright, P. H. (2003), Highway
Engineering, 7th edition, John Wiley & Sons Inc.
3. Garber, N.J. & Hoel, L.A. (2001), Traffic & Highway Engineering,
3rd edition, Thomson-Engineering
4. Ethiopian Road Authority Manual, 2003
5. Addis Ababa City Road Authority Manual, 2004
6. AASHTO design manual, 1993
Chapter One
Overview of pavement structures

INTRDUCTION
• The field of pavement design is dynamic
– the concepts are changing with time as technology develops and
new equipment emerges for
• site investigation,
• material testing
• traffic data collection, and
• new data become available
• In the early stage, pavement design was carried out by a rule-
of-thumb procedure based on entirely past experience.
• Through the observation of performances of the already
constructed roads, highway engineers became aware that
pavement performance is dependent on the subgrade soils.
• Pavements constructed over plastic soils showed higher
distress than those constructed over granular deposits.
• With the knowledge of soil mechanics, pavement design was
made with soil classification.
Samuel K. 8
Overview of Pavement Structures
• A pavement structure
– is a layer structure which supports the vehicle load
on its surface and
– transfers and spreads the load to the subgrade
without exceeding either the strength of the sub-
grade or the internal strength of the pavement
itself.
Samuel K. 9
• The basic idea in building a pavement for all-
weather use by vehicles is to prepare a suitable
sub-grade, provide necessary drainage, and
construct a pavement that will:
– Have sufficient total thickness and internal strength to
carry expected traffic loads, and distribute them over
the subgrade soil without overstressing;
– Have adequate properties to prevent or minimize the
penetration or internal accumulation of moisture; and
– Have a surface that is reasonably smooth and skid
resistant at the same time, as well as reasonably
resistant to wear, distortion and deterioration by
vehicle loads and weather.
Samuel K. 10
• For a very low traffic where the soil can be trafficable and
when there is economic limitation
– the natural subgrade soil can be made to carry the traffic load after
clearing and shaping.
– Such earth roads give seasonal services and require reshaping after
seasonal changes.
• Better than earth roads, gravel surfaced roads are also
constructed
– by spreading gravel over the subgrade, shaping and compacting to
avoid excessive strain at the subgrade level
– to give services usually in all seasons.
• In the case of gravel-surfaced roads
– reshaping is necessary, but not as frequent as in earth roads.
• Gravel roads follow selected routes and designed to carry low
to medium traffic and serve as stage construction.
• The surface material should be kept to a certain standard such
as grading and plasticity. Samuel K. 11
Flexible Pavements
• A flexible pavement is one
– which has low flexural strength, and
– the load is largely transmitted to the subgrade soil
through the lateral distribution of stresses with
increasing depth as shown in Figure 1-1.
• The pavement trickiness is designed such that
– the stresses on the subgrade soil are kept within its
bearing capacity and
– the subgrade is prevented from excessive deformation.
• The strength and smoothness of flexible
pavement structure depends to a large extent on
the deformation of the subgrade soil.
Samuel K. 12
Flexible Pavements
Samuel K. 13
Flexible Pavements
• Generally, two types of construction have
been used for flexible pavements:
– conventional flexible pavement, and
– full-depth asphalt pavement
• A third type, known as contained rock asphalt
mat (CRAM) construction is still in the
experimental stage and has not been widely
accepted for practical use.
Samuel K. 14
Conventional Flexible pavements
• Conventional flexible pavements
– are multilayered structures with
• better materials on top where the intensity of stress is high and
• inferior materials at the bottom where the intensity is low.
• This design principle makes possible to use local materials and usually
results in a most economical design.
• This is particularly true in regions where high-quality materials are
expensive but local materials of inferior quality are readily available.
• Starting from the top, a conventional flexible pavement normally
consists of
– surface course,
– base course,
– subbase course,
– compacted subgrade, and
– natural subgrade.
• The use of the various courses is based on either necessity or economy
and some of the courses may be omitted.
Samuel K. 15
Conventional Flexible pavements
Samuel K. 16
Surface Course
• The surface course is the top course of an asphalt
pavement, sometimes called the wearing course.
• It is usually constructed by dense graded hot-mix
asphalt.
• It is a structural part of the pavement, which must
be tough to resist distortion under traffic and
provide a smooth and skid-resistant riding
surface.
• The surface course must be waterproof to protect
the entire pavement and subgrade from the
weakening effect of water.
Samuel K. 17
Base Course
• The base course is the layer of material
immediately beneath the surface course.
• It may be composed of
– well-graded crushed stone (unbounded),
– granular material mixed with binder, or
– stabilized materials.
• It is the main structural part of the pavement and
provides a level surface for laying the surface
layer.
• If constructed directly over the subgrade,
– it prevents intrusions of the fine subgrade soils into the
pavement structure.
Samuel K. 18
Sub-base Course
• The subbase course is the layer of material
beneath the base course constructed using
– local and cheaper materials for economic reason on top
of the subgrade.
• It provides additional help to the base and the
upper layers in distributing the load.
• It facilitates drainage of free water that might get
accumulated below the pavement.
• If the base course is open graded, the subbase
course with more fines can serve as a filter
between the subgrade and the base course.
Samuel K. 19
Sub-grade
• Sub-grade is the foundation on which the
vehicle load and the weight of the pavement
layers finally rest.
• It is an in situ or a layer of selected material
compacted to the desirable density near the
optimum moisture content.
• It is graded into
– a proper shape,
– properly drained, and
– compacted to receive the pavement layers
Samuel K. 20
Full-Depth Asphalt Pavements
• Full-depth asphalt pavements are constructed by
– placing one or more layers of hot-mix asphalt
directly on the sub-grade or improved sub-grade.
• This concept was conceived by the Asphalt
Institute and is generally considered
– the most cost-effective and
– dependable type of asphalt pavement for heavy
traffic
– quite popular in areas where local materials are not
available
Samuel K. 21
Rigid Pavements
• Rigid pavements
– are pavement structures constructed of cement concrete slabs,
– which derive their capacity to withstand vehicle loads from
flexural strength or beam strength due to high modulus of
elasticity.
• Because of high flexural strength,
– the vehicle load on cement concrete slab is distributed over a
relatively wider area of the soil than flexible pavements and
– thus, variation in the subgrade soil strength has little influence.
• The flexural strength also permits the slab to bridge over
minor irregularities under it.
– Thus, the performance of rigid pavements is more governed by
the strength of the concrete slab than the subgrade supports.
• Hence, the major factor considered in the design of rigid
pavement is the structural strength of the concrete.
Samuel K. 22
Rigid Pavements
• The sub-grade may provide a uniform support for the slab.
• However, where the sub-grade soil cannot provide a
uniform support, or for one or more of the following
reasons described here under, there is always a necessity to
build a base course under cement concrete slab and it is
widely used for rigid pavements.
– Control pumping
– Control of frost action
– Improvement of drainage
– Control of shrinkage and swell
– Expedition of construction
Samuel K. 23
Control of pumping
• Pumping
– is defined as the ejection of water and subgrade soil
through
• joints, cracks, and along the edges of the pavements caused
by the downward movements of due to heavy axle loads.
• Pumping occurs
– when there is void space under the slab due to
temperature curling of the slab,
– deformation of the subgrade or both and
– erodible material under the slab is saturated.
• It leads to faulting and cracking of the slab if not
corrected in time.
Samuel K. 24
Control of frost action
• Heave caused by the increase in volume of
freezed water and the formation and continuing
expansion of ice lenses causes the concrete slab
to break and softens the subgrade during frost
melts period.
• This occurs when the soil within the depth of
frost penetration is frost susceptible (e.g. clay),
there is supply of moisture, and the temperature
freezes for a sufficient period of time.
Samuel K. 25
Improvement of drainage
• When the water table is high and close to the
ground surface,
– a base course can raise the pavement to a desirable
elevation above the water table.
• An open-graded base course
– provides an internal drainage system capable of
rapidly removing water that seeps through
pavement cracks and joints carry it away to the
roadside.
• A dense-graded or stabilized base course
– can also serve as a waterproofing layer.
Samuel K. 26
Control of shrinkage and swell
• When the change in moisture causes subgrade
to shrink or swell,
– the base course can serve as a surcharge load to
reduce the amount of shrinkage and swell in
addition to its use of improving drainage.
• Measures that are taken to reduce entering
water into the sub-grade further reduce the
shrinkage and swell potentials.
Samuel K. 27
Expedition of Construction
• A base course can be used as a working
platform for heavy construction equipment.
• Under severe weather conditions,
– a base course can keep the surface clean and dry
and facilitate the construction work.
Samuel K. 28
Types of Concrete Pavement
• Concrete pavements can be classified into four
types:
– jointed plain concrete pavement (JPCP),
– jointed reinforced concrete pavement (JRCP),
– continuous reinforced concrete pavement (CRCP),
– Pre-stressed concrete pavement (PCP)
Samuel K. 29
• Jointed plain concrete pavements
– are plain concrete pavements constructed with closely
spaced contraction joints.
– Dowels or aggregate interlocks may be used for load
transfer across the joints.
• In jointed reinforced concrete pavements
– are concrete pavements with steel reinforcements in
the form of wire mesh or deformed bars mainly to
allow the use of longer joint spacing but do not
increase the structural capacity of pavements.
– Because of the longer panel length, dowels are
required for load transfer across the joints.
– The amount of distributed steel increases with the
increase in joint spacing and is designed to hold the
slab together after cracking.
Samuel K. 30
• Continuous reinforced concrete pavements
– are reinforced concrete pavements designed joint-free
for the purpose of eliminating joints, which are the
weak spots in rigid pavements.
– The elimination of joints would decrease the thickness
of pavement required.
– Concrete is weak in tension but strong in compression.
– The thickness of concrete pavement required is
governed by its modulus of rupture, which varies, with
the tensile strength of the concrete.
– The pre-application of a compressive stress to the
concrete greatly reduces the tensile stress caused by
the traffic loads and thus decreases the thickness of
concrete required.
Samuel K. 31
• The pre-stressed concrete pavements
– have less probability of cracking and fewer transverse
joints and therefore result in less maintenance and
longer pavement life.
• Pre-stressed concrete has been used more
frequently for airport pavements than for
highway pavements because the saving in
thickness for airport pavements is much greater
than that for highway pavements.
Samuel K. 32
Composite Pavements
• Composite pavements
– are pavements composed of cement concrete as a bottom layer and
hot-mix asphalt as a top layer to obtain an ideal pavement with the
most desirable characteristics.
– The cement concrete slab provides a strong base and the hot-mix
asphalt provides a smooth and non-reflective surface.
• However, this type of pavement is very expensive and is rarely
used as a new construction.
• Composite pavements include rehabilitated concrete pavements
using asphalt overlays, and asphalt pavements with stabilized
bases.
• For flexible pavements with untreated bases, the most critical
tensile stress or strain is located at the bottom of asphalt layer,
while for composite pavements the most critical location is at the
bottom of the cement concrete slab or stabilized bases.
• A disadvantage of this construction
– the occurrence of reflection cracks on the asphalt surface due to the
joints and cracks in the rigidSamuel
baseK.layer. 33
Comparison of Rigid and Flexible
Pavements
• The following main differences between rigid and
flexible pavements can be cited:
– The manner in which vehicle loads are transmitted to
the sub-grade soil,
– Design precision,
– Design life,
– Maintenance requirements,
– Initial cost,
– Suitability for stage construction,
– Surface characteristics,
– Permeability, and
– Traffic dislocation during construction.
Samuel K. 34
Highway and Airport Pavements
• The principles used for the design of highway pavements can be applied
to those of airport pavements.
• However, due to the following differences airport pavements are
generally thicker than highway pavements and require better surfacing
materials.
– The gross-weight of an airplane is usually greater than that of a heavy truck,
but the number of load repetitions on airport pavements is usually smaller
that that on highway pavements.
– The arrangement and spacing of wheel loads on airport and highway
pavements are different.
– A typical tyre pressure on highway pavements is in the vicinity of 0.5 MPa
while aircrafts use a tyre pressure up to 3 MPa which is an important factor in
the design of the materials in the upper layer of the pavements.
– Vehicle loads are applied near to the edge of highway pavements but far
away from the outside edge of airport pavements.
– Unlike highway pavements, airfield pavements are subjected to an impact
loading.
– The design load of airport pavements is the wheel load of the largest aircraft
during takeoff time due to heavy fuel weight. Although wheel loads can be
used as design loads, number of repetitions of standard axles is the
commonly used design parameter for highway pavements.
Samuel K. 35
Basic Design Factors
• Design factors can be divided into four broad
categories:
– traffic loading,
– environment,
– materials,
– failure criteria
Samuel K. 36
Traffic Loading
• The loading applied by traffic is one of the major factors affecting the design and
performance of pavements.
• It is fundamental to estimate the structural wear produced by traffic quantitatively
both
– for the purposes of pavement design and maintenance and
– for making comparisons of the structural effects of different traffic loading conditions.
• The configuration, magnitude, and repetitions of axle loads are important aspects
of traffic loading that are considered in the analysis and design of pavements.
• The principal function of pavement structure
– to protect the sub-grade from the loading imposed by traffic.
• The primary loading factors that are important in the structural analysis and
design of pavements are
– the magnitude,
– configuration, and
– repetition of traffic loading
• The magnitude of maximum loading is commonly controlled by legal load limits.
• Traffic surveys and load-meter studies are often used to establish the relative
magnitude and occurrence of the various loadings to which a pavement is
subjected.
• Prediction or estimation of the total traffic that will use a pavement during its
design life is a very difficult but obviously important task.
Samuel K. 37
Axle-loads and configurations
• Axle loads
– affect primarily the stresses and strains developed in the lower
layers of the pavement.
• Although much publicity is given to the physical size and
gross weight of vehicles,
– it is the individual wheel or axle-load that is critical in
pavement design and performance.
• Vehicle size and weight may have a devastating effect on
the environment but will not necessarily increase pavement
wear.
• Heavier loads are usually carried on a larger numbers of
axles or wheels thereby maintaining or even reducing the
individual wheel or axle loads, although their number may
be increased.
• Most countries limit, by law, the maximum axle-load of
vehicles that may use the roads without special permission.
Some of these limits are:Samuel K. 38
Country Gross vehicle weight Maximum Axle load limit
(tones) (tones)
France 38 12
Germany 38 10
Italy 44 12
United Kingdom 38 10.2
EU proposal 44 11
• It has been found difficult to enforce such limits and

considerable effort is being directed to the development
of:
– An on-board axle-load indicator, at reasonable cost, which
could work in a similar way to the tachograph; and
– A relatively cheap axle-load measuring sensor to monitor
the magnitude of dynamic axle-loads at the roadside.
Samuel K. 39
• The spacing and configuration of wheels and axles vary with
the purpose of the motor vehicle.
• The arrangement of wheels and axles affect
– the stress distribution and deflection within and below the pavement
structure.
• Unless an equivalent single-axle or single-wheel load is used,
– the consideration of multiple axles or multiple wheels is not a simple
matter.
• Rholdes (1996) cites literatures to substantiate the effect of
multiple-axle and multiple wheel loads.
• The AASHO Road Test showed that
– an 80 kN single axle-load produced about the same level of
pavement wear as a 142 kN tandem axle-load.
• More recently, it has also been shown by Atkinson and
Blackman that
– a single wheel load would cause 2.5 times as much fatigue wear per
wheel pass as a dual wheel applying the same total load.
Samuel K. 40
• The design may, thus, be unsafe if
– the tandem and tridem axles are treated as a group and
considered as one repetition.
• The design is too conservative if
– each axle is treated independently and considered as one
repetition.
• Similarly, if the pavement is to be designed for a
fixed traffic, the case for airport pavements or
highway pavements with heavy wheel loads but light
traffic volume,
– multiple wheels must be converted to an equivalent single
wheel load.
Samuel K. 41
Tyre pressure
• For most problems, the wheel load is assumed to be uniformly distributed
over the contact area.
• If a given load is applied via a number of different tyre pressures,
– the structural effects in the upper layers are affected,
– whereas those deeper in the pavement are relatively unaffected.
• This is due primarily to
– the area of the tyre pavement surface contact patch varying with tyre pressure.
• As indicated in Figure 1- 3,
– the contact pressure is greater than the tyre pressure for low-pressure tyres,
• because the wall of tyres is in compression and
• the sum of vertical forces due to wall and tyre pressure must be equal to the force due
to the contact pressure;
– the contact pressure is smaller than the tyre pressure for high pressure tyres.
– However, in pavement design,
• the contact pressure is generally assumed to be equal to the tyre pressure.
– Because, heavier axle loads have higher tyre pressures and more destructive
effects on pavements, the use of tyre pressure as the contact pressure is
therefore on the safe side.
Samuel K. 42
Tyre pressure
• As will be seen in the coming sections, it is only the commercial

vehicle that is important in structural pavement design and a typical
tyre pressure would be 0.5 MN/m2.
• Aircraft tyres use pressures up to nearly 3.0 MN/m2 which can cause
serious problems in the design of the materials employed in the upper
layers of the pavement. Samuel K. 43
Tyre pressure
• The approximate shape of contact area for each tyre, which
is composed of a rectangle and two semicircles with the
dimensions shown Figure 1- 4a.
• Based on the finite element analysis of rigid pavements,
– a rectangular contact area is also assumed with a length of
0.8712L and a width of 0.6L, which has the same area of
0.5227L2, as shown Figure 1- 4b.
Samuel K. 44
Tyre pressure
• These contact areas are not axisymmetric and cannot be
used with the layered theory.
• When the layered theory is used for flexible pavement
design,
– it is assumed that each tyre has a circular contact area.
– This assumption is not correct, but the error incurred is
believed to be small.
• To simplify the analysis of flexible pavements,
– a single circle with the same contact area as the duals is
frequently used to represent a set of dual tyres, instead of
using two circular areas.
– This practice usually results in a more conservative design,
but may become unconservative for thin asphalt surface.
Samuel K. 45
Number of repetitions
• A succession of loads has a cumulative effect on the
behaviour of pavements.
• It is therefore necessary
– to design the pavement for a specified number of years and
– to estimate the total number and magnitude of loads that will
be applied during the periods specified.
• A widely accepted procedure of considering traffic load is
– the use of equivalent factor and convert each load into an
equivalent 80 kN single axle load.
• The equivalency between two different loads
– depends on the failure criteria used
• Equivalent factor based on permanent deformation
– may be different from those based on fatigue cracking
• Generally, an empirical approximation of a single
equivalent factor is used for practical purposes.
Samuel K. 46
Speed of traffic loading
• Another factor related to traffic loading is the speed of
traveling vehicles.
• Studies showed that the stresses and deflections tend to
decrease as the vehicle speed increases.
• Speed is directly used as the duration of loading on
pavements.
• Generally, the greater the speed, the larger the modulus,
and the smaller the strains in the pavement.
• Because of this, for a given volume of traffic, greater
thickness and quality of paving materials are required
for pavements in urban areas than those in rural areas.
• Similarly, such requirements are considered for up hill
roads and bus stops.
Samuel K. 47
Environment
• The environmental factors that influence
pavement design include
– temperature, and
– precipitation.
• Different standards of pavement design
consider the effects of these factors in various
ways.
Samuel K. 48
Temperature
• The effect of temperature on asphalt
pavements is different from that on concrete
pavements.
• Temperature affects the resilient modulus of
bituminous layers and creates thermal stresses
in cement concrete slabs.
• In cold climates, the resilient modulus of
unstabilised materials also varies with freeze-
thaw cycles.
Samuel K. 49
Temperature
• The elastic and visco-elastic properties of bituminous
materials are affected significantly by pavement
temperature.
• When the temperature is low,
– the bituminous layer becomes rigid and has less fatigue
life.
• The stiffness is also influenced by
– the condition of the mix and
– the hardness of the binder used
• To minimize thermal contraction cracking
– at low temperature,
• a relatively soft binder and
• high binder content would be used,
– whereas for hot conditions
• the hardness of the binder would be increased and
• the binder content reduced to minimize plastic flow in the material.
Samuel K. 50
Temperature
• The warping stresses in rigid pavements
– are generated principally by temperature changes.
• Warping stresses some times of the year or the day are additive
to the traffic stresses and can influence the slab thickness
requirements.
• Shortening the slab length reduces the effect of these stresses.
• Another effect of temperature on pavement design in cold
climate is
– the frost penetration, which results in a frost heave, and stronger
subgrade in the winter but a much weaker subgrade in the spring.
• Frost heave causes differential settlements and pavement
roughness.
• The most detrimental effect of frost penetration occurs during
the spring period when the ice melts and the subgrade is in a
saturated condition.
• It is desirable to protect the subgrade by using non-frost-
susceptible materials or the design should take into account the
weakening of the subgrade.Samuel K. 51
Precipitation
• Precipitation is important in the design,
construction, and performance of roads in
three main aspects:
– The construction of earth works
– Strength of pavement structure
– Surface water drainage
Samuel K. 52
The construction of earthworks
• The specification of earthworks is normally defined
either
– directly or in terms of a standard compaction test.
• The design engineer should ensure that
– the contractor meets the requirements of such specification.
• Such requirements in areas where there are
considerable dry months, such as in arid regions,
transporting water can considerably increase the
construction cost.
• In contrary, there are areas where excess rainfall over
evaporation falls and cause difficulty for earthworks,
and attaining the specification will only be possible in
few dry months.
Samuel K. 53
Strength of pavement structure
• The natural moisture content of the soil determines the
– subgrade strength to be used in the design of the pavement
structure.
• It is the responsibility of the design engineer
– to estimate the natural moisture content
– the corresponding strength of the subgrade and
– ensure that this moisture content is maintained through out
the service life of the pavement structure.
• Further more, if the surface of a pavement is not
impervious,
– water in the form of rainfall percolates easily and can
degrade the structural performance of each layer of the
pavement structure.
Samuel K. 54
Surface water drainage
• The maximum intensity of rainfall is required for
the design of the
– surface water drainage system of the road.
• This may include
– from open side ditches to crossing structures like
bridges.
• Minimum grades of roads are also decided based
on the
– criterion of pavement drainage.
• The link between rainfall and surface drainage is
also important in the design and construction of
roads.
Samuel K. 55
Materials
• Pavement materials include
– soils,
– aggregates,
– bituminous binders, and
– cement
• The properties of these materials under traffic loading in a
given environmental conditions is
– fundamental for the proper design of pavement structures.
• Moreover, if economically constructed facilities are to be
obtained,
– locally available materials are to be used efficiently.
• The materials used in the construction of a highway are of
interest to
– the highway engineer to many other branches of Civil
Engineering where the engineer need not to be very deeply
concerned with the properties of the materials being used.
Samuel K. 56
Performance and Failure Criteria
• Pavements are normally designed and constructed to
provide, during the design life,
– a riding quality acceptable for both private and commercial
vehicles with acceptable maintenance.
• The assumption is often made that
– road pavements begin to deteriorate as soon as they are
open to traffic, particularly, when they are underdesigned.
• But, where the design life is of the order of 20 years or
more, there should no visible deformation for the first
five years.
• Fatigue cracking, rutting, and thermal cracking are the
three principal types of distress generally considered
for flexible pavement design.
• The fatigue cracking of flexible pavements
– is due to the horizontal tensile strains at the bottom of
bituminous layer.
Samuel K. 57
• Rutting
– is a permanent deformation that occurs on flexible pavements
along the wheel path.
• Thermal cracking includes low-temperature and thermal
fatigue cracking.
• Low temperature cracking
– is usually associated with flexible pavements in cold regions
where temperature fall below –23oC.
• Thermal fatigue cracking can occur in much milder
regions if an excessive hard bituminous binder is used or
the binder becomes hardened due to ageing.
• Different methods of pavement design consider these
pavement failures as design criteria, but differently.
• In the AASHTO method of pavement design,
– a rating system known as the present serviceability index
(PSI) is used to account for performance of pavements.
Samuel K. 58
• Others, such as the Asphalt Institute and Shell relate the
allowable number of load repetitions to control fatigue cracking
and permanent deformation.
• Several models are also available to estimate cumulative damage
of thermal cracking for a specified time after construction.
• Fatigue cracking, pumping and other distresses such as faulting
and joint deterioration are recognised failures in rigid pavements.
• Fatigue cracking is most likely caused by the edge stress at the
midslab.
– It has long been considered the major criterion for rigid pavement
design.
• Although permanent deformation is not considered in rigid
pavements design, the resilient deformation under repeated wheel
loads cause pumping.
• The resulting corner deflection has been used as a criterion in
addition to the fatigue.
Samuel K. 59
Chapter Two
Stresses in pavement structures

Stresses in Pavements
Stresses in Flexible Pavements
 Stresses in Homogeneous Mass
• Boussinesq formulated models for the stresses inside an elastic
half-space due to a concentrated load applied on the surface.
• A half-space has an infinitely large area and an infinite depth
with a top plane on which the loads are applied.
• The simplest way to characterize the behaviour of a flexible
pavement under wheel loads is to consider
– the subgrade,
– the subbase,
– base, and
– the surfacing layers to form a homogeneous half-space
• If the modulus ratio between the pavement and the subgrade is
close to unity, as exemplified by a thin asphalt surface and a
thin granular base,
– the Boussinesq theory can be applied to determine the stresses,
strains, and deflections in the subgrade.
61
Stresses in Homogeneous Mass
• Figure 2-1 shows a homogeneous half-space
subjected to a circular load with a radius a and a
uniform pressure q.
• The halfspace has an elastic modulus E and a
Poisson ratio, v.
• A small cylindrical element with centre at a
distance z below the surface and r from the axis of
symmetry is shown.
• Due to axisymmetry,
– there are only three normal stresses, σz, σr, and σt, and
– one shear stress, τrz, which is equal to τrz.
• These stresses are functions of q, r/ a, and z/ a.
62
63
• Foster and Ahlvin have developed charts as provided
here from Figure 2-2 to Figure 2-6 for determining
– vertical stress σz,
– radial stress σr,
– tangential stress σt,
– shear stress τrz, and
– vertical deflection w, assuming the half-space is
incompressible with a Poisson ratio of 0.5.
• After the stresses are obtained from the charts, the
strains can be computed from
If the contact area consists of two circles, the stresses and strains
can be computed by superposition.
64
Cont...
65
Cont...
66
Cont...
67
Cont...
68
Cont...
69
Cont...
• When a wheel load is applied over a single
contact area,
– the most critical stress, strain, and deflection occur
under the centre of the circular area on the axis of
symmetry, where τrz = 0 and σr = σt, so σz and σr are
the principal stresses.
• The stresses, strain, and deflection on the axis of
symmetry of a wheel load applied to a
pavement, which is similar to a load applied to a
flexible plate with radius a and a uniform
pressure q, can be computed by:
70
Cont...
71
Cont...
When ν = 0.5, the equation is simplified to
On the surface of the loaded half-space, z = 0, the deflection is
If the load is applied on a rigid plate such as that used in a plate

loading test, the deflection is the same at all points on the plate,
but the pressure distribution under the plate is not uniform and
is expressed as:
72
Cont...
The smallest pressure is at the centre and equal to one-half of the

average pressure. The pressure at the edge is infinity. The deflection
of the rigid plate is given by
73
Cont...
• All the above analyses are based on the assumption that
the flexible pavement is
– homogenous,
– isotropic and semi-infinite, and
– that elastic properties are identical in every direction
throughout the material.
• With these assumptions, Bousinesq theory has the
following drawbacks:
– Flexible pavements are multilayered structures each layer
with its own modulus of elasticity.
– The pavement layers and the subgrade soil are not perfectly
elastic.
– The assumption that the load is uniformly distributed may
not be true.
74
Stresses in Layered Systems
• In actual case, flexible pavements are layered systems with better
materials on top and cannot be represented by a homogeneous mass.
• Various multilayer theories for estimating stresses and deflections have
been proposed.
• However, basic theories that utilize assumptions close to actual
conditions in a flexible pavement are those proposed by Burmister.
• Burmister first developed solutions for a two-layer system and then
extended them to a three-layer system with the following basic
assumptions:
1. Each layer is homogeneous, isotropic, and linearly elastic with an elastic
modulus E and a Poisson ratio, ν.
2. The material is weightless and infinite in the lateral direction, but of
finite depth, h, whereas the underlying layer is infinite in both the
horizontal and vertical directions.
3. A uniform pressure q is applied on the surface over a circular area of
radius a.
4. The layers are in continuous contact and continuity conditions are
satisfied at the layer interfaces, as indicated by the same vertical stress,
shear stress, vertical displacement, and radial displacement.
75
Two-Layer Systems
• The exact case of a two-layer system is
– the full-depth asphalt pavement construction in which a thick layer of
hot-mix asphalt is placed directly on the subgrade.
• If a pavement is composed of three layers (e.g., surface course,
base course, and subgrade)
– the stresses and strains in the surface layer can be computed by
combining the base course and the subgrade into a single layer.
– Similarly, the stresses and strains in the subgrade can be computed by
combining the surface course and base course.
• Vertical stress: The stresses in a two-layer system depends on the
modulus ratio E1/E2, and the thickness-radius ratio h1/a.
• Figure 2-8a shows the effect of pavement layer on the distribution
of vertical stresses under the centre of a circular loaded area when
the thickness h1 of layer 1 is equal to the radius of contact area, or
h1/a = 1 and a Poisson ratio of 0.5 for all layers.
• Figure 2-8b also shows the effect of pavement thickness and
modulus ratio on the vertical stress, σc, at the pavement-subgrade
interface. 76
Cont...
Figure 2-8 (a) Vertical stress distribution in a two-layer system

(Burmister, 1958) and (b) effect of pavement thickness and modulus
ratio on pavement–subgrade interface vertical stresses (Haung, 1969)
77
Cont...
• Deflection: Surface and interface deflections have been
used as criteria of pavement design.
• The surface deflection, w0, under a uniformly circular
loaded area is given in terms of the deflection factor F2 as:
• The deflection factor, F2, can be obtained from Figure

2-9 for the corresponding E1/E2 and h1/a.
78
Cont...
79
Cont...
• If the load is applied on a rigid plate, then
• The interface deflection, w, between the two

layers is expressed in terms of the deflection
factor F as:
• The deflection factor, F, is different from F2 and

provided in Figure 2-10 as a function of E1/E2,
h1/a, and r/a, where r is the radial distance from
the centre of loaded area.
80
Cont...
81
82
Cont...
• Critical tensile strain:
– The tensile strains at the bottom of the asphalt
layer have been used as a design criterion to
prevent fatigue cracking.
• The critical tensile strain, e, at the bottom of
the first layer for a two-layer system can be
determined by
83
Cont...
Where, Fe is the strain factor that can be obtained in Figure 2-12 as a

function of E1/E2, and h1/a. The critical tensile strain under dual
wheels or dual-tandem wheels is obtained from the same equation, but
the strain factor needs to be corrected. 84
Three-Layer System
• With quick computational facilities available,
the analysis of three or more layers is no more
a difficult task. The three-layer system can be
conceived as follows:
– Top layer, representing all the bituminous layers
taken together,
– Second layer, representing the unbound base and
subbase courses, and
– Third layer, representing the subgrade.
85
Cont...
• At the axis of symmetry, tangential and radial stresses

are identical and the shear stress is equal to 0.
86
Cont...
• Jones has developed a series of tables for
determining the stresses in a three-layer system
for the following dimensionless parameters
87
Cont...
• Part of Jones‘s tables is presented here as Table 2.1,
from which four sets of stress factors,
– ZZ1, ZZ2, ZZ1-RR1, and ZZ2-RR2, can be obtained.
• The product of these factors and the contact pressure
gives the stresses as:
88
Cont...
• From the continuity of horizontal displacement
at the interfaces, σ‘r1 and σ‘r2 can be computed
from
• Once the stresses at the interfaces are

calculated, strains can be computed from the
equations of strains.
89
90
91
Stresses in Rigid Pavements
• Stresses in rigid pavements result from variety
of sources, of which
– the applied vehicle loads,
– changes in temperature of the slab,
– friction between the slab and the subgrade or base
course are the most important
• These factors tend to result in deformations of
the concrete slab,
– which cause tensile, compression, and flexural
stresses of varying magnitude.
92
Stresses Due to Vehicle Loading
• Three methods can generally be used to determine
the stresses and deflections in concrete pavements
due to vehicle loading:
– Westergaard‘s formulas
– Influence charts
– Finite element analysis
• Westergaard‘s formulas derived to examine three
critical conditions of loading:
– corner loading,
– interior loading, and
– edge loading far from any corner
93
Westergaard’s assumptions:
• The concrete slab acts as a homogenous isotropic,
elastic solid in equilibrium.
• The reactions of the subgrade are vertical only
and they are proportional to the deflections of the
slab.
• The thickness of the concrete slab is uniform.
• The load at the interior and the corner is
distributed uniformly over a circular area of
contact and the circumference of the contact area
at the corner is tangent to the edges of the slab.
• The edge loading is distributed uniformly over a
semi-circular area, the diameter of the semi-circle
being at the edge of the slab.
94
Corner Loading:
• when a circular load is applied near the corner of the
concrete slab, the stress, σc, and the deflection, Δc, at
the corner are given by
in which P is the load, l is the radius of relative stiffness defined as
, k is the modulus of the subgrade reaction, and a is the contact radius.

95
Corner Loading:
• The results obtained applying the finite
element method of analysis are:
where, c is the side length of the a square contact area, c = 1.772a
96
Cont...
• Modulus of subgrade reaction, k,
– is the constant that defines the subgrade in classical works of rigid
pavements as shown in Figure 2-14 and defined as:
p = kΔ
• where, p is the reactive pressure, and Δ is the deflection of the slab.
• The value of k is determined by means of the plate-loading tests.
97
Interior Loading:
• The formula developed by Westergaard for the
stress in the interior of a slab under a circular
loaded area of radius a is
in which l is the radius of relative stiffness and

b=a when a ≥ 1.724h
The deflection due to interior loading is
98
Edge Loading:
• The stresses and deflections due to edge
loading as formulated by Westergaard are:
For circular contact area
99
Edge Loading:
For semicircular contact area
When a load is applied over a set of dual tyres, the equations

can be used after converting the contact area of the dual tyres
into a radius, a, of equivalent circular contact area as:
100
where, Pd is the load on dual tyres, q is the contact pressure, Sd is the spacing of the tyres.
Stresses Due to Curling
• Changes in temperature through the slab cause differential
expansion or contraction between the top and bottom which
results
– curling of the slab upward or downward.
• The weight of the slab restrains the slab from curling upward or
downward.
• Consequently, stresses known as curling or warping stresses
develop in the slap.
• During the day when the temperature on the top of the slab is
greater than that of the bottom,
– the top tends to expand with respect to the neutral axis while the
bottom tends to contract.
• Because the weight of the slab restrains the downward curling,
– compressive stresses are induced at the top while tensile stresses
occur at the bottom.
• At night, when the temperature on the top of the slab is lower
than that at the bottom, the effect is the reverse. 101
• The strain in the x-direction in the infinite slab
curled upward as shown in Figure 2-15 due to
the stresses in the two directions can be
determined by the generalized Hook‘s law as:
Figure 2-15: Upward curling of elastic slab due to temperature

102
103
Cont...
• Let Δt represents the temperature differential between top
and bottom of the slab, and αt represents the coefficient of
thermal expansion of concrete.
• If the temperature at top is greater than at the bottom and
the slab is completely restrained and prevented from
moving,
– the strain developed at the top will be compressive and at the
bottom tensile as shown in Figure 2-16 assuming the distribution
of temperature is linear through out the slab depth.
104
Cont...
• The stress in x-direction due to bending in the
in x-direction is
• and the stress in the x-direction due to bending

in y-direction is
• The total stress in the x-direction is then,
105
Cont...
• For a finite slab with length Lx and Ly in the x-
and y-directions respectively, the total stress in
the x-direction can be expressed as:
• where Cx and Cy are correction factors for a finite

slab in the x- and y-directions respectively.
Similarly, the stress in the y-direction is
106
Cont...
• Based on Westergaard‘s analysis,
– Bradbury developed a simple chart shown as Figure 2-
17 here for determining the correction factors
depending on Lx/l and Ly/l in the respective
directions.
• In the above equations, σx and σy are the
maximum interior stresses at the centre of the
slab.
• The edge stress at the midspan of the slab can be
determined by
in which σ may be σx or σy depending on

whether C is Cx or Cy. 107
Cont...
108
Stresses Due to Friction
• The friction between a concrete slab and its foundation
causes
– tensile stresses in the concrete,
– in the steel reinforcements,
– if any, and in the tie bars.
• It is the criteria for
– The spacing of plain concrete contraction joints
– Steel reinforcements for longer spaced concrete pavements
– The number of tie bars required as shown in Figure 2-18
109
Cont...
• The volume change caused by the variation of temperature and
moisture:
– Induces tensile stresses and causes the concrete to crack
– Causes the joint to open and decreases the efficiency of load transfer
• Figure 2-19 shows a concrete pavement subject to a decrease in
temperature.
• Due to symmetry,
– the slab tends to move from both ends toward the centre, but the subgrade
prevents it from moving;
– thus, frictional stresses are developed between the slab and the subgrade.
• The amount of friction depends on the relative movement,
– being zero at the centre where no movement occurs and
– maximum at some distance from the centre where the movement is fully
mobilized, as shown in Figure 2-19b.
• The tensile stress in the concrete is greatest at the centre and can be
determined by
– equating the frictional force per unit width of slab to the tensile force as
shown in Figure 2-19
110
Cont...
111
Cont...
• in which σc is the stress in the concrete, γc is

the unit weight of the concrete, L is the length
of the slab, and f is the average coefficient of
friction between slab and subgrade, usually
taken as 1.5.
112
Cont...
• The spacing of joints in plain concrete pavements depends
more on
– the shrinkage characteristics of the concrete rather than on the
stress in the concrete.
• Longer joint spacing causes the joint to open wider and
decrease the efficiency of load transfer.
• The opening of a joint can be computed approximately by
113
Cont...
• The design of longitudinal and transverse
reinforcements and the tie bars across longitudinal
joints is determined based on
– the stresses due to friction assuming that all tensile
stresses are taken by the steel alone.
• Wire fabric or bar mats are used
– to increase the joint spacing
– to tie the cracked concrete together and
– maintain load transfers through aggregate interlock,
but not to increase the structural capacity of the slab.
in which As is the area of steel required per unit width and fs is the allowable stress in steel.
114
Cont...
• The steel is usually placed at the middepth of the slab and
discontinued at the joint.
• However, in actual practice the same amount of steel is used
throughout the length of the slab.
• Tie bars are placed along the longitudinal joint
– to tie the two slabs together so that
• the joint will be tightly closed and
• the load transfer across the joint can be ensured.
• The amount of steel required for tie bars can be determined
in the same way as the longitudinal or transverse
reinforcements as:
in which As is the area of steel required per unit length of slab

and L' is the distance from the longitudinal joint to the free edge
where no tie bars exist. 115
Cont...
• For two- or three-lane highways, L‘ is the lane
width.
– If tie bars are used in all three longitudinal joints of
a four-lane highway,
• L‘ is equal to the lane width for the two outer joints and
twice the lane width for the inner joint.
• The length of tie bars is governed by the
allowable bond stress.
116
Chapter Three
Traffic volume and loading

traffic loading and volume
• traffic loading is the most important factor in pavement
analysis and design.
• among the most important traffic loading factors to be
included in the structural design of pavement design
are:
– loading magnitude
– loading configuration
– number of repetations
• there are three different procedures for considering
vehicular and traffic effects in pavement design:
– fixed traffic
– fixed vehicle
– variable traffic and vehicle
Samuel K. 118
fixed traffic
• design thickness of pavements is determined
by sinfle wheel load magnitude independent of
load repetations.
– any wheel configurations are converted to
equivalent single wheel load (ESWL).
– design is performed based on the largest equivalent
single wheel load within all configurations.
– commonly used for airport and heavy-wheel load,
but light traffic volume highways.
– not commonly used today.
Samuel K. 119
fixed vehicle
• design thickness of pavement is determined by the
number of reptations of a standard single axle
load (80 KN).
– any axle configuration is converted to equivalent
single axle load (80 KN) by multiplying the number of
repetitions of each configuration by its equivalent axle
load factor (EALF).
– design is performed based on combined effects of all
types of axle loads in terms of equivalent single axle
loads (ESAL).
– because of the great variety of axle loads and traffic, it
is the most commonly used method for design today.
Samuel K. 120
variable traffic and vehicle
• design is performed based on individual effect
of each traffic and vehicle.
– most commonly used in the mechanistic design
approach
– no need to convert equivalent axle load factor
– it has been used by the portland cement association
with design charts.
Samuel K. 121
fixed traffic procedure
• in a fixed traffic procedure
– a single wheel load governs the thickness of the
pavement
– the number of load repetitions is not considered as a
variable.
• it involves converting multiple wheel loads to an
equivalent single wheel load (ESWL).
• ESWL defined as
– the load on a single tyre that will cause an equal
magnitude of stress, strain, defelection or distress at a
given location within a specific pavement system to
that resulting from multiple-wheel load.
Samuel K. 122
fixed traffic procedure
• the method most frequently used for
– airport pavements or
– for highway pavement with heavy wheel loads but
light traffic volume.
• usually the heaviest wheel load anticipated is used
for design purposes.
• criteria used for converting multiple wheel loads
to single wheel load include:
– equal vertical stress
– equal vertical defelection
– equal contact pressure
– equal tensile strain
Samuel K. 123
equal vertical stress ESWL
• working from a theoretical consideration of the
vertical stress in an elastic half-space.
• the method assumes that the ESWL varies with
the pavement thickness as shown in the fig.
below.
• the total load, Pd, is applied on dual tyres
assembled with center to center spacing Sd and
clear distance b/n tyre edges d (d=Sd – 2a).
• for pavement thickness, z, smaller than half the
clearance b/n dual tyres, i.e. d/2, no stress overlap
occurs and thus, the stress at these depths is due to
only one wheel of the dual.
Samuel K. 124
Samuel K. 125
• for thickness greater than twice the center to center
spacing of tyres, i.e. 2Sd, the subgrade stress due to the
two wheels overlap completely.
• by assuming a straight-line relationship b/n pavement
thickness and wheel load on logarithmic scales, the
ESWL for any intermediate thicknesses can be easily
determined.
• after the ESWL for dual wheel is found, the procedure
can be applied to tandem wheels.
• instead of plotting the pavement thickness and wheel
load, it is more convenient to compute the ESWL by
Samuel K. 126
• the vertical stress factor σz/q can also be used to
determine the theoretical ESWL.
• for the same vertical subgrade stress, σz,
• for the same contact radius, the contact pressure is

proportional to the wheel load or
• in which
– Ps = the single wheel load, which is the ESWL to be
determined
– Pd = the load on each of the dual
– qs and qd = the contact pressures under single and dual
wheels respectively
Samuel K. 127
equal deflection ESWL
• in this method
– the pavement system is considered as a homogeneous half space
– the vertical deflections at a deph equal to the thickness of the pavement
can be obtained from Boussinesq solutions.
• a single wheel load that has the same contact radius as one of the dual
wheels and results in a maximum deflection equal to that caused by
the dual wheels is the ESWL.
• using the vertical deflection factor F pressented in other chapter-stess,
the deflections due to single wheel load and dual wheel loading are
expressed as:
• inwhich the subscript s indicate single wheel and d dual wheels.

• the deflection factor Fd is obtained by superposition of the duals.
• to obtain the same deflection, Ws = Wd,
• for the same contact radius, the contact pressure is proportional to

wheel load:
Samuel K. 128
equal contact pressure ESWL
• the above analysis of ESWL are based on the assumption
that the single wheel has the same contact radius as each
of the dual wheels.
• another assumption, which has been frequantly made, is
that the single wheel has a different contact radius but the
same contact pressure as the dual wheels.
• the interface defelection for single and dual wheels with
the same contact pressure can be written as
• for equal defelection, Ws = Wd
• where,
• which results that

Samuel K. 129
fixed vehicle procedure
• in the fixed vehicle procedure
– the number of repetations of a standard vehicle or
– number standard single axle load governs the
thickness of the pavement.
• axle loads which are not equal to the standard
single axle load or consist of tandem or tridem
axles are converted to
– the standard single axle load by
• multiplying them with the corressponding EALF to
obtain the equivalent effect of the standard single axle
load.
Samuel K. 130
Equivalent axle load factor-EALF
• EALF is defined as
– the damage per pass to a pavement by the axle in question
relative to the damage per pass of a standard axle load, (80KN).
• The number of repetitions under each single or multiple
axle load must be multiplied by its EALF to obtain
– the equivalent effect based on an 80kN single axle load.
• A summation of the equivalent effects of all axle loads
during the design period results in
– an equivalent single axle load (ESAL).
• ESAL is the design parameter to be used in pavement
thickness design.
• Due to the great varieties of axle loads and traffic volumes
and their intractable effects on pavement performance,
– most of the design methods in use today are based on the fixed
vehicle procedure.
Samuel K. 131
Determination of EALF
• The EALF depends on:
– Type of pavement,
– Thickness or structural capacity
– Terminal condition at which the pavement is considered
failed,
– Failure criterion
– The condition of the deterioration of pavement at the time
of evaluation, etc.
• The most widely used method for determining EALF is
– based on empirical equations developed from the AASHO
road test (AASHTO 1972).
• EALF can also be determined theoretically
– based on the critical stresses and strains in the pavement
and the failure criteria.
Samuel K. 132
EALF for Flexible Pavement
• AASHTO Equivalency Factors
– The following regression equation is one of the most
widely used methods for determining EALF obtained
from the AASHTO Road Test:
Where, Wtx = the number of x-axle load application at the end of time t,
Wt18 = the number of 18kip (80KN) single axle load application to time t,
Lx = the load in kip on one single axle, one set of tandem axles, or one set of tridem
axles,
Samuel K. 133
Cont...
L2 is the axle code: = 1 for single axles, 2 for tandem axles, and 3
for tridem axles,
SN = structural number - a function of thickness, modulus of each
layer, and drainage condition of base and subbase.
pt= terminal serviceability – which indicates the pavement
conditions to be considered as failures,
β18= the value of βx when Lx = 18 and L2=1
and
• Practically, EALF is not very sensitive to pavement

thickness and SN equal to 5 may be used for most cases
and a pt value of 2 or 2.5 can be used.
Samuel K. 134
Theoretical Analysis
• In mechanistic analysis,
– fatigue cracking and permanent deformation of pavements
are employed as failure criteria.
• To limit the failure due to fatigue cracking,
– the allowable number of load repetition is expressed as:
Where,
• Nf = the allowable number of load repetitions to prevent fatigue
cracking,
• εt = the tensile strain at the bottom of the asphalt layer,
• E1 = modulus of the asphalt layer, and
• f1, f2, and f3 are constants to be determined from laboratory fatigue
tests (f1 modified to correlate with field observations).
Samuel K. 135
Cont...
• If Nfx and Nf80 are the allowable number of x-
kN and 80 kN axle load repetitions, then
Where, εtx and εt80 are the tensile strains at the

bottom of asphalt layer due to x kN and 80 kN
axle load repetitions respectively.
Samuel K. 136
Cont...
• The constant f2 was determined by Asphalt institute and
Shell
– the values are 3.291 and 5.671 respectively.
• A theoretical analysis of EALF was also conducted by
Deacon based on
– an assumed f2 value of 4, which is in the range determined by
Asphalt institute and Shell.
• For single axles,
– it is reasonable to assume that tensile strains due to the axles in
question and the standard single-axle are directly proportional to
axle loads.
• Using 4 as the value of f2,
– the EALF can be approximated by what is known as the fourth
power rule as:
Samuel K. 137
Cont...
• For tandem and tridem axles, a more general equation is
• Where, Ls is the load on standard axles which have the same

number of axles as Lx.
• If the EALF for one set of tandem or tridem axles is known,
that for other axles can be determined by the above equation.
• The other failure criterion is to control permanent
deformation
– by limiting the vertical compressive strain on top of the subgrade,
which can be expressed as:
Samuel K. 138
Cont...
• Suggested values of f5 are
– 4.477 by the Asphalt Institute,
– 4.0 by Shell, and
– 3.71 by the University of Nottingham.
• The use of 4 for f5 is also reasonable.
• Therefore, when Ls and Lx are of the same axle
configuration,
– the EALF based on fatigue cracking may not be much different
from that based on permanent deformation and similar equation
with the power of 4 can be applied.
• ERA pavement design manual, which is based on TRL Road
Note 31,
– relates the damaging effect of axle loads to the standard 80kN
axle using a power of 4.5 instead of 4.
• For multiple axle vehicles, i.e. tandem or tridem axles,
– each axle in the multiple-group is considered separately.
Samuel K. 139
EALFs for Rigid Pavements
• AASHTO Equivalency Factors
– The AASHTO equations for determining the EALF of rigid
pavements are:
• Where Wtx, Wt18, Lx, L2, pt, and β18, are as defined for
flexible pavements and D is the slab thickness in
inches.
• Value of pt = 2.5 and D = 9 inches can be used for
unknown cases.
Samuel K. 140
Cont...
• Theoretical Analysis
– Based on fatigue cracking the allowable number of
repetitions can be expressed as:
• in which, Nf is the allowable number of load repetitions for

fatigue cracking,
• σ is the flexural stress in slab,
• Sc is modulus of rupture of concrete, and f1 and f2 are
constants.
– In the design of zero maintenance jointed plain
concrete pavements, f1 = 16.61 and f2 = 17.61 are
recommended (Darter and Barenberg (1977).
Samuel K. 141
Cont...
• The Portland Cement Association (USA)
recommends the following fatigue equations
Samuel K. 142
Traffic Analysis
• The deterioration of paved roads is caused by traffic
results from both
– the magnitude of the individual wheel loads and
– the number of times these loads are applied.
• Hence, to design a paved highway,
– it is necessary to consider not only the traffic volume or
the total number of vehicles that will use the road but
also to predict the number of repetitions of each axle
load group (or wheel load group) during the design
period.
• To convert the traffic volumes into cumulative
equivalent standard axle loads (ESAL or CESAL
which is one design parameter in pavement design)
– equivalency factors are used.
Samuel K. 143
Cont...
• On the other hand, the mechanism of deterioration of
gravel roads differs from that of paved roads.
• Design of thickness of gravel roads is directly related to
– the number of vehicles using the road rather than the
number of equivalent standard axles as that for paved
roads.
• The traffic volume is therefore used in the design of
unpaved roads (gravel roads), as opposed to the paved
roads which require
– the conversion of traffic volumes into the appropriate
cumulative number of equivalent standard axles.
• In this section, method of determining the traffic
volume and CESAL with reference to Ethiopian Roads
Authority (ERA) Pavement Design Manual will be
discussed.
Samuel K. 144
Traffic Analysis
• The deterioration of paved roads by traffic results both from
– Magnitude of Load
– Repetition of Load
• Hence, to design a paved highway, it is necessary to consider
– the traffic volume or the total number of vehicles that will use the road
&
– to predict the number of repetitions of each axle load group (or wheel
load group) during the design period.
– The traffic volume is converted into cumulative equivalent standard
axle loads (ESAL or CESAL) using equivalency factors (EALF).
– CESAL is one design parameter in pavement design
• Gravel Roads - mechanism of deterioration of gravel roads
different from that of paved roads.
– Design of thickness of gravel roads is more related to the number of
vehicles using the road rather than the CESAL.
– The Traffic Volume in terms of initial AADT is used in the design of
unpaved roads (gravel roads),
• The following Parameters and Considerations/Steps are involved
in Traffic Analysis for pavement
Samuel K.design. 145
Design Period
• The length or duration of time during which the
pavement structure is expected to function
satisfactorily
– without the need for major intervention (rehabilitation
such as overlays or reconstruction) or
– the duration in time until the pavement structure
reaches its terminal condition (failure condition).
• Selecting appropriate design period depends on
– Functional importance of the road
– Traffic volume
– Location and terrain of the project
– Financial constraints
– Difficulty in forecasting traffic
Samuel K. 146
Cont...
• Longer Design Period –
– for important roads,
– high traffic volume,
– roads in difficult location and
– terrain where regular maintenance is costly and
– difficult due to access problems or lack of construction material
• Short Design Period – if there is problem in traffic forecasting,
financial constraints, etc.
ERA recommended: Design Period
Road classification Design period (year)
Trunck road 20
Link road 20
Main access road 15
Other road 10
Samuel K. 147
Determine Traffic Volume (ADT, AADT)
 Vehicle classification
• Small axle loads from private cars and other light
vehicles do not cause significant pavement damage.
• Damage caused by heavier vehicles (commercial
vehicles)
• Hence, important to distinguish
– the proportion of vehicles which cause pavement damage
(commercial vehicles) from total traffic
• To do this, we need to have a vehicle classification
system –
– To distinguish between commercial vehicles and small cars
– Distinguish between the different types of commercial
vehicles and group them according to their type, size
(loading), configuration, etc.
• ERA vehicle classification system
Samuel K. 148
Cont...
• Table : ERA Vehicle Classification
Vehicle Type of Vehicle Description
Code
1 Small car Passenger cars, minibusses (up to 24-passenger
seats), taxis, pick-ups, and land cruisers, land rovers
etc..
2 Bus Medium and large size buses above 24 passenger
seats
3 Medium Truck Small and medium size trucks including tankers up to
7 tons load
4 Heavy Truck Trucks above 7 tons load
5 Articulated Truck Trucks with trailer and semi-trailer and Tanker
Trailers
Samuel K. 149
Cont...
 Traffic Count
• Traffic Count necessary
– To assess the traffic-carrying capacity of different types of roads
– Examine the distribution of traffic between the available traffic
lanes
– In the preparation of maintenance schedules for in-service roads
– In the forecasting of expected traffic on a proposed new road
from traffic studies on the surrounding road system
• Traffic volume data determined from
– Historical traffic data available in relevant authorities (ERA
conducts regular 3 times a year (Feb., Jul., Nov.) traffic counts
on its major road network) and/or
– By conducting classified traffic counts:
• On the road to be designed – if the road is an existing road and the
project is Upgrading, Rehabilitation, Maintenance, reconstruction, etc.
• On other parallel routes and/or adjacent roads – for new roads
Samuel K. 150
Cont...
• Traffic volume data may vary daily, weekly,
seasonally.
• Hence to avoid error in traffic analysis and
capture the average yearly trend, minimum 7 days
count recommended
• ERA recommended procedure
– Conduct 7 days classified traffic count
• 5 days for 16 hrs
• Minimum 2 days for 24 hrs (one week day and one weekend)
• For long projects, there may be large difference in
traffic volume along the road and hence it is
necessary to make the traffic counts at several
locations.
Samuel K. 151
Cont...
ADT (Average Daily Traffic)
• ADT is determined from the traffic count data
as follows
– Adjust the 16hrs traffic count data into 24hr data
by multiplying with the average night adjustment
factor
• Night adjustment factor = (24hr traffic)/(16hr traffic) :-
obtained from the two days 24hr count data.
– (ADT)o = the current Average Daily Traffic=
Average of the 7 days 24 hr traffic volume data
Samuel K. 152
Cont...
 (AADT)o (Annual Average Daily Traffic = total
annual traffic in both directions divided by 365)
• In order to capture the average annual traffic flow trend,
– adjustment must be made for seasonal traffic variation,
• Hence traffic count as above must be made at different
representative seasons
– ERA conducts traffic counts on February, July and
November
• Make adjustment to (ADT)o –
– based on the season at which the current traffic count
belongs to and
– based on seasonal adjustment factors for the road (or similar
roads) derived from historic traffic data (ERA or other
regional/national sources)
• (AADT)o = (ADT)o adjusted for seasonal variation
Samuel K. 153
Traffic Forecast –
• determining traffic growth rate over the
design period
– Very uncertain process
– Requires making analysis and forecast of past and
future traffic growth trends, social and economic
development trends, etc
– In forecasting, Traffic categorized into the
following:
Samuel K. 154
Normal Traffic:
• Traffic that would pass along the existing road
or track even if no new or improved pavement
were provided.
• Forecasted by extrapolating data on traffic levels
and assume that growth will remain either
– Constant in absolute terms i.e. a fixed number of
vehicles per year, or
– Constant in relative terms i.e. a fixed percentage
increase.
• Growth rate can also be related linearly to
anticipated Gross Domestic Product (GDP).
Samuel K. 155
Diverted Traffic:
• Traffic that changes from another route (or
mode of transport) to the project road
• because of the improved pavement, but still travels
between the same origin and destination.
– Origin and destination surveys (O/D survey) should
preferably be carried out to provide data on the traffic
diversions likely to arise.
Samuel K. 156
Generated Traffic:
• Additional traffic which occurs in response to
– the provision or improvement of the road.
• It may arise either because
– a journey becomes more attractive by virtue of a cost
or time reduction or
– because of the increased development that is brought
about by the road investment.
• Generated traffic is also difficult to forecast
accurately and can be easily overestimated.
From thorough analysis of economic, social and
development trends,
determine overall growth rate r for all vehicle
categories or separate growth rate ri for each vehicle
category.
Samuel K. 157
Axle Load Survey
• Carried out together with the traffic count
• Portable vehicle(wheel) weighing devices or weigh in
motion (WIM) devices can be used for survey
• Each axle of the vehicle is weighed and EALF computed
for each axle
• Each axle of a tandem axle or tridem axle assembly is

considered as one repetition and EALF calculated for each
axle i.e. a tandem axle constitutes 2 load repetitions and a
tridem axle constitutes 3 load repetitions. (according to
ERA Pavement design manual)
• AASHTO pavement design procedure considers each
passage of a tandem or tridem axle assembly as one
repetition and EALF calculated
Samuel K.
correspondingly. 158
Truck factor
• Truck factor can be computed for each vehicle by
summing up the number of ESAL per vehicle
• Average truck factor can be computed for each vehicle
category (for example for Buses, Light Trucks, Medium
Trucks, etc.), by summing up the ESAL of all the
vehicles in each category and dividing by the number of
vehicles (of that category) weighed:
Where
– TFi = Truck factor for the ith vehicle category
– n = number of vehicles weighed (of the ith vehicle category)
during the axle load survey
– ESALj = number of equivalent standard axle loads for the jth
vehicle
Samuel K. 159
Design Traffic Loading
• The data and parameters obtained from the studies
discussed in the preceding sections can now be used to
estimate the design cumulative design traffic volume and
loading.
i. Adjustment for Lane and Directional Distribution of
Traffic – the AADT should be adjusted as follows
• Lane Distribution Factor (P):
– accounts for the proportion of commercial vehicles in the
design lane.
– For two lane highways,
• the lane in each direction is the design lane, so the lane distribution
factor is 100%.
– For multilane highways,
• the design lane is the heavily loaded lane (outside lane).
Samuel K. 160
Cont...
• Table : Lane Distribution Factors (ERA/AASHTO)
Number of Lanes in each direction Percent Traffic (ESAL) in design lane
1 100
2 80-100
3 60-80
4 50-75
• Directional Distribution Factor (D):

– factor that accounts for any directional variation in
• total traffic volume or loading pattern.
– It is usually 0.5 (50%).
– However, could be adjusted
• based on actual condition (if there is directional tendency to commercial
vehicle distribution (volume or loading);
• for example if the heavy vehicles in one direction are loaded and come
back empty in the other direction).
Samuel K. 161
Cont...
ii. Calculating (AADT)1
• AADT1 = Annual Average Daily Traffic (both
directions) at year of Road Opening (year at
which construction works are completed and the
whole road is made open for traffic).
• If time between traffic count year (design time)
and estimated year of road opening = x, then
AADT1 = AADT0 (1+r)x
• Note that AADT1 is used as the Design Traffic
Parameter for Gravel Roads (ERA Pavement
Design Manual)
Samuel K. 162
Cont...
iii. Cumulative Traffic Volume (T) – can be
computed for all traffic (T) or for each vehicle
class (Ti)
Ti = 365 (P) (D) AADT1i [(1+ri)N – 1] / ( ri )
• Ti = cumulative volume of traffic for the ith commercial
vehicle class in the design lane
• over the design period (adjusted for lane distribution and
direction).
• ri = annual growth rate for the ith commercial vehicle class
• P = Lane distribution factor; D = Directional distribution
factor
• N = Design Period in years
Samuel K. 163
Cont...
iv. Design Traffic (Cumulative Equivalent
Standard Axle Load - CESAL) –
– is computed by multiplying the total traffic volume
for each vehicle category (Ti) by its corresponding
truck factor (TFi)
Design Traffic Load = CESAL=Σ(Ti x TFi)
Samuel K. 164
Cont...
v. The CESAL is used to determine
– the traffic class to be employed for pavement design.
Table: ERA Traffic Classes for Flexible Pavement Design
Traffic Classes Range (106 ESAs)
T1 < 0.3
T2 0.3 – 0.7
T3 0.7 – 1.5
T4 1.5 – 3
T5 3–6
T6 6 – 10
T7 10 – 17
T8 17 - 30
Samuel K. 165
Chapter Four
Sub-grade soils
Introduction
• The subgrade constitutes
– the foundation material for the pavement structure as highway
pavements ultimately rest on the native soil (subgrade).
• Hence the performance of the pavement is affected by
– the characteristics of the subgrade.
• one of the major functions of a highway pavement is
– to reduce the stresses transmitted to the subgrade to a level
which the soil will accept without significant deformation.
• Soil is also used as construction materials for highway
construction
– fill, capping layer, subbase, etc.
• Hence it is important to study the characteristics and
engineering properties of soils for highway engineers.
Samuel K. 167
Introduction
• It is important to note here that the basic characteristics
of soils and the engineering properties of the soils
depend among others
– on the geological processes and mechanics of soil formation
i.e. origin & formation of soils - parent material;
– mode of deposition/transport (residual or transported soil);
– mechanism of transport – ice (glacial soils), water (alluvial
soils), wind (aeoline soils), gravity (colluvial soils));
– climate; topography; time/age; vegetation, etc.
• Hence a comprehensive study and analysis of soils will
also involve a study of soils
– on a parent material basis, which relies on the geological
concepts of rock and soil formation as well as a good grasp
of the basic principles of soil mechanics.
Samuel K. 168
Overview of Soil Survey and Site Investigation
• In the evaluation of an area for construction of road including
structures, or as a source of construction materials,
– the soil condition must be investigated before any detailed designs
are made.
• A soil survey forms
– an essential part of the preliminary engineering survey for a road and
– its purpose is to furnish the design engineer with all required
information regarding
• the soil and ground water condition so that a rational and economical design
can be obtained.
• The information most often required from soil investigation
include
– depth,
– thickness,
– properties of each soil layer (the characteristics of the soil profile),
– location of groundwater table,
– availability of suitable local construction materials, etc.
Samuel K. 169
cont...
• Information obtained from the soil survey enables to make
decision on one or more of the following design requirements:
– stability of the proposed location, both horizontally and vertically,
and thereby selection of roadway alignment;
– suitability of local materials for use as a construction materials for
embankments and pavement layers;
– subsurface and surface drainage requirements;
– need for treatment of subgrade and type of treatment required;
– thickness of pavement required; and
– design of foundations for bridges and other structures.
• In general, a soil investigation work involves the following
steps:
– Desk study & Site Reconnaissance - (Feasibility Stage
Investigations)
– Ground investigation (field sampling and testing) and Laboratory
testing - (Detail Investigations)
– Reporting - preparation of factual and interpretative report/reports
pertinent for engineering design. 170
Samuel K.
Desk study:
• refers to work taken up prior to commencing the
ground investigation.
• It is the first step in any soil survey work and
involves the collection and review of existing
information on
– the general soil characteristics of the area in which the
highway is to be located.
• Relevant data to be collected and studied/analyzed
include:
– general features such as geological formations, climate
(temperature and rainfall) and topography of the area.
– Specific data from previous soil investigation works for
road design/construction purposes or other purposes in
the project area.
Samuel K. 171
cont....
• During the desk study, one can obtain and investigate
relevant information/data from such sources as given
below:
– General data –
• geological and pedological (soil) maps,
• land use maps,
• topographical maps and aerial photos, and
• climate maps, meteorological data (temperature and rainfall data);
– Specific data –
• test pit/borehole logs and laboratory test results of previous
investigations on adjacent roads (or roads in the general project
area),
• construction histories of such roads,
• other subsurface investigation data (example – mineralogical
explorations, water well drilling logs, or investigations for any
other civil engineering structures in the general project area)
Samuel K. 172
cont...
• Data and information obtained from the desk
study:
– will provide preliminary information about
• the depth and characteristics of the soil profile, as the soil
profile in a particular local generally depend on
– the climate,
– topography,
– origin (parent material) and
– mode of formation (geological process), time and vegetation cover.
– will enable delineation of approximate limits of
• geological formations in the project area (a map showing
such limits can also be prepared).
– can also furnish indications of potential material
sources.
Samuel K. 173
Site Reconnaissance -
• The data inferred above is normally
supplemented with field reconnaissance survey
– to assess or visually inspect actual soil condition
and any problems to be anticipated (may involve
collection of few samples and field/laboratory
testing).
• This is a walk over survey of the site involving
visual inspection of alignment soils and other
pertinent geotechnical, topographic and
hydrologic/hydraulic features.
Samuel K. 174
cont...
• It aims at:
– verifying the assumptions made regarding the limits of
geological formations,
– confirm the indications relative to sources of materials
identified during the office review,
– visual inspection of the general and particular features
including
• vegetation cover,
• identifying in general terms potential problem areas like
– embankments on compressible soils,
– expansive soils,
– deep or potentially unstable cuts,
– major rock excavations,
– slope instability,
– marshy ground,
– springs or seepage,
– ponds or streams, etc,
Samuel K. 175
Detail Investigation (Ground investigation and laboratory testing)
• Detail soil survey is carried out on the selected route

– either in one stage or in stages, example: Preliminary soil survey and Final
soil survey.
• This will involve
– obtaining and investigating enough soil samples along the selected route,
– carrying out field tests as required,
– transporting the sampled soils to a central laboratory and laboratory testing.
• These will involve visual assessment, soil sampling and field and
laboratory testing.
– Visual assessment on type and extent of soil along the route (soil extension
survey)
– Identification of stretches with rock outcrops, potentially unsuitable
(problematic) soil formations (expansive soils, swamps, etc.)
– assessment of drainage patterns and potential drainage problems
– assessment of potential slope stability hazards
– assessment of foundation conditions for bridges or other structures,
embankments, etc.
– identifying and investigation of potential suitable sources of materials for use
in the road construction works – fill, capping, subbase, etc.
Samuel K. 176
cont...
• The results of the investigation will be used to
– characterize the different soil types along the road
and map their boundaries (homogeneous sections),
– derive pertinent design parameters required –
• deformation parameters (E),
• shear strength parameters (C, ),
• consolidation parameters (Cc, Cv, mv),
• empirical design parameters (CBR), etc.
• The investigation has to be carried out
– at sufficient intervals and
– to depths below which ground conditions cease to
affect the works.
Samuel K. 177
cont...
• The field investigation and sampling are carried out
by the following methods:
– Test pits or trenches:
• suitable for shallow depths only;
• enables disturbed/undisturbed soil sampling, and
• direct inspection and register of soil profiles.
– Hand augers:
• suitable for shallow depths only,
• enable inspection of disturbed and mixed samples of soil
(mostly for visual inspection purposes).
– Boring test holes and sampling with drill rigs:
• using borehole rigs for advancing boreholes and obtaining
disturbed/ undisturbed samples,
• allow greater depths of investigation.
– Geophysical methods (Seismic or electrical)
Samuel K. 178
cont....
• The routine tests normally carried out on subgrade
soils include:
– Soil classification and index tests:
• gradation,
• Atterberg limits,
• moisture content;
– Compaction and Strength tests:
• compaction test (standard or modified),
• CBR test
– Field tests (on existing road):
• field density and moisture content,
• DCP test
– Other tests (not so common):
• Modulus of deformation (Resilient Modulus – E),
• plate bearing test (field test)
Samuel K. 179
Reporting:
• The process and findings of the investigations/survey works are
presented in one report or series of reports.
• The procedures followed,
– the detail plan/program of investigation and actual investigations carried
out,
– the types and procedures (standards) followed in conducting
field/laboratory tests,
– the actual test results obtained,
– analysis and interpretation of field observations/assessments & test data
and final recommendations for design (design parameters and
recommendations) are compiled and presented in Factual reports and
interpretative (Engineering) Reports (Soils and Materials Report,
Engineering Report, Geotechnical Report, etc.).
• The reporting shall include among others
– a summery of the test program,
– a general description of the soil conditions,
– a detailed analysis of each type of soil found, and
– recommendations for design (as required).
–
Samuel K. 180
A copy of the test-hole logs and the soil profile is also included.
Depth of Investigation
• Depth of investigation
– should normally extend to the level below which ground conditions
cease to affect the works.
• In this respect, it will be important to predefine
– the required depth of investigation (or design depth) and
– carry out investigations accordingly at least to this depth.
• The design depth is defined as
– the depth from the finished road level to the depth that the load
bearing strength of the soil no longer has an effect on the pavement‘s
performance in relation to traffic loading.
• Properties of soil below the design depth may indirectly affect
pavement performance, but are generally unrelated to traffic
loading.
• A preliminary vertical alignment may be required at the time of
the soil survey
– in order to ensure that soil samples are actually taken at levels that fall
within the design depth of the road.
Samuel K. 181
cont...
• The following table shows the depth of test pits
for soil sampling as given in Tanzania Pavement
Design Manual, 1999.
• Table : Design Depth (Tanzania Pavement Design
Manual, 1999)
Road Type Design Depth (m)
General requirements Heavy traffic class roads
Paved trunk roads 0.8 1.2

Other roads 0.6 1.0
* Heavy traffic class roads are roads with proportion of traffic loading
as a result of axles loaded to above 13 tonnes is > (greater than) 50%
of the total design traffic loading (and CESAL > 0.2 x 106)
Samuel K. 182
cont...
• The Ethiopian Roads Authority Site
Investigation manual (ERA, 2002) stipulates that
for the purpose of taking representative samples,
– pits shall be dug mostly in anticipated cut areas
(since these cuts will expose the sub-grade support of
the future pavement and provide embankment
materials),
– if possible down to at least 30 cm below the expected
sub-grade level.
– Further, in the case of a new alignment,
• the depth of any pit should in no case be less than 1.5m
unless rock or other material impossible to excavate by
hand is encountered.
Samuel K. 183
cont...
• The engineer in charge of planning the investigations
should make every effort to locate the test pits (along the
alignment as well as within the lateral extent of the
anticipated excavation)
 in order to optimize the representatively of the material
excavated from the test pit.
• When required, investigations should be extended
to below design depth
– to detect problems that need special considerations
such as presence of
• problem soils,
• unfavorable subgrade conditions, and
• features associated with slope and embankments stability.
– If necessary, sub surface investigation is carried out
using field or in-place testing techniques.
Samuel K. 184
Sampling and Frequency
• The frequency and spacing of the test pits should
depend on
– sound engineering judgment
– on the field conditions
– be guided by a prior review of all possible documents
as well as a preliminary visual survey of the entire
road alignment and
– the results of the investigations during preliminary
design.
• Although it is neither possible nor desirable to
specify rigid rules for spacing,
– it is necessary to set adequate average guidelines for
homogeneity of design and reliability.
Samuel K. 185
cont...
• Tables below, give recommended sampling frequency
and the corresponding tests.
• This frequencies may be altered depending on
– the variations in soil types along the alignment.
• The identification tests
– include Atterberg limits and gradation tests.
Table : Sampling Frequency (ERA, Site Investigation Manual, 2001)
Investigation Stage Test Description Frequency of Cumulative

Sampling
Feasibility/Preliminary Identification 1 km
CBR 2 – 5 km
Final Identification 0.5 km
CBR 1 km
Samuel K. 186
cont...
Table : Sampling Frequency (Tanzanian Pavement Design Manual, 1999)
Type of Road Test Description Frequency of Cumulative

Sampling
Paved Trunk Road Identification 0.25km
CBR 0.5km
Other Paved Roads Identification 0.5km
CBR 1km
Gravel Roads Identification 0.5km
CBR 2km
Samuel K. 187
Laboratory Tests
• The subgrade (the foundation material) must possess
sufficient strength and stiffness to provide
– adequate support for the pavement structure and associated traffic
load, without shear failure or excessive deformation.
• Owing to the fact that the nature of stresses imposed on
pavements (transient loads of short duration) and their
relatively lower magnitude,
– shear strength of the soil is not normally anticipated to be a critical
factor in pavement thickness design.
• The elastic properties (E, ) of the subgrade
– are the major foundation parameters needed (assuming normal
traffic loading regimes).
• However, these are relatively complex properties to measure
and bearing in mind
– the variability of soils within relatively short distances,
– it may not be economically feasible to use them directly to
evaluate subgrade properties.
Samuel K. 188
cont...
• Hence, if pavements are to be designed empirically based on past
performance/experience records or fundamentally using the elastic theory
(analytical methods),
– relatively simple test procedures are required, the result of which can be
related by experiment to the structural properties.
• The shear strength parameters – cohesion, c & angle of internal friction υ-
– are normally required in the stability analysis of road embankments and deep
cuts.
• Also consolidation parameters Cv, mv, Cc will be
– required for settlement analysis of high embankment on soft clayey soils.
• General desirable properties of a subgrade soil (or any foundation
material) include:
– Stability – good strength and stiffness under adverse loading and climatic
(moisture) conditions,
– incompressibility,
– Good drainage properties,
– ease of compaction,
– volume stability (no/minimum shrink / swell characteristics with change in
moisture content).
Samuel K. 189
Gradation Test (AASHTO T88)
• The type of soil can be described in terms of the particle sizes
present.
• The particles in soil may range
– from granular fractions (boulders and cobbles - >75mm in size, Gravels
75 – 4.75mm in size; Sand – 4.75 – 0.075mm) to
– fine fractions which are too small to measure dierectly (silt – 0.075mm
– 0.002mm & clay - < 0.002mm and colloids - <0.001mm in size).
• Gradation test is conducted in order to obtain
– the maximum size and the grain size distribution of particles in the soil. .
• it is expressed in terms of particles by weight finer than specified
sizes.
• Gradation test (sieve Analysis) is carried out
– for soil particles larger than 0.075(0.063)mm.
• Sedimentation tests (Hydrometer test) is conducted
– for smaller (finer) particles.
• Sedimentation tests are relatively complex (for site laboratories) and
the properties of the silt and clay fraction for road projects are
generally assessed by plasticity tests.
Samuel K. 190
cont...
• Depending on the sample at hand, different types of gradation
tests may be carried out:
– Dry Sieve Analysis
• for pure coarse or granular materials with out fines,
– Wet sieve analysis with/without sedimentation test on the fine
fractions
• for mixture of coarse and fine grained soils and
– Hydrometer analysis
• for fine grained soils.
• The gradation of soils influences many properties of the soil
such as
– density/compactibility,
– strength/stability/deformability,
– voids content,
– permeability, etc.
• Also particle shape, mineral composition and degree of
compaction have an effect on the above properties.
Samuel K. 191
Atterberg Limits
• Soils containing clay exhibit a property called plasticity.
• Plasticity
– is the ability of a material to be moulded (irreversibly deformed)
with out fracturing.
• This behavior is unique to clays and arises due to
– the electrochemical behavior of clay minerals.
• The stiffness or consistency of fine grained soils depends on
– their moisture content, and
– varies with variations in the amount of moisture present.
• Depending on its moisture content, a soil can exist in one of
the following states:
– viscous liquid,
– plastic solid,
– semi solid and
– solid.
Samuel K. 192
cont...
• Atterberg in 1911 proposed a series of tests,
mostly empirical, for the determination of
– the consistency properties/states of fine grained soils.
• Atterberg limits define the moisture contents at
which the soil changes from one state to another.
These include
– the liquid limit (LL),
– the plastic limit (PL),
– shrinkage limit (SL).
• They are determined by tests carried out on
– the fine soil fraction passing the 425μm (No. 40) sieve.
Samuel K. 193
cont...
• Liquid limit (AASHTO T89) may be defined as
– the minimum water content at which the soil will start to flow
under the application of a standard shearing force (dynamic
loading).
• Plastic limit (AASHTO T90) –
– measure of toughness – the moisture content at which the soil
begins to fracture when rolled into a 3mm diameter thread.
• Shrinkage limit 9AASHTO T92)
– is the maximum moisture content after which further reduction in
water content does not cause reduction in volume.
– It is the lowest water content at which a clayey soil can occur in
a saturated state.
• Plasticity index (PI=LL-PL)
– is the numerical difference between the liquid and plastic limits.
– Thus, it indicates the range of moisture content over which the
soil remains deformable (in plastic state).
Samuel K. 194
cont...
Consistency limits of soils
• Another index that is used to reflect the properties of

the natural soil is the liquid index (LI) and is defined as:
where, wn is the natural moisture content of the soil.
Samuel K. 195
cont...
• Consistency limits and the plasticity index are used in
– the identification and classification of soils.
• Generally, soils having high values of liquid limit and plasticity index
– are poor as subgrades/engineering materials.
• Both the liquid limit and plastic limit depend on
– the type and amount of clay in the soils.
• In soils having same values of liquid limit, but with different values of
plasticity index; it is generally found that
– rate of volume change and dry strength increases and permeability decreases
with increase in plasticity index.
• On the other hand, in soils having same values of plasticity index but
different values of liquid limit,
– it is seen that compressibility and permeability increase, and dry strength
decreases with increase in liquid limit.
• Soils that cannot be rolled to a thread at any water content are termed as
Non-Plastic (NP).
Samuel K. 196
Compaction Test
• Compaction
– is the process by which air is excluded from a soil mass to
bring the particles closer together and
– thus increase its density (dry density).
• The state of compaction of a soil is appropriately
expressed in terms of
– the dry density (d) which is a measure of the state of
packing of soil particles.
• In-situ soils (foundation soils) in highway
construction or other structures, and imported soils
used in embankments, subbases, bases in roads or
other types of construction projects are placed in
layers and compacted to a higher density.
Samuel K. 197
cont...
• Increasing the density of a soil
– improves its strength,
– lowers its permeability, and
– reduces deformability (settlement, volume change).
• Compaction is achieved in the field by using
– hand-operated tampers,
– sheep-foot rollers,
– rubber-tired rollers, or
– other types of roller.
• The maximum density achieved because of compaction with
rollers, and other types of compaction equipment is measured in
the field and compared with the maximum dry density of the
soil previously determined in laboratory compaction tests.
• This is the most common method of quality control at
construction sites.
Samuel K. 198
cont...
• If a loose soil is compacted by the application of a fixed
amount of energy,
– then the dry density achieved is related to the moisture
content.
• The moisture-density relationship of soils was first
studied by
– Procter, and the test is sometimes known as Procter test.
• The dry density that can be obtained by compaction
varies with
– the moisture content,
– type of soil being compacted, and
– the compaction effort
• The dry density of a soil will vary with its water content.
Samuel K. 199
cont...
• At low moisture content,
– the soil is dry and stiff and
– friction between adjacent particles prevents/limits
relative movement between particles to assume denser
configuration.
• As water is added,
– larger films of water form around the particles,
• causing lubrication effects and facilitating relative movements
between particles to assume denser configuration (high density
of soil mass).
• Thus, the density increase and the air content
decreases as the moisture increases.
• At some moisture content,
– the soil attains the maximum practical degree of
saturation (S<100%).
Samuel K. 200
cont...
• The degree of saturation, S, cannot be increase further
due to
– entrapped air in the void spaces and around the particles.
• Hence any further addition of water will result in
– the voids being overfilled with water causing separation of
particles and
– reduction of density
• the additional water taking the space of the solid particles
• The moisture content at which maximum dry density is
obtained is known as optimum moisture content (OMC).
• At moisture content higher than the OMC,
– the air and water in the soil mass tend to keep particles apart
and prevent compaction.
• The dry density at higher moisture contents than OMC,
thus, decreases and the total voids increase.
Samuel K. 201
cont...
Typical compaction curve

Samuel K. 202
cont...
• The zero-air void (ZAV) curve shown in the figure
above
– is the theoretical moisture-density curve for a saturated
soil (where volume of voids filled with air is 0% (zero-
air voids) or degree of saturation S=100%).
• This curve is not attained in the field,
– since zero-air void cannot be attained
• Points on the ZAV curve may be calculated by
where, γd= Dry density of soil,

γw= unit weight (density) of water,
Gs=specific gravity of soil particles, and
w= moisture content of the soil.
Samuel K. 203
cont...
• The distance between the zero-air void (ZAV)
curve and the test moisture-density curve
– is an indication of the amount of air voids
remaining in the soil at different moisture contents.
• The farther away a point on the moisture-
density curve is from the ZAV curve,
– the more air voids remaining in the soil and
– the higher is the like hood of expansion or swelling
if the soil is exposed to water.
• Swelling of such soil can be reduced
– by compacting at higher moisture content.
Samuel K. 204
cont...
• Soil type and gradation heavily affect
– the density that can be achieved by compaction.
• Granular, well-graded soils generally have
– fairly high maximum densities at lower optimum moisture
contents,
• while clayey soils have
– lower densities and higher OMC.
• The edge-to-side bonds between clay particles
– resist compactive efforts preventing attainment of denser
structure.
• With granular soils,
– the more well-graded soils have spaces between large particles
that are filled with smaller particles when compacted, leading
to a higher density than with uniform or poorly graded soils.
Samuel K. 205
cont...
• Typical moisture-density curves for different
types of soils are shown in Figure below.
• Note that a line joining the peak points of the
density curves would be approximately
parallel to the ZAV curve.
– This is due to the fact that most soils at their
maximum density still contain about 2-3% air.
• The OMC and the maximum dry density that
can be attained on a given soil also depend on
– the compactive effort used.
Samuel K. 206
Compaction curves
for different types
soils
207
Samuel K.
cont...
• Compactive effort
– is a measure of the mechanical energy imposed on the
soil mass during compaction (energy per unit volume).
• For a given soil, increase in compactive effort
generally results in
– an increase in dry density and
– a decrease in optimum moisture content.
Effect of compactive effort on dry density
Samuel K. 208
Laboratory compaction test:
• is a standard method of compaction using a standard amount
of compactive effort
– to produce a soil density against which site density values can be
compared.
• The original test involved
– compacting the soil in three approximately equal layers in a
standard mould,
– Using a 2.5kg hammer falling through a height of 305mm
(standard compaction test).
• However, with the advent of heavier compaction equipment,
– greater densities were now achievable in the field.
• A modified version of the test was developed to allow
– the application of greater compactive effort (and achieve greater
density) – i.e.
• compacting the soil of the same height in five approximately equal layers
using a 4.5kg hammer falling through 457mm height (modified or heavy
compaction test). Samuel K. 209
cont...
• The soil sample is first air dried and sieved
– usually through the 4.75-mm (No.4) sieve or 19mm
sieve,
• mixed thoroughly with water and then compacted
in layers.
• The mass of the compacted sample is measured
(W), and a small sample taken to measure the
corresponding moisture content (w).
• More water is then added to the soil, and the
procedure repeated until the dry density obtained
decreases.
• Comparison of standard and modified compaction
tests is given in the following table;
Samuel K. 210
Table : Standard and Modified compaction tests
Items Standard Compaction Test Modified (Heavy)

(AASHTO T99) Compaction Test
(AASHTO T180)
Diameter of mould (mm) 101.6/152.4 101.6/152.4
Height of sample (mm) 117 117
Number of lifts (layers) 3 5
Number of blows per lift 25/56 25/56
Weight of hammer 2.5kg 4.5kg
Diameter of end face of 51 51
hammer (mm)
Free fall height (mm) 305 457
Net volume of mould (cm3) 944/2124 944/2124
Note: larger diameter mould (152.5mm) is used for gravelly

soils (soils with a significant amount of gravel).
Samuel K. 211
cont...
• The bulk density of the soil for each trial is
obtained
– by dividing the weight of the soil by the total
volume (γb=W/V).
• The dry density of the soil is determined by:
Where γb = bilk unit weight, w = moisture

content
Samuel K. 212
Field density test:
• Since the compatibility of soils varies considerably,
– the construction requirements for roads are usually
specified as a percentage of the maximum dry density
found in a laboratory compaction test for each soil type
encountered on the project.
• For example, a project specification might require
that the soil be compacted to 95% of the maximum
dry density found by the standard compaction test.
• Quality control of compaction on a construction
project involves
– conducting standard field compaction tests on each soil
type and constructed layer after compaction,
– comparing the result with the laboratory maximum dry
density value for the soil, to ascertain if the specifications
have been met.
Samuel K. 213
cont...
• If the maximum dry density from the test was
2000 kg/m3 at an optimum water content of
11%,
– the required field density would be 95% of 2000, or
1900 kg/m3.
• The moisture content of the soil should be as
close as possible to 11%,
– which reduces the required compactive effort
• for example, number of passes of the roller
• Field density tests are made using either
destructive or nondestructive methods.
Samuel K. 214
cont...
• Destructive methods:
– the simplest is the core-cutter method.
– This method can be used only on
• cohesive soils free from coarse-grained material.
– It involves driving a hollow metal cylinder, which
has a cutting edge,
• into the soil to remove an undistributed sample on which
dry density and moisture content determinations can be
made.
Samuel K. 215
cont...
• The other commonly used methods are
– the Sand Replacement method and
– Rubber Balloon method.
• In these methods, a sample of compacted material is dug
out of a test hole in the soil layer whose density is being
checked.
• The bulk mass of the soil removed is immediately
weighed (making sure that it does not loose any
moisture) and the sample transported to the laboratory
for measuring the moisture content or the oven dried
mass.
• The volume originally occupied by the sample (the test
hole) is then measured.
• The two methods differ in the method used to measure
the volume of the test hole.
Samuel K. 216
cont...
• In the rubber balloon method, the volume is determined
by forcing a liquid-filled balloon into the test hole.
• The rubber membrane allows the fluid to fill all the
cavities in the test hole.
• The volume of fluid required to do this is read on a
scale on the apparatus.
• In the sand replacement method (using a sand cone
apparatus), the weight of a standard dry sand (Ws), of
known unit weight, γs, required to fill the test hole is
measured.
• The volume of the test hole is then determined from the
known unit weight of the sand as follows:
Samuel K. 217
cont...
• The quick and nondestructive method of measuring the
in situ density and moisture content of the compacted
soil is the nuclear method.
• Using the nuclear equipment,
– the density is obtained by measuring the scatter of gamma
radiation by the soil particles
• since the amount of rays is proportional to the bulk density of the
soil.
• The moisture content is also obtained by measuring the
scatter of
– neutrons emitted in the soil due to the presence of hydrogen
atoms.
• The detector in the nuclear equipment measures
– the amount of rays and the neutrons that passes through the
soil, and
– thus the density and the moisture content can be calculated.
Samuel K. 218
California Bearing Ratio (CBR) Test
• The CBR test was originally developed by
– the California Division of Highways in the 1930s,
• as part of a study of pavement failures.
• Its purpose was to provide an assessment of
– the relative stability of fine crushed rock base materials.
• The test has been modified since then and extended to
subgrades.
• It is now widely used for evaluating
– the stability or strength of subgrade soil and other flexible
pavement materials for pavement design throughout the world.
• The CBR values obtained from either laboratory tests or
in-situ (field tests) have been correlated with
– flexible pavement thickness requirements for highways and
airfields.
Samuel K. 219
cont...
• In this test, a plunger is made to penetrate the soil,
– which is compacted to the prevalent dry density and moisture
content anticipated in the field (or to MDD and OMC as
specified) in a standard mould (CBR mould) at a specified rate
of penetration.
• The resulting load-penetration curve is compared with that
obtained for
– a standard crushed rock material, which is considered an
excellent base course material.
• A load is applied
– by cylindrical metal plunger of 56 mm diameter
– to penetrate the specimen at a rate of one mm per minute and
– readings of the applied load are taken at intervals of penetration
of
• 0.25 mm up to a total penetration of not more than 7.5 mm. 220
Samuel K.
cont...
• Depending upon the prevailing climatic conditions of
the site,
– CBR specimens are immersed in water for four days before
the test
• to obtain a saturation condition similar to what may occur in the
field.
• During this period,
– the sample is loaded with a surcharge load
• that simulates the estimates weight of pavement layers over the
material tested.
• Any swell due to soaking is also measured.
• Experience suggests that the CBR test is of poor
reproducibility, particularly with granular soils.
Samuel K. 221
cont...
• The CBR value is reported as a percentage of a standard value
which is intended to represent
– the value that would be obtained with compacted crushed stone.
• Typical test results are illustrated in Figure below.
• It sometimes happens that
– the plunger is still not perfectly bedded in the specimen and,
– as a result of this and other factors, a load-penetration curve with a
shape similar to that of curve for Test 2 in Figure below may be
obtained instead of the more normal shaped curve illustrated by the
curve for Test 1.
– When this happens
• the curve must be corrected by drawing a tangent at the point of greatest slope
and
• then transposing the axis of load so that
– zero penetration is taken as the point where the tangent cuts the axis of penetration.
• The corrected load-penetration curve
– is the tangent from the new origin to the point of tangency and
– then the curve itself as illustrated in Figure below.
Samuel K. 222
Figure: Typical results from the CBR test Samuel K. 223
cont...
• The CBR is then determined by reading off
from the curve
– the load that causes a penetration of 2.54 mm and
– dividing this value by the standard load (6.9 Mpa)
required to produce
• The same penetration in the standard crushed stone as
Samuel K. 224
cont...
• Similarly, the CBR at 5.08 mm penetration is
obtained by
– dividing the load causing a penetration of 5.08 mm with
– the load of 10.34 MPa required to produce the same
penetration in standard crushed stone.
• The two values are then compared
– if the 2.54 mm value is greater than the 5.08 value,
• it is the CBR of the material and used for design purposes.
– If on the other hand the 5.08 mm value is larger,
• the test is entirely repeated on a fresh specimen.
– If the new percentage valve at 5.08 mm penetration is still
greater,
• then this is taken as the CBR value.
Samuel K. 225
Design Sub-grade CBR:
• The strength of subgrade soils is dependent on
– the type of soil,
– density, and
– moisture content.
• Hence to determine the subgrade strength, which would be
used for design of the road pavement structure,
– it is apparent to ascertain the density-moisture content-strength
relationship specific to the subgrade soils encountered along the
project road.
• The design CBR of the subgrade soil, therefore,
– should be evaluated at the moisture content and density
representative to the subgrade condition during the service time
of the pavement structure.
• For wet or moderate climatic zones and where the ground
water influences the subgrade moisture content,
– the CBR test is carried out after 4 days of soaking.
Samuel K. 226
cont...
• A road section for which a pavement design is undertaken should
be
– subdivided into subgrade areas where
• the subgrade CBR can be reasonably expected to be uniform, i.e. Without
significant variations.
• Identification of sections deemed to have homogenous subgrade
conditions is carried out by
– desk studies on the basis of
• geology,
• pedology,
• drainage conditions and topography, and
• considering soil categories which have fairly consistent geotechnical
characteristics (e.g. grading, plasticity, CBR).
• Usually, the number of soil categories and the number of uniform
subgrade areas will not exceed 4 or 5 for a given road project.
• The design subgrade CBR for homogenous section is usually
taken as
– the 90 %-ile value of the CBR test results as shown in Figure below.
Samuel K. 227
Figure: Design CBR as the 90 % -ile value
228
Samuel K.
Resilient Modulus Test
• The resilient modulus, MR,
– is the elastic modulus obtained from repetitive load test that
simulates the actual pavement loading.
• MR is defined as
– the ratio of the repeated deviator stress sd to the recoverable
axial strain er.
• It is well known that most paving materials are not
elastic but experience some permanent deformation
after each load application.
• However, if the load is small compared to the strength
of the material and is repeated for a large numbers of
times,
– the deformation under each load repetition is nearly
completely recoverable and
– proportional to the load and can be considered as elastic.
Samuel K. 229
Figure: Strains under repeated loads
230
Samuel K.
cont...
• The above figure shows the straining of a specimen under a repeated load
test.
• At the initial stage of load applications,
– there is considerable permanent deformation, as indicated by the plastic strain.
• As the number of repetitions increases,
– the plastic strain due to each load repetitions decreases.
• After 100 to 200 repetitions,
– the strain is practically all recoverable, as indicated by εr in the figure.
• A triaxial device equipped for repetitive load condition is used to carry out
resilient modulus test.
• The test may be conducted on all types of unbounded pavement materials
– ranging from cohesive to stabilized materials.
• However this test is not available in most cases and hence some
recommendations are made to correlate the CBR values with the resilient
modulus.
• The asphalt Institute recommends the following approximate relationships
in their design method:
MR (MPa) = 10.35 X CBR value
Samuel K. 231
Other soil Tests
• There are other tests likely to be used for soil surveys, design
and control of construction depending on
– the site conditions encountered and structures to be constructed.
• These include field and laboratory tests such as those
conducted to determine
– the shear strength,
– settlement, and
– permeability of soils.
• These include tests required for special investigations relating
to
– deep cuts,
– embankments over soft and compressible soils,
– expansive soils and
– natural slopes (Refer to ERA, Site Investigation Manual, 2001).
• Procedures for these and those described above are defined in
– BS, ASTM, AASHTO and other equivalent standards.
Samuel K. 232
Soil Classification for Highway Use
• The purpose of soil classification system
– is to group soils with similar properties or attributes.
• As a means of obtaining general behavior, soils are
systematically categorized on the basis of
– some common characteristics obtained from visual
inspection/description and laboratory tests.
• In highway engineering,
– soils are classified by conducting relatively simple tests on
disturbed samples to serve as
• a means of identifying suitable materials and
• predicting the probable behavior when used as subgrade or
subbase material.
• The two most important soil characteristics used in
classifying soils are
– their grain size distribution and plasticity.
Samuel K. 233
cont...
• The most commonly used classification systems
for highway purposes are
– the American Association of State Highway and
Transportation Officials (AASHTO) Classification
System and
– the Unified Soil Classification System (USCS).
• These classification systems only help engineers
to predict
– how the soil will behave if used as a subgrade or
subbase material, however,
– the information obtained should not be regarded as a
substitute for the detailed investigation of the soil
properties.
Samuel K. 234
AASHTO Classification System
• The AASHTO Classification System is based on
– the Public Roads Classification System that was
developed from the results of extensive research
conducted by the Bureau of Public Roads, now known
as
• the Federal Highway Administration of the United States.
• The system has been described by AASHTO as a
means for determining the relative quality of soils
for use in
– embankments,
– subgrades,
– subbases, and
– bases.
Samuel K. 235
cont...
• In this system of classification,
– soils are categorized into seven groups, A-1 through A-7,
with several subgroups, as shown in table below.
• The classification of a given soil is
– based on its particle size distribution, LL, and PI.
• Soils are evaluated within each group by using
– an empirical formula to determine the group index (GI) of
the soils, given as
GI = (F - 35)[0.2 + 0.005(LL - 40)] + 0.01(F - 15)(PI - 10)
where,
GI = group index
F = % of soil particles passing 0.075 mm (No. 200) sieve in whole
number based on material passing 75 mm (3 in.) sieve,
LL = liquid limit expressed in whole number, and
PI = plasticity index expressed in whole number.
236
Samuel K.
Table: AASHTO soil classification system
237
Samuel K.
cont...
• The GI is determined to the nearest whole number.
• A value of zero should be recorded when a negative
value is obtained for the GI.
• Also, in determining the GI for A-2-6 and A-2-7
subgroups,
– the LL part is not used, that is, only the second term of the
equation is used.
• Classifying soils under the AASHTO system is
finding the correct group for the particle size
distribution and atterberg limits of the soil from the
classification Table above.
• The group is then designated using the GI value.
Samuel K. 238
cont...
• Granular soils fall into classes A-1 to A-3.
– A-1 soils consist of well-graded granular materials,
– A-2 soils contain significant amounts of silts and clays, and
– A-3 soils are clean but poorly graded sands.
• A-4 soils cover non-plastic or moderately plastic soils,
and
• A-5 contains similar material to Group A-4
– but exhibits high LL.
• A-6 soils are typical plastic clays
– exhibiting high volume change between wet and dry states.
• Group A-7 covers plastic clays,
– having high values of LL and PI and show high volume
change.
Samuel K. 239
cont...
• In general, according to the AASHTO system of classification,
– the suitability of a soil deposit for use in highway construction can
be summarized as follows.
1. Soils classified as A-1-a, A-1-b, A-2-4, A-2-5, and A-3 can be
used satisfactorily as
• subgrade or subbase material if properly drained.
• In addition, such soils must be properly compacted and covered with an
adequate thickness of pavement for the surface load to be carried.
2. Materials classified as A-2-6, A-2-7, A-4, A-5, A-6, A-7-5, and A-
7-6
• will require a layer of subbase material if used as subgrade.
• If these are to be used as embankment materials,
– special attention must be given to the design of the embankment.
3. Generally, as the GI of a soil increases
• its value as subgrade material decreases.
• For example, a soil with a GI of 0 (an indication of a good subgrade
material)
– will be better as a subgrade material than one with GI of 20 (an indication of a
poor subgrade material).
Samuel K. 240
Unified Soil Classification System (USCS)
• Originally developed by
– Casagrande during World War II for use in airfield construction,
• The fundamental premise used in the USCS system is that,
– the engineering properties of any coarse-grained soil depend on
• its particle size distribution,
– whereas those for a fine-grained soil depend on
• its plasticity
• Thus, the system classifies
– coarse-grained soils on the basis of
• grain size characteristics
– fine-grained soils
• according to plasticity characteristics.
• In this system of classification,
– material that is retained in the 75 mm (3 in.) sieve is recorded, but
– only that which passes is used for the classification of the sample.
Samuel K. 241
cont...
• Soils are designated by letter symbols with each letter
having a particular meaning as defined as follows:
– Coarse-grained soils.
• Soils with more than 50 percent of their particles being
retained on the No. 200 sieve
– are classified as coarse-grained soils.
• The coarse-grained soils are subdivided into
– gravels (G) ¾ soils having more than 50 percent of their particles
larger than4.75 mm (i.e., retained on No. 4 sieve) and
– sands (S) ¾ those with more than 50 percent of their particles
smaller than 4.75 mm (i.e., passed through No. 4 sieve).
Samuel K. 242
cont...
• The gravels and sands are further divided into four subgroups,
– each based on grain size distribution and the nature of the fine
particles in them as
» well graded (W),
» poorly graded (P),
» silty (M), or
» clayey (C).
• Gravels can be described as either
– well-graded gravel (GW),
– poorly graded gravel (GP),
– silty gravel (GM), or
– clayey gravels (GC),
• sands can be described as
– well-graded sand (SW),
– Poorly graded sand (SP),
– silty sand (SM), or
– clayey sand (SC).
Samuel K. 243
Samuel K. 244
cont...
• A gravel or sandy soil is described as well
graded or poorly graded,
– depending on the values of two shape parameters
known as
• the coefficient of uniformity, Cu, and
• the coefficient of curvature, Cc given as
where,
D = grain diameter at 60% passing
60

30

10
Samuel K. 245
cont...
• Accordingly, gravels are described as well graded
– if Cu is above 4, and Cc is between 1 and 3.
• Sands are also described as well graded
– if Cu is above 6, and Cc is between 1 and 3.
• Moreover, coarse-grained soils with more than 12
percent fines (i.e., passes No. 200 sieve) are classified as
– silty or clayey depending on their LL plots.
• Those soils with plots below the ―A‖ line (defined as
below) or with a PI less than 4 are
– silty gravel (GM) or silty sand (SM),
• Those with plots above the "A‖ line with a PI greater
than 7 are
– classified as clayey gravels (GC) or clayey sands (SC).
Samuel K. 246
cont...
• Fine-grained soils
– Soils with less than 50 percent of their particles
retained on the No. 200 sieve are classified as
• fine-grained soils
– The fine-grained soils are subdivided into clays (C)
or silt (M) based on
• a plasticity chart plotted PI versus LL of the soil from
which
– a dividing line known as the "A" line separates the more clayey
materials from the silty materials.
• The equation of the ―A‖ line is
PI = 0.73(LL - 20)
Samuel K. 247
cont...
• Soils
– that fall below the ―A‖ line are silty soils, whereas
– those with plots above the "A‖ line are clayey soils.
• Organic clays are an exception to this general rule
– since they plot below the "A" line.
• Organic clays, however, generally behave similarly to soils of lower
plasticity.
• The organic, silty, and clayey soils are further divided into two groups,
– one having a relatively low LL (L) and
– the other having a relatively high LL (H).
• The dividing line between high LL soils and low LL soils is arbitrarily set
at 50 percent.
• Fine-grained soils are, thus, further classified as either
– silt with low plasticity (ML),
– silt with high plasticity (MH),
– clays with high plasticity (CH),
– clays with low plasticity (CL), or
– organic with high plasticity (OH).
Samuel K. 248
Chaper Five
PAVEMNET MATERIALS
UNBOUND GRANULAR MATERIALS
Introduction
• Pavement design requires
– the efficient use of locally available materials if economically constructed
roads are to be built.
• Pavement materials include:
– Granular materials (aggregates)
– Binders
• Granular materials (aggregates)
– which includes crushed rock aggregates obtained from
• hard rock sources,
• natural (pit-run) gravels,
• gravel-sand-soil mixtures either as dug or semi-processed (i.e. screening, crushing of
oversized stones, mixing with other materials (mechanical stabilization) and
• other artificial or modified materials
• Binders –
– surfacing binders such as
• bitumen and cement;
– binders used for stabilizing or modifying the properties of
subgrade/subbase/base such as lime, cement, foam bitumen, etc.
Samuel K. 250
cont...
• Granular materials (aggregates) make up
– the bulk (by volume and weight) of the pavement structure and
– are used in different layers of pavement structure.
• They may be used alone or in combination with various types of
cementing materials.
• They provide a number of functions depending on the layer in
which they are used.
• In general,
– they have to be stable and hard to carry the loads by traffic and
construction equipment without failure, excessive deformation and
other undue effects,
– they have to be able to resist wear due to abrasion by traffic and
– they have to be durable to resist undue environmental effects (like
freezing and thawing, moisture variations (wetting and drying).
– The manner in which they do so depends on
• the inherent properties
• qualities of the individual particles and
• on the means by which theySamuel
are held
K.
together (i.e. interlocking, binders,251or
both).
cont...
• In Gravel Roads, soil-aggregates form the entire
pavement structure;
– have to be well graded to furnish adequate stability
(strength and stiffness) to carry traffic stresses,
– should possess adequate amount of fines and plasticity of
fines to bind the coarse aggregates.
• Subbase aggregates in flexible pavements are specified
mainly by their gradation:-
– to furnish adequate load-bearing capacity to carry
construction traffic and
– further reduce traffic stresses on subgrade,
– prevent the intrusion of fine particles (filtration as
required),
– improve the subsurface drainage characteristics of the
roadway, etc.
Samuel K. 252
cont...
• Base layer aggregates should have such properties
and be graded in such a manner that
– they have high stability (strength and stiffness),
• which is the factor of primary importance.
• Base layers may also be used for subsurface
drainage.
• Base/Subbase Aggregates, under rigid pavements,
– are not specified mainly by their loadbearing capacity,
– but emphasis is also placed on achieving a gradation
which will prevent
• pumping of the subgrade or
• intrusion of frost-susceptible materials while at the same time
improving the subsurface drainage characteristics of the roadway.
Samuel K. 253
cont...
• In high-quality bituminous road surfacing,
– aggregates comprise of up to about 95 per cent of the weight of the surfacing.
• The surfacing aggregates
– should have adequate stability
• as they are primarily responsible for any load-carrying capacity which the surfacing
may have
– must be resistant to abrasion and durable
• resistant to adverse weather conditions
• Although there are very many types of bituminous surfacing, in general,
the ideal aggregates should have
– Adequate strength and toughness,
– ability to crush into chunky particles,
– free from dust,
– unduly thin and elongated particles,
– hydrophilic (water loving) characteristics,
– particle size and gradation appropriate to the type of construction.
• These criteria are also important for concrete, particularly those relating to
particle shape and size distribution, since they affect water requirements
and workability of concrete mixes as well as other important concrete
properties. Samuel K. 254
cont...
• A wide range of materials can be used as unbound base and subbase
courses including
– crushed quarried rock,
– crushed and screened,
– mechanically stabilized,
– modified or naturally occurring ―as dug‖ or ―pit run‖ gravels.
• Their suitability for use depends primarily on
– the design traffic level of the pavement and climate.
• However, such materials must have a particle size distribution and particle
shape
– which provide high mechanical stability and
– should contain sufficient fines (amount of material passing the 0.425 mm
sieve) to produce a dense material when compacted.
• The use of locally available materials is encouraged, particularly at low
traffic volumes.
• Their use should be based on the results of performance studies and should
incorporate any special design features which ensure their satisfactory
performance.
Samuel K. 255
Sources of Aggregates
• Sources of aggregates for use in pavement works
include:
– Hard rock sources (crushed quarried rock) –
• hard sound bed rock exposures that need blasting and
crushing
– Naturally occurring gravels –
• which includes alluvial deposits, and highly weathered and
fractured residual formations (rippable or can be worked
using earth moving machinery such as Dozers).
• These may be used as is (pit-run) or may need further
processing to be suitable for use such as crushing over sized
stones and screening and/or other modifications such as
mechanical stabilization.
• The principal sources of road aggregates in Ethiopia include
natural sand and gravelSamuel
deposits,
K. and crushed rock. 256
cont...
– Crushed aggregates –
• Hard rocks are important sources of aggregates.
• There are different types of rocks, all composed of
grains of crystalline minerals held together in a variety
of ways.
• The Property of a rock depends upon the properties of its
constituent minerals and nature of bond between them
(i.e. composition, grain size and texture of the rock)
which in turn depends on its mode of origin.
• Geologists classify rocks into three major types
according to their mode of origin/formation.
• These are Igneous, Sedimentary, and Metamorphic
rocks.
Samuel K. 257
cont...
• Igneous rocks
– are formed from the cooling of molten material (magma).
– Some classifications/definitions related to igneous rocks are given below:
• Extrusive (volcanic) igneous rocks (Basalts, Rhyolite, Trachyte) –
– which formed on or near the surface of the earth‘s crust due to
• rapid cooling of magma and hence rocks are fine grained (glassy or vitreous/like a
glass (without crystal) or partly vitreous and partly crystalline (with small grain sizes)).
• Intrusive (Plutonic) igneous rocks (Granite, Gabbro) –
– formed by the cooling of magma below the earth‘s surface, and hence are
crystalline (the crystals may be big enough to be visible by the naked eye).
• Acidic Rocks –
– igneous rocks with high silica (Sio2) content > 63%. (granite, rhyolite)
• Intermediate rocks –
– igneous rocks with intermediate silica content, Sio2 between 52% - 62%.
(andesite, diorite)
• Basic rocks –
– igneous rocks with low silica content, Sio2 between 45 – 52%. (Basalt,
gabbro).
Samuel K. 258
cont...
• Generally speaking, igneous rocks with medium grain sized particles are
preferable as source for aggregates.
• Coarse grained rocks (grain size >1.25mm)
– are liable to be brittle and to break down under compacting roller (not
tough).
• While rocks with too fine grain sizes (<0.125mm) and if especially
vesicular,
– the aggregates are liable to be brittle and splittery (flaky/elongated shapes).
• Acidic rocks tend to be negatively charged and aggregates containing
large amounts of feldspar and quartz in large crystals
– do not bind well with bitumen (bitumen has a slight –ve charge).
• Basic rocks bind well with bitumen.
• Most common igneous rocks used for production of aggregate are
– basalt and granite which are both strong and resistant to wear.
• But one may need to check the bonding property of granitic aggregates
with bitumen,
– which may need anti-stripping agent.
Samuel K. 259
cont...
• Sedimentary rocks are formed from
– the solidification of chemical or mineral sediments deposited under ancient
seas.
• They are usually layered since the original material was deposited in this
manner.
• Sedimentary rocks may be siliceous rocks –
– formed from disintegrated rock sediments transported by wind or water, re-
deposited as sediments, then consolidated or cemented in to a new rock type
• e.g. conglomerate (consolidated gravel), sand stone (consolidated sand), shales
(consolidated mud/clay, rich in organic matter: silt stone or clay stone).
• Calcareous rocks – rocks formed by
– chemical deposition of organic remains in water (Gypsum, chalk, Limestone).
• Indurated sediementary rocks (Fine grained well cemented sand stones,
limestones) are used as aggregate sources.
• However, they are not entirely satisfactory as high quality aggregates due
to
– their variable cementation (hence variable strength and durability),
– their tendency to polish (not suitable for surfacing aggregates) and
– risk of alkali aggregate reaction (concrete aggregates).
Samuel K. 260
cont...
• Metamorphic rocks
– are igneous or sedimentary rocks that have been changed (metamorphosed)
due to intense heat and pressure into new rocks by the recrystallization of
their constituents (e.g. Quartzite, Gneiss, Schist, Slate, marble, etc.).
• Schist and slate are highly foliated rocks
– which are not desirable as they are fissile and liable to be crushed when
compacted with rollers.
• Quartzite and gneisses can furnished good aggregates.
• The properties of aggregates produced in quarries from bedrock depend
on the type of bedrock.
• Igneous and metamorphic rocks are usually very hard and make
excellent aggregates for most purposes.
• Limestone and dolomite are quite common sedimentary rocks.
– They are softer than igneous rocks, but are still acceptable as aggregates for
most purposes.
• Shale, being composed of clay grains,
– is very weak and disintegrates easily when exposed to the weather and
– is a poor aggregate material.
Samuel K. 261
cont...
• Natural sand and gravel pits
– have been used extensively as sources of road aggregates.
• Sand or gravel pit is first stripped of topsoil, vegetation, and other
unsuitable material from the surface of the deposit to obtain
– pit run materials.
– The material obtained is loose, and is usually excavated with power
shovels or front-end loaders.
– Often it is crushed, especially if there are cobbles or boulders in the
deposit.
– The smaller sizes go through the crusher without change, whereas
– Larger particles are broken down to the desired size.
• Crushed gravel, as this is called, is a high-quality aggregate used for
many purposes.
• Sand or gravel deposits might be composed of many different types
of mineral particles-such as
– limestone,
– sandstone, and
– granite--depending on the original bedrock source of the particles.
Samuel K. 262
cont...
• Recycled material –
– the use of pulverized concrete from pavements,
sidewalks, and buildings being demolished is growing in
other countries both due to
• the increased cost of natural aggregates and
• the desire to recycle rather than landfill these materials.
– Recycled concrete is crushed, processed, and used as
base material and in concrete and asphalt paving
mixtures.
– Asphalt pavements can be recycled and reused in
pavements.
– Pulverized asphalt mixtures are also used as aggregates
in base courses, but the proportion may be limited to
• about 30-50% as the strength of the layer can be reduced due to
the lubricating effect of the asphalt film on the particles.
Samuel K. 263
Aggregate tests
• Aggregates are obtained from different sources
– consequently differ considerably in their constitutions;
– inevitably they differ also with regard to their engineering properties.
• The properties of aggregate that are important for road construction
include
– its cleanliness (contamination with dust and other deleterious materials),
– particle size and shape,
– gradation,
– toughness - resistance to crushing,
– abradability - wearing/abrasion resistance,
– durability/soundness,
– specific gravity and water absorption,
– surface texture,
– tendency to polish,
– bonding property with bitumen.
• Aggregate tests are necessary
– to determine the suitability of the material for a specific use and
– to make sure that the required properties are consistently within specification
limits. Samuel K. 264
Particle Size and Shape
• Gradation test:
– Gradation is the characteristic of aggregates on which
perhaps the greatest stress is placed in specifications for
highway bases, cement concretes, and asphalt mixes.
– Hence, gradation test, also called sieve analysis, screen
analysis or mechanical analysis,
• is the most common test performed on aggregates to evaluate the
suitability of the aggregate materials with respect to their grain size
distribution for a specific use.
– Gradation is determined by separating the aggregates into
portions,
• which are retained on a number of sieves or screens having
specified openings,
• which are suitably graded from coarse to fine.
– The results obtained may be expressed
• either as total percentage passing or retained on each sieve or as the
percentages retained between successive sieves.
Samuel K. 265
cont...
• The theoretical maximum density of aggregates is obtained
– when the grain size distribution follow the Fuller maximum
density equation of the form
• in which,
– p is the percent passing sieve size "d",
– "D' represents the maximum aggregate size in the material, and
– n is a constant which varies between 0.45 and 0.5 for maximum
density.
• The assumption in this relationship is that
– the voids between the larger particles are filled with still smaller
particles,
– until the smallest voids are filled with a small amount of fines.
• Strength, or resistance to shear failure, in road bases and
other aggregate layers that carry load is increased greatly
– if the mixture is dense graded.
Samuel K. 266
cont...
• The larger particles are in contact with each other,
– developing frictional resistance to shearing failure, and
– tightly bound together due to the interlocking effect of
the smaller particles.
• When aggregate particles are to be bound together
by cement or bitumen,
– a variation in the grading of an aggregate will result in
a change in the amount of binder required to produce a
material of given stability and quality.
• Proper aggregate grading contributes to
– the uniformity,
– workability and
– plasticity of the material as it is mixed.
Samuel K. 267
cont...
• Often the fines content must be limited, because
– they are relatively weak, and
– require an excessive amount of binder to cover them.
• If fines are present as dust on larger particles,
– they weaken the bond between the cement and those particles.
• Fines in highway bases may lead to
– drainage and frost- heaving problems.
• Also, excessive amounts of fines may result in
– weak mixtures, as the large particles are not in contact with each other.
• The strength of the mixture would then depend
– only on friction between the small particles, which is much less than
between large particles.
• In practice, the required gradation is not found naturally, particularly, if
the aggregates are pit-run materials.
• In such cases, combining two or more aggregates of different sources
– satisfies the gradation requirement for a specific use.
Samuel K. 268
Aggregate Crushing Value (ACV) Test
• Aggregate crushing test evaluates the resistance of
– aggregates against the gradually applied load.
• The test is used to evaluate
– the crushing strength of available supplies of rock, and
– in construction, to make sure that minimum specified values are maintained.
• The test is undertaken using
– a metal plunger to apply gradually a standard load of 400kN to a sample of
the aggregate (10 – 14 mm) contained in a standard test mould.
• The amount of material passing 2.36 mm sieve in percentage of the total
weight of the sample is referred to as the Aggregate Crushing value
(ACV).
• Over the range of normal road making aggregates,
– ACVs vary from 5 percent for hard aggregates to 30 percent for weaker
aggregates.
• For weaker aggregates than this, the same apparatus is used to evaluate
– the Ten Percent Fines value i.e. the load which produces 10 percent of fines
passing 2.36 mm sieve.
– The value is obtained by interpolating of the percentage of fines produced
over a range of test loads.
Samuel K. 269
Aggregate Impact Test
• This test is a means of evaluating
– the resistance of aggregates to sudden impact loading.
• It is carried out
– by filling a steel test mould with a sample of aggregate (10 –
14mm) and
– then the impact load applied is by dropping hammer at a height
of 380 mm.
• The Aggregate Impact Value (AIV)
– is the percentage of fines passing 2.36 mm sieve after 15 blows.
• This test produces results
– that are normally about 105 per cent of the ACV and
– it can be used for the same purposes.
• Both tests give results
– which are sufficiently repeatable and reproducible for contract
specifications.
Samuel K. 270
Abrasion Test
• Abrasion test is the test used to know how the
aggregate is sufficiently hard to
– resist the abrasive effect of traffic over its service life.
• The most widely used abrasion test is
– the Los Angeles Abrasion Test which involves
• the use of a steel drum, revolving on horizontal axis, into
which the test sample of chippings is loaded together with steel
balls of 46.8 mm diameter.
• The Los Angeles Abrasion Value (LAV)
– is the percentage of fines passing the 1.7 mm sieve after
a specified number of revolutions of the drum at
specified speed.
• The drum is fitted with
– internal baffles causing the aggregate and the steel balls
to be lifted and then fall as the drum revolves.
Samuel K. 271
cont...
• The test therefore gives an indication of
– the impact strength in combination with the
abrasion resistance of the aggregate.
• For bituminous surface dressings,
– chippings with an ACV less than 30 are desirable
and
– the stronger they are the more durable will be the
dressings.
• The repeatability and reproducibility of this
test are satisfactory and appropriate for use in
contract specifications.
Samuel K. 272
Soundness Test
• This test procedure is useful in both survey and design for
the evaluation of
– aggregates to resist disintegration due to weathering.
• A sample of aggregate is saturated in a
– solution of magnesium sulphate or sodium sulphate, and
– then removed and dried in an oven.
• This process is repeated for five cycles.
• On completion, the percentage lost gives
– the durability of the material.
• The test is not suitable for
– providing a single criterion for the susceptibility of aggregates
to rapid weathering but
– it may find a place as part of the evaluation procedure of
aggregates suspected of containing minerals that are weakened
by chemical alteration.
Samuel K. 273
Specific Gravity and Water Absorption
• The tests are likely to be used both in surveys
of
– aggregate resources and
– in design, particularly in the interpretation of
compaction tests and in the design of bituminous
mixtures.
• They may also be used as part of quality
control during construction,
– particularly when the survey has indicated that
aggregate from the chosen source is subject to
variations in density.
Samuel K. 274
cont...
• Most rocks absorb less than one per cent by weight
of water and, up to this level, water absorption is of
no great consequence.
• However, some rocks can absorb up to 4 percent of
water.
• This suggests that the rock may be of low
mechanical strength and will be difficult to dry and
heat during processing to make bituminous mixtures.
• Inadequate drying will cause difficulty
– in securing good adhesion between bitumen and stone,
and
– in hot process mixtures, where the stone must be heated to
about 180oc, it causes a large waste of energy.
Samuel K. 275
cont...
• In the tests,
– a 4 kilogram sample of the crushed rock of specific nominal
size chippings is soaked in distilled water for 24 hours,
– weighed in water (WW),
– surface dried and weighed in air (WS).
– It is then oven dried at 105oc for 24 hours and
– weighed again in air (WD).
• The specific gravity and the water absorption are then
obtained as follows:
Samuel K. 276
Shape Tests
• Three mechanical measures of particle shape which may be
included in the specifications for aggregates for road
construction, are
– the flakiness index, elongation index and angularity number.
• The flakiness index of an aggregate
– is the percentage by weight of particles whose least thickness is less
than three-fifths of their mean dimension.
• The mean dimension, as used in each instance,
– is the average of two adjacent sieve aperture sizes between which the
particle being measured is retained by sieving.
• The elongation index of an aggregate
– is the percentage by weight of particles whose greatest length is
greater than 1.8 times their mean dimension.
• The angularity number of an aggregate
– is the amount, to the nearest whole number, by which the percentage
of voids exceeds 33 when an aggregate is compacted in a specified
manner in a standardized metal cylinder.
Samuel K. 277
cont...
• Use of the shape tests in specifications is based on the
view that the shapes of the particles
– influence both the strength of aggregate particles and internal
friction that can be developed in the aggregate mass.
• Since, other factors being equal, an aggregate composed
of smooth rounded particles of a certain gradation
– will contain less voids than one of the same grading but
composed of angular particles,
• the angularity of an aggregate can be reflected in terms of the volume
of contained voids when the aggregate is compacted.
• Measurements show that the angularity number may
range
– from zero for a material of highly rounded beach-gravel
particles
– to 10 or more for newly crushed rock aggregate.
Samuel K. 278
Blending aggregates
• To meet the gradation requirements of aggregates
for particular uses in pavement construction,
– it is often necessary to blend two or more aggregates
together.
• Charts and diagrams are available to do this
blending,
– but the trial-and-error method is simpler and just about as
fast as more complex methods.
• Consider two aggregates graded and designated as
aggregate A and B, and let the specification limit be
as given in Table below.
• The use of the trail-and-error method for blending is
then illustrated as follows:
Samuel K. 279
Table : Aggregate gradation to be combined to meet
specification limits
% passing
Sieve
Aggregate Aggregate Specification Mid-point Combined
A B aggregate
12.5mm 100 100 90-100 95 100
No. 10 0 100 40-55 48 48
No. 200 0 14 5-10 8 7
It is clear in the above table that all the material passing a No.10 sieve
must come from aggregate B, i.e., approximately 48% which leaves 52
% for aggregate A. Or consider the retained percentage on No.10 sieve
for alternative solution.
All materials retained on No.10 must come from aggregate A, which is
52 % require in the specification, i.e. 52 % from A and 48% from B.
In both cases, the proportion which best fits the specification limits can
be satisfied. Samuel K. 280
cont...
• The following equation may be written to apply the
procedure to any given sieve:
aA + bB = T
where,
• A and B are percentages from aggregates A and B to be blended for
satisfying the specification limits.
• a and b are the respective sieve analysis values for a given sieve X,
expressed as a decimal fraction, and
• T is the sieve analysis value in the blended aggregate.
• The equation can be used for gradation expressions
– Percentage retained on a given sieve,
– Percentage passing on a given sieve, and
– Percentage retained on two or more sieves.
Samuel K. 281
cont...
• The result of this equation is used to proportion the 1st trial
blend for the trial-and-error method.
• The second and the subsequent blends are proportioned by
observation until the specification is satisfied.
• In the above illustration, the equation can be written for the
No.10 sieve, % passing, as:
apAp + bpBp = Tp
• in which the subscript p indicates the percentage passing.
– The known variables here are ap = 0, bp = 1, and Tp = 48%,
which implies that B = 48%.
• For percent retained, the equation can be written as:
ar Ar + br Br = Tr
• in which the subscript r indicates the percentage retained.
– The known variables here are ar = 1, br = 0, and Tr = 52%, which
implies that A = 52%.
Samuel K. 282
cont...
• Example: Three aggregates are to be blended to meet a
specification. The aggregates, gradations, and the
specification are given in Table below.
• Table: Aggregate gradation and specification for the
example
Seive Aggregate Aggregate Aggregate specification Spec. Combination
size A B C Mid-point gradation 1st trial
Passing 100 100 100 100
12.5mm
9.5mm 62 100 72-88 80 79
4.75mm 8 100 78 45-65 55 46
2.36mm 2 91 52 30-60 45 34
1.18mm 0 73 36 25-55 40 25
600µm 51 29 16-40 28 18
300µm 24 24 8-25 16.5 11
150µm 4 20 4-12 8 6
75µm 1 18 3-6 4.5 5
Samuel K. 283
Solution:
• Most of coarse aggregate will come from aggregate A and most of the
fines will be obtained from aggregate C.
• To obtain a mixture that is approximately in the middle of the
specification,
– we first use the equation and continue with more trials.
• The equation can be written to blend aggregate A, B, and C for retained
on 9.5 mm sieve and passing 75 μm sieve as follows:
aA + bB + cC = T
• For retained materials on 9.5 mm sieve, the known variables are ar =
0.38, br = 0, cr = 0 and Tr = 20%, which implies that A = 53%.
• Similarly, for passing 75 μm, the known variables are ap = 0, bp = 0, cp =
0.18 and Tp = 4.5%, which results C = 25%, and B = 100 – 53 – 25 = 22.
• The first trial blend as seen in Table above is within the specification
limit, but on the coarse side.
• Reducing the contribution of aggregate A and increasing B, or C or both
for the second and the subsequent trials can result in a blend more close
to the middle of the specification.
Samuel K. 284
Unbound Base and Subbase Materials
(ERA Pavement Design Manual Requirements)
• Unbound base and subbase courses in pavement structures
are
– granular materials from sand or gravel deposits or crushed rock
from quarries without admixtures.
• The required properties of these materials vary
– with the type of pavement and
– the depth of the material in the pavement structure.
• Different standard methods of design specify materials of
construction differently considering
– the traffic load,
– locally available materials, and
– environmental conditions.
• The following describes the requirements set for different
unbound pavement materials for base and subbase courses
as specified in ERA pavement design manual (2002).
Samuel K. 285
Base course
• Graded crushed aggregate:
– This material is produced by crushing fresh, quarried rock usually
termed a 'crusher-run', or alternatively the material may be separated
by screening and recombined to produce a desired particle size
distribution, as per the specifications.
• The rock used for crushed aggregates should be hard and durable.
• Laboratory and field experiences have shown that
– crushed particles have, in general, more stability than rounded
materials due to primarily to added grain interlock.
• In addition, crushed materials possess high coefficient of
permeability.
• After crushing, the material should be angular in shape with
• a Flakiness Index of less than 35%, and preferably of less than
30%.
• In constructing a crushed stone base course,
– the aim should be to achieve maximum impermeability compatible
with good compaction and high stability under traffic.
Samuel K. 286
Table: Grading limits for graded crushed stone base
course materials
Samuel K. 287
cont...
• To ensure that the materials are sufficiently durable, they
should satisfy the criteria given in next Table.
• These are a minimum Ten Per Cent Fines Value (TFV) and
limits on the maximum loss in strength following a period of
24 hours of soaking in water.
• Alternatively, if requirements expressed in terms of the
results of the Aggregate Crushing Value (ACV) are used, the
ACV should preferably be less than 25 and in any case less
than 29.
• Other simpler tests e.g. the Aggregate Impact Test may be
used in quality control testing provided a relationship
between the results of the chosen test and the TFV has been
determined.
• Unique relationships do not exist between the results of the
various tests but good correlations can be established for
individual material types and these need to be determined
Samuel K. 288
locally.
cont...
• The in situ dry density of the placed material should
be a minimum of 98% of the maximum dry density
obtained in the Heavy Compaction.
• The compacted thickness of each layer should not
exceed 200 mm.
• Crushed stone base materials described above should
have CBR values well in excess of 100 per cent, and
fines passing 0.425 mm sieve should be nonplastic.
Table: Mechanical strength requirements for crushed stone base defined by TFV
Typical annual raifall Minimum 10% fines Minimum ratio
values (KN) wet/dry Test (%)
>500 110 75
<500 110 60
Samuel K. 289
cont...
• Requirements for natural gravels and weathered
rocks:
– A wide range of materials including
• lateritic, calcareous and quartzitic gravels, river gravels,
boulders and
• other transported gravels, or granular materials resulting
from the weathering of rocks can be used successfully as
base course materials.
– Table below contains three recommended particle size
distributions for suitable materials corresponding to
• maximum nominal sizes of 37.5 mm, 20 mm and 10 mm.
– When the traffic is in excess of 1.5x106 ESA, only the
two larger sizes should be considered.
Samuel K. 290
Table: Recommended particle size distributions for base
course material
Samuel K. 291
cont...
• For materials whose stability decreases with
breakdown,
– an aggregate hardness based on a minimum soaked TFV of
50 kN may be specified.
• The fines of these materials should preferably
– be nonplastic but should normally never exceed a PI of 6.
– If the PI approaches the upper limit of 6,
• it is desirable that the fines content be restricted to the lower end of
the range.
• To ensure this, a maximum Plasticity Product (PP) of
60 is recommended or alternatively a maximum
Plasticity Modulus (PM) of 90 where:
– PP = PI x (percentage passing the 0.075 mm sieve)
– PM = PI x (percentage passing the 0.425 mm sieve)
Samuel K. 292
cont...
• When used as a base course,
– the material should be compacted to a density equal to or greater
than 98 per cent of the maximum dry density achieved in the Heavy
Compaction.
• When compacted to this density in the laboratory,
– the material should have a minimum CBR of 80% after four days
immersion in water.
• In low rainfall areas, typically with a mean annual rainfall of
less than 500 mm, and where evaporation is high,
– moisture conditions beneath a well sealed surface are unlikely to rise
above the optimum moisture content.
– In such conditions, high strengths (CBR>80 %) are likely to develop
even when natural gravels containing a substantial amount of plastic
fines are used.
• In these situations, for traffic loading within 0.7 million
equivalent standard axles,
– the maximum allowable PI can be increased to 12 and the minimum
soaked CBR criterion reduced to 60% at the expected field density.
Samuel K. 293
cont...
• Naturally occurring gravels which do not normally meet the
normal specifications for base course materials have
occasionally been used successfully.
• They include lateritic, calcareous and volcanic gravels.
• In general their use should be confined to the lower traffic
roads.
• Laterite gravels with plasticity index in the range of 6-12
and plasticity modulus in the range of 150-250 is
recommended for use as base course material for of traffic
volume up to 1.5 million equivalent standard axles.
• The values towards higher range are valid for semi-arid and
arid areas of Ethiopia, i.e. with annual rainfall less than 500
mm.
• Cinder gravels can also be used as base course materials in
lightly trafficked (below 0.7x106 ESA) surface dressed
roads.
Samuel K. 294
Sub-base course materials
• The sub-base
– is an important load spreading layer which enables
traffic stresses to be reduced to acceptable levels on
the subgrade.
• It also acts as a working platform for
– the construction of the upper pavement layers
separating the subgrade and base course.
• Under special circumstances,
– it may serve as a filter or as a drainage layer.
• The selection of sub-base materials depends on
– the design function of the layer and
– the anticipated moisture regime, both in service and at
construction.
Samuel K. 295
cont...
• Bearing capacity:
– A minimum CBR of 30 per cent is required at the highest
anticipated moisture content when compacted to the specified
field density, usually a minimum of 95 per cent of the MDD
achieved in the Heavy Compaction.
– Under conditions of good drainage and when the water table is
not near the ground surface the field moisture content under a
sealed pavement will be equal to or less than the optimum
moisture (Light Compaction).
– In such conditions, the sub-base material should be tested in
the laboratory in an unsaturated state.
– If saturation of the sub-base is likely, the bearing capacity
should be determined on samples soaked in water for a period
of four days.
– Materials which meet the recommendations of Tables below
will usually be found to have adequate bearing capacity.
Samuel K. 296
cont...
• Use as a construction platform:
– In many circumstances the requirements of a sub-base are
governed by
• its ability to support construction traffic without excessive deformation
or ravelling.
– A high quality sub-base is therefore required where loading or
climatic conditions during construction are severe.
– Suitable material should possess properties similar to those of a
good surfacing material for unpaved roads.
– The material should be well graded and have a plasticity index
at the lower end of the appropriate range for an ideal unpaved
road wearing course under the prevailing climatic conditions.
– These considerations form the basis of the criteria given in the
Tables below.
– Material meeting the requirements for severe conditions will
usually be of higher quality than the standard sub-base
material.
Samuel K. 297
cont...
• In the construction of low-volume roads, where cost
savings at construction are particularly important,
– local experience is often invaluable and
– a wider range of materials may often be found to be
acceptable.
• In Ethiopia, laterite is one of the widely available
materials and can be used as a sub-base material.
• Laterite meeting the gradation requirements of Table
below can be used for traffic levels up to 3x106 ESA
provided the following criteria is satisfied:
• Plasticity Index (%) < 25
• Plasticity Modulus (PM) < 500
• CBR (%) > 30
Samuel K. 298
cont...
Table: Recommended plasticity characteristics for granular sub-bases
Table: Typical particle size distribution for sub-bases
Samuel K. 299
cont...
• Filter or separating layer:
– This may be required
• to protect a drainage layer from blockage by a finer
material or
• to prevent migration of fines and the mixing of two
layers.
– The two functions are similar except that
• for use as a filter the material needs to be capable of
allowing drainage to take place and
• therefore the amount of material passing the 0.075 mm
sieve must be restricted.
Samuel K. 300
cont...
• The following criteria should be used to evaluate a
subbase as a separating or filter layer:
• where D15 is the sieve size through which 15% by weight of the
material passes and D85 is the sieve size through which 85%
passes.
• For a filter to possess the required drainage

characteristics a further requirement is:
• These criteria may be applied to the materials at both

the base course/sub-base and the subbase/subgrade
interfaces.
Samuel K. 301
cont...
• Selected subgrade materials and capping layers
– These materials are often required to provide sufficient cover on weak
subgrades.
– They are used in the lower pavement layers as a substitute for a thick
sub-base to reduce costs, and a cost comparison should be conducted
to assess their cost effectiveness.
– The requirements are less strict than for sub-bases.
– A minimum CBR of 15 per cent is specified at the highest anticipated
moisture content measured on samples compacted in the laboratory at
the specified field density.
– This density is usually specified as a minimum of 95 per cent of the
MDD in the Heavy Compaction.
– Recommended gradings or plasticity criteria are not given for these
materials.
– However, it is desirable to select reasonably homogeneous materials
since overall pavement behaviour is often enhanced by this.
– The selection of materials which show the least change in bearing
capacity from dry to wet is also beneficial.
Samuel K. 302
Gravel surface roads
• Gravel surface roads
– are generally roads which are constructed and maintained at
low cost using locally available materials in the near vicinity of
the site.
• Coarse well graded gravel is a very satisfactory material
for constructing cheap all-weather roads.
• This type of construction is designed
– for AADT between 350 and 400 and
– when the weight of the individual vehicle is in the order of 10
ton.
• Beyond these, they often become not economical.
• At higher traffic the following problems such as
– surface pitting,
– the formation of transverse corrugation,
– high cost of replacing or grading, and
– dust may occur. Samuel K. 303
cont...
• The general soil-aggregate mixture used for
constructing gravel roads should be
– stable (support the loads without detrimental
deformation which is the function of particle size
distribution and particle shape, density, and
internal friction and cohesion),
– abrasion resistant,
– shed a large portion of the rain which falls on the
surface,
– posses capillarity properties to replace the moisture
lost by surface evaporation, and
– low-cost.
Samuel K. 304
cont...
• Type 1 (Table below) is recommended for gravel wearing course material
in the new construction of roads having
– an AADT greater than 50 and for all routine and periodic maintenance
activities.
• According to the Tanzanian Design Manual (1999),
– gravel wearing course for major roads require a minimum CBR of 25 %.
• Type 4 materials may be used in the new construction of roads having an
AADT less than 50.
• Minor gravel roads (AADT design less than 20) which are normally
community roads
– are usually unsurfaced (earth roads) and constructed by labor-based methods.
• However, for subgrade CBR values less than 5% and longitudinal
gradients of greater than 6%,
– a gravel wearing course is recommended.
• Materials for gravel wearing course shall comply with
– the requirements for Type 4 material for new construction and Type 1 for
maintenance activities.
• The CBR requirements may be reduced to 20%
– if other suitable material is not locally available.
Samuel K. 305
Table: Gradation requirements for gravel wear course (ERA,
2001)
306
Samuel K.
cont...
• Type 1:
– The grading of the gravel after placing and compaction
shall be a smooth curve within and approximately
parallel to the envelopes detailed in above Table.
– The material shall have a LAV of not more than 50 at
500 revolutions.
– The material shall be compacted to a minimum in-situ
density of 95% of the maximum dry density.
– The plasticity index should be not greater than 15 and
not less than 8 for wet climatic zones and should be not
greater than 20 and not less than 10 for dry climatic
zones.
– The linear Shrinkage should be in a range of 3-10%.
Samuel K. 307
cont...
• Type 2 & 3:
– These materials may be more rounded particles
fulfilling the following:
• the Plasticity Index lies in a range of 5-12% in wet areas,
and in any case less than 16% in other areas,
• a minimum crushing under traffic in percentage by weight
of particles with at least one fractured face of 40%,
• the CBR should be in excess of 20 after 4 days of soaking
at 95% of maximum dry density under Heavy
Compaction.
• For very low traffic, the requirement may be relaxed to a
CBR of 15.
Samuel K. 308
cont...
• Type 4:
– This material gradation allows for larger size
material and corresponds to the gradation of a base
course material.
– The use of this gradation of materials is subject to
the local experience and shall be used with PIs in a
range of 10-20.
• Type 5 & 6:
– These materials gradations are recommended for
smaller size particles.
– They may be used if sanctioned by experience with
plasticity characteristics as for material Type 1.
Samuel K. 309
Chapter Six
Stabilized pavement materials

STABILIZED PAVEMENT MATERIALS
• The term ‗soil stabilisation‘ may be defined as
– the alteration of the properties of an existing soil
• either by blending (mixing) two or more materials and improving particle
size distribution or
• by the use of stabilizing additives to meet the specified engineering
properties.
• Quite often soils are stabilized for road construction in most
parts of the world for the following one or more objectives:
– Improve the strength (stability and bearing capacity) for
subgrade, subbase, base, and low-cost road surfaces,
– Improve the volume stability – undesirable properties such as
swelling, shrinkage, high plasticity characteristics, and difficulty
in compaction, etc. caused by change in moisture,
– Improve durability – increase the resistance to erosion,
weathering or traffic, and
– Improve high permeability, poor workability, dust nuisance, frost
susceptibility, etc.
Samuel K. 311
cont...
• Due to their mineralogical composition, soils may be rather
complex materials.
• Stabilization is therefore not a straightforward application of a
given stabilizing agent;
– a number of aspects should be taken into account in the selection of
the proper stabilization technique.
• The factors that should be considered include
– physical and chemical composition of the soil to be stabilized,
– availability and economical feasibility of stabilising agents,
– ease of application,
– site constraints,
– climate,
– curing time, and
– safety.
• Such factors should be taken into account in order to select the
proper type of stabilisation.
Samuel K. 312
cont...
• Basically four techniques of soil stabilization are
commonly practiced in pavement construction.
• These are: -
– Mechanical stabilization,
– Cement stabilization,
– Lime stabilization, and
– Bitumen stabilization.
• Mechanical stabilization is a method by which a soil or
gravel is mixed with the original soil in order to improve
– the grading and
– mechanical characteristics of the soil.
• Other methods of stabilization use additives such as
– cement,
– lime and
– bitumen; to improve strength, workability or waterproofing.
Samuel K. 313
cont...
• Portland cement has been used with great success to
improve
– existing gravel roads, as well as to stabilize natural soils.
• It can be used for base courses and subbases of all
types.
• It can be used in
– granular soils,
– silty soils, and
– lean clays,
– but it cannot be used in organic materials.
• Since soil cement shows strength gains over that of
the natural material,
– it is very often used for basecourse construction.
Samuel K. 314
cont...
• Another cementing agent, which is often used, is lime.
• Lime increases soil strength primarily by pozzolanic action,
– which is the formation of cementatious silicates and aluminates.
• It is the most effective agent to stabilize clayey materials.
• Bituminous materials are used as stabilizers
– to retard or completely stop moisture absorption by coating soil
or aggregate grains in the soil-aggregate mixture.
• Bituminous stabilization is best for semigranular soils.
• As will be seen in the coming sections, the suitability of
these methods depends on
– site constraints,
– materials,
– climate, and
– economic feasibility.
Samuel K. 315
cont...
• The stabilizing process with admixture
involves
– the addition of a stabilizing agent to the soil,
– mixing with sufficient water to achieve the
optimum moisture content,
– compaction of the mixture, and final
– curing to ensure that the strength potential is
developed.
Samuel K. 316
Mechanical Stabilization
• mechanical stabilization
– is an improvement of an available material by
• blending it with one or more materials in order to improve the particle size
distribution and plasticity characteristics.
• Typical materials used for mechanical stabilization include
– river deposited sand,
– natural gravel,
– silty sands,
– sand clays,
– silt clays,
– crushed run quarry products and waste quarry products,
– volcanic cinders and scoria,
– poorly graded laterites and
– beach sands, etc.
• Materials produced by blending have properties similar to
conventional unbounded materials and can be evaluated by
ordinary methods.
Samuel K. 317
cont...
• The principal properties affecting the stability
of compacted base or sub-base materials are
– internal friction and cohesion.
• Internal friction is chiefly dependent on
– the characteristics of the coarser soil particles, i.e.
gravel, sand and silt sizes.
• The cohesion, shrinkage, swelling and
compressibility are mainly associated with
– the quantity and nature of the clay fraction as
indicated by plastic properties.
Samuel K. 318
cont...
• Preliminary mix design of mechanical
stabilization is based on
– particle size distribution and
– plastic properties.
• It is desirable also that strength tests (CBR, etc.)
be carried out to verify that
– the required improvement has been achieved.
• When unconventional materials are used,
– more detailed testing and investigation will often be
needed and
– may include the modification of the accepted design or
specification criteria.
Samuel K. 319
cont...
• Particle Size Distribution:
– While maximum frictional strength does not
necessarily coincide with maximum density,
• the achievement of a high density will generally provide a
high frictional strength.
– A particle size distribution that results maximum dry
density, obtained with the closest packing and
minimum voids,
• has been shown experimentally to follow Fuller‘s
equation with the value of the exponent 'n' usually 0.45 to
0.50 for most soils.
Samuel K. 320
cont...
– However, with some materials, e.g. gravel-sand-
clays,
• high densities can be achieved with ‗n’ values as low as
0.33.
– For materials with a maximum size of 19 mm,
• the amount of fines passing the 75 μm sieve will be 6 and
8 percent for ‗n’ values of 0.5 and 0.45 respectively.
– In certain cases higher percentages of material
passing the 75 μm sieve may provide the best
performance.
Samuel K. 321
cont...
• When the pavement design relies on a relatively low
permeability in the pavement courses,
– the materials used should be of particle size distribution within the
limits established by substituting values of 0.50 and 0.33 for ‗n’ in
the above equation.
• These limits are sufficiently wide to allow for variations that
will inevitably occur in field mixing.
• However, if plant mixing is undertaken, more restrictive limits
may be set.
• Where the value of the exponent ‗n’ is less than 0.33,
– the fines content of the material may be excessive.
• A high fine content will result in reduced permeability, but
– may lead to the development of pore pressures and consequent
instability during compaction or in service.
• Where ‗n’ is greater than 0.5,
– the material tends to be harsh, and may be prone to segregation and
ravelling and therefore more difficult to work.
Samuel K. 322
cont...
• Liquid Limit and Plasticity Index:
– The plasticity limits indicated in Chapter 5 for
different layers of pavement structure can generally be
used as satisfactory design criteria for mechanical
stabilized materials.
– Moreover, Plastic Index and Linear Shrinkage of a
material passing 0.425 mm are normally related to
one another.
– The permissible values of shrinkage may be
determined by test or estimated from the permissible
values of PI.
– Typical values are 2 % for sealed and 3 % for
unsealed pavements.
Samuel K. 323
cont...
– When the percentage of soil binder is low, as a rough rule,
• the Plasticity Modulus (PI x the percentage passing the 425 mm
sieve relative to the whole material) should not exceed 200 for
gravel to receive bituminous surface treatment.
– In arid climates, consideration could be given to relax the
PM to about 400, provided road formations are well
drained.
– Slightly wider limits of Plasticity Index may prove
satisfactory with some ironstone gravels and limestone
rubbles, if the soil binder has some natural setting
properties.
– This should not, however, be taken as a general rule - each
case should be treated on its merits and caution should be
exercised in dealing with new and unfamiliar materials.
– In the case of major works it is advisable to construct trial
sections of pavement for evaluation at least two years
before embarking upon their large-scale use. 324
Samuel K.
Figure: General properties of mechanically stabilised gradings
Samuel K. 325
cont...
• Strength Tests:
– Stabilized materials may be assessed by strength tests
suitable for this purpose at the density and moisture
conditions prevailing in the pavement during the service
life.
– It is important, in the testing of a potential base material,
to be able to predict its moisture condition.
– By this means, the failure envelopes at moisture
conditions bracketing the equilibrium moisture conditions
and at the required density anticipated in the proposed
pavement may be derived.
– The equilibrium moisture conditions to be expected in a
pavement may be obtained by examining existing roads
constructed from materials similar to those being
investigated and assembling such information for future
use.
Samuel K. 326
cont...
– One of the most commonly used strength tests is the
laboratory CBR test.
– The values given in Table below have generally been
found to be applicable.
– A 4-day soaking of compacted specimens before testing is
generally used.
– Conditions adopted for the test may be altered in respect
of
• the degree of compaction and moisture content, to simulate the
worst conditions expected in service.
– In some circumstances, conditioning the specimens by
soaking for 4 days might be too conservative, and in other
cases a period longer than 4 days might be more
appropriate for relatively impermeable materials.
– In this case the adoption of a minimum CBR value
different to those tabulated above should be considered.
Samuel K. 327
cont...
• The selection of suitable criteria should take account of
local experience,
– especially that related to the performance of local materials.
• Design of stabilised mixtures involves
– characterising the individual materials,
– proportioning them to fit the selected criteria, and
– making up a trial mixture to check that the preferred
proportions do provide the desired qualities.
• In addition to adequate investigation and design,
– good construction and control testing techniques are essential if
a satisfactory road pavement is to result.
• This involves careful proportioning and thorough mixing
of the constituent materials to produce a uniform
unsegregated final product which can be compacted and
finished in accordance with the specification.
Samuel K. 328
Table: California bearing ratio limits for mechanical
stabilised base materials
pavement Minimum CBR values

High class, high traffic volume 100
Rural roads, wet areas 80
Rural roads, dry areas 60
Samuel K. 329
Cement Stabilization
• Cement is an effective stabilizing agent applicable to
a wide range of soils and situations.
• It has two important effects on soil behaviours:
– Reduces the moisture susceptibility of soils ⎯
• cement binds the particles greatly and reduces moisture induced
volume change (shrinkage and swell) and
• it also improve strength stability under variable moisture
– Develop inter-particle bonds in granular materials ⎯
• increased tensile strength and
• elastic modulus.
• Soil properties progressively change with increasing
cement contents.
• For practical reasons, two categories of cement
stabilised materials have been identified.
Samuel K. 330
cont...
• Cement modified materials ⎯
– cement is used to reduce plasticity, volume-change, etc, and
the inter-particle bonds are not significantly developed.
– Such materials are evaluated in the same manner as
conventional unbound flexible pavement materials.
• Cement bound materials ⎯
– cement is used to sufficiently enhance modulus and tensile
strength.
– Cement bound materials have practical application in
stiffening the pavement.
• There are no established criteria to distinguish
between modified and bound materials,
– but an arbitrary limit of indirect tensile strength of 80 kN or
unconfined compressive strength of 800 kPa after seven
days moist curing has been suggested.
Samuel K. 331
cont...
• A number of factors influence the quality of the cement-soil
interactions. The most important factors can be categorized into
four groups:
• Nature and type of soil: This include:
• clay content (max 5 %),
• plasticity of the soil (max LL of 45) ,
• gradation,
• content of organic materials (max 2 %),
• sulphate content (max 0.25 % for cohesive soils and 1 % for non-cohesive
soils), and
• PH content.
– Soils with high clay content and high plasticity are difficult to mix
and high additive contents are required for an appreciable change in
properties.
– Pre-treatment with lime however is good method to allow the soil to
be cement-stabilized later on.
– Basically well graded material requires less cement content than
poorly graded one.
– The requirements with respect to the organic matter, PH, and sulphate
contents are in fact the same as those which are used for concrete.
Samuel K. 332
cont...
• Cement content:
– The cement required to stabilize soils effectively vary with the
nature and type of soils.
– The criteria used are the compressive strength (about 1.7 MPa)
after seven days.
– The quantity required for gravely soils is generally much less
than required for silty and clayey soils.
– Generally, a soil is regarded to be suited for cement-stabilised
if,
• the soil has a maximum grain size less than 75 mm,
• percents passing and retained 0.075 mm sieve is less than 35 %, and
greater than 55 % respectively, and
• liquid and plastic limits less than 50 and 25 respectively.
– Based on vast experiences on cement stabilization, the general
guidelines shown in Table below have been provided regarding
the amounts of cement that are needed to stabilize a soil. 333
Samuel K.
Table: General guidelines on cement requirement to stabilise soil
334
Samuel K.
cont...
• Moisture content:
– Moisture is required for hydration of cement to take place,
to improve the
• workability, and
• facilitate the compaction of the soil-cement mixture.
– The soil-cement mixture exhibit the same type of
moisture-density relationship as an ordinary soil.
– Thus, for a given compaction effort,
• there is an optimum moisture content at which the maximum
density is obtained.
– It is, however, seen that the highest compressive strength
can be obtained with
• specimens compacted slightly below the optimum for maximum
density.
Samuel K. 335
cont...
• Pulverization, mixing, compaction, and curing conditions:
– Many procedures of construction are available, but can be
categorised into
• mixing in plant (in a travelling plant and stationary plant for dry mixing),
and
• in place mixing.
– The methods are principally the same except mixing in the first is
done in mixing plants and in the later is in-place.
– Regardless of the type of machine used, the procedure of mix-in-
place construction involves
• initial preparation of the subgrade,
• pulverization of the soil,
• spreading of the soil,
• dry-mix the soil and the cement,
• adding water and wet mix,
• compact and finish, and
• protect and cure (place a curing membrane to keep moist). 336
Samuel K.
cont...
• The influence of the degree of mixing and compaction is
self explaining.
• One should however be aware of the fact that any delay in
compaction after mixing will have a negative effect.
• As with concrete, curing is an important factor influencing
on the end result.
• The temperature should be high enough and the stabilised
material should be prevented from drying out in order to
obtain the best result.
• Since cement stabilized materials constitute in most cases
the main structural part of pavements, much attention is
given to their mechanical characteristics such as:
– Tensile and compressive strength,
– Deformation behaviour, and
– Fatigue characteristics
Samuel K. 337
cont...
• Tensile and compressive strengths:
– After rapid strength gains in the first one to two days,
• cement stabilised materials continue to gain strength, providing curing is
sustained.
– The compressive strength based on the unconfined compressive
test increases with the cement content in the mixture depending
on the nature and types of soil as shown in Figure below.
– It has been used to determine the strength of stabilized materials,
but has little direct application to pavement design.
– CBR can be used to evaluate the strength of cement modified
materials, but not for bound materials.
– Tensile strength is important in the design of cement bound
materials.
– Density is also an important parameter which has a direct
relationship with the UCS.
Samuel K. 338
cont...
Figure: Effect of cement content on UCS and density of various stabilized soils
Samuel K. 339
cont...
– Curing temperature and curing time, compaction,
and degree of pulverization are important factors
which affect
• the strength gained by cement stabilization.
– Curing time is meant
• the time during which evaporation of moisture is
prevented.
– The method of compaction is also important for
clayey soils.
– High degree of pulverization achieved in a shorter
period of time leads to
• more intensive reaction between soils and cement and
results high strength.
Samuel K. 340
cont...
• Deformation behaviour:
– In order to make proper stress-strain analyses,
• information on the elastic modulus of the materials
should be known.
– It is well known that clays, sands, and gravels
• show different elastic deformation behaviour under
repetitive loading.
– The addition of cement on these materials changes
• the elastic deformation properties, but not completely.
– The parent material will have a great influence on
the properties of the soil-cement mixture.
Samuel K. 341
cont...
– It has been shown that cemented clayey materials
• have a different behaviour in compression (about 1.5 times of
modulus) than in tension.
– However, a more or less linear behaviour is observed up to
75 % of the failure load.
– Cemented clayey material also exhibits some degree of
• permanent deformation under repeated loading and a certain
amount of creep under steady loads.
– Cemented sand and gravel exhibit a similar performance but
• permanent deformation and creep are less than in cemented clayey
soils.
– The less fines are present in the soil mixture
• the more the cement-treated soil behave like concrete.
– Altogether, this means that the influence of the parent soil is
still noticeable in the performance of the stabilized material.
Samuel K. 342
cont...
• Fatigue characteristics:
– Cement stabilized materials cracks
• either due to hydration and drying shrinkage and fatigue at the result of
repeated tensile stresses (strains).
– Knowledge of fatigue characteristics of cement-treated materials is
essential for design purposes.
– It has been apparent that the parent soil has a great influence on the
fatigue characteristics of cement stabilized materials.
– Although there seems a great variation, there is indeed something like
a threshold strain level under which no fatigue will occur.
– Since durability and UCS are strongly related, the durability test is
normally used in the soil-cement mix design procedure.
– However, UCS test is also used as an additional test to the durability
test.
– Durability is defined as a loss in weight of a specimen after 12 freeze-
thaw cycles or 12 wet-dry cycles.
– The material loss is generated by brushing the samples after each
cycle.
Samuel K. 343
Lime stabilization
• Lime is a broad term which is used to describe
– calcium oxide (CaO) – quick lime;
– calcium hydroxide Ca(OH)2 – hydrated lime, and
– calcium carbonate (CaCO3) – carbonate of lime.
• Out of these, calcium oxide and calcium hydroxide react with
soil and calcium carbonate is of no value for stabilization.
• In practice, various forms of quick lime and hydrated lime have
been successfully utilized as a soil stabilizing agent.
• The most commonly used products are
– hydrated calcitic lime (Ca (OH2)),
– monohydrated dolomitic lime (Ca (OH2) MgO),
– calcitic quick lime (CaO),
– dolomitic quick lime (CaO MgO).
• They are available as commercial and waste lime.
• Lime can be applied as dry hydrated lime, quick lime or slurry
lime.
Samuel K. 344
cont...
• As has been indicated above, lime is an effective
stabilizing agent
– for clayey materials to improve both workability and
strength.
• Lime is not effective with cohesionless or low cohesion
materials
– without the addition of secondary (pozzolanic-fine
materials which react with lime to form cementitious
compounds) additives.
• The cementitious products resulting from cement and
lime stabilization
– are with comparable behaviour and may follow fairly
similar evaluation, design, and construction considerations.
• The significant difference in the nature and rate of
cementitious reactions, however,
– is a basis for the choice between cement and lime.
Samuel K. 345
cont...
• The reaction between soil and lime are complex and still not completely
understood.
• Basically four different factors are involved in the soil-lime reaction
which are:
– cation exchange,
– flocculation,
– pozzolanic reaction, and
– carbonation.
• Cation exchange is an immediate reaction and unlike pozzolanic reaction,
– it is not significantly dependent on temperature in which cations such as
sodium and hydrogen are replaced by calcium ions for which the clay mineral
has a greater affinity.
• It has been shown that the thickness of the water layer around the clay
particles
– decrease substantially as the result of cation exchanges.
• This condition in turn promotes the development of flocculent structures.
• This means that plasticity, shrinkage and swelling and other normal clay –
water interactions are distinctly inhibited.
Samuel K. 346
cont...
• The effects of lime on the plasticity properties of soils are
primarily due to
– cation exchange reactions.
• An immediate reduction in plasticity results in an immediate
increase in shear strength.
• The effect of lime on clay minerals of high cation exchange
capacity, such as montmorillonite clays,
– is therefore more apparent than it is on clay minerals of low
cation exchange capacity such as koalinite clays.
• Chemically equivalent amounts of quick lime and hydrated
lime have the same effect on plasticity.
• However, quick lime has an additional drying effect since,
– the chemical reaction between the lime and the water in the soil
removes free water from the sol and the heat produced by the
reaction assists in drying.
Samuel K. 347
cont...
• The change in plasticity is accompanied by an immediate
change in the strength of the soil as measured by the CBR.
• Figure below shows how the effect of lime on the CBR
value increases with time as the pozzolanic reactions take
effect.
• Siliceous and aluminous materials in the soil react with lime
to produce a gel of calcium silicates and aluminates.
• This gel cements the soil particles together in a manner that
is similar to that of hydrated cement.
• Minerals in the soil that react with lime to produce a
cementing compound are known as pozzolans.
• Lime-cementing action in a soil is usually a slow process;
depending on the type of pozzolans, it takes considerably
more time than required for hydration of Portland cement.
Samuel K. 348
cont...
• This long term effect on strength, causing continuing
strength improvements with time, often called
pozzolanic reactions.
• The cementing action also depends on climatic
conditions and a thorough compaction of the mixture.
• High curing temperatures have a positive effect on the
pozzolanic reactions.
• Temperatures lower than 13 and 16oC retard the
reaction; from this point of view it is obvious that lime
stabilization is especially popular in tropical countries.
• Carbonation occurs when the hydrated lime reacts with
the CO2 from the air.
• Carbonates (CaCO3) add some strength but the
carbonation reaction ―eats‖ the lime and will therefore
deter pozzolanic reactions.
Samuel K. 349
cont...
Figure: Effect of lime content and time on the CBR values of lime stabilised soil
350
Samuel K.
cont...
• Other factors that are of influence on the soil-lime
reaction are:
– The presence of excessive quantities of organic carbon
retards the lime-soil reaction,
– Moderately weathered and unweathered soils with
high pH display good reactivity,
– Poorly drained soils exhibit a higher degree of lime-
reactivity than better drained soils,
– All calcareous soils react satisfactorily with lime, and
– A minimum amount of clay approximately 15 % is
required to insure an adequate source of silica and/or
alumina for the lime-soil pozzolanic reaction.
Samuel K. 351
cont...
• The strength of lime stabilized materials is dependent on
– the amount of lime,
– the curing time,
– curing temperature and
– compaction.
• In addition,
– the quality of water,
– type of stabilizing lime, and
– uniformity of mixing are important factors affecting the quality of production
as they are in cement stabilization.
• Although lime modifies or bonds soil as in cement stabilization, the
tendency to form bound products is less with lime than it is with cement.
• Lime has more tendencies to produce granular materials and consequently
its major applications are in the modification of
– clays,
– plastic sands, and
– plastic gravels.
Samuel K. 352
cont...
• Mix design procedures for lime stabilisation are
– the determination of the maximum amount of lime that can be taken by
the soil before free lime occurs (the lime content above which further
increases do not produce significant additional strength) or
– the lime requirement to attain a specific strength levels.
• The usually used minimum strength requirements for mix design
are
– 0.69 MPa for subbase and 1.03 MPa for base courses.
• These minimum strengths are
– related to the AASHTO coefficients of relative strength of 0.12 for
subbase materials and 0.11 for base-course materials.
• When lime is used for subgrade improvements, the design lime
content may be designated as
– the lime content above which no further appreciable reduction in PI
occurs or
– a minimum lime content which produces an acceptable PI reduction.
• For field construction, the lime content is increased 0.5 to 1.0 %
to offset the effect of field variability.
353
Samuel K.
Bituminous Stabilization
• Bituminous stabilization is used
– with non-cohesive granular materials ⎯
• where the bitumen adds cohesive strength;
– with cohesive materials ⎯
• where the bitumen ―waterproofs‖ the soil thus reducing loss of strength
with increase in moisture content.
• Both effects take place partly
– from the formation of bitumen film around the soil particles
which bonds them together,
– prevents the absorption of water, and
– partly from simple blocking of the pores, preventing water
from entering the soil mass.
• Because more care is necessary in bituminous stabilization
to achieve satisfactory mixing,
– its use has not been as widespread as cement and lime
stabilizations.
Samuel K. 354
cont...
• Bituminous materials.
– The bituminous materials that are used for stabilization works are
mostly
• penetration grade bitumen,
• cutback bitumen and
• bitumen emulsion.
– The characteristics of cutbacks depended on
• the particle size distribution of the soil,
• the temperature of application, and
• the type of mix plant.
– The more viscous binders are normally used soils having only a small
proportion of material passing the 0.075 mm sieve and for plat mixes,
while the lighter binders are used for mix-in-place methods and with
soils containing a larger proportion of fines.
– Emulsions are generally suitable for soil stabilization in climate where
rapid drying conditions occur,
• since this is equivalent to adding water to the soil as well as bituminous binder.
– In the tropics, where the temperature is high the use of emulsions may
be an advantage since it helps to
• provide part of the optimum moisture content for compaction, thereby
reducing the amount of water necessary for this purpose. 355
Samuel K.
cont...
• Soil requirements:
– Bituminous materials are used for the stabilization of
both cohesive and noncohesive granular soils.
– Soils which can readily pulverized by construction
equipment are satisfactory for bituminous stabilization.
– Cohesive soils usually have satisfactory bearing
capacity at low moisture content.
– The purpose of using bitumen as a stabilizer in such
soils is
• to waterproof them as a means to maintain them at low
moisture contents and high bearing capacities.
– In the non-cohesive granular materials,
• bitumen serves as a bonding or cementing agent between
particles.
Samuel K. 356
cont...
– Depending on the particle size distribution and
physical properties of the available soil materials
and the function of the stabilising bitumen, there
are four types of soil-bitumen mixtures in highway
engineering:
• Soil-bitumen:
– this is a mixture of cohesive soil and bitumen for
waterproofing purposes.
– The maximum grain size should preferably not greater than
one-third of the compacted layer.
– The best result has been obtained with soils that fall within the
grain size limits shown in next Table .
– The bitumen requirements commonly range from 4-7% of the
dry weight of the soil.
Samuel K. 357
cont...
• Sand bitumen:
– sands such as beach, river, pit, or existing roadway sand may be
stabilized with bitumen if they are substantially free from
vegetable matter, lumps or balls of clay or adherent films of clay.
– Some times it may require admixture of filler material to meet
mechanical stability requirements.
– It is recommended that the sand contain less than 12 % of 0.075
mm.
– however, in the case of windblown sands up to 25 % finer than
0.075 mm may be allowed provided that the portion of the sand
passing the No. 40 sieve has a field moisture less than 20 % and
linear shrinkage less than 5 %.
– The required amount of bitumen content ranges from 4-10 %, the
optimum should be determined by compaction, strength, and
water resistance testing and should not exceed the pore space of
the compacted mineral mix.
Samuel K. 358
cont...
• Waterproofed granular stabilization:
– This is a system in which a soil material possessing
» good gradation of constituent particles from coarse to fine,
and
» having high potential density is waterproofed by uniform
distribution of small amount (1-2 %) of bitumen.
– Recommended gradations of the soil aggregate materials are
shown in the next Table.
• Oiled earth:
– This is a soil surface, consisting of silt-clay material made water
and abrasion resistant by slow or medium curing bitumen
cutbacks or emulsions.
Samuel K. 359
Table : Characteristics of soils empirically found suitable for bitumen stabilization
360
Samuel K.
cont...
• The mechanism of stabilization with bituminous materials
consists of
– adding cohesive strength and reducing the percolation of water;
• no chemical interaction is taking place.
• Waterproofing occur
– by coating the surface of particles or aggregated lumps of particles
or
– by blocking the pores of the soil mass, and a strength comes from
the presence of a continuous film of bitumen, giving cohesion.
• There are two opposing effects –
– the thinner the film of bitumen the stronger the material; however,
– thick films or filled pores are the most effective in preventing
ingress of water.
• Too much bitumen, however,
– causes loss of strength by lubricating the particles and preventing
interlock.
Samuel K. 361
cont...
• The mix design procedure for bituminous treatments of soils may
be considered under four headings:
– mix design for stability in non-cohesive material;
– mix design for waterproofing in non-cohesive or cohesive materials;
– mix design for sand-bitumen mixes, and
– mix design for oiled earth roads.
• For the first three types of mix,
– a series of tests should be made with varying bitumen contents and
grades using
• hot bitumen,
• cutback and emulsion, and
• the appropriate mix is selected giving due weight to the need for stability or
water resistance as required.
• Compaction, compressive, and water absorption test are normally
used to select the optimum amount of bitumen content.
• Many difficulties in construction and poor pavement
performance may be attributed to a lack of appreciation of this
additive effect. Samuel K. 362
Chapter Seven
Bituminous materials
Introduction
• Binders used in pavement construction are mainly of two types:
– cement and
– bitumen.
• Cement:
– the most commonly used cementing agent in the concrete building industry.
– In road construction, it is used as a binder for rigid pavement structures and
a stabilizing agent.
• Bituminous material (or bitumen), also known as asphalt cement in the
US:
– a viscous liquid or solid material, black or dark brown in colour, having
adhesive properties, consisting essentially of hydrocarbons which are
soluble in carbon disulphate.
– They are usually fairly hard at normal temperatures.
– When heated, they soften and flow.
– When mixed with aggregates in their fluid state, and then allowed to cool,
they solidify and bind the aggregates together, forming a pavement surface.
– They are used on all types of roadway – from multiple layers of asphalt
concrete on the highest class of highways to thin, dust-control layers on
seldom-used roads.
Samuel K. 364
Types of Bituminous Materials
• Bituminous materials are derived from
– petroleum or occur in natural deposits in different parts of the
world.
• Based on their sources there are two main categories of
bitumens, namely,
– those which occur naturally and
– those which are by-products of the fractional distillation of
petroleum at refinery.
• Refinery bitumens are by far
– the greater proportion of road bitumen used all over the
world.
• Of the possible types falling into these categories,
– the ones that are used for highway paving purposes are
illustrated in Figure below.
Samuel K. 365
cont...
Figure: Commonly used types of road bitumen
Bituminous Materials
Natural Bitumen Refinery Bitumen
Penetration grade Liquid bitumen

Lake asphalt Rock asphalt
bitumen
Cutbacks Emulsions
Slow curing Medium curing Rapid curing Anionic Cationic

cutback cutback cutback emulsion emulsion
Samuel K. 366
Natural Bitumen
• Native or natural Bitumens relate to a wide variety of materials
and refer to
– those bitumens that are found in nature as native asphalts or rock
asphalts associated with appreciable quantities of mineral matter.
• Native asphalts are obtained from
– asphalt lakes in Trinidad and other Caribbean areas, and
– were used in some of the earliest pavements in North America after
softening with petroleum fluxes.
• The properties depend on the insoluble materials (organic and
inorganic) the asphalt contains.
• Some natural asphalts are soft and adhesive; others are very
hard and brittle.
• Some exist on the surface of the earth in lakes or pools, while
others occur at depth and must be mined.
• Rock asphalts
– are natural rock deposits containing bituminous materials that have
been used for road surfaces in localities where they occur.
Samuel K. 367
Refinery Bitumen
• Bitumens artificially produced by the industrial
refining of crude petroleum oils are known under a
number of names depending on the refining method
used such as
– residual bitumens,
– straight-run bitumens,
– steam-refined bitumens and ⎯
– as is now most commonly accepted ⎯ refinery bitumens.
• Petroleum crudes
– are complex mixtures of hydrocarbons differing in
molecular weight and consequently in boiling range.
• Before they can be used, crudes have to be
– separated, purified, blended, and sometimes chemically or
physically changed.
Samuel K. 368
cont...
• Not all petroleum crudes contain a sufficient
quantity of bitumen
– to enable straight reduction to specification road
bitumen.
• Those which do are known as asphaltic-base
crudes.
• Crudes which contain high proportions of simpler
paraffinic compounds, with little or no bituminous
bodies present, are known as paraffinic-base
crudes.
• Some petroleum crudes exhibit characteristics of
both the previous categories, and these are known
as mixed-base crudes.
Samuel K. 369
cont...
• The primary processing involved in the production of
bitumen from petroleum is fractional distillation.
• This is carried out in tall steel towers known as
fractionating or distillation columns as schematically
shown in Figure below.
• The inside of the column is divided at intervals by
horizontal steel trays with holes to allow vapour to rise
up the column.
• In this process, part of the hydrocarbon materials in the
crude oil are vaporized by heating them above their
boiling points under pressure.
• The lightest fractions of the crude remain as a vapour and
are taken from the top of the distillation column, heavier
fractions are taken off the column as side-streams with
the heaviest fractions remaining as a liquid and therefore
left at the base of the column.
Samuel K. 370
cont...
• The lightest fractions produced by the crude distillation
process include
– propane and butane which are gases under atmospheric
conditions.
• Moving down the column a slightly heavier product,
naphtha, is produced
– which is a feedstock for gasoline production and the chemical
industry.
• Then there is kerosine, which is used primarily for aviation
fuel and to a lesser extent for domestic fuel.
• Heavier again is gas oil, which is used as a fuel for diesel
engines and central heating.
• The heaviest fraction taken from the crude oil distillation
process is long residue which is a complex mixture of high
molecular weight hydrocarbons.
• Such refining process is known as straight-run distillation,
and the residue is straight-run bitumen.
Samuel K. 371
Figure: Flow chart of the manufacture of refinery bitumens
Samuel K. 372
cont...
• To remove high boiling temperature constituents such as
those contained in the non-volatile oils, refining is carried
out, without changing them chemically by the use of
– reduced pressures and steam injection in the fractionating
column.
• This type of distillation is known as vacuum or steam
distillation, and bitumen produced by such means are said
to be vacuum reduced or steam refined.
• On the other hand, when the objective is primarily to
increase the yield of fuels,
– the petroleum oil undergoes cracking distillation.
• In general, cracking process consists in exposing the
petroleum crude to a temperature of 475-600oC at pressure
varying from 3 to 75 atmospheres.
• This process produces heavier residues as a consequence of
forming the lighter materials.
Samuel K. 373
cont...
• These residues are known as "cracked oil" or "cracked asphalt".
• They are characterized by relatively
– high specific gravity,
– low viscosity, and
– poor temperature susceptibility.
• They are generally regarded as
– Less durable or weather resistant than straight run materials.
• In a few cases, a selective solvent, such as propane, is used to treat
the topped crude to separate paraffinic crude oils of high viscosity
index
– for use in the manufacture of lubricating oils and special products.
• This separation method is based on chemical type and molecular
weight rather than
– by boiling point as in the usual distillation.
• In the process, the paraffinic oils are dissolved by the solvent and
come afloat in the fractioner vessel.
• The residual asphalt, which is relatively insoluble, is drawn off at
the bottom.
Samuel K. 374
cont...
• These residual asphalts produced by the different
methods of refining described above are of various
grades asphalt cement,
– depending upon the degree to which distillates are removed
as determined by the conditions of distillations.
• They are further processed by
– air-blowing,
– blending,
– compounding, and
– admixing with other ingredients to make variety of asphalt
products used in
• paving,
• roofing,
• waterproofing,
• coating and sealing materials, and
• materials for industrial applications. 375
Samuel K.
Penetration Grade Bitumen
• In the preparation of paving binders, it is common
to blend two or more different asphaltic residues
to produce
– a material possessing desirable physical properties.
• Additive materials may also be used to improve
properties such as
– adhesion to solid surfaces and flowing characteristics.
• By varying the ingredients and the amounts used,
– it is possible to exercise considerable control over the
properties of the finished asphalt.
• The major paving products are
– penetration grade bitumens (also known as asphalt
cements),
– cutback asphalts, and
– asphalt emulsions. Samuel K. 376
cont...
• Penetration grade bitumen or asphalt cements are in
consistency from semi-solid to semi-liquid at room
temperature.
• Such bitumen are graded according their viscosity (mainly
used in the US) and penetration.
• Penetration is the depth in 0.1 mm that a specified needle is
able to penetrate the samples when standard penetration
tests are carried out.
• They are produced in various viscosity grades,
– the most common being AC 2.5, AC 5, AC 10, AC 20, and AC
40.
• These roughly correspond to penetration grades 200-300,
120-150, 85-100, 60-70, and 40-50, respectively.
• The viscosity grades indicate the viscosity in hundreds of
poises ± 20% measured at 60oC (140oF).
• For example, AC 2.5 has a viscosity of 250 poises ± 50. AC
40 has a viscosity of 4000 poises ± 800. 377
Samuel K.
cont...
• The majority of penetration grade bitumens is used in
road construction,
– the harder grades, 35 pen to 100 pen, being used in asphalt
where bitumen stiffness is of primary importance and
– the softer grades, 100 pen to 450 pen, in macadams where
the lubricating properties during application and bonding
of the aggregate in service are more important.
• During construction
– asphalt cements require to be heated to varying degrees
before mixing with hot or warm aggregates and
– the mixed material must be laid while hot within a few
hours of mixing.
Samuel K. 378
Liquid Bitumen
• Sometimes it is uneconomical or inconvenient to use
hot asphalt in road construction.
• In such situations, liquid binders are preferable to
handle at relatively low temperatures and mixed with
aggregates either when cold or only warmed
sufficiently to make them surface-dry.
• For the suitability of such construction methods,
– asphalt cements are frequently modified by preparation as
liquid products.
• The two forms of liquid bitumen generally,
– are those which are prepared by dissolving the asphalt
cement in a suitable volatile solvent and known as cutback
bitumen, and
– those which are prepared by emulsifying the asphalt
cement in an aqueous medium and called bitumen
emulsions.
Samuel K. 379
Cutback Bitumen
• Cutback bitumen are prepared by
– dissolving penetration grade bitumen in suitable
volatile solvents
• to reduce their viscosity to make them easier to use at
ordinary temperatures.
• They are commonly heated and then sprayed on
aggregates.
• Upon curing by evaporation of the solvent,
– the cured-out asphalt cement will be in approximately
the same condition as before being taken into solution
and
– bind the aggregate particles together.
• The curing period depends on the volatility of
solvents. Samuel K. 380
cont...
• Cutback bitumen are grouped into three types based
on
– the type of solvent, which governs the rates of evaporation
and curing, namely,
• slow-curing (SC),
• medium-curing (MC), and
• rapid-curing (RC).
• Each type of cutback bitumen is subdivided into
several grades characterized by
– their viscosity limits.
• The viscosity is controlled by the quantity of cutback
solvent
– to make the various grades from very fluid to almost semi-
solid at ambient temperatures.
Samuel K. 381
Slow-Curing (SC) Cutbacks:
• Slow-curing asphalts can be obtained directly as
slow-curing straight-run asphalts through
– the distillation of crude petroleum or
– as slow-curing cutback asphalts by "cutting back" asphalt
cement with a heavy distillate such as diesel oil.
• They have lower viscosities than asphalt cement and
are very slow to harden.
• Slow-curing asphalts are usually designated as
– SC-70, SC-250, SC-800, or SC-3000,
• where the numbers are related to the approximate kinematic
viscosity in centistokes at 60oC (140oF).
• They are used with
– dense-graded aggregates and
– on soil-aggregate roads in warm climates to avoid dust.
Samuel K. 382
Medium-Curing (MC) Cutbacks:
• Medium-curing asphalts are produced by
– fluxing, or cutting back, the residual asphalt (usually 120-150 penetration)
with light fuel oil or kerosene.
• The term medium refers to the medium volatility of the kerosene-type
dilutent used.
• Medium-curing cutback asphalts harden faster than slow-curing liquid
asphalts,
– although the consistencies of the different grades are similar to those of the
slow-curing asphalts.
• However, the MC-30 is a unique grade in this series as
– it is very fluid and
– has no counterpart in the SC and RC series.
• The fluidity of medium-curing asphalts depends on
– the amount of solvent in the material.
• MC- 3000, for example, may have only 20 percent of the solvent by
volume, whereas MC-70 may have up to 45 percent.
• These medium-curing asphalts can be used for the construction of
– pavement bases,
– surfaces, and
– surface treatments. Samuel K. 383
Rapid-Curing (RC) Cutbacks:
• Rapid-curing cutback asphalts are produced by
– blending asphalt cement with a petroleum distillate that will
easily evaporate,
• thereby facilitating a quick change from the liquid form at time of
application to the consistency of the original asphalt cement.
• Gasoline or naphtha generally is used as the solvent
for this series of asphalts.
• The grade of rapid-curing asphalt required
– dictates the amount of solvent to be added to the residual
asphalt cement.
• For example, RC-3000 requires about
– 15 percent of distillate, whereas RC-70 requires about 40
percent.
– These grades of asphalt can be used for jobs similar to those
for which the MC series is used, but where there is a need
for immediate cementing action or colder climates. 384
Samuel K.
Asphalt emulsions
• Emulsified asphalts are produced by
– breaking asphalt cement, usually of 100-250 penetration
range, into minute particles and
– dispersing them in water with an emulsifier.
• These minute particles have like electrical charges
and therefore do not coalesce.
• They remain in suspension in the liquid phase as
long as
– the water does not evaporate or
– the emulsifier does not break.
• Asphalt emulsions therefore consist of asphalt,
– which makes up about 55 percent to 70 percent by weight,
– up to 3% emulsifying agent, water and
– in some cases may contain a stabilizer.
Samuel K. 385
cont...
• Two general types of emulsified asphalts are produced,
depending on the type of emulsifier used:
– cationic emulsions,
• in which the asphalt particles have a positive charge;
– anionic,
• in which they have a negative charge.
• Each of the above categories is further divided into three
subgroups,
– based on how rapidly the asphalt emulsion will return to the state of
the original asphalt cement.
• These subgroups are
– rapid setting (RS),
– medium-setting (MS), and
– slow setting (SS).
• A cationic emulsion is identified by
– placing the letter "C" in front of the emulsion type; no letter is placed
in front of anionic and nonionic emulsions.
– For example, CRS-2 denotes a cationic emulsion, and RS-2 denotes 386
either anionic or nonionic emulsion. Samuel K.
cont...
• The anionic and cationic asphalts generally are used in
– highway maintenance and construction.
• Note, however, that since anionic emulsions contain
negative charges,
– they are more effective in adhering aggregates containing
electropositive charges such as limestone,
– whereas cationic emulsions are more effective with
electronegative aggregates such as
• those containing a high percentage of siliceous material.
• Cationic emulsions also work better with wet aggregates
and in colder weather.
• Bitumen emulsions break
– when sprayed or mixed with mineral aggregates in a field
construction process;
• the water is removed, and
• the asphalt remains as a film on the surface of the aggregates.
• In contrast to cutback bitumens,
– bitumen emulsions can be applied to a damp surface. 387
Samuel K.
The Air-Blown Bitumen
• The physical properties of the short residue are further modified by air-
blowing.
• Air-blowing is
– a process in which a soft asphaltic residue is heated to a high temperature in
an oxidation tower where air is forced through the residue either on a batch
or a continuous basis.
• The process results in a dehydrogenation and polymerization of the
residue.
• The hard asphaltic material produced by this process is known as
oxidized or air-blown asphalt and is usually specified and designated by
both softening point and penetration tests.
• If proper precautions are not taken,
– the temperature can rise to the point where the physical characteristics of
the product are seriously affected.
• However, by controlling the conditions in the process a large variety of
blown asphalts can be produced.
• Oxidised bitumens are used almost entirely for industrial applications,
such as
– roofing, flooring, mastics, pipe coatings, paints, etc, but
– their use in road construction is limited. Samuel K.
388
Road Tars
• Tars are obtained from
– the destructive distillation of such organic materials as
coal.
• Their properties are significantly different from
petroleum asphalts.
• In general, they are more susceptible to
– weather conditions than are similar grades of asphalts,
and
– they set more quickly when exposed to the
atmosphere.
• Tars are rarely used now for highway pavements.
Samuel K. 389
Tests on Bituminous Materials
• Several tests are conducted on bituminous
materials to determine both
– their consistency and
– their quality to ascertain whether materials used in
• highway construction meet the prescribed
specifications.
• Some of these specifications are provided in
– standards of AASHTO, ASTM, and Asphalt
Institute.
• Procedures for testing and selecting
representative samples of asphalt have also
been standardized. Samuel K. 390
Consistency Tests
• The consistency of bituminous materials is important in pavement
construction because
– the consistency at a specified temperature will indicate the grade of the
material.
• It is important that the temperature at which the consistency is determined
be specified,
– since temperature significantly affects the consistency of bituminous
materials.
• As stated earlier, asphaltic materials can exist in either liquid, semisolid,
or solid states.
• This necessitates for more than one method for determining consistency of
asphaltic materials.
• The property generally used to describe the consistency of asphaltic
materials in the liquid state is
– the viscosity, which can be determined by conducting either
• the Saybolt Furol viscosity test or
• the kinematic viscosity test.
• Tests used for asphaltic materials in the semisolid and solid states include
– the penetration test and the float test.
• The ring-and-ball softening point test may also be used for blown asphalt.
391
Samuel K.
Saybolt Furol Viscosity Test
• Saybolt Furol viscosity test is
– a test carried out by the Saybolt Furol Viscometer
• which has a standard viscometer tube, 12.7 cm (5 in) long and about
2.54 cm (1 in) in diameter with an orifice of specified shape and
dimensions provided at the bottom of the tube.
• When testing, the orifice is closed with a stopper, and the tube is
filled with a quantity of the material to be tested.
• The material in the tube is brought to the specified temperature
by
– heating in a water bath and
– when the prescribed temperature is reached the stopper is removed,
and the time in seconds for exactly 60 milliliters of the asphaltic
material to flow through the orifice is recorded.
• This time is the Saybolt Furol viscosity in units of seconds at the
specified temperature.
• Temperatures at which asphaltic materials for highway construction
are tested include 25oC (77oF), 50oC (122oF), and 60oC (140oF).
• It is apparent that the higher the viscosity of the material, the longer it
takes for a given quantity to flow through the orifice. 392
Samuel K.
Kinematic Viscosity Test
• The test uses a capillary viscometer tube
– to measure the time it takes the asphalt sample to flow
at a specified temperature between timing marks on
the tube.
• Three types of viscometer tubes, namely
– Zeitfuch's cross-arm viscometer,
– Asphalt Institute vacuum viscometer, and
– Cannon- Manning vacuum viscometer are used.
• When the cross-arm viscometer is used,
– the test is started by placing the viscometer tube in a
thermostatically controlled constant temperature bath,
and
– a sample of the material to be tested is then preheated
and poured into the large side of the viscometer tube
until the filling line level is reached.
Samuel K. 393
cont...
• The temperature of the bath is then brought to 135oC
(275oF), and some time is allowed for the viscometer and
the asphalt to reach a temperature of 135oC (275oF).
• Flow is then induced by applying a slight pressure to the
large opening or a partial vacuum to the efflux (small)
opening of the viscometer tube.
• This causes an initial flow of the asphalt over the siphon
section just above the filling line.
• Continuous flow is induced by the action of gravitational
forces.
• The time it takes for the material to flow between two
timing marks is recorded.
• The kinematic viscosity of the material in units of
centistokes is obtained by multiplying the time in seconds
by a calibration factor for the viscometer used.
Samuel K. 394
cont...
• The calibration of each viscometer is carried out by
using standard calibrating oils with known viscosity
characteristics.
• The factor for each viscometer is usually furnished by
the manufacturer.
• The test may also he conducted at a temperature of
60oC (140oF) using either
– the Asphalt Institute vacuum viscometer or
– the Cannon-Manning vacuum viscometer.
• In this case, flow is induced by applying a prescribed
vacuum through
– a vacuum control device attached to a vacuum pump.
• The product of the time interval and the calibration
factor in this test gives
– the absolute viscosity of the material in poises.
Samuel K. 395
Penetration Test
• The penetration test gives
– an empirical measurement of the consistency of a
semi-solid asphaltic material in terms of
• the depth a standard needle penetrates into that material
under a prescribed loading and time.
• It is the bases for
– classifying semi-solid bituminous materials into
standard grades.
Figure: Standard
penetration test
Samuel K. 396
cont...
• Figure above shows a schematic of the standard penetration
test.
• A sample of the asphalt cement to be tested is placed in a
container,
– which in turn is placed in a temperature-controlled water bath.
• The sample is then brought to
– the prescribed temperature of 25oC (77oF), and
– the standard needle, loaded to a total weight of 100 gm,
• is left to penetrate the sample of asphalt for the prescribed time of exactly 5
sec.
• The penetration is given as
– the diepth in units of 0.1 mm that the needle penetrates the sample.
• For example, if the needle penetrates a depth of exactly 20 mm,
– the penetration is of the material is said to be 200.
• When carried out at different temperature
– penetration test can indicate temperature susceptibility of the binder.
Samuel K. 397
Float Test
• The float test is used to determine
– the consistency of semisolid asphalt materials that are
• more viscous than grade 3000 or
• have penetration higher than 300, since these materials cannot be
tested conveniently using either
– the Saybolt Furol viscosity test or the penetration test.
Figure: Float test 398

Samuel K.
cont...
• The float test is conducted with the apparatus schematically shown in
Figure above.
• It consists of an aluminum saucer (float), a brass collar that is open at
both ends, and a water bath.
• The brass collar is filled with a sample of the material to be tested and
then is attached to the bottom of the float and chilled to a temperature
of 5oC (41oF) by immersing it in ice water.
• The temperature of the water bath is brought to 50oC (122oF), and the
collar (still attached to the float) is placed in the water bath, which is
kept at 50oC (122oF).
• The head gradually softens the sample of asphaltic material in the
collar until the water eventually forces its way through the plug into
the aluminium float.
• The time in seconds that expires between the instant the collar is
placed in the water bath and that at which the water forces its way
through the bituminous plug is the float test value, and it is a measure
of consistency.
• It is apparent that the higher the float-test value, the stiffer the
material.
Samuel K. 399
Ring-and-Ball Softening Point Test
• The ring-and-ball softening point test is used to measure
– the susceptibility of asphaltic maertails to temperature changes by
determining
• the temperature at which the material will be adequately softened to allow a
standard ball to sink through it.
• Figure below shows the apparatus commonly used for this test.
• It consists principally of
– a small brass ring of 15.875 mm (5/8 in) inside diameter and 6.35 mm
(1/4 in) high,
– a steel ball 9.525 mm (3/8 in) in diameter, and
– a water or glycerin bath.
• The test is conducted
– by first placing a sample of the material to be tested in the brass ring,
– which is cooled and immersed in the water or glycerin bath that is
maintained at a temperature of 5oC (41oF).
• The ring is immersed to a depth such that its bottom is exactly 2.54
mm (1 in) above the bottom of the bath.
400
Samuel K.
cont...
• The temperature of the bath is then gradually increased,
– causing the asphalt to soften and
– permitting the ball to sink eventually to the bottom of the bath.
• The temperature in at which the asphaltic material touches the bottom of the
bath is recorded as the softening point.
Figure: Ring-and-ball softening point test

Samuel K. 401
Durability Tests
• When asphaltic materials are used in the construction of
roadway pavements,
– they are subjected to changes in temperature and other weather
conditions over a period of time.
• These changes cause natural weathering of the material,
which may lead to
– loss of plasticity,
– cracking,
– abnormal surface abrasion, and
– eventual failure of the pavement.
• This change, known as weathering, is caused by
– chemical and physical reactions that take place in the material.
• One test used to evaluate the susceptibility characteristics
of asphaltic materials to changes in temperature and other
atmospheric factors is the thin-film oven test.
Samuel K. 402
Thin-Film Oven Test (TFO)
• This is a procedure that measures
– the changes that take place in an asphalt during the
hot-mix process by
• subjecting the asphaltic material to hardening conditions
similar to those in a normal hot-mix plant operation.
• The consistency of the material is determined
before and after the TFO procedure,
– using either the penetration test or a viscosity test,
• to estimate the amount of hardening that will take place
in the material when used to produce plant hot-mix.
Samuel K. 403
cont...
• The procedure is performed by pouring
– 50 cc of the material into a cylindrical flat-bottom pan,
14 cm (5.5 in) inside diameter and 1 cm (3/8 in) high.
• The pan containing the sample is then placed on
– a rotating shelf in an oven and rotated for five hours at
a temperature of 163oC (325oF).
• The amount of penetration after the TFO test is
then expressed as
– a percentage of that before the test to determine
percent of penetration retained.
• The minimum allowable percent of penetration
retained is usually specified for different grades of
asphalt cement.
Samuel K. 404
Rate of Curing
• Tests for curing rates of cutbacks and emulsions
are based on inherent factors, which can be
controlled.
• The test is conducted to determine
– the time required for a liquid asphaltic material to
increase in its consistency on the assumption that the
external factors are held constant.
• Volatility and quantity of solvent for cutbacks are
commonly used to indicate the rate of curing.
• The curing rates for both cutbacks and emulsions
may be determined from the distillation test.
Samuel K. 405
Distillation Test for Cutbacks
• In the distillation test for cutbacks, the apparatus used consists
principally of
– a flask in which the material is heated,
– a condenser, and
– a graduated cylinder for collecting the condensed material.
• A sample of 200 cc of the material to be tested is measured and poured
into the flask.
• The material is then brought to boiling point by heating it with the
burner.
• The evaporated solvent is condensed and collected in the graduated
cylinder.
• The temperature in the flask is continuously monitored and the amount
of solvent collected in the graduated cylinder recorded
– when the temperature in the flask reaches 190oC (374oF), 225oC (437oF),
260oC (500oF), and 316oC (600oF).
• The amount of condensate collected at the different specified
temperatures gives
– an indication of the volatility characteristics of the solvent.
• The residual in the flask is the base asphaltic material used in preparing
406
the cutback. Samuel K.
Distillation Test for Emulsions
• The distillation test for emulsions is similar to that described for cutbacks.
• A major difference, however, is that
– the glass flask and Bunsen burner are replaced with an aluminum alloy still and a
ring burner.
• This equipment prevents potential problems that may arise from
– the foaming of the emulsified asphalt as it is being heated to a maximum of 260oC
(500oF).
• The results obtained from the use of this method to recover the asphaltic residue
and to determine the properties of the asphalt base stock used in the emulsion
may not always be accurate because of
– significant changes in the properties of the asphalt due to concentration of inorganic
salts, emulsifying agent, and stabilizer.
• These changes, which are due mainly to the increase in temperature, do not
occur in field application of the emulsion
– since the temperature in the field is usually much less than that used in the
distillation test.
• The emulsion in the field, therefore,
– breaks either electrochemically or by evaporation of the water.
• An alternative method to determine the properties of the asphalt after it is cured
on the pavement surface is
– to evaporate the water at subatmospheric pressure and lower temperatures.
Samuel K. 407
General Tests
• Several other tests are routinely conducted on
asphaltic materials used for pavement
construction either
– to obtain specific characteristics for design
purposes (for example, specific gravity) or
– to obtain additional information that aids in
determining the quality of the material.
• Some of the more common routine tests are
described briefly hereunder.
Samuel K. 408
Specific Gravity Test
• The specific gravity of asphaltic materials is used mainly
– to determine the weight of a given volume of material, or vice versa,
– to determine the amount of voids in compacted mixes and
– to correct volumes measured at high temperatures.
• Specific gravity is defined as
– the ratio of the weight of a given volume of the material to the
weight of the same volume of water.
• The specific gravity of bituminous materials, however, changes
with temperature,
– which dictates that the temperature at which the test is conducted
should be indicated.
• For example, if the test is conducted at 25oC (77oF) which is
usually the case and the specific gravity is determined to be
1.41, this should be recorded as 1.41/25oC.
• Note that both the asphaltic material and the water should be at
the same temperature.
Samuel K. 409
cont...
• The test is normally conducted with
– the dry weight (W1) of the pycnometer and stopper is obtained,
and then
– the pycnometer is filled with distilled water at the prescribed
temperature.
• The weight (W2) of the water and pycnometer together is
determined.
• If the material to be tested can flow easily into the
pycnometer, then
– the pycnometer must be completely filled with the material at the
specified temperature after pouring out the water.
• The weight W3 is then obtained.
• The specific gravity of the asphaltic material is then given
as
• Where Gb is the specific gravity of the asphaltic material

and W1, W2, and W3 are in grams.
410
Samuel K.
cont...
• If the asphaltic material cannot easily flow,
– a small sample of the material is heated gradually to facilitate
flow and
– then poured into the pycnometer and left to cool to the
specified temperature.
• The weight W4 of pycnometer and material is then
obtained.
• Water is then poured into the pycnometer to completely fill
the remaining space not occupied by the material.
• The weight W5 of the filled pycnometer is obtained.
• The specific gravity is then given as
Samuel K. 411
Ductility Test
• Ductility:
– the distance in centimeters a standard sample of asphaltic material will
stretch before breaking when tested on standard ductility test equipment at
25oC (77oF).
• The result of this test indicates
– the extent to which the material can he deformed without breaking.
• It also indicates the temperature susceptibility of binders.
• Bitumens possessing high ductility are usually
– highly susceptible to temperature while low ones are not.
• The test is used mainly for semisolid or solid materials,
– which first are gently heated to facilitate flow and then are poured into a
standard mold to form a briquette of at least 1 cm2 in cross section.
• The material is then allowed to cool to 25oC (77oF) in a water bath.
• The prepared sample is then placed in the ductility machine and
extended at a specified rate of speed until the thread of material joining
the two ends breaks.
• The distance (in centimeters) moved by the machine is the ductility of
the material.
Samuel K. 412
Solubility Test
• The solubility test is used to measure
– the amount of impurities in the asphaltic material.
• Since asphalt is nearly 100 percent soluble in certain
solvents,
– the portion of any asphaltic material that will be effective in
cementing aggregates together can be determined from the
solubility test.
• Insoluble materials include free carbon, salts, and other
inorganic impurities.
• The test is conducted
– by dissolving a known quantity of the material in a solvent,
such as trichloroethylene, and
– Then filtering it through a Gooch Crucible.
• The material retained in the filter is dried and weighed.
• The test results are given in terms of the percent of the
asphaltic material that dissolved in the solvent. 413
Samuel K.
Flash-Point Test
• The flash point of an asphaltic material
– is the temperature at which its vapors will ignite instantaneously in the
presence of an open flame.
• Note that the flash point is normally lower than the temperature at
which the material will burn.
• The test can be conducted by using either
– the Tagliabue open-cup apparatus or
– the Cleveland open-cup apparatus.
• The Cleveland open-cup test is more suitable for
– materials with higher flash points,
• Whereas the Tagliabue open-cup is more
– suitable for materials with relatively low flash points, such as cutback
asphalts.
• The test is conducted by
– partly filling the cup with the asphaltic material and gradually
increasing its temperature at a specified rate.
• A small open flame is passed over the surface of the sample at
regular intervals as the temperature increases.
Samuel K. 414
cont...
• The increase in temperature will cause
– evaporation of volatile materials from the material
being tested, until a sufficient quantity of volatile
materials is present to cause
• an instantaneous flash when the open flame is passed
over the surface.
• The minimum temperature at which this occurs
is the flash point.
• It can be seen that this temperature gives an
indication of
– the temperature limit at which extreme care should
be taken, particularly when heating is done over
open flames in open containers.
Samuel K. 415
Loss-on-Heating Test
• The loss-on-heating test is used to determine
– the amount of material that evaporates from a sample of asphalt under a
specified temperature and time.
• The result indicates
– whether an asphaltic material has been contaminated with lighter
materials.
• The test is conducted by
– pouring 50 g of the material to be tested into a standard cylindrical tin
and
– leaving it in an oven for 5 hr at a temperature of 163oC (325oF).
• The weight of the material remaining in the tin is determined, and
the loss in weight is expressed as a percentage of the original
weight.
• The penetration of the sample may also be determined before and
after the test to determine
– the loss of penetration due to the evaporation of the volatile material.
• This loss in penetration may be used as an indication of
– the weathering characteristics of the asphalt. 416
Samuel K.
Types of Asphalt Mixtures
• There are different types of asphalt mit. Some
of the common mixes are the following:
• Hot mix asphalt concrete (HMAC)
– is produced at 160 degrees Celsius.
– This high temperature serves to decrease viscosity
and moisture during the manufacturing process,
resulting in a very durable material.
– HMAC is most commonly used for high-traffic
areas, such as busy highways and airports.
Samuel K. 417
cont....
• Warm mix asphalt concrete (WAM or WMA)
– reduces the temperature required for manufacture by
adding asphalt emulsions, waxes, or zeolites.
– This process benefits both the environment and the
workers,
• as it results in less fossil fuel consumption and reduced
emission of fumes.
• cold mix asphalt concrete,
– the asphalt is emulsified in soapy water before mixing
it with the aggregate,
• eliminating the need for high temperatures altogether.
– However, the asphalt produced is not nearly as durable
as HMAC or WAM, and cold mix asphalt is typically
used for low traffic areas or to patch damaged HMAC.
Samuel K. 418
cont....
• Cut-back asphalt concrete
– has been illegal in the United states since the 1970s,
but many other countries around the world still use it.
– This type of concrete is the least environmentally
friendly option,
• resulting in significantly more air pollution than the other
forms.
– It is made by dissolving the asphalt binder in kerosene
before mixing it with the aggregate,
• reducing viscosity while the concrete is layered and
compacted.
– The lighter kerosene later evaporates, leaving a
hardened surface.
Samuel K. 419
cont...
• Mastic asphalt, also called sheet asphalt
– has a lower bitumen content than the rolled asphalt
forms discussed above.
– It is used for some roads and footpaths, but also in
roofing and flooring.
– Stone mastic asphalt (SMA), another variety, is
becoming increasingly popular as an alternative to
rolled asphalt.
– Its benefits include an anti-skid property and the
absence of air pockets, but if laid improperly,
• it may cause slippery road conditions.
Samuel K. 420
Asphalt Concrete
• Asphalt concrete
– A uniformly mixed combination of
• asphalt cement
• course aggregate
• fine aggregate and
• other materials, depending on the types of asphalt concrete
• the different types of asphaltic concretes commonly used in
pavement construction are
– hot mix, hot laid
– cold-mix, cold laid
• when used in the construction of highway pavements
– it must resist deformation from imposed traffic loads
– be skid resistant even when wet and
– not be easly affected by weathering forces.
• the degree to which an asphalt concrete achieves these
characteristics is mainly dependent on the design of the mix
used in producing the concrete.
Samuel K. 421
Hot-Mix, Hot-Laid Asphalt Concrete
• Produced by properly blending
– asphalt cement
– coarse aggregate
– fine aggregate and
– filler (dust) at tempratures ranging from about 175oF to 325oF,
depending on the type of asphalt cement used.
• Suitabe types of asphaltic materials include AC-20, AC-10
and AR-1000 with penetration grades of 60-70, 85-100,
120-150 and 200-300.
• Hot-mix, hot laid asphaltic concrete is normally used for
high-type pavement construction and the mixture can be
described as
– open graded
– course graded
– dense graded and
– fine graded Samuel K. 422
Dense-Graded Mixes
• A dense-graded mix
– is a well-graded HMA intended for general use.
– When properly designed and constructed, a dense-graded mix is
relatively impermeable.
– Dense-graded mixes are generally referred to by their nominal
maximum aggregate size.
– They can further be classified as either fine-graded or coarse-graded.
– Fine-graded mixes have more fine and sand sized particles than
coarse-graded mixes.
– Dense-graded mixes are used extensively in Washington State for all
purposes.
• Purpose:
– Suitable for all pavement layers and for all traffic conditions.
– They work well for structural, friction, leveling and patching
needs.
• Materials:
– Well-graded aggregate, asphalt binder (with or without modifiers),
RAP Samuel K. 423
Figure. Dense-graded mix cross-section and typical gradation curve for a
dense-graded mix.
Samuel K. 424
Stone Matrix Asphalt (SMA)
• Stone matrix asphalt (SMA), sometimes called stone mastic asphalt, is a gap-
graded HMA originally developed in Europe to maximize rutting resistance and
durability.
• The mix goal is to create stone-on-stone contact.
• Since aggregates do not deform as much as asphalt binder under load, this stone-
on-stone contact greatly reduces rutting.
• SMA is generally more expensive than a typical dense-graded HMA because it
requires more durable aggregates, higher asphalt content, modified asphalt binder
and fibers.
• In the right situations it should be cost-effective because of its increased rut
resistance and improved durability.
• SMA, has been used in the U.S. since about 1990, although it has only been used
in Washington State on several pilot projects.
• Purpose:
– Improved rut resistance and durability.
– SMA is almost exclusively used for surface courses on high volume interstates and
U.S. roads.
• Materials:
– Gap-graded aggregate, modified asphalt binder, fiber filler
– Other reported SMA benefits include wet weather friction (due to a coarser surface
texture) and less severe reflective cracking.
– Mineral fillers and additives are used to minimize asphalt binder drain-down during
construction, increase the amount of asphalt binder used in the mix and to improve
mix durability. Samuel K. 425
Figure . Gap-graded (Performance-Designed or SMA) mix and typical gradation curve
for a gap-graded mix.
Samuel K. 426
cont...
Figure: SMA Surface Figure: SMA Lab Sample
Samuel K. 427
Open-Graded Mixes
• Unlike dense-graded mixes and SMA, an open-
graded HMA mixture is designed to be water
permeable.
• Open-graded mixes use only crushed stone (or
gravel) and a small percentage of manufactured
sands.
• The two most typical open-graded mixes are:
– Open-graded friction course (OGFC).
• Typically 15 percent air voids and no maximum air voids
specified.
– Asphalt treated permeable bases (ATPB).
• Less stringent specifications than OGFC since it is used only
under dense-graded HMA, SMA or PCC for drainage. 428
Samuel K.
Figure . Open-graded (Permeable Friction Course) mix cross-section and typical
gradation curve for open-graded mix.
Samuel K. 429
cont...
Figure: OGFC Surface Figure: OGFC Lab Samples
Samuel K. 430
HMA - FUNDAMENTALS
• HMA consists of two basic ingredients:
– aggregate and asphalt binder.
• HMA mix design is the process of determining
– what aggregate to use,
– what asphalt binder to use and
– what the optimum combination of these two ingredients ought
to be.
• HMA mix design has evolved as a laboratory procedure
that uses several critical tests to make key characterizations
of each trial HMA blend.
• Although these characterizations are not comprehensive,
– they can give the mix designer a good understanding of how a
particular mix will perform in the field during construction and
under subsequent traffic loading.
• This section covers mix design fundamentals common to
all mix design methods. Samuel K. 431
Variables
• HMA is a rather complex material upon which many
different, and sometimes conflicting, performance
demands are placed.
• It must
– resist deformation and cracking,
– be durable over time,
– resist water damage,
– provide a good tractive surface, and yet be inexpensive,
– readily made and easily placed.
• In order to meet these demands, the mix designer can
manipulate all of three variables:
– Aggregate.
• Items such as type (source),
• gradation and size, toughness and abrasion resistance,
• durability and soundness,
• shape and texture as well as cleanliness can be measured, judged and
altered to some degree. Samuel K. 432
cont...
– Asphalt binder.
• Items such as type,
• durability,
• rheology,
• purity as well as additional modifying agents can be
measured, judged and altered to some degree.
– The ratio of asphalt binder to aggregate.
• Usually expressed in terms of percent asphalt binder by total
weight of HMA,
• this ratio has a profound effect on HMA pavement
performance.
• Because of the wide differences in aggregate
specific gravity,
– the proportion of asphalt binder expressed as a
percentage of total weight can vary widely even
though the volume of asphalt binder as a percentage of
total volume remains quite constant. 433
Samuel K.
Objectives
• Before embarking on a mix design procedure it is important to
understand what its objectives.
• This section presents the typical qualities of a well-made HMA
mix.
• By manipulating the variables of aggregate, asphalt binder and
the ratio between the two, mix design seeks to achieve the
following qualities in the final HMA product:
– Deformation resistance (stability)
– Fatigue resistance
– Low temperature cracking resistance
– Durability
– Moisture damage resistance
– Skid resistance.
– Workability
• Knowing these objectives, the challenge in mix design is then
to develop a relatively simple procedure with a minimal amount
of tests and samples that will produce a mix with all the above
434
HMA qualities. Samuel K.
Deformation resistance (stability)
• HMA should not distort (rut) or deform (shove)
under traffic loading.
• HMA deformation is related to one or more of the
following:
– Aggregate surface and abrasion characteristics.
• Rounded particles tend to slip by one another causing HMA
distortion under load while angular particles interlock with
one another providing a good deformation resistant structure.
• Brittle particles cause mix distortion because they tend to
break apart under agitation or load.
• Tests for particle shape and texture as well as durability and
soundness can identify problem aggregate sources.
• These sources can be avoided, or at a minimum, aggregate
with good surface and abrasion characteristics can be blended
in to provide better overall characteristics.
Samuel K. 435
cont...
– Aggregate gradation.
• Gradations with excessive fines (either naturally occurring
or caused by excessive abrasion) cause distortion because
the large amount of fine particles tend to push the larger
particles apart and act as lubricating ball-bearings between
these larger particles.
• A gradation resulting in low VMA or excessive asphalt
binder content can have the same effect.
• Gradation specifications are used to ensure acceptable
aggregate gradation.
Samuel K. 436
cont...
– Asphalt binder content.
• Excess asphalt binder content tends to lubricate and
push aggregate particles apart making their
rearrangement under load easier.
• The optimum asphalt binder content as determined by
mix design should prevent this.
– Asphalt binder viscosity at high temperatures.
• In the hot summer months, asphalt binder viscosity is at
its lowest and the pavement will deform more easily
under load.
• Specifying an asphalt binder with a minimum high
temperature viscosity (as can be done in the Superpave
asphalt binder selection process) ensures adequate high
temperature viscosity.
Samuel K. 437
Fatigue resistance
• HMA should not crack when subjected to repeated loads over time.
• HMA fatigue cracking is related to
– asphalt binder content and stiffness.
• Higher asphalt binder contents will result in a mix that has a
greater tendency
– to deform elastically (or at least deform) rather than fracture under
repeated load.
• The optimum asphalt binder content as determined by
– mix design should be high enough to prevent excessive fatigue cracking.
• The use of an asphalt binder with a lower stiffness will
– increase a mixture's fatigue life by providing greater flexibility.
• However, the potential for rutting must also be considered in the
selection of an asphalt binder.
• Note that fatigue resistance is also highly dependent upon the
relationship between structural layer thickness and loading.
• However, this section only addresses mix design issues.
Samuel K. 438
Low temperature cracking resistance.
• HMA should not crack when subjected to low
ambient temperatures.
• Low temperature cracking is primarily
– a function of the asphalt binder low temperature
stiffness.
• Specifying asphalt binder with adequate low
temperature properties (as can be done in the
Superpave asphalt binder selection process)
– should prevent, or at least limit, low temperature
cracking.
Samuel K. 439
Durability
• HMA should not suffer excessive aging during production and
service life.
• HMA durability is related to one or more of the following:
– The asphalt binder film thickness around each aggregate particle.
• If the film thickness surrounding the aggregate particles is insufficient, it is
possible that the aggregate may become accessible to water through holes in
the film.
• If the aggregate is hydrophilic, water will displace the asphalt film and
asphalt-aggregate cohesion will be lost.
• This process is typically referred to as stripping.
• The optimum asphalt binder content as determined by mix design should
provide adequate film thickness.
– Air voids.
• Excessive air voids (on the order of 8 percent or more) increase HMA
permeability and allow oxygen easier access to more asphalt binder thus
accelerating oxidation and volatilization.
• To address this, HMA mix design seeks to adjust items such as asphalt content
and aggregate gradation to produce design air voids of about 4 percent.
• Excessive air voids can be either a mix design or a construction problem and
this section only addresses the mix design problem.
Samuel K. 440
Moisture damage resistance.
• HMA should not degrade substantially from moisture
penetration into the mix.
• Moisture damage resistance is related to one or more of the
following:
– Aggregate mineral and chemical properties.
• Some aggregates attract moisture to their surfaces, which can cause
stripping.
• To address this, either stripping-susceptible aggregates can be avoided
or an anti-stripping asphalt binder modifier can be used.
– Air voids.
• When HMA air voids exceed about 8 percent by volume, they may
become interconnected and allow water to easily penetrate the HMA
and cause moisture damage through pore pressure or ice expansion.
• To address this, HMA mix design adjusts asphalt binder content and
aggregate gradation to produce design air voids of about 4 percent.
• Excessive air voids can be either a mix design or a construction problem
and this section only addresses the mix design problem. 441
Samuel K.
Skid resistance
• HMA placed as a surface course should provide
– sufficient friction when in contact with a vehicle's tire.
• Low skid resistance is generally related to one or more of
the following:
– Aggregate
• characteristics such as texture, shape, size and resistance to polish.
• Smooth, rounded or polish-susceptible aggregates are less skid resistant.
• Tests for particle shape and texture can identify problem aggregate
sources.
• These sources can be avoided, or at a minimum, aggregate with good
surface and abrasion characteristics can be blended in to provide better
overall characteristics.
• Excessive asphalt binder can cause HMA bleeding.
• Using the optimum asphalt binder content as determined by mix design
should prevent bleeding.
Samuel K. 442
Workability.
• HMA must be capable of being placed and compacted with
reasonable effort.
• Workability is generally related to one or both of the
following:
– Aggregate
• texture, shape and size.
• Flat, elongated or angular particles tend to interlock rather than slip by one
another making placement and compaction more difficult (notice that this
is almost in direct contrast with the desirable aggregate properties for
deformation resistance).
• Although no specific mix design tests are available to quantify workability,
tests for particle shape and texture can identify possible workability
problems.
– Aggregate gradation.
• Gradations with excess fines (especially in the 0.60 to 0.30 mm (No. 30 to
50) size range when using natural, rounded sand) can cause a tender mix.
• A gradation resulting in low VMA or excess asphalt binder content can
have the same effect.
• Gradation specifications are used to ensure acceptable aggregate gradation.
443
Samuel K.
cont...
• At laydown temperatures (above about 120 °C (250 °F))
asphalt binder works as a lubricant between aggregate
particles as they are compacted.
• Therefore, low asphalt binder content reduces this lubrication
resulting in a less workable mix.
• Note that a higher asphalt binder content is generally good
for workability but generally bad for deformation resistance.
– Asphalt binder viscosity at mixing/laydown
temperatures.
• If the asphalt binder viscosity is too high at mixing and
laydown temperatures, the HMA becomes difficult to dump,
spread and compact.
• The Superpave rotational viscometer specifically tests for
mixing/laydown temperature asphalt binder viscosity.
Samuel K. 444
Basic Procedure
• HMA mix design is the process of determining
– what aggregate to use,
– what asphalt binder to use and
– what the optimum combination of these two ingredients ought to
be.
• In order to meet the demands placed by the preceding
desirable HMA properties, all mix design processes involve
three basic steps:
– Aggregate selection.
• No matter the specific method, the overall mix design procedure begins
with evaluation and selection of aggregate and asphalt binder sources.
• Different authorities specify different methods of aggregate
acceptance.
• Typically, a battery of aggregate physical tests is run periodically on
each particular aggregate source.
• Then, for each mix design, gradation and size requirements are
checked.
• Normally, aggregate from more than one source is required to meet
gradation requirements.
Samuel K. 445
cont...
– Asphalt binder selection.
• Although different authorities can and do specify different
methods of asphalt binder evaluation, the Superpave asphalt
binder specification has been or will be adopted by most State
DOTs as the standard (NHI, 2000).
– Optimum asphalt binder content determination.
• Mix design methods are generally distinguished by the method
with which they determine the optimum asphalt binder content.
• This process can be subdivided as follows:
– Make several trial mixes with different asphalt binder contents.
– Compact these trial mixes in the laboratory.
» It is important to understand that this step is at best a rough
simulation of field conditions.
– Run several laboratory tests to determine key sample characteristics.
» These tests represent a starting point for defining the mixture
properties but they are not comprehensive nor are they exact
reproductions of actual field conditions.
– Pick the asphalt binder content that best satisfies the mix design
objectives.
Samuel K. 446
The Job Mix Formula
• The end result of a successful mix design is
– a recommended mixture of aggregate and asphalt binder.
• This recommended mixture, which also includes aggregate
gradation and asphalt binder type is often referred to as
– the job mix formula (JMF) or recipe.
• For HMA manufacturing,
– target values of gradation and asphalt binder content are
specified based on the JMF along with allowable specification
bands to allow for inherent material and production variability.
• It bears repeating that these target values and specification
bands are based on the JMF and not any general HMA
gradation requirements.
• Thus, the mix designer is allowed substantial freedom
– in choosing a particular gradation for the JMF and then the
manufacturer is expected to adhere quite closely to this JMF
gradation during production.
Samuel K. 447
HMA - MARSHALL METHOD
• The basic concepts of the Marshall mix design
method were originally developed by
– Bruce Marshall of the Mississippi Highway Department
around 1939 and then refined by the U.S. Army.
• Procedure
– The Marshall mix design method consists of 6 basic steps:
• Aggregate selection.
• Asphalt binder selection.
• Sample preparation (including compaction).
• Stability determination using the Marshall stability and flow test.
• Density and voids calculations.
• Optimum asphalt binder content selection.
Samuel K. 448
Aggregate Evaluation
• Although neither Marshall nor Army Waterways
Experiment Station (WES) specifically developed
an aggregate evaluation and selection procedure,
– one is included here because it is integral to any mix
design.
• A typical aggregate evaluation for use with either
the Hveem or Marshall mix design methods
includes three basic steps (Roberts et al., 1996):
– Determine aggregate physical properties.
• This consists of running various tests to determine properties
such as:
– Toughness and abrasion
– Durability and soundness
– Cleanliness and deleterious materials
– Particle shape and surface texture 449
Samuel K.
cont...
– Determine other aggregate descriptive physical properties.
• If the aggregate is acceptable according to step #1, additional tests are run
to fully characterize the aggregate.
• These tests determine:
– Gradation and size
– Specific gravity and absorption
– Perform blending calculations to achieve the mix design aggregate
gradation.
• Often, aggregates from more than one source or stockpile are used to obtain
the final aggregate gradation used in a mix design.
• Trial blends of these different gradations are usually calculated until an
acceptable final mix design gradation is achieved.
• Typical considerations for a trial blend include:
– All gradation specifications must be met.
» Typical specifications will require the percent retained by weight on particular
sieve sizes to be within a certain band.
– The gradation should not be too close to the FHWA's 0.45 power maximum density
curve.
» If it is, then the VMA is likely to be too low.
» Gradation should deviate from the FHWA's 0.45 power maximum density
curve, especially on the 2.36 mm (No. 8) sieve.
450
Samuel K.
Asphalt Binder Evaluation
• The Marshall test does not have
– a common generic asphalt binder selection and evaluation
procedure.
• Each specifying entity uses their own method with
modifications to determine
– the appropriate binder and, if any, modifiers.
• Binder evaluation can be based on local experience,
– previous performance or a set procedure.
• Perhaps the most common set procedure now in use is
– based on the Superpave PG binder system.
• However, before this system
– there was no nationally recognized standard for binder
evaluation and selection.
• Once the binder is selected, several preliminary tests are
run to determine
– the asphalt binder's temperature-viscosity relationship. 451
Samuel K.
Sample Preparation
• The Marshall method, like other mix design
methods,
– uses several trial aggregate-asphalt binder blends
(typically 5 blends with 3 samples each for a total of 15
specimens),
– each with a different asphalt binder content.
– Then, by evaluating each trial blend's performance, an
optimum asphalt binder content can be selected.
• In order for this concept to work,
– the trial blends must contain a range of asphalt contents
both above and below the optimum asphalt content.
• Therefore, the first step in sample preparation is
– to estimate an optimum asphalt content.
• Trial blend asphalt contents are then determined
from this estimate. Samuel K. 452
cont...
• Optimum Asphalt Binder Content Estimate
– The Marshall mix design method
• can use any suitable method for estimating optimum
asphalt content and
• usually relies on local procedures or experience.
• Sample Asphalt Binder Contents
– Based on the results of the optimum asphalt binder
content estimate,
• samples are typically prepared at 0.5 percent by weight
of mix increments,
• with at least two samples above the estimated asphalt
binder content and two below.
Samuel K. 453
cont...
• Compaction with the Marshall Hammer
– Each sample is then heated to the anticipated compaction
temperature and compacted with a Marshall hammer,
• a device that applies pressure to a sample through a tamper foot
(see Figure ).
– Some hammers are automatic and some are hand operated.
– Key parameters of the compactor are:
• Sample size = 102 mm (4-inch) diameter cylinder 64 mm (2.5
inches) in height (corrections can be made for different sample
heights)
• Tamper foot = Flat and circular with a diameter of 98.4 mm
(3.875 inches) corresponding to an area of 76 cm2 (11.8 in2).
• Compaction pressure = Specified as a 457.2 mm (18 inches) free
fall drop distance of a hammer assembly with a 4536 g (10 lb.)
sliding weight.
• Number of blows = Typically 35, 50 or 75 on each side
depending upon anticipated traffic loading.
Samuel K. 454
cont...
• Simulation method = The tamper
foot strikes the sample on the top
and covers almost the entire
sample top area.
– After a specified number of blows,
the sample is turned over and the
procedure repeated.
• The standard Marshall method
sample preparation procedure
is contained in:
– AASHTO T 245: Resistance to
Plastic Flow of Bituminous
Mixtures Using the Marshall
Apparatus
Figure : Marshall Drop Hammers
Samuel K. 455
The Marshall Stability and Flow Test
• The Marshall stability and flow test provides
– the performance prediction measure for the Marshall mix
design method.
• The stability portion of the test measures
– the maximum load supported by the test specimen at a loading
rate of 50.8 mm/minute (2 inches/minute).
• Basically, the load is increased
– until it reaches a maximum
• then when the load just begins to decrease, the loading is
stopped and the maximum load is recorded.
• During the loading,
– an attached dial gauge measures the specimen's plastic flow as
a result of the loading (see Figure ).
– The flow value is recorded in 0.25 mm (0.01 inch) increments
at the same time the maximum
Samuel K.
load is recorded. 456
cont...
Figure: Marshall Testing Apparatus
457
Samuel K.
cont...
• Typical Marshall design stability and flow criteria
are shown in Table.
• One standard Marshall mix design procedure is:
– AASHTO T 245: Resistance to Plastic Flow of
Bituminous Mixtures Using Marshall Apparatus
Table: Typical Marshall Design Criteria (from Asphalt Institute, 1979)
Samuel K. 458
Density and Voids Analysis
• All mix design methods use density and voids to
determine basic HMA physical characteristics.
• Two different measures of densities are typically taken:
– Bulk specific gravity (Gmb).
– Theoretical maximum specific gravity (TMD, Gmm).
• These densities are then used to calculate the
volumetric parameters of the HMA.
• Measured void expressions are usually:
– Air voids (Va), sometimes expressed as voids in the total
mix (VTM)
– Voids in the mineral aggregate (VMA) - see Table below.
– Voids filled with asphalt (VFA)
• Generally, these values must meet local or State
criteria.
Samuel K. 459
cont...
Table: Typical Marshall Minimum VMA (from Asphalt Institute, 1979)
Samuel K. 460
Selection of Optimum Asphalt Binder Content
• The optimum asphalt binder content is finally
selected based on the combined results of
– Marshall stability and flow,
– density analysis and
– void analysis (see Figure below).
• Optimum asphalt binder content can be arrived at in
the following procedure (Roberts et al., 1996):
– Plot the following graphs:
• Asphalt binder content vs. density. Density will generally
increase with increasing asphalt content, reach a maximum, then
decrease. Peak density usually occurs at a higher asphalt binder
content than peak stability.
• Asphalt binder content vs. Marshall stability. This should follow
one of two trends:
– Stability increases with increasing asphalt binder content, reaches a
peak, then decreases.
– Stability decreases with increasing asphalt binder content and does not
show a peak. This curve is common for some recycled HMA mixtures.
461
Samuel K.
cont...
• Asphalt binder content vs. flow.
• Asphalt binder content vs. air voids. Percent air voids should
decrease with increasing asphalt binder content.
• Asphalt binder content vs. VMA. Percent VMA should decrease
with increasing asphalt binder content, reach a minimum, then
increase.
• Asphalt binder content vs. VFA. Percent VFA increases with
increasing asphalt binder content.
– Determine the asphalt binder content that corresponds to
• the specifications median air void content (typically this is 4
percent).
• This is the optimum asphalt binder content.
– Determine properties at this optimum asphalt binder
content by referring to the plots.
• Compare each of these values against specification values and if
all are within specification, then the preceding optimum asphalt
binder content is satisfactory.
• Otherwise, if any of these properties is outside the specification
462
range the mixture should be redesigned.
Samuel K.
Figure: Selection of Optimum Asphalt Binder Content Example (from Roberts et al., 1996)
Samuel K. 463
Analysis of Results from Marshal Test
• The first step in the analysis of the results in
the determination of the average bulk specific
gravity for all test specimens having the same
asphalt content.
• The average unit weight of each mixture is
then obtained by multiplying its average
specific gravity by the density of water .
• A smooth curve that represents the best fit of
the plots of unit weight versus percentage of
asphalt is determined.
• This curve is used to obtain the bulk specific
gravity.
Samuel K. 464
Bulk specific gravity of aggregate
• Bulk specific gravity is defined as
– The weight in air of a unit volume (including all normal voids) of a
permeable material at a selected temperature, divided by the weight in
air of the same density of gas free distilled water at the same selected
temprature.
• Since the aggregate mixture consists of different fractions of coarse
aggregate, fine aggregate and mineral fillers with different specific
gravity, the bulk specific gravity of the total aggregate in the paving
mixture is given as
Where
Ggam = bulk specific gravity of aggregate in the paving mixture
Pw, Pfa, Pmf = percent of weight of aggregate, fine aggregate and mineral
filler respectivelly in the paving mixture.
Gbca, Gbfa, Gbmf = bulk specific gravities of course aggregate, fine
aggregate and mineral filler respectivelly.
Samuel K. 465
Apparent specific gravity of aggregates
• The apparent specific gravity is defined as
– The ratio of the weirht in air of an impermeable material to the
weigth of an equal volume of distilled water at a specific
temperature.
• The apparent specific gravity of the aggregate mix is
therefore obtained as
Where
Gama = bulk specific gravity of aggregate in the paving mixture
Pw, Pfa, Pmf = percent of weight of aggregate, fine aggregate and
mineral filler respectivelly in the paving mixture.
Gaca, Gafa, Gamf = bulk specific gravities of course aggregate, fine
aggregate and mineral filler respectivelly.
Samuel K. 466
cont...
• Fig. bulk, effective and apparent specific gravities; air void
and effective asphalt content in compacted asphalt mixture.
Samuel K. 467
Effective specific gravity aggregate
• Effective specific gravity of the aggregates is normally
based on the maximum specific gravity of the paving
mixture.
• It is therefore the specific gravity of the aggregates when all
void spaces in the aggregate particles are included, with the
expection of those that filled with the asphalt.
• It is given as
Where
Gea = effective specific gravity of the aggregates
Gmp = maximum specific gravity of the paving mixture (no air
viods)
Pac = asphalt percent by total weight of paving mixture (thus 100-
Pac is the percent by weight of the base mixture that is not
asphalt)
468
Gac = spacific gravity of asphalt Samuel K.
Maximum specific gravity of the paving mixture
• The maximum specific gravity of the paving mixture Gmp assumes
that there are no air voids in the asphalt concrete.
• The effective specific gravity obtained above is then used to
determine the maximum specific gravity of the paving mixtures with
different asphalt cement content using:
Where
Gea = effective specific gravity of the aggregates (assumed to be constant
for different asphalt cement contents)
Gmp = maximum specific gravity of the paving mixture (asphalt concrete)
Pta = percent by weigth of aggregates in paving mixtures (asphalt concrete)
Pac = percent by weight of asphalt in paving mixture (asphalt concrete)
Gac = spacific gravity of asphalt
• this can be determined in the lab by using standard test – ASTM
Designation D2041
Samuel K. 469
cont...
• Once these different specific gravities have been determined, the
asphalt absorption, the effective asphalt content, the percent voids in
mineral aggregates (VMA), and the percent voids in the compacted
mixture can also be determined.
• Asphalt absorption
– the percent by weight of the asphalt that is absorbed by the aggregates
based on the total weight of the aggregates.
– This is given as
Where
Gea = effective specific gravity of the aggregates
Gbam = bulk specific gravity of aggregate
Paa = amount of asphalt absorbed as percentage of the total weight of the
aggregate
Gac = spacific gravity of asphalt
Samuel K. 470
Effective asphalt content
• The effective asphalt content is the difference between
the total amount of asphalt in the mixture and that
absorbed into the aggregate particles.
• The effective asphalt content is thereforethat which
coats the outside of the aggregate particles and
influences the pavement performance.
• It is given as
Where
Peac = effetive asphalt content in paving mixture (percent by weight)
Pac = percent by weight of asphalt in paving mixture
Pta = aggregate percent by weight of paving mixture
Paa = amount of asphalt absorbed as a percentage of the total weight
of the aggregate
Samuel K. 471
Percent voids in compacted mineral aggregates
• The percent voids in compacted mineral aggregates, or
VMA, is
– the percentage of void spaces between the granular particles in
the compacted paving mixture, including the air voids and the
volume occupied by the effective asphalt content.
• It is usually calculated as a percentage of the bulk volume
of the compacted mixture, based on the bulk specific
gravity of aggregates.
• It is given as
Where
VMA = percent voids in compacted mineral aggregate
Gbcm = bulk specific gravity of compacted mixture (asphalt concrete)
Gbam = bulk specific gravity of aggregate
Pta = aggregate percent by weight of total paving mixture (asphalt
concrete)
Samuel K. 472
Percent air voids in compacted mixture
• This is the ratio, expressed as
– a percentage between the volume of the small air voids
between the coated particles and the total volume of
the mixture.
• It can be obtained from
Where
Pav = percent air voids in compacted paving mixture
Gmp = maximum specific gravity of the compacted paving
mixture
Gbcm = bulk specific gravity of the compacted paving mixture
Samuel K. 473
Surface Treatments
• Various types of surface treatment are available for
– improving the quality of an existing pavement.
• Typically, a surface treatment is
– a thin layer of material (about ½ to ¾ in thick) applied to the surface of a
road in single or multiple lifts.
• Surface treatments generally consist of a bituminous material
• applied to crushed stone by the inverted penetration method.
• Since the surface treatment is relatively thin, it is usually not intended to
support loads by itself.
• Surface treatments can be used to achieve
– a seal coat,
– armor coat,
– dust palliative, or
– prime or tack coat for a new wearing course.
• A surface treatment is applied to a granular-type base by a pressure
distributor truck.
• This type of vehicle is equipped with a tank containing the surfacing
material and a spray bar with nozzles that spread the binder over a given
width of roadway. Samuel K. 474
cont...
• A surface treatment
– is a simple,
– highly effective and
– inexpensive road surfacing if adequate care is
taken in the planning and execution of the work.
• The process is used for
– surfacing both medium and lightly trafficked
roads, an also as a maintenance treatment for roads
of all kinds.
Samuel K. 475
cont...
• A surface treatment comprises
– a thin film of binder, generally bitumen or tar, which is
sprayed onto the road surface and then covered with a
layer of stone chippings.
• The thin film of binder acts as
– a waterproofing seal preventing the entry of surface water
into the road structure.
• The stone chippings protect this film of binder
– from damage by vehicle tires,
– form a durable,
– skid-resistant and
– dust-free wearing surface.
• In some circumstances the process may be repeated
to provide double or triple layers of chippings.
Samuel K. 476
cont...
• A surface treatment can provide an effective and economical
running surface for newly constructed road pavements.
• For sealing new roadbases,
– traffic flows of up to 500 vehicles/lane/day are appropriate,
although this can be higher if the roadbase is very stable or if a
triple seal is used.
• Roads carrying in excess of 1000 vehicles/lane/day,
– have been successfully surfaced with multiple surface treatments.
• A correctly designed and constructed surface treatment
should last at least 5 years before resealing with another
surface treatment becomes necessary.
• If traffic growth over a period of several years necessitates a
more substantial surfacing or increased pavement thickness,
– a bituminous overlay can be laid over the original surface
treatment when the need arises. Samuel K. 477
Types of Surface Treatment
• Surface treatments can be constructed in a
number of ways to suit site conditions.
• The common types of surface treatments are
illustrated in Figure.
Samuel K. 478
Samuel K. 479
SINGLE SURFACE TREATMENT
• A single surface treatment would not normally be used on
a new roadbase because of
– the risk that the film of bitumen will not give complete
coverage.
• It is also particularly important to minimize the need for
future maintenance and a double dressing should be
considerably more durable than a single dressing.
• However, a ‗racked-in‘ dressing (see below) may be
suitable for use on a new roadbase which has a tightly knit
surface
– because of the heavier applications of binder which is used
with this type of single dressing.
• When applied as a maintenance operation to an existing
bituminous road surface
– a single surface treatment can fulfill the functions required of a
maintenance re-seal, namely waterproofing the road surface,
arresting deterioration, and restoring skid resistance. 480
Samuel K.
DOUBLE SURFACE TREATMENT
• Double surface treatments should be used when:
– A new roadbase is surface treated.
– Extra ‗cover‘ is required on an existing bituminous road surface
because of its condition (e.g. when the surface is slightly cracked
or patched).
– There is a requirement to maximize durability and minimize the
frequency of maintenance and resealing operations.
• The quality of a double surface treatment will be greatly
enhanced of traffic is allowed to run on the first treatment for
a minimum period of 2-3 weeks (and preferable longer) before
the second treatment is applied.
• This allows the chippings of the first treatment to adopt a
stable interlocking mosaic, which provides a firm foundation
for the second treatment.
• Sand may sometimes be used as an alternative to chippings for
the second treatment.
Samuel K. 481
TRIPLE SURFACE TREATMENT
• A triple surface treatment may be used to
advantage where
– a new road is expected to carry high traffic
volumes from the outset.
• The application of a small chipping in the third
seal will reduce
– noise generated by traffic and
– the additional binder will ensure a longer
maintenance-free service life.
Samuel K. 482
RACKED-IN SURFACE TREATMENT
• This treatment is recommended for use where traffic is
particularly heavy or fast.
• A heavy single application of binder is made and a layer of
large chippings is spread to give approximately 90 per cent
coverage.
• This is followed immediately by the application of smaller
chippings which should ‗lock-in‘ the larger aggregate and
form a stable mosaic.
• The amount of bitumen used is more than would be used
with a single seal but less than for a double seal.
• The main advantages of the racked-in surface treatment
are:
– Less risk of dislodged large chippings.
– Early stability through good mechanical interlock.
– Good surface texture.
Samuel K. 483
OTHER TYPES OF SURFACE TREATMENT
• ‗Pad coats‘ are used where
– the hardness of the existing road surface allows very
little embedment of the first layer of chippings,
• such as on a newly constructed cement stabilized roadbase or
a dense crushed rock base.
• A first layer of nominal 6mm chippings will
adhere well to the hard surface and will provide a
‗key‘ for larger 10mm or 14mm chippings in the
second layer of the treatment.
• ‗Sandwich‘ surface treatments
– are principally used on existing binder rich surfaces
and
– sometimes on gradients to reduce the tendency for the
binder to flow down the slope.
Samuel K. 484
Chippings for Surface Treatments
• The selection of chipping sizes is based on
– the volume of commercial vehicles having unladen weight of more
than 1.5 tonnes and the hardness of the existing pavement.
• Ideally, chippings used for surface treatment should be
– single sized,
– cubical in shape,
– clean and free from dust,
– strong,
– durable, and
– not susceptible to polishing under the action of traffic.
• In practice the chippings available usually fall short of this
ideal.
• It is recommended that chippings used of surface treatment
should
– comply with the requirements of Table1 for higher levels of traffic,
and
– to the requirements of Table 2 for lightly trafficked roads of up to
Samuel K. 485
250 vehicles per day:
Table1: Grading Limits, Specified Size and Maximum Flakiness Index for
Surface Treatment Aggregates
Samuel K. 486
Table 2: Grading Limits, Specified Size and Maximum Flakiness Index for
Surface Treatment Aggregates for Lightly Trafficked Roads
Samuel K. 487
cont...
• Samples of the chippings should be tested for
– grading,
– flakiness index,
– Aggregate crushing value and,
– when appropriate, the polished stone value and aggregate abrasion value.
• Specifications for maximum aggregate crushing value (ACV) for
surface treatment chippings typically
– lie in the range 20 to 35.
• For lightly trafficked roads the higher value is likely to be adequate but
on more heavily trafficked roads a maximum ACV of 20 is
recommended.
• The polished stone value (PSV) of the chippings is important if the
primary purpose of the surface treatment is to restore or enhance the
skid resistance of the road surface.
• The PSV required in a particular situation is related to the nature of the
road site and the speed and intensity of the traffic.
• The resistance to skidding is also dependent upon the macro texture of
the surface which, in turn, is affected by the durability of the exposed
aggregate.
Samuel K. 488
cont...
• The nominal sizes of chippings normally used
for surface treatment
– 6, 10, 14 and 20 mm.
• Flaky chippings are those with a thickness
(smallest dimension)
– less than 0.6 of their nominal size.
• The proportion of flaky chippings clearly
affects
– the average thickness of a single layer of the
chippings, and
– it is for this reason that the concept of the ‗average
least dimension‘ (ALD) of chippings was
introduced. Samuel K. 489
cont...
• The most critical period for a surface treatment
– occurs immediately after the chippings have been spread on
the binder film.
• At this stage the chippings have yet to become
– an interlocking mosaic and are held in place solely by the
adhesion of the binder film.
• Dusty chippings can seriously impede adhesion and
can cause immediate failure of the dressing.
• The effect of dust can sometimes be mitigated
– by dampening them prior to spreading them on the road.
• The chippings dry out quickly in contact with the
binder.
• When a cutback bitumen or emulsion is used,
– good adhesion develops more rapidly than when the coating
Samuel K. 490
of dust is dry.
cont...
• Most aggregates have a preferential attraction for
water rather than for bitumen.
• Hence if heavy rain occurs within the first few
hours when adhesion has not fully developed,
– loss of chippings under the action of traffic is possible.
• Where wet weather damage is considered to be a
severe risk, or the immersion tray test shows that
the chippings have poor affinity with bitumen,
– an adhesion agent should be used.
• An adhesion agent can be added to the binder or,
used in a dilute solution to precoat the chippings.
• For detail see ERA manual
Samuel K. 491
Bitumen
• It is essential that good bonding is achieved

between the surface treatment and the existing
road surface.
• This means that non-bituminous materials
must be primed before surface treatment is
carried out.
Samuel K. 492
PRIME COATS
• Where a surface treatment is to be applied to a previously
untreated road surface
– it is essential that the surface should be
• dry,
• clean and
• as dust-free as possible.
• The functions of a prime coat can be summarized as follows:
– It assists in promoting and maintaining adhesion between the roadbase
and a surface treatment
• by pre-coating the roadbase and penetrating surface voids.
– It helps to seal the surface pores in the roadbase
• thus reducing the absorption of the first spray of binder of the surface
treatment.
– It helps to strengthen the roadbase near its surface
• by binding the finer particles of aggregate together.
– If the application of the surface treatment is delayed for some reason
• it provides the roadbase with a temporary protection against rainfall and light
traffic until the surfacing can be laid.
Samuel K. 493
cont...
• The depth of penetration of the prime
– should be between 3-10mm and
– the quantity sprayed should be such that the surface is dry within a
few hours.
• The correct viscosity and application rate are dependent
primarily on
– the texture and density of the surface being primed.
• It is usually beneficial to spray the surface lightly with water
before applying the prime coat
– as this helps to suppress dust and
– allows the primer to spread more easily over the surface and to
penetrate.
• Bitumen emulsions are not suitable for priming
– as they tend to form a skin on the surface.
• Low viscosity, medium curing cutback bitumens such as MC-
30, MC-70, or in rare circumstances MC-250,
– can be used for prime coats.
Samuel K. 494
BITUMENS FOR SURFACE TREATMENTS
• The correct choice of bitumen for surface treatment
work is critical.
• The bitumen must fulfill a number of important
requirements. It must:
– be capable of being sprayed;
– ‗wet‘ the surface of the road in a continuous film;
– not run off a cambered road or form pools of binder in
local depressions;
– ‗wet‘ and adhere to the chipping at road temperature;
– be strong enough to resist traffic forces and hold the
chippings at the highest prevailing ambient temperatures;
– remain flexible at the lowest ambient temperature, neither
cracking nor becoming brittle enough to allow traffic to
‗pick-off‘ the chippings; and
– resist premature weathering and hardening.
Samuel K. 495
cont...
• Figure below shows the permissible range of binder viscosity for
– successful surface treatment at various road surface temperatures.
• In Ethiopia, daytime road temperatures typically lie between about 25oC
and 50oC,
– normally being in the upper half of this range unless heavy rain is falling.
• For these temperatures the viscosity of the binder
– should lie between approximately 104 and 7x105 centistokes.
• At the lower road temperatures
– cutback grades of bitumen are most appropriate
• At higher road temperatures
– penetration grade bitumens can be used
• The temperature/viscosity relationships shown in Figure below do not
apply to bitumen emulsions.
• These have a relatively low viscosity and ‗wet‘ the chippings readily,
after which
– the emulsion ‗breaks,‘
– the water evaporates and
– particles of high viscosity bitumen adhere to the chippings and the road
surface. Samuel K. 496
Figure: Surface Temperature/Choice of Binder for Surface Treatments
Samuel K. 497
cont...
• Depending upon availability and local
conditions at the time of construction, the
following types of bitumen are commonly
used:
– Penetration grade
– Emulsion
– Cutback
– Modified bitumens
Samuel K. 498
PENETRATION GRADE BITUMENS
• Penetration grade bitumens vary between
– 80/100 to approximately 700 penetration.
• The softer penetration grade binders are usually produced
at the refinery but can be made in the field by
– blending appropriate amounts of kerosene, diesel, or a blend of
kerosene and diesel.
• With higher solvent contents the binder has too low a
viscosity to be classed as being of penetration grade and is
then referred to as a cutback bitumen which,
– for surface treatment work, is usually an MC or RC 3000
grade.
• In very rare circumstances a less viscous grade such as MC
or RC 800 may be used
– if the pavement temperature is below 15oC for long periods of
the year. Samuel K. 499
BITUMEN EMULSION
• Cationic bitumen emulsion with a bitumen
content of 70 to 75 per cent
– is recommended for most surface treatment work.
• This type of binder can be applied
– through whirling spray jets at a temperature between
70 and 85oC and,
– once applied, it will break rapidly on contact with
chippings of most mineral types.
• The cationic emulsifier is normally
– an anti-stripping agent and
– this ensures good initial bonding between chippings
and the bitumen.
Samuel K. 500
CUTBACK BITUMENS
• Except for very cold conditions,
– MC or RC 3000 grade cutback is normally the most fluid binder used for
surface treatments.
• This grade of cutback is basically
– an 80/100 penetration grade bitumen blended with approximately 12 to
17 percent of cutter.
• In Ethiopia, the range of binders available to the engineer may be
restricted.
• In this situation it may then be necessary to blend
– two grades together or to ‗cut-back‘ a supplied grade with diesel oil or
kerosene in order to obtain a binder with the required viscosity
characteristics.
• Diesel oil, which is less volatile than kerosene and is generally
more easily available, is preferable to kerosene for blending
purposes.
• Only relatively small amounts of diesel oil or kerosene are required
to modify a penetration grade bitumen such that
– its viscosity is suitable for surface treatment at road temperatures in
Samuel K. 501
Ethiopia.
• For example, Figure 1 below shows that between 2 and 10 per cent of diesel oil was
required
– to modify 80/100 pen bitumen to produce binders with viscosities within the range of
road temperatures of between 40°– 60°, which prevail in Ethiopia (Figure above).
Figure 1: Blending Characteristics of 80/100 Pen Bitumen with Diesel Fuel
Samuel K. 502
• Figure 2 below shows the temperature/viscosity relationships
for five of the blends made for trials in Kenya.
Figure 2: Viscosity/Temperature Relationahips for Blends of 80/100 Pen Bitumen with
Diesel Fuel
Samuel K. 503
POLYMERMODIFIED BITUMENS
• Polymers can be used in surface treatment to modify
– penetration grade,
– cutback bitumen and
– emulsions.
• Usually these modified binders are used at locations where
– the road geometry, traffic characteristics or the environment dictate that the
road surface experiences high stresses.
• Generally the purpose of the polymers is to reduce
– binder temperature susceptibility so that variation in viscosity over the
ambient temperature range is as small as possible.
• Polymers can also improve
– the cohesive strength of the binder so that it is more able to retain chippings
when under stress from the action of traffic.
• They also improve the early adhesive qualities of the binder allowing the
road to be
– reopened to traffic earlier than may be the case with conventional
unmodified binders.
• Other advantages claimed for modified binders are
– improved elasticity in bridging hairline cracks and overall improved
Samuel K. 504
durability.
ADHESION AGENTS
• Fresh hydrated lime can be used to enhance adhesion.
• It can be mixed with the binder in the distributor before spraying
(slotted jets are probably best suited for this) or the chippings can be
pre-coated with the lime just before use, by spraying with lime slurry.
• The amount of lime to be blended with the bitumen should be
determined in laboratory trials
– but approximately 12 per cent by mass of the bitumen will improve
bitumen Pavement aggregate adhesion and it should also improve the
resistance of the bitumen to oxidative hardening.
• Proprietary additives, known as adhesion agents, are also available
for adding to binders to help
– to minimize the damage to surface treatments that may occur in wet
weather with some types of stone.
• When correctly used in the right proportions, these agents can
enhance
– adhesion between the binder film and the chippings even though they may
be wet.
• Cationic emulsions inherently contain an adhesion agent and lime
505
should not be used with this type of binder. Samuel K.
Design
• The key stages in the surface treatment design

procedure are illustrated in Figure below.
Samuel K. 506
Samuel K. 507
EXISTING SITE CONDITIONS
• Selection of a suitable surface treatment
system for a road and the nominal size of
chippings to be used is based on
– the daily volume of commercial vehicles using
each lane of the road and
– the hardness of the existing pavement surface.
Samuel K. 508
SELECTING THE BINDER
• The selection of the appropriate binder for a surface treatment
is usually constrained by
– the range of binders available from suppliers, although it is
possible for the user to modify the viscosity of penetration grade
and cutback binders to suit local conditions.
• The factors to be taken into account in selecting an
appropriate binder are:
– The road surface temperature at the time the surface treatment is
undertaken.
• For penetration grade and cutback binders the viscosity of the binder should
be between 104 and 7x105 centistokes at the road surface temperature.
– The nature of the chippings.
• If dusty chippings are anticipated and no pretreatment is planned, the
viscosity of the binder used should be towards the lower end of the
permissible range.
• If the binder selected is an emulsion it should be borne in mind that anionic
emulsions may not adhere well to certain acidic aggregates such as granite
509
and quartzite. Samuel K.
cont...
– The type of binder handling and spraying equipment
available.
• The equipment must be capable of maintaining an adequate quantity
of the selected binder at its appropriate spraying temperature and
spraying it evenly at the required rate of spread.
– The available binders.
• There may be limited choice of binders but a balanced choice should
be made where possible.
• Factors which may influence the final selection of a binder include
cost, ease of use, flexibility with regard to adjusting binder viscosity
on site and any influence on the quality of the finished dressing.
• Consideration of these factors will usually narrow the
choice of binder to one or two options.
• The final selection will be determined by other factors
such as the past experience of the surface treatment
team.
Samuel K. 510
CHOICE OF BINDER AND TIMING OF CONSTRUCTION WORK
• The choice of cutback grade or penetration grade bitumen for
surface treatment work is largely controlled by road temperatures
at and shortly after the time of construction.
• However, there are relative advantages and disadvantages
associated with the use of penetration grade binders or cutback
bitumen.
• MC 3000 cutback binder typically contains 12 to 17 per cent of
cutter.
• Under warm road conditions this makes the binder very tolerant
of short delays in the application of chippings and of the use of
moderately dusty chippings.
• It is therefore a good material to use.
• Penetration grade bitumens as hard as 80/100 are often used for
surface treatment work when road temperatures are high.
• The most difficult situations occur when it is required to start
work early in the day and temperatures are considerably lower
than they will be in the afternoon.
Samuel K. 511
DESIGNING THE SURFACE TREATMENT
• Having selected the nominal size of chipping and the
type of binder to be used, the next step in the design
of a surface treatment is
– to determine the rate of spread of the binder.
• General approach to the determination of the rate of
spread of the binder for application in Ethiopia
– Differences in climate,
– uniformity of road surfaces,
– the quality of aggregates,
– traffic characteristics and
– construction practice are necessary
• The method of design relates the voids in a layer of
chippings to the amount of binder necessary to hold
the chippings in place.
Samuel K. 512
cont...
• In a loose single layer of chippings such as is spread
for a surface treatment,
– the voids are initially about 50 per cent, decreasing to
about 30 per cent after rolling and subsequently to 20 per
cent by the action of traffic.
• For best results, between 50 and 70 per cent of the
voids in the compacted aggregate should be filled
with binder.
• Hence it is possible to calculate the amount of binder
required to retain
– a layer of regular, cubical chipping of any size.
• However, in practice chippings
– are rarely the ideal cubical shape (especially when
unsuitable crushing plant has been used) and
– this is why the ALD concept was originally introduced.
513
Samuel K.
DETERMINING THE AVERAGE LEAST DIMENSION (ALD) OF CHIPPINGS
• The ALD of chippings is a function of both

– the average size of the chippings, as determined by normal
square mesh sieves, and
– the degree of flakiness.
• The ALD may be determined in two ways:
– Method A:
• A grading analysis is performed on a representative sample of the
chippings in accordance with ASTM C136.
• The sieve size through which 50 per cent of the chippings pass is
determined (i.e. the ‗median size‘).
• The flakiness index is then also derived from the nomograph shown in
Figure below.
– Method B:
• A representative sample of the chipping is carefully subdivided (in
accordance with British Standard 812: 1985) to give approximately 200
chippings.
• The least dimension of each chipping is measured manually and the
mean value, or ALD, is calculated.
Samuel K. 514
Figure: Determination of Average Least Dimension
Samuel K. 515
DETERMINING THE OVERALL WEIGHTING FACTOR
• The ALD of the chippings is used with an overall weighting
factor to determine
– the basic rate of spray of bitumen.
• The overall weighting factor ‗F‘ is determined by adding
• together four factors that represent;
– the level of traffic,
– the condition of the existing road surface,
– the climate and
– the type of chippings that will be used.
• Factors appropriate to the site to be surface dressed are selected
from Table below.
• For example, if flaky chippings (factor –2) are to be used at a
road site carrying medium to heavy traffic (factor –1) and
which has a primed base surface (factor +6) in a wet tropical
climate (factor +1) the overall weighting factor ‗F‘ is:
-2 –1 +6 +1 = +4
Samuel K. 516
Table: Weighting Factors for Surface Treatment Design
Samuel K. 517
DETERMINING THE BASIC BITUMEN SPRAY RATE
• Using the ALD and ‗F‘ values in equation 1 will give the required basic
rate of spread of binder.
R = 0.6250 + 0 (F*0.023) + [0.0375 + (F*0.0011)] ALD ---------- (1)
Where
F = Overall weighting factor
ALD = The average least dimension of the chippings (mm)
R = Basic rate of spread of bitumen (kg/m2)
• Alternatively, the values for F and ALD can be used in the design chart
given in Figure below.
• The intercept between the appropriate factor line and the ALD line is
located and the rate of spread of the binder is then read off directly at the
bottom of the chart.
• The basic rate of spread of bitumen (R) is the mass of MC 3000 binder
per unit area on the road surface immediately after spraying.
• The relative density of MC 3000 can be assumed to be 1.0 and the
spread rate can therefore also be expressed in 1iters/m2; however,
calibration of a distributor is easier to do by measuring spray rates in
terms of mass.
Samuel K. 518
Figure: Surface Treatment Design Chart
Samuel K. 519
SPRAY RATE ADJUSTMENT FACTORS
• Best results will be obtained if the basic rate of
spread of binder is adjusted to take account of traffic
speed and road gradient as follows:
– For slow traffic or climbing grades with gradients steeper
than 3 per cent,
• the basic rate of spread of binder should be reduced by
approximately 10 per cent.
– For fast traffic or down grades steeper than 3 per cent
• the basic rate of spread of binder should be increased by
• The definition of traffic speed is not precise but
– is meant to differentiate between roads with a high
proportion of heavy vehicles and those carrying mainly
cars traveling at 80km/h or more.
Samuel K. 520
cont...
• The basic rate of spread of binder must also be
modified to allow for the type of binder used.
• The following modifications are appropriate:
– Penetration grade binders: decrease the rate of spread by
10 per cent.
– Cutback binders: for MC/RC 3000 no modification is
required (in the rare cases when cutbacks with lower
viscosity are used the rate of spread should be increased to
allow for the additional percentage of cutter used).
• Suggested adjustment factors for different binders
and different site conditions are given in Table
below.
• The adjustment factors reflect the amount of cutter
used in the base 80/100 penetration grade bitumen
but must be regarded as approximate values. 521
Samuel K.
Table: Typical Bitumen Spray Rate Adjustment Factors
• The amount of cutter required for ‗on-site‘ blending should be

determined in the laboratory by making viscosity tests on a
range of blends of bitumen and cutter.
• MC 3000 can be made in the field by blending 90 penetration
bitumen with 12 to 14 per cent by volume of a 3:1 mixture of
kerosene and diesel.
• If a different grade of binder is required then the adjustment
factor should reflect the different amount of cutter used.
Samuel K. 522
ADJUSTING RATES OF SPRAY FORMAXIMUM DURABILITY
• The spray rate which will be arrived at after applying the
adjustment factors in Table above
– will provide very good surface texture and use an ‗economic‘
quantity of binder.
• However, because of the difficulties experienced in carrying out
effective maintenance,
– there is considerable merit in sacrificing some surface texture for
increased durability of the seal.
• For roads on flat terrain and carrying moderate to high-speed
traffic it is possible
– to increase the spray rates obtained from Table above by
• The heavier spray rate may result in the surface having a
‗bitumen-rich‘ appearance in the
– wheel paths of roads carrying appreciable volumes of traffic.
• However, the additional binder should not result in bleeding
and it can still be expected that more surface texture
– will be retained than is usual in an asphalt concrete wearing course.
Samuel K. 523
SURFACE TREATMENT DESIGN FOR LOW VOLUME ROADS
• If a low volume road, carrying less than about 100 vehicles per
day, is surface dressed
– it is very important that the seal is designed to be as durable as
possible to minimize the need for subsequent maintenance.
• A double surface treatment should be used on new road-bases
and the maximum durability of the seal can be obtained by
using
– the heaviest application of bitumen that does not result in bleeding.
• Where crushing facilities are put in place solely to produce
chippings for a project,
– it will be important to maximize use of the crusher output.
• This will require the use of different combinations of chipping
sizes and correspondingly different bitumen spray rates.
• The normally recommended sizes of chippings for different
road hardness and low commercial traffic volumes are given in
Table below.
Samuel K. 524
Table : Nominal Size of Chippings for Different Hardness of Road Surface
• It may be desirable to use chippings of a larger size than those recommended

in Table above for reasons of economy.
• It is likely that the rate of application of bitumen determined in the normal
way will be too low to obtain good durability.
• Low volumes of traffic are also unlikely to cause the chippings to be
‗rotated‘ into a tight matrix and this will result in the layer being of greater
depth than the ALD of the chippings, which is assumed in the design
process.
• It should therefore be safe to increase bitumen spray rates on low volume
roads to compensate for the reduced embedment of ‗oversize‘ chipping and
the increased texture depth that results from less re-orientation of the
chippings under light traffic. 525
Samuel K.
cont...
• Ideally the ALD of the two aggregate sizes used in a double
surface treatment should differ by at least a factor of two.
• If the ALD of the chippings in the second seal is more than half
the ALD of the chippings in the first seal then
– the texture depth will be further increased and
– the capacity of the aggregate structure for bitumen will be increased.
• It is suggested that on low volume roads the bitumen spray rates
should be increased above the basic rate of spread of bitumen
indicated above by up to the percentages given in Table below.
• It is important that these increased spray rates are adjusted on
the basis of trial sections and local experience.
Table: Suggested Maximum Increases in Bitumen Spray Rate for Low Volume Roads
Samuel K. 526
SPREAD RATE OF CHIPPINGS
• An estimate of the rate of application of the

chippings, assuming that the chippings have a
loose density of 1.35Mg/m3, can be obtained
from the following equation:
Chipping application rate (kg/m2) = 1.364*ALD
Samuel K. 527
Example of a Surface Treatment Design
• Site Description
– A two-lane trunk road at an altitude of
approximately 1500m.
– Vehicle count averaged 3370 per day/lane (i.e.
‗Heavy‘ rating).
– Bitumen to be used is 400 penetration grade (made
by cutting back 80/100 pen bitumen with 6.7 per
cent by mass or approximately 7.5 per cent by
volume) of a 3:1 mixture of kerosene and diesel.
Samuel K. 528
cont....
• The determination of spread rates of 80/100 and 400

pen bitumen for an F factor of –5 and an ALD of 12 on
a site where maximum durability is required are
summarized in Table below. 529
Samuel K.
cont...
Table: Determination of Spread Rates for 400 Penetration Grade Bitumen
Samuel K. 530
reading ass.
• other surface treatment

– ERA manual
– AACRA manual
– other books
Samuel K. 531
Chapter Eight
Structural design of flexible pavements

FLEXIBLE PAVEMENT BASICS
• Flexible pavements are so named because
– the total pavement structure
• deflects, or flexes, under loading.
• A flexible pavement structure is typically
composed of several layers of material.
• Each layer receives the loads from the above
layer,
– spreads them out, then passes on these loads to the
next layer below.
• Thus, the further down in the pavement structure
a particular layer is,
– the less load (in terms of force per area) it must carry
(see Figure below).
Samuel K. 533
Figure: Flexible Pavement Load
Distribution
Samuel K. 534
cont...
• In order to take maximum advantage of this property,
material layers are usually arranged in order of descending
load bearing capacity with
– the highest load bearing capacity material (and most expensive)
on the top and
– the lowest load bearing capacity material (and least expensive)
on the bottom.
• This section describes the typical flexible pavement
structure consisting of:
– Surface course.
• This is the top layer and the layer that comes in contact with traffic.
• It may be composed of one or several different HMA sub-layers.
– Base course.
• This is the layer directly below the HMA layer and generally consists of
aggregate (either stabilized or unstabilized).
– Subbase course.
• This is the layer (or layers) under the base layer.
535
• A subbase is not always needed. Samuel K.
Basic Structural Elements
• A typical flexible pavement structure (see Figure below)
consists of
– the surface course and the underlying base and subbase
courses.
• Each of these layers contributes to structural support and
drainage.
• The surface course (typically a HMA layer)
– is the stiffest (as measured by resilient modulus) and
– contributes the most to pavement strength.
• The underlying layers
– are less stiff
– but are still important to pavement strength as well as drainage
and frost protection.
• A typical structural design results in a series of layers that
gradually decrease in material quality with depth.
Samuel K. 536
Figure: Basic Flexible
Pavement Structure
Samuel K. 537
Surface Course
• The surface course
– is the layer in contact with traffic loads and
– normally contains the highest quality materials.
• It provides characteristics such as
– friction,
– smoothness,
– noise control,
– rut and shoving resistance and
– drainage.
• In addition, it serves to prevent
– the entrance of excessive quantities of surface water
into the underlying base, subbase and subgrade
(NAPA, 2001).
Samuel K. 538
cont...
• This top structural layer of material is sometimes
subdivided into two layers (NAPA, 2001):
– Wearing Course.
• This is the layer in direct contact with traffic loads.
• It is meant to take the brunt of traffic wear and can be
removed and replaced as it becomes worn.
• A properly designed (and funded) preservation program
should be able to identify pavement surface distress while it
is still confined to the wearing course.
• This way, the wearing course can be rehabilitated before
distress propagates into the underlying intermediate/binder
course.
– Intermediate/Binder Course.
• This layer provides the bulk of the HMA structure.
• It's chief purpose is 2.Samuel
to K.distribute load. 539
Base Course
• The base course is immediately beneath the surface
course.
• It provides additional load distribution and
contributes to drainage and frost resistance.
• Base courses are usually constructed out of:
– Aggregate.
• Base courses are most typically constructed from durable
aggregates (see Figure below) that will not be damaged by
moisture or frost action.
• Aggregates can be either stabilized or unstabilized.
– HMA.
• In certain situations where high base stiffness is desired, base
courses can be constructed using a variety of HMA mixes.
• In relation to surface course HMA mixes,
– base course mixes usually contain larger maximum aggregate sizes,
– are more open graded and
– are subject to more lenient specifications. 540
Samuel K.
Figure: Limerock Base Course Undergoing Final Grading
Samuel K. 541
Subbase Course
• The subbase course
– is between the base course and the subgrade.
• It functions primarily as structural support but it can also:
– Minimize the intrusion of fines from the subgrade into the pavement structure.
– Improve drainage.
– Minimize frost action damage.
– Provide a working platform for construction.
• The subbase generally consists of
– lower quality materials than the base course but better than the subgrade soils.
• A subbase course is not always needed or used.
– For example, a pavement constructed over a high quality, stiff subgrade may
not need the additional features offered by a subbase course so it may be
omitted from design.
• However, a pavement constructed over a low quality soil such as a
swelling clay may require
– the additional load distribution characteristic that a subbase course can offer.
• In this scenario the subbase course may consist of
– high quality fill used to replace poor quality subgrade (over excavation).
Samuel K. 542
Perpetual Pavements
• "Perpetual Pavement" is a term used to describe
– a long-lasting structural design, construction and maintenance concept.
• A perpetual pavement can last 50 years or more
– if properly maintained and rehabilitated.
• As Michael Nunn pointed out in 1998,
– flexible pavements over a minimum strength are not likely to exhibit structural
damage even when subjected to very high traffic flows over long periods of time.
• He noted that existing pavements over about 370 mm (14.5 inches) should be
able to withstand
– an almost infinite number of axle loads without structural deterioration due to either
fatigue cracking or rutting of the subgrade.
• Deterioration in these thick, strong pavements was observed to initiate in the
pavement surface as
– either top-down cracking or rutting.
• Further, Uhlmeyer et al. (2000) found that
– most HMA pavements thicker than about 160 mm (6.3 inches) exhibit only surface-
initiated top-down cracking.
• Therefore, if surfaceinitiated cracking and rutting can be accounted for before
they impact the structural integrity of the pavement,
– the pavement life could be greatly increased.
Samuel K. 543
cont...
• Researchers have used this idea as well as pavement materials research
to develop a basic perpetual pavement structural concept.
• This concept uses a thick asphalt over a strong foundation design with
three HMA layers, each one tailored to resist specific stresses (TRB,
2001):
– HMA base layer.
• This is the bottom layer designed specifically to resist fatigue cracking.
• Two approaches can be used to resist fatigue cracking in the base layer.
• First, the total pavement thickness can be made great enough such that the tensile
strain at the bottom of the base layer is insignificant.
• Alternatively, the HMA base layer could be made using an extra-flexible HMA.
• This can be most easily accomplished by increasing the asphalt content.
• Combinations of the previous two approaches also work.
– Intermediate layer.
• This is the middle layer designed specifically to carry most of the traffic load.
• Therefore it must be stable (able to resist rutting) as well as durable.
• Stability can best be provided by using stone-on-stone contact in the coarse
aggregate and using a binder with the appropriate high-temperature grading.
– Wearing surface.
• This is the top layer designed specifically to resist surface-initiated distresses such as
top-down cracking and rutting.
• Other specific distresses of concern would depend upon local experience. 544
Samuel K.
cont...
• In order to work, the above
pavement structure must be built on
a solid foundation.
• Nunn (1998) notes that rutting on
roads built on subgrade with a CBR
greater than 5 percent originates
almost solely in the HMA layers,
– which suggests that a subgrade
with a CBR greater than 5 percent
(resilient modulus greater than
about 7,000 psi (50 MPa)) should
be considered adequate.
• As always, proper construction
techniques are essential to a
perpetual pavement's performance.
• The figure shows an example
cross-section of a perpetual
pavement design
– to be used in California on I-710 Figure: Example I-710 Long Beach Freeway
(the Long Beach Freeway) in Los
Angeles County. Perpetual Pavement Design
545
Samuel K.
(from Monismith and Long, 1999)
Unique Properties of Flexible Pavements
• Pavement is unique when compared to other

civil engineering structures.
• Some of the unique properties of flexible
pavement are discussed below.
Samuel K. 546
Fast Deterioration with Time
• Each traffic load application contributes to some extent
to pavement distresses.
• Different types of distress could happen and accumulate
over the years such as
– rutting,
– fatigue cracking,
– material disintegration,
– roughness and
– bleeding.
• When one or more of these distresses reach a certain
unacceptable level,
– the pavement is considered as failed.
• The typical life of a flexible pavement varies from case
to case,
– with an average value of 10 to 15 years.
Samuel K. 547
cont...
• A good method of pavement design should include
– the designed life, or
– how long the pavement is expected to last before failure.
• The incorporation of the designed life in the design process
– is one of the hardest tasks faced by the pavement designer.
• In many cases, the expected designed life does not match
with the actual service life.
• Unlike pavements, the design of other civil engineering
structures does not consider the factor of designed life.
• In such cases the designer assumes that
– if the structure is safe under the maximum possible load, it will
be safe for an extended period of time.
• This concept does not work with pavements because of
– their fast deterioration and short service lives. 548
Samuel K.
cont...
FIGURE: Load distribution in flexible pavement.
Samuel K. 549
Repeated Loads
• When a traffic wheel moves on the pavement
surface
– it creates a stress pulse.
• This stress pulse creates a dynamic pavement
response,
– which is harder to analyze as compared to static
response.
• Dynamic waves propagate throughout
– the pavement layers and sub-grade, and
– involve reflections and refractions at the layer
interfaces.
Samuel K. 550
Variable Load Configuration
• Different vehicular axle configurations are available with a
different number of wheels at the end of each axle.
• Axles can be
– single,
– tandem,
– tridem, or
– multiple,
• While wheels can be
– either single or dual.
• Passenger cars have FIGURE: Schematic of common axle
– single axles and single wheels. and wheel configurations.
• However, trucks can take
– different combinations of axle and wheel configurations as shown in
Figures.
• Different axle and wheel configurations result in
– stress interactions within the pavement structure,
• which in turn influence pavement performance. 551
Samuel K.
FIGURE: Trucks with different axle and wheel configurations .
Samuel K. 552
Variable Load Magnitude
• Traffic loads vary
– from light to heavy for passenger cars and loaded
trucks respectively.
• Since pavement materials have non-linear
response,
– doubling the load magnitude does not result in
doubling the stress or strain.
• More importantly, doubling the load magnitude
– does not result in doubling the rate of pavement
deterioration.
• In fact, increasing the load magnitude
exponentially
– increases the rate of pavement deterioration.
Samuel K. 553
Variable Tyre Pressure
• Trucks have much higher tyre pressures than passenger
cars.
• Typical tyre pressures of
– passenger cars are in the order of 30 – 35 psi,
– while trucks have tyre pressures of 100 – 115 psi.
• Higher tyre pressures result in
– higher contact pressures at the surface of the pavement and,
in turn,
– faster deterioration of the surface layer.
• Truck tyre pressures have been increasing over the
years,
– challenging pavement engineers to improve the quality of
the HMA material in order to reduce premature pavement
failure.
Samuel K. 554
Traffic Growth
• Pavement is designed to carry future traffic,
– which usually increases over the years.
• Predicting future traffic growth is not always
accurate.
• This inaccuracy in predicting future traffic
affects
– the accuracy of predicting pavement performance
and
– consequently pavement designed life.
Samuel K. 555
Change of Material Properties with Environmental Conditions
• Environmental conditions have large effect on
– the properties of pavement materials.
• For example, HMA gets
– softer at high temperatures resulting in rutting, and
– harder at low temperatures resulting in thermal cracking.
• Also, rain and freeze – thaw cycles
– weaken the HMA materials and
– reduce the load carrying capacity of
• base,
• subbase and
• subgrade.
• In addition, HMA ages with time resulting in
increasing
– its stiffness and
– its susceptibility to cracking. 556
Samuel K.
Change of Subgrade Properties with Distance
• Since pavement is built to cover a large
distance,
– the same road might be built over different types
of subgrade materials with different properties.
• Moreover, the road could be built
– over cut or fill subgrade sections having different
material properties.
• The change of subgrade properties requires
different thicknesses of pavement layers
– in order to support the same traffic load and
– produce the same performance.
Samuel K. 557
Channelized Traffic Load
• Traffic load is applied in the wheel path.
• This channelization of traffic load results in
– faster deterioration in the wheel path as compared to
the area between wheel paths.
• The design process should consider
– the proper stress and strain distributions within the
pavement structure
• to determine critical locations and possible deteriorations.
Samuel K. 558
Multi-Layer System
• The pavement structure consists of several
layers built over the subgrade.
• These layers have different materials with
different properties.
• The distribution of stresses and strains within
the multi-layer pavement system depends on
– the thickness and material properties of these
layers.
Samuel K. 559
Unconventional Failure Definition
• Failure of typical civil engineering structures is defined as break or
fracture.
• This usually happens when the applied stress exceeds the maximum
allowable value, or the strength of the material.
• Unlike other civil engineering structures, the applied stresses in
pavement are usually much smaller than the strength of the material.
• Therefore, one load application does not fail the pavement, but causes an
infinitesimal amount of deterioration.
• This deterioration gradually increases until it reaches an unacceptable
level, or failure.
• Because of this failure mechanism, different types of distresses can
occur in asphalt pavements as discussed in the next section.
• Thus, pavement failure does not happen because of a collapse of the
pavement structure, but when one or more of the distresses reach an
unacceptable level.
• These unique properties of pavement call for a design concept that is
different than that used for other structures.
• The designer has to incorporate these factors to ensure that failure is not
reached until the end of the intended designed life.
560
Samuel K.
Pavement Distresses and Performance
• Common Pavement Distresses
• Different types of distress can occur in asphalt
pavement.
• These distresses could be developed due to
– traffic load repetitions,
– temperature,
– moisture,
– aging,
– construction practice, or
– combinations.
Samuel K. 561
Fatigue Cracking
• Fatigue cracks
– are a series of longitudinal and interconnected cracks
• caused by the repeated applications of wheel loads.
• This type of cracking generally
– starts as short longitudinal cracks in the wheel path and
– progress to an alligatorcracking pattern (interconnected cracks)
as shown in Figure.
• This type of cracking happens because of
– the repeated bending action of the HMA layer when the load is
applied.
• This generates tensile stresses
– that eventually create cracks at the bottom of the asphalt layer.
• Cracks gradually propagate to the top of the layer and later
progress and interconnect.
• This type of distress will eventually lead to
– a loss of the structural integrity of pavement system. 562
Samuel K.
cont...
FIGURE: Advanced stage of fatigue cracking.

Samuel K. 563
Rutting
• Rutting is defined as
– permanent deformation in the wheel path as shown in Figure A.
• Rutting can occur due to:
– unstable HMA,
– densification of HMA,
– deep settlement in the subgrade as demonstrated in Figure B.
• Unstable HMA can occur because of one or more of
different reasons such as
– too much asphalt binder,
– too soft asphalt binder,
– rounded aggregate particles,
– smooth aggregate texture, or
– too many fines in the HMA mix.
• Densification of HMA can occur because of
– the poor compaction during construction.
• Deep settlement can happen because of
– poor drainage or weak subgrade. 564
Samuel K.
cont...
FIGURE B: Rutting due to: (a)

Unstable asphalt concrete, (b)
Densification of asphalt concrete,
and (c) Deep settlement.
FIGURE A: Rutting.
Samuel K. 565
Roughness
• Roughness is defined as
– the irregularities in the pavement profile which causes
uncomfortable, unsafe, and uneconomical riding.
• Roughness affects
– the dynamics of moving vehicles,
– increasing the wear on vehicle parts and the handling of
vehicles.
• Thus, road roughness has an appreciable impact on
– vehicle operating costs and the safety,
– comfort, and
– speed of travel.
• It also increases
– the dynamic loading imposed by vehicles on the surface,
– accelerating the deterioration of the pavement structure.
Samuel K. 566
Thermal Cracking
• As the temperature decreases
– the HMA material contracts.
• Since the material is restrained from movement
due to the friction with the underlying material,
– tensile stresses develop within the HAM
material.
• If the tensile stress exceeds the tensile strength
of the material,
– thermal cracks develop as shown in Figure.
• Thermal cracks typically occur
– in the transverse direction perpendicular to the
direction of traffic.
• This type of cracking is usually equally spaced.
• This is a non-load associated type of cracking
and it starts during the winter season.
• The width of the thermal cracks usually
changes from summer to winter.
• In some cases, small cracks heal during the
summer season. FIGURE: Thermal cracking.
• In other cases, the width of the crack increases
from one year to another.
Samuel K. 567
Shoving
• Shoving is a form of plastic
movement resulting in
– a localized bulging of the
pavement surface.
• Shoving can take a number of
different forms such as
– upheaval (Figure),
– ―wash-boarding‖ or ripples across
the pavement surface, or
– crescentshaped bulging.
• Shoving occurs in the asphalt
layers that lack stability because
of
– too much asphalt binder in the
HMA mix,
– too soft asphalt binder,
– rounded aggregate particles, FIGURE: Shoving.
– smooth aggregate texture, or
– too many fines in the mix.
Samuel K. 568
Bleeding or Flushing
• Bleeding or flushing
– the upward movement of
the asphalt binder resulting
in the formation of a film
of asphalt on the surface as
shown in Figure 8.10.
• Bleeding occurs when
– the HMA mix is too rich
with asphalt binder that is
forced to the surface when
traffic load is applied
especially in hot weather.
• Bleeding could be
hazardous because FIGURE: Bleeding or flushing.
– it makes the pavement

slippery when wet.
Samuel K. 569
Raveling
• Raveling
– is the progressive separation of
aggregate particles in a pavement
from the surface downward or
from the edge inward as shown in
Figure.
• Usually fine aggregate particles
are separated first followed by
coarse aggregates.
• Raveled surfaces are aged and
typically look dry and
weathered.
• Raveling is caused by one or
more of several reasons such as
– lack of compaction, FIGURE: Raveling.
– dirty or disintegrating aggregate,
– too little asphalt in the mix, or
– overheating of the mix. Samuel K. 570
Polished Aggregate
• Aggregate particles in the HMA may get polished
smooth and create a slippery pavement surface when
wet (Figure).
• Some aggregates, particularly some types of
limestone, become polished rather quickly under
traffic. FIGURE: Polished aggregate.
571
Samuel K.
Reflection Cracking
• Cracks in the underneath
layer might reflect in the
overlay as shown in Figure.
• Reflection cracking occurs
frequently in asphalt
overlays
– on concrete pavement and
cement treated basis.
• They also occur when
cracks in
– the old asphalt layer are not
properly repaired before
overlay.
• Reflect cracks may take
several forms depending on
FIGURE: Reflection cracking.
– the pattern of the crack in
the underneath layer.
Samuel K. 572
Pavement Performance
• Figure below shows how the pavement condition varies with
– time or traffic applications.
• When the road is first built it typically has a good condition.
• With time and with the continuous applications of traffic loads
– the pavement gradually deteriorates and the condition gets worse.
• The change of pavement condition with time or traffic is defined
as performance.
• Performance is affected by several factors as discussed in the
next section and cannot easily be predicted.
FIGURE: Change of pavement

condition versus time.
Samuel K. 573
cont...
• When pavement condition reaches a certain
unacceptable level
– the pavement reaches the end of its serviceable life.
• Performance prediction is important in order to
ensure that
– the pavement reaches the unacceptable condition at
the end of its designed life.
• Currently, there is no completely mechanistic (or
theoretical) method to predict pavement
performance.
• Empirical or mechanistic-empirical methods are
currently being used to predict performance.
Samuel K. 574
Serviceability
• User‘s opinion of how well they are being served by the road is largely
subjective.
• The serviceability of a given road may be expressed as
– the average evaluation given by all users of the road.
• Performance, therefore, is the overall appraisal of
– the serviceability history of a pavement.
• Present serviceability rating (PSR)
– is the average of user assessment or rating of the quality of the pavement.
• Present serviceability index, (PSI)
– is an estimate of the PSR of a pavement based on objective measures of the
pavement quality and
– a correlation equation for relating these measures to the PSR.
• PSR and PSI follow a scale from zero to five,
– where zero is for an impassable road condition and five for an excellent
condition.
• A well-constructed new pavement has a PSI of 4.5 – 4.6.
• Agencies typically define failure as a terminal serviceability index, pt, of
2.0, 2.5, or 3.0 as shown in Figure on the previous slid.
Samuel K. 575
Factors Affecting Performance of Flexible Pavements
• Pavement performance is affected by several

factors, which are
– traffic,
– soil and pavement materials,
– environment, and
– construction and maintenance practice.
Samuel K. 576
Traffic
• Traffic has a major effect on pavement performance.
• Traffic characteristics that affect performance are
– traffic load,
– traffic volume,
– tyre pressure, and
– vehicle speed.
• Traffic load produces stresses and strains within the
pavement structure and the subgrade,
– which gradually contribute to the development of pavement
distresses.
• For example, heavier loads result in higher potential
for fatigue cracking and rutting.
Samuel K. 577
cont...
• Traffic volume affects pavement performance
– since larger number of load repetitions increases the chance for
fatigue cracking.
• Also, higher tyre pressure produces higher stress
concentrations at the pavement surface
– that could result in rutting and shoving in the HMS layer.
• Finally, vehicle speed affects the rate of applying the load.
• Since asphalt concrete is a visco-elastic plastic material,
– its response is affected by the rate of load application.
• Slow or stationary vehicles have more chances of
developing
– rutting and shoving than high-speed vehicles.
• On the other hand, high travel speeds
– cause more severe bouncing of vehicles, and result in larger
dynamic loading and increased roughness.
Samuel K. 578
Soil and Pavement Materials
• Soil and pavement materials significantly affect pavement
performance.
• Of course, high quality materials are needed to provide
– good support to traffic loads under various environmental
conditions.
• Important material properties include mechanical
properties such as
– elasticity,
– visco-elasticity,
– plasticity,
– temperature susceptibility,
– durability and
– aging characteristics.
• These properties affect how the material responds to traffic
loads and environmental conditions such as temperature,
freeze-thaw effect, and rain. 579
Samuel K.
Environment
• Environmental conditions that affect pavement
performance include
– moisture, temperature, and their interaction.
• For example, moisture may reduce subgrade
support and weakens various pavement layers.
• High temperatures soften asphalt concrete and
could create rutting within the surface layer.
• Temperatures below freezing have a bad effect
on pavement performance, especially cycles of
freeze and thaw.
Samuel K. 580
Construction and Maintenance Practice
• In many cases, defects in pavement start during construction and
propagate during service.
• In fact, poor construction procedure will almost always ensure poor
pavement performance.
• For example, poor compaction of subgrade or any pavement layer
allows excessive further compaction by traffic,
– which appears in the form of rutting and surface cracking.
• Poor placement of HMA during construction may result in
– weak transverse or longitudinal construction joints that are susceptible to
early cracking and deterioration.
• Excessive air voids in the HMA layer due to poor compaction
– will result in fast aging followed by cracking.
• In contrast, too much compaction of HMA
– will result in too small amount of air voids that could create rutting or
bleeding.
• Lack of smoothness of the pavement during construction increases
– the dynamic impact of traffic, and
– consequently, speeds up the rate of developing roughness during service.
Samuel K. 581
Design Objectives, Constraints, and Evolution
• It is important to define the basic objectives

and constraints of the design process
– so that the procedure would be efficient and
successful.
Samuel K. 582
Design Objectives and Constraints
• The objectives of pavement design can be listed
as follow (Haas et al., 1994).
– Maximum economy, safety, and serviceability over the
design period
– Maximum or adequate load-carrying capacity in terms
of load magnitude and repetitions
– Minimum or limited deteriorations over the design
period
– Minimum or limited noise or air pollution during
construction
– Minimum or limited disruption of adjoining land use
– Maximum or good aesthetics
Samuel K. 583
cont...
• The pavement designer typically faces several
economic, physical, and technical design constraints
such as,
– Availability of time and fund for design and construction
– Minimum allowable level of serviceability before
rehabilitation
– Availability of materials
– Minimum and maximum layer thickness
– Capabilities of construction and maintenance personnel and
equipment
– Testing capabilities
– Capabilities of structural and economic models available
– Quality and extent of the design data available
Samuel K. 584
AASHTO, 1993 Design Method
• Background
– The AASHTO, 1993 design method is based the
statistical evaluation of the performance data
obtained from the AASHO Road Test.
– The 1993 version (AASHTO, 1993) was preceded
by several versions and followed by the proposed
AASHTO mechanistic-empirical design method.
– Although the 1993 method is not the latest, it is
currently the most commonly used method of
design of asphalt pavement and is expected to be
used for several more years until agencies adopt
the proposed AASHTO M-E method.
Samuel K. 585
cont...
• Equivalent Single Axle Load (ESAL)
– Traffic loads applied on the pavement surface
• range from light passenger cars to heavy trucks.
– Heavy traffic loads have more harmful effect to
pavement than light loads.
– Also, as the number of repetitions of the same load
increases the effect on the pavement increases.
– To design a pavement section the damage caused
by all axle loads that will be applied on the
pavement during its designed life has to be
considered.
Samuel K. 586
cont...
– According to the AASHTO (1993) design method axles with
different magnitudes and different numbers of repetitions are
converted to
• an equivalent number of repetitions of a standard axle load that causes the
same damage to the pavement.
– A standard axle load was selected as 18000 Lb (80 kN) applied on a
single axle with a dual wheel at each end.
– The ESAL is the equivalent number of repetitions of the
• 18-kip (80 kN) standard axle load that causes the same damage to the
pavement caused by one-pass of the axle load in question.
– The 1993 AASHTO guide developed Equivalent Axle Load Factors
(EALF) to
• relate the damage caused by different load magnitudes and axle
configurations to the standard axle load as shown in Equation below.
where Wt18 is the number of 18-kip (80-kN) single-axle load

applications to time t (failure) and Wtx is the number of x-axle load
applications to time t (failure). 587
Samuel K.
cont...
– Based on the data obtained at the AASTO Road Test,
the following regression equation was developed.
where
Lx is the load in kips on one single axle, one set of tandem axles, or one set of
triple axles;
L2 is the axle code (1 for single, 2 for tandem axles, and 3 for triple axles);
pt is the terminal serviceability index;
ß18 is the value of ßx when Lx is equal to 18 and L2 is equal to one.
SN is the structural numbers, which is an index that combines the effect of
material properties, layer
thicknesses and drainage quality according to Equation on the next slid. 588
Samuel K.
cont...
• Tables 1 and 3 show numerical values of EALF for single, tandem, and
triple axles, respectively.
• In order to get the EALFs the structural number has to be known or
assumed.
SN = a1D1 + a2D2m2 + a3D3m3
• where:
– a1, a2 and a3 = Structural layer coefficients (Defined in the next section, Step 8).
– D1, D2 and D3 = Thicknesses of surface, base and subbase, respectively.
– m2, m3 = Drainage coefficients (Table 4).
• Since the EALFs are not very sensitive to SN, a SN value of 5 may be
assumed in most cases.
• Unless the design thickness is significantly different, no iterations will be
needed.
• As shown in Table 1, the EALF corresponding to a single axle of 18000 Lb
is 1, since it is the standard axle load. For tandem and triple axles (Tables 2
and 3), an EALF of 1 corresponds to an axle load of about 34 kips and 48
kips, respectively.
• Note that the EALF is very sensitive to the magnitude of the axle load
regardless of the axle configuration.
• In fact, the EALF exponentially increases as the load increases indicating a
significant increase in pavement damage. Samuel K. 589
TABLE 1 Axle Load Equivalency Factors for Flexible Pavements, Single Axles
Samuel K. 590
TABLE 2 Axle Load Equivalency Factors for Flexible Pavements, Tandem Axles
Samuel K. 591
TABLE 3 Axle Load Equivalency Factors for Flexible Pavements, Triple Axles
Samuel K. 592
TABLE 4 Recommended Drainage Coefficients for Untreated Bases and Subbases in Flexible
Pavements (AASHTO, 1993)
Samuel K. 593
cont...
• Knowing the EALF and the estimated number of
repetitions of axle loads for different axle groups in the
first day of opening the road to traffic, the initial daily
ESAL (ESALo) is computed as:
where
Ni is the number of repetitions of axle group i,
EALFi is the equivalency factor for axle group i, and
m is number of axle groups.
• The cumulative ESAL during the designed life of the
pavement (W18) is then calculated as:
• where n is the designed life of the pavement in years and i

is the expected annual traffic growth rate. 594
Samuel K.
Design Procedure
• The main requirement is to determine
– the thicknesses of various pavement layers to
satisfy the design objectives.
• Assuming that the pavement section consists
of surface, base and subbase,
– three thicknesses: D1, D2 and D3 are required for
the three layers, respectively.
• The design procedure can be divided into 12
steps as presented below.
Samuel K. 595
cont...
• Step 1 — Reliability
– A reliability level (R) is selected depending on
• the functional classification of the road and
• whether the road is in urban or rural area.
– The reliability is the chance that pavement will last for the
design period without failure.
– A larger reliability value will ensure better performance,
but it will require larger layer thicknesses.
– Table 5 shows reliability levels suggested by the 1993
AASHTO design guide.
– The reliability levels shown in Table 5 have a wide range
to accommodate different field conditions.
– Different agencies typically select reliability values from
the table that match their local conditions.
Samuel K. 596
cont...
TABLE 5 Suggested Levels of Reliability for Various Functional Classifications
(AASHTO, 1993)
Samuel K. 597
cont...
• Step 2 — Overall Standard Deviation
– The overall standard deviation (So) takes into
consideration the variability of all input data.
– The 1993 design guide recommends an
approximate range of 0.4 to 0.5 for flexile
pavements.
– An overall standard deviation value (So) is selected
by the designer within this range.
Samuel K. 598
cont...
• Step 3 — Cumulative Equivalent Single Axle Load
– In this step, the designer assumes a designed life,
• typically in the range of 10 to 20 years.
– The cumulative expected 18-kip (80-kN) ESAL (W18) during the
designed life in the design lane is then determined as discussed earlier.
– If the cumulative two-directional 18-kip ESAL is known,
• the designer must factor the design traffic by directions by multiplying by the
directional distribution factor (D) to get the ESAL in the predominate direction.
– For example, if the traffic split during the peak hour is 70 – 30%, D is
taken as 0.7.
– To get the ESAL in the design (right) lane,
• the design traffic in the predominant direction is multiplied by the lane
distribution factor (L) shown in Table 6.
TABLE 6 Lane Distribution
Factor (AASHTO, 1993)
Samuel K. 599
cont...
• Step 4 — Effective Roadbed Soil Resilient Modulus
– Determine the resilient modulus (MR) of the roadbed soil in the
laboratory according to AASHTO T307 method (AASHTO, 2004).
– Since the resilient modulus of the soil depends on the moisture
content,
• different resilient moduli will be obtained in different seasons depending on
the amount of rain or snow in each season.
– Thus, an effective roadbed soil resilient modulus is needed to
represent a weighted average value for the whole year.
– Figure 21 can be used to estimate the effective roadbed soil resilient
modulus.
– In this method, the year is divided into a number of distinct seasons
where the resilient modulus is significantly different.
– The relative damage (uf ) corresponding to each MR value is
determined using the scale in Figure 21 and recorded in the table.
– The uf values are averaged and the corresponding MR value is
obtained from the same scale and reported as the effective roadbed
soil resilient modulus.
Samuel K. 600
FIGURE 21 Worksheet for
estimating effective roadbed
soil resilient modulus
(AASHTO, 1993).
601
Samuel K.
cont...
• Step 5 — Resilient Moduli of Pavement Layers
– The resilient moduli (MR) of the surface, base, and
subbase layers are either determined using laboratory
testing or estimated using previously developed
correlations.
• Step 6 — Serviceability Loss
– The serviceability loss is the difference between the initial
serviceability index (po) and the terminal
– serviceability index (pt) (See Figure on the previous slid).
– PSI = po + pt
– The typical Po value for a new pavement is 4.6 or 4.5.
– The recommended values of pt are 3.0, 2.5 or2.0 for major
roads, intermediate roads and secondary roads,
respectively. Samuel K. 602
cont...
• Step 7 — Structural Numbers
– As shown in Equation above, the structural number (SN) is an
index value that combines
• layer thicknesses,
• structural layer coefficients, and
• drainage coefficients.
– In this step the structural numbers required above the subgrade,
subbase, and base layers are determined.
– The required structural number above the subgrade (SN3) is
determined first using either Equation below or Figure 22.
where:
W18 = Cumulative expected 18-kip ESAL during the designed life in the design lane
ZR = Normal deviate for a given reliability R (3)
So = Standard deviation
MR = Effective roadbed soil resilient modulus (step 4)
Samuel K. 603
cont...
FIGURE 22 Design chart for flexible pavements based on using mean values for
each input (AASHTO, 1993).
Samuel K. 604
cont...
• Step 8 — Structural Layer Coefficients
– The structural layer coefficient
• is a measure of the relative ability of a unit thickness of a given
material to function as a structural component of the pavement.
– Three structural layer coefficients (a1, a2 and a3) are
required for the surface, base and subbase, respectively.
– These coefficients can be determined from road tests, as
was done in the AASHO Road Test, or from correlations
with material properties as shown in Figures 23, 24 and 25
(Van Til et al., 1972).
– It is recommended that the structural layer coefficients be
based on the resilient modulus, which a more fundamental
material property.
– A typical a1 value for the dense-graded HMA is 0.44,
which corresponds to a resilient modulus of 450,000 psi as
shown in Figure 23.
Samuel K. 605
cont...
FIGURE 23 Chart for estimating structural layer coefficient of dense-graded asphalt
concrete based on the elastic (resilient) modulus (Van Til et al., 1972).
606
Samuel K.
FIGURE 24 Correlation charts for
Samuel K.
estimating resilient modulus607of
bases (Van Til et al., 1972).
cont...
FIGURE 25 Correlation chart for estimating resilient modulus of

subbases (Van Til et al., 1972).
Samuel K. 608
cont...
• Step 9 — Drainage Coefficients
– Drainage coefficients are measures of the quality of
drainage and the availability of moistures in the
granular base and subbase.
– Two equal drainage coefficients (m2 and m3) are needed
for the base and subbase, respectively.
– The drainage coefficient values for the untreated base
and subbase recommended by the AASHTO 1993
design guide are shown in Table 4.
Samuel K. 609
cont...
• Step 10 — Layer Thicknesses
– Using the structural numbers required above the base,
subbase and the subgrade (SN1, SN2 and SN3)
obtained in Step 7, the layer thicknesses of the
surface, base and subbase (D1, D2 and D3) can be
obtained from Equations *, ** and ***, respectively.
– First, Equation * is used to solve for D1 and the value
is round up to the next 1/2 in. increment.
– The rounded value of D1 is used in Equation ** to
solve for D2 and the value is rounded up to the next 1
in. increment.
– Finally, the rounded values of D1 and D2 are used in
Equation *** to solve for D3 and the value is rounded
up to the next 1 in. increment. 610
Samuel K.
cont...
SN1  a1D1 *
SN2  a1D1 + a2D2m2 **
SN3  a1D1 + a2D2m2 + a3D3m3 ***
– The values of D1, D2 and D3 have to meet certain
minimum practical thicknesses as shown in Table 7.
– Note that Equations * to *** may allow for thickness
compensations among layers.
– For example, a larger value for D1 may be used that
would allow for a smaller value of D2.
– Since the costs of materials at different locations are
different,
• the designer can make use of the thickness compensation
concept to obtain the most economic pavement section.611
Samuel K.
cont...
TABLE 7 Minimum Thickness (in.) (AASHTO, 1993)
Samuel K. 612
cont...
• Step 11 — Freeze or Thaw and Swelling
– If the pavement is located in an area where freeze or thaw and
soil swelling exists,
• the AASHTO (1993) design guide recommends additional procedure to
estimate the reduction in the service life due to this environmental effect
(AASHTO, 1993).
• Step 12 — Life-Cycle Cost
– In step 3, a pavement design period was assumed, which may
not produce the least life-cycle cost.
– In this step, the designer assumes a few other design periods
and repeats the design process for each design period.
– A life cycle cost analysis is then performed to obtain the most
economic design strategy.
– In this analysis all the costs included in the analysis period are
considered such as the costs of initial construction,
maintenance, rehabilitation, and the salvage value of the
pavement section at the end of the analysis period.
– The users cost may also be considered. 613
Samuel K.
ERA design procedure
• FLEXIBLE PAVEMENT DESIGN CATALOG
– Description of the Catalog
• The design of flexible pavements, as given in this manual,
is based on the catalog of pavement structures of TRL Road
Note 31.
• Before the catalog is used, the elements described regarding
traffic and subgrade should be considered.
• Simultaneously, the information regarding availability, costs
and past experience with materials should be gathered.
• The catalog offers, in eight different charts, alternative
pavement structures for combinations of traffic and
subgrade classes.
• The various charts correspond to distinct combinations of
surfacing and roadbase materials, as shown in Table below:
Samuel K. 614
Table: Summary of Material Requirements for the Design Charts
• All the charts provide alternate pavement structures for all subgrade classes
(S1 through S6).
• They are not however suitable for all classes of traffic, as some structures
would be neither technically appropriate nor economically justified. 615
Samuel K.
Use of the Catalog
• Although the thicknesses of layers should follow
the design charts whenever possible,
– some limited substitution of materials between
subbase and selected fill is allowable
• based on the structural number principles outlined in the
AASHTO Guide for Design of Pavement Structures.
• Where substitution is allowed, a note is included
with the design chart.
Samuel K. 616
cont....
– The choice of chart will depend on a variety of factors
but should be based on minimizing total transport
costs.
– Factors that will need to be taken into account in a full
evaluation include:
• the likely level and timing of maintenance
• the probable behavior of the structure
• the experience and skill of the contractors and the
availability of suitable equipment
• the cost of the different materials that might be used
• other risk factors
Samuel K. 617
Design Example
• An example of traffic calculations was given in
Chapter 3
– for a particular section of a trunk road.
• In the example, a traffic class T8 has been derived
– with a total of ESAs on the order of 20 millions over the
design period.
• From Table given above, for that class of traffic,
– it is readily apparent that the use of the design charts in
the catalog of structures is narrowed down to Charts 4
through 7.
• From the same table, without further information
regarding the subgrade and the materials,
– it would also appear that any type of surfacing is possible,
as well as several types of roadbase. 618
Samuel K.
cont...
• The subgrade strength has reasonably been
ascertained to be represented by
– CBRs in the range of 5 to 7, considering that
• some portions of the alignment which might exhibit higher
strength are so limited in number and
• extent that it makes it impractical to consider several
designs.
• The subgrade strength class to be assigned to this
project is therefore S3.
Samuel K. 619
cont...
• The following preliminary information has been derived
from the investigations and simple cost comparison:
– The materials which may be considered for cement- or lime-
stabilization have relatively low percentages of fines and low
plasticity,
• thus making cementstabilization more promising.
– Granular subbase materials are available in sufficient quantities
and cement stabilization of the subbase is uneconomical when
compared to bank-run materials.
• Stabilization of subbase materials will not be further considered.
– All other materials entering the composition of the possible
pavement structures are available,
• albeit in various quantities and associated transport/construction costs.
• Based on the above, and with the T8/S3 combination of
traffic and subgrade strength classes,
– the design charts 4 through 7 indicate the possible alternate
pavement structures given in Table below.
Samuel K. 620
Table: Design Example: Possible Pavement Structures
Samuel K. 621
cont...
• Further analyses of recent contracts, production costs
hauling distances and associated costs have established
relative costs for the various alternate pavement layers (all
costs per m2 and expressed as a ratio to the highest cost
element) as shown in Table A below.
• With these elements, the relative costs of the possible
alternate pavement structures are evaluated as follows in
Table B below.
• Based on the above, the alternate structures including
cement stabilized layers (Nos. 1 and 3) appear prohibitive,
and the alternate (No. 2) including only crushed stone
roadbase and subbase also appear at a disadvantage.
• The preferred solutions (Nos. 4a and 4b) are only
marginally different. It may be advisable to present both
alternatives for bidding purposes. 622
Samuel K.
Table A: Design Example: Relative Unit Costs of Materials
Samuel K. 623
Table B: Relative Costs of the Possible Alternate Pavement Structures
Samuel K. 624
Figure: Key to
Structural Catalog
Samuel K. 625
Samuel K. 626
Samuel K. 627
Samuel K. 628
Samuel K. 629
Samuel K. 630
Samuel K. 631
Samuel K. 632
Samuel K. 633
Chapter Nine
Design of gravel surfaced road

General
• Much of the information presented in this Section of the
Pavement Design is based on
– the "Pavement and Materials Design Manual" prepared by the
United Republic of Tanzania Ministry of Works 1999, and on
relevant ERA and TRL publications.
• Available information has been modified to provide
– a simple procedure to design gravel wearing courses and low
standard roads, which is appropriate to Ethiopian conditions.
• Gravel road pavements are generally utilized for roads
where
– design traffic flow Annual Average Daily Traffic (AADT) is
less than 200.
• This Section sets out the standards for pavement design,
and specifies the materials which may be used for gravel
roads. Samuel K. 635
Design Principles
• STEPS TO BE CONSIDERED IN THE
DESIGN PROCESS
– Traffic (Baseline flow and forecast)
– Material and geotechnical information (Field
survey and material properties)
– Subgrade (Classification, foundation for expansive
soils and material strength)
– Thickness design (Gravel wearing coarse
thickness)
– Materials design
Samuel K. 636
cont...
• ALL-WEATHER ACCESS
– An essential consideration in the design of gravel
roads is
• to ensure all-weather access.
– This requirement places particular emphasis on
• the need for sufficient bearing capacity of the pavement
structure,
• provision of drainage and
• sufficient earthworks in flood or problem soil areas (e.g.
black cotton).
Samuel K. 637
cont....
• SURFACE PERFORMANCE
– The performance of the gravel surface mainly depends on
• material quality,
• the location of the road, and
• the volume of traffic using the road.
– Gravel roads passing through populated areas in particular require
• materials that do not generate excessive dust in dry weather.
– Steep gradients place particular demands on gravel wearing course
materials;
• this must not become slippery in wet weather or erode easily.
– Consideration should therefore be given to
• the type of gravel wearing course material to be used in particular locations
such as towns or steep sections.
– Gravel loss rates of about 25-30mm thickness a year per 100
vehicles per day is expected,
• depending on rainfall and materials properties (particularly plasticity).
– Performance characteristics that will assist in identifying suitable
material are shown in Figure 1.
Samuel K. 638
cont...
Figure -1. Expected Performance of Gravel Wearing Course Materials
Samuel K. 639
cont...
• MAINTENANCE
– The material requirements for the gravel wearing
course include
• provision of a gravel surface that is effectively
maintainable.
– Adherence to the limits on oversize particles in the
material is of particular importance in this regard
and will normally necessitate the use of crushing
or screening equipment during material production
activities.
Samuel K. 640
Design Method
• The required gravel thickness shall be
determined as follows:
– Determine the minimum thickness necessary to
avoid
• excessive compressive strain in the subgrade (D1).
– Determine the extra thickness needed
• to compensate for the gravel loss under traffic during
the period between regravelling operations (D2).
– Determine the total gravel thickness required
• by adding the above two thicknesses (D1+ D2).
Samuel K. 641
cont...
• MINIMUM THICKNESS REQUIRED
– It is necessary to limit the compressive strain in the
subgrade
• to prevent excessive permanent deformation at the surface
of the road .
– Figure 3 gives the minimum gravel thickness required
for each traffic category
• with the required thickness of improved subgrade materials
for upper and lower subgrade layers.
Samuel K. 642
cont...
• GRAVEL LOSS
– According to TRL Laboratory Report 673, an estimate of
the annual gravel loss is given by the following equation:
GL = f T2 / ( T2 + 50) ( 4.2 + 0.092 T + 3.50 R2 + 1.88V)
Where
GL = the annual gravel loss measured in mm
T = the total traffic volume in the first year in both directions,
measured in
thousands of vehicles
R = the average annual rainfall measured in m
V = the total (rise + fall) as a percentage of the length of the road
f = 0.94 to 1.29 for lateritic gravels
= 1.1 to 1.51 for quartizitic gravels
= 0.7 to 0.96 for volcanic gravels (weathered lava or tuff)
= 1.5 for coral gravels
= 1.38 for sandstone gravels
Samuel K. 643
cont...
• TOTAL THICKNESS REQUIRED
– The wearing course of a new gravel road shall have a
thickness D calculated from:
D = D1 + N. GL
Where
D1 is the minimum thickness from Figure 3
N is the period between regravelling operations in years
GL is the annual gravel loss
– Regravelling operations should be programmed to
ensure that
• the actual gravel thickness never falls below the minimum
thickness D1.
Samuel K. 644
Pavement and Materials
• Depending on the CBR design of the sub grade,

– improved subgrade layers shall be constructed as
required,
• on which the gravel wearing course is placed.
Samuel K. 645
Crossfall and Drainage
• The crossfall of carriageway and shoulders for
gravel roads
– shall be ― 4%‖ as indicated in ERA‘s Geometric
Design Manual - 2002.
• This is to ensure that
– potholes do not develop by rapidly removing surface
water and
– to ensure that excessive crossfall does not cause
erosion of the surface.
• Provision of drainage is extremely important
– for the performance of gravel roads.
Samuel K. 646
Material Requirements
• EXPERIENCE WITH LOCAL MATERIALS
– Knowledge of past performance of locally occurring
materials for gravel roads is essential.
– Material standards may be altered to take advantage
of available gravel sources provided
• they have proved to give satisfactory performance under
similar conditions.
Samuel K. 647
cont...
• MARGINAL MATERIALS
– Figure -1 illustrates the performance
characteristics
• to be expected of materials that do not meet the
requirements for gravel wearing course.
– Refinements and amendments of the standard
material specification may be necessary
• to overcome problem areas such as towns (dust
nuisance) or steep hills (slipperiness).
Samuel K. 648
cont...
• IMPROVED SUBGRADE LAYER
– In General the use of improved subgrade layers has
the following advantages:
• Provision of extra protection under heavy axle loads;
• Protection of underlying earthworks;
• Provides running surface for construction traffic;
• Assists compaction of upper pavement layers;
• Provides homogenous subgrade strength;
• Acts as a drainage filter layer;
• More economical use of available materials.
Samuel K. 649
cont...
• SUBGRADE CBR
– All subgrade materials shall be brought to strength
of
• at least a minimum CBR of 7% for minor gravel roads
and
• at least a minimum CBR 25 % for major gravel roads.
– The different types of gravel roads are explained in
the next slids.
Samuel K. 650
cont...
• TREATMENT OF EXPANSIVE FORMATIONS
– The following treatment operations should be applied
on Expansive Formations for higher class roads of
AADT-design greater than 50:
• Removal of Expansive Soil
– Where the finished road level is designed to be less than 2 metres
above ground level,
» remove the expansive soil to a minimum depth of 600 mm over
the full width of the road, or
– Where the finished road level is designed to be greater than 2
metres above ground level,
» remove the expansive soil to a depth of 600 mm below the
ground level under the unsurfaced area of the road structure, or
– Where the expansive soil does not exceed 1 meter in depth,
» remove it to its full depth. 651
Samuel K.
cont...
• Stockpile the excavated material on either side of the
excavation
– for subsequent spreading on the fill slopes so as to produce as flat a
slope as possible.
• The excavation formed as directed in paragraph (i)
– should be backfilled with a plastic non-expansive soil of CBR value
3 - 4 or better, and
– compacted to a density of 95% modified AASHTO.
• After the excavated material has been replaced with non-
expansive material in 150mm lifts to 95% modified
AASHTO density,
– bring the road to finished level in approved materials, with a side
slope of 1:2, and
– ensure that pavement criteria are complied with; the previously
stockpiled expansive soil excavated as directed under (i) should
then be spread over the slope. 652
Samuel K.
cont...
• Do not construct side drains unless they are absolutely
essential to stop ponding;
– where side drains are necessary, they should be as shallow as
possible and
– located as far from the toe of the fill as possible.
• Ideally, construction over expansive soil should be done
– when the in-situ moisture content is at its highest, i.e. at the
end of rainy season
Samuel K. 653
cont...
– The following treatment operations may be applied
on Expansive Formations for light traffic class
roads of AADT design less than 50:
• Remove 150mm of expansive topsoil and stockpile
conveniently for subsequent use on shoulder slopes
• Shape road bed and compact to 90% modified AASHTO
• The excavation formed as directed in paragraph (i)
should be backfilled with a plastic non-expansive soil of
CBR value 3 - 4 or better, and compacted to a density of
95% modified AASHTO in each 150mm layer; the
subgrade material may be plastic but non-expansive.
Samuel K. 654
• MATERIAL CHARACTERISTICS
– Soils used for improved subgrade layers shall be
• non-expansive,
• non-dispersive and
• free from any deleterious matter.
– They shall comply with the requirements shown in Table below.
Samuel K. 655
Gravel Wearing Course
• PERFORMANCE CHARACTERISTICS OF
GRAVEL WEARING COURSE
– The materials for gravel wearing course should satisfy
the following requirements that are often somewhat
conflicting:
• They should have sufficient cohesion to prevent ravelling
and corrugating (especially in dry conditions)
• The amount of fines (particularly plastic fines) should be
limited to avoid a slippery surface under wet conditions.
– Figure -1 shows the effect of the Shrinkage Product (SP) and
Grading Coefficient (GC) on the expected performance of gravel
wearing course materials.
– Excessive oversize material in the gravel wearing course affects the
riding quality in service and makes effective shaping of the surface
difficult at the time of maintenance. Samuel K. 656
cont...
– For this reason the following two types of gravel wearing course
material are recommended.
» Type 1 gravel wearing course which is one of the best material
alternatives which shall be used on all roads which have AADT-
design greater than 50.
» Type 1 material shall also be used for all routine and periodic
maintenance activities for both major and minor gravel roads.
» Type 1 or Type 4 gravel wearing course material may be used
on new construction of roads having AADT-design less than 50.
Samuel K. 657
cont...
• GRAVEL WEARING COURSEMATERIAL SPECIFICATION
– Selected material shall consist of hard durable angular particles of fragments
of stone or gravel.
– The material shall be free from vegetable matter and lumps or balls of clay.
• Type 1
– The grading of the gravel after placing and compaction shall be a smooth
curve within and approximately parallel to the envelopes detailed in Table -2.
– The material shall have a percentage of wear of not more than 50 at 500
revolutions, as determined by AASHTO T96.
– The material shall be compacted to a minimum in-situ density of 95% of the
maximum dry density determined in accordance with the requirements of
AASHTO T 180.
– The plasticity index should be not greater than 15 and not less than 8 for wet
climatic zones and should be not greater than 20 and not less than 10 for dry
climatic zones.
– The linear Shrinkage should be in a range of 3-10%.
– Note that the above gradation and plasticity requirements are only to be used
with angular particles and that crushing and screening are likely to be required
in many instances for this purpose.
Samuel K. 658
cont...
• Type 2 & 3
– These materials may be more rounded particles fulfilling the
following:
• The Plasticity Index lies in a range of 5-12% in wet areas, and in any case
less than 16% in other areas
• The materials have the sanction of local experience .
– Use of more rounded particles may allow the use of river gravel.
– Trials should nevertheless be conducted to verify whether crushing
occurs under traffic or whether crushing should be considered prior
to use.
– Subject to trials, a minimum percentage by weight of particles with
at least one fractured face of 40% may be considered.
– This requirement may also be expressed in terms of crushing ratio.
– Except for very low traffic (less than 15 vehicles per day),
• the CBR should be in excess of 20 after 4 days of soaking at 95% of
maximum dry density under Heavy Compaction.
– For very low traffic, the requirement may be relaxed to a CBR of 15.
Samuel K. 659
cont...
• Type 4
– This material gradation allows for larger size material
and corresponds to the gradation of a base course
material.
– The use of this gradation of materials is subject to the
local experience and shall be used with PIs in a range
of 10-20.
• Type 5 & 6
– These materials gradations are recommended for
smaller size particles.
– They may be used if sanctioned by experience with
plasticity characteristics as for material Type 1.
Samuel K. 660
cont...
Table - 2
Samuel K. 661
cont...
• MAJOR GRAVEL ROADS (AADT DESIGN =20
TO 200)
– Major gravel roads are roads which have a design
AADT greater than 20 and less than 200.
– These will generally fall within the design category of
DS5 to DS8 (See ERA Geometric Design Manual -
2002, Chapter 2.
– It is recommended to use a gravel wearing course
material of grading Type 1
• in the new construction of roads having an AADT greater
than 50 and for all routine and periodic maintenance
activities.
– Type 4 material may be used in the new construction
of roads having an AADT less than 50.
Samuel K. 662
cont...
• MINOR GRAVEL ROADS (AADT DESIGN <20)
– Minor gravel roads are roads which have a design AADT
(AADT design) less than 20.
– They are normally community roads, which are constructed by
labor-based methods.
– These roads generally fall within the design category of DS9 to
DS10 (Refer to ERA Geometric Design Manual-2002).
– Usually these roads are unsurfaced (earth roads).
– However, for subgrade CBR values less than 5% and
longitudinal gradients of greater than 6%, a gravel wearing
course is recommended.
– Materials for gravel wearing course shall comply with the
requirements for
• Type 4 material for new construction and Type 1 for maintenance
activities.
– The CBR requirements may be reduced to 20% if other
suitable material is not locally available.
Samuel K. 663
Determination of CBRdesign
• GENERAL
– The CBR-design is the CBR value of a
homogenous section,
• for which the subgrade strength is classified into S5, S4
or S2 for the purpose of pavement design.
– The procedure to determine CBR-design is shown
in the flow chart in Figure-2.
– Figure-2: Flow Chart for Design
Samuel K. 664
cont...
Figure-2: Flow Chart for Design
Samuel K. 665
cont...
• HOMOGENOUS SECTIONS
– Identification of sections deemed to have homogenous
subgrade conditions is carried out by desk studies of
appropriate documents such as
• geological maps, followed by site reconnaissance that
includes excavation of inspection pits and initial indicator
testing for confirmation of the site observations.
– Due regard for localized areas that require individual
treatment is an essential part of the site reconnaissance.
– Demarcation of homogenous sections shall be
reviewed and changed as required when the CBR test
results of the centerline soil survey are available.
Samuel K. 666
cont...
• STATISTICAL ANALYSIS
– The flow chart in Figure - 2 shows the procedure to
determine CBR-design.
– The CBR-design for cuttings is the lowest CBR value
encountered for the homogenous section.
– The CBR-design for sections that do not require special
assessment or are not within cuttings are determined by
the 90%-ile value of the CBR test results.
– The 90%-ile value for a section of this type is the CBR
value which 10% of the test results fall below.
– The following example shows how this is calculated.
• CBR values are plotted in ascending order (number of tests on
the "x axis" and the CBR test result values on the "y axis");
• Calculate d = 0.1 x (n-1),
where n = number of tests;
• d is measured along the "x axis" and the CBR-design is
determined from the "y axis".
Samuel K. 667
cont...
• LABORATORY TESTING
– Each CBR value shall be determined by laboratory
measurement carried out for a minimum of three
density values to give a CBR - Density relationship
for the material.
– The CBR value is determined at the normal field
density specified for the respective operation (i.e.
A minimum in-situ density of 95% of the
maximum dry density determined in accordance
with the requirements of AASHTO T 180).
Samuel K. 668
Improved Subgrade and Pavement Design
• MAJOR GRAVEL ROADS
– Pavement and improved subgrade for major gravel
roads shall be constructed in accordance with
Figure -3.
– This includes all design categories DS5, DS6, DS7
and DS8 as defined in ERA Geometric Design
Manual -2002.
Samuel K. 669
cont...
• MINOR GRAVEL ROADS
– Pavement and improved subgrade for minor gravel
roads shall also be constructed in accordance with
Figure -3.
– This includes design categories DS9 and DS10 as
defined in ERA Geometric Design Manual -2002.
– The desired properties of the gravel wearing course
material, GW, are given in the previous slid section.
– However, the CBR may be reduced to 20%, and the
LA abrasion value may be increased to 55% for minor
roads, if better quality material is not locally available.
Samuel K. 670
cont...
Figure -3: Pavement and Improved Subgrade for Gravel Roads for ADDTs < 200
Samuel K. 671
Climatic Zones
• ZONES
– For the purposes of gravel wearing course design,
Ethiopia is divided into two climatic zones.
– All places with elevations over 2,000 meters
(average rainfall 80mm/month) are considered to
be wet zones and all places with elevations 2,000
meters or less (average rainfall 20mm/month) are
considered to be dry zones.
– However, engineering judgment should be made
for individual projects as to which category the
design falls.
Samuel K. 672
cont...
• ARID AREAS
– It is acknowledged that, in many arid areas, rates
of rainfall may be extremely high over short
durations.
– Pavement design techniques, unlike drainage
design techniques, do not take this into account as
they are based on annual rates of rainfall.
Samuel K. 673
Samuel K. 674

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WOLLO UNIVERSITY

Highway Engineering II – CENG 4302

Overview of pavement structures

• It has been found difficult to enforce such limits and

• As will be seen in the coming sections, it is only the commercial

Stresses in pavement structures

When ν = 0.5, the equation is simplified to

On the surface of the loaded half-space, z = 0, the deflection is

If the load is applied on a rigid plate such as that used in a plate

The smallest pressure is at the centre and equal to one-half of the

Figure 2-8 (a) Vertical stress distribution in a two-layer system

• The deflection factor, F2, can be obtained from Figure

• The interface deflection, w, between the two

• The deflection factor, F, is different from F2 and

Where, Fe is the strain factor that can be obtained in Figure 2-12 as a

• At the axis of symmetry, tangential and radial stresses

• Once the stresses at the interfaces are

in which P is the load, l is the radius of relative stiffness defined as

, k is the modulus of the subgrade reaction, and a is the contact radius.

where, c is the side length of the a square contact area, c = 1.772a

in which l is the radius of relative stiffness and

The deflection due to interior loading is

When a load is applied over a set of dual tyres, the equations

Figure 2-15: Upward curling of elastic slab due to temperature

• and the stress in the x-direction due to bending

• The total stress in the x-direction is then,

• where Cx and Cy are correction factors for a finite

in which σ may be σx or σy depending on

• in which σc is the stress in the concrete, γc is

in which As is the area of steel required per unit length of slab

Traffic volume and loading

• for the same contact radius, the contact pressure is

• inwhich the subscript s indicate single wheel and d dual wheels.

• for the same contact radius, the contact pressure is proportional to

• for equal defelection, Ws = Wd

• which results that

• Practically, EALF is not very sensitive to pavement

Where, εtx and εt80 are the tensile strains at the

• Where, Ls is the load on standard axles which have the same

• in which, Nf is the allowable number of load repetitions for

• Each axle of a tandem axle or tridem axle assembly is

• Directional Distribution Factor (D):

• Detail soil survey is carried out on the selected route

Paved trunk roads 0.8 1.2

Table : Sampling Frequency (ERA, Site Investigation Manual, 2001)

Investigation Stage Test Description Frequency of Cumulative

Type of Road Test Description Frequency of Cumulative

• Another index that is used to reflect the properties of

where, wn is the natural moisture content of the soil.

Typical compaction curve

where, γd= Dry density of soil,

Effect of compactive effort on dry density

Items Standard Compaction Test Modified (Heavy)

Note: larger diameter mould (152.5mm) is used for gravelly

Where γb = bilk unit weight, w = moisture

D = grain diameter at 30% passing

D = grain diameter at 10% passing

Table: Typical particle size distribution for sub-bases

• For a filter to possess the required drainage