Flexible Pavement Thickness

Design / Asphalt Institute Method

Elements of Thickness Design

1. Traffic Loading
2. Climate or Environment
3. Material Characteristics
4. Others: Cost, Construction,
Maintenance, Design period.
 Traffic Loading
 Pavement must withstand the large umber of
repeated loads of variable magnitudes
 Primary loading factors:
1. Magnitude of axle loads (controlled by legal load limits).
2. Volume & composition of axle load (Traffic survey, load
meters, & growth rate).
3. Tire pressure & contact area.

 Equivalent Standard Axle Load ESAL (80 kN

(18,000 lb or 18 kips) single axle load.
 The total no. of ESAL is used as a traffic loading
input in the design of pavement structure.
 Climate or Environment
 Climate or environment affect the behavior &
performance of materials used in pavements
1. Temperature: high temp. cause asphalt to
loose stability, low temp. cause asphalt to
become hard & stiff, and frost heave.
2. Moisture: Frost related damage, volume
changes due to saturation, chemical stability
problems with moisture existence (Stripping).
 Material Characteristics
 Required materials characteristics:
1. Asphalt surface: Material should be strong & stable
to resist repeated loading (fatigue).
2. Granular base & subbase: gradation, stable & strong
to resist shears from repeated loading.
3. Subgrade: soil classification, strong & stable.
 Various standard tests are available for
determination of desired properties.
 CBR, Marshal stability, Resilient Modulus, Shear
strength.
 Mr (psi) = 1500 CBR or Mr (Mpa) = 10.3 CBR
 Asphalt Institute Method
 Method is based on two assumed stress –strain
conditions:

1. Wheel load (W) is transmitted to the pavement

surface through the tire at a uniform vertical
pressure (Po). The stresses are then spread through
the pavement structure to produce a reduced max.
vertical stress (P1) at the subgrade surface.
(See Figure 16-4 in textbook )

2. The wheel load (W) causes the pavement structure

to deflect creating both compressive & tensile
stresses in the pavement structure.
(See Figure 16-5 & 16-6 in textbook ).
Asphalt Institute Method Cont.
 This method considers the following strains
as being responsible for the most common
traffic related distresses:
1. Max. Horizontal tensile strains (Et) on the
bottom of the asphalt layer (causes fatigue
cracking).
2. Max. Vertical compressive strains (Ec) on
the top of subgrade (causes permanent
deformation).
 Et & Ec are used as failure criteria
 Design Procedure for Asphalt
Institute Method
 Asphalt Institute thickness design manual was
prepared using a computer program and suitable data.
 The charts have been prepared for a range of
traffic load, which are usually adequate for normal
traffic volume encountered in practice, when this
range is exceeded the computer version should be
used.
 The manual includes charts for six types of pavement
structures, and three sets of environmental conditions
based on the mean annual air temp. (45o, 60o, and 75o
F).
 Example design chart is shown in the coming slide.
Pavement Types in The Asphalt
Institute Method
1. Full depth asphalt concrete.
2. Asphalt concrete surface and emulsified asphalt
base.
1. Type I: Emulsified asphalt mixes made with processed
dense-graded aggregates.
2. Type II: Emulsified asphalt mixes made with semi-
processed aggregates.
3. Type III: Emulsified asphalt mixes made with sands or silty
sands.
3. Asphalt concrete and untreated aggregate base.
1. Base thickness of 6”.
2. Base thickness of 12”.
 Design Procedure for Asphalt
Institute Method
1. Select or determine Input data.
2. Select surface and base material.
3. Determine minimum thickness required for
input data.
4. Evaluate feasibility of stage construction and
prepare stage construction plan.
5. Carry out economic analysis of alternative
design and select the best design.
Step 1: Determine Design Inputs

 Design Inputs are:

 Traffic characteristics.
 Subgrade engineering properties.

 Subbase and base engineering properties.

 Traffic Characteristics

 Determined in terms of number of

repetitions of an 18,000 lb (80)kN single
axle load applied to the pavement on two
sets of dual tires (Equivalent Single
Axle Load (ESAL)).
 See next slides for the determination of
the ESAL.
 Traffic Analysis

1. Estimate the number of vehicles of

different types (Passenger cars, single
unit trucks, multi unit trucks of various
sizes) expected to use the pavement
over the design period.
 Traffic Analysis Cont.
2. Estimate the (%) of total truck traffic expected to use the
design lane.
Design lane: Lane expected to receive the severe service.
 % of trucks is found by observation

 In case data are not available, estimates can be made

from Table 16.1 which gives representative values for
truck distribution in the united states.
 Traffic Analysis Cont.
3. When the axle load of each vehicle type is known,
these can be converted to ESAL using the equivalency
factors given in Table 16.3

If the axle load is unknown, the ESAL can also be

found from the vehicle types by using a truck factor for
that vehicle type.

Truck Factor (TF): The no. of ESALs contributed by

passage of a vehicle.

For each weight class, determine the truck factor.

 Traffic Analysis Cont.

TF = [SUM (No. of axles in each wt. class X EALF)] / Total No. of

vehicles

 Truck factor can be estimated Using Table16.2.

 Equivalent Axle Load factor or Load equivalency factor (EALF)
presented in Table 16.3.
 EALF: Defines the damage per pass to a pavement by the axle of
question relative to the damage per pass of a standard axle load
(80 kN or 18-kip)
 EALF depends on type of pavement, thickness or structural
capacity, and failure conditions (based on experience).
 See Truck Factor Example provided in Figure 16.8 in text.
 Axle & Wheel Configurations
Single Axle with
Tandem Axles with Dual Single Tire
Tires

Single Axle with Dual Tires

Tridem Axles with Dual
Tires
Truck Factor Example
 Traffic Analysis Cont.

4. Multiply (Tf) by the no. of vehicles in

each group and get the sum for all
groups.
ESAL = Sum (TF X No. of vehicles) all
groups.
See Example provided in next slides.
Example 16.1… on Computation of ESAL
 Total ESAL Calculation
 The total ESAL applied on the highway during its
design period can be determined only if the following
are known:
 Design period
 Traffic growth factor
 Traffic growth factor is estimated using historical
records or comparable facilities or obtained from
studies made by specialized agencies.
 It is advisable to determine annual growth rates for
trucks and passenger cars separately.
 Design period: Number of years the pavement will
effectively continue to carry the traffic load without
requiring an overlay. (usually 20 years).
Expected Traffic Volume During
Design Period
calculate growth factors using:

r: Rate of growth.
n: Design period (yrs)

T1 : Initial volume during first year (ESAL)

T : Total volume during design period “n” (Design ESAL)
Example 16.2: Computing Design ESAL
(Projected)
Total ESAL Calculation Cont.
 the portion of the ESAL acting on the design lane is
used in the determination of pavement thickness.
 Either lane of a two-lane highway is a design lane.
 In multilane highways the outer lane is the design
lane.
 See Table 16.1 for percentage of total truck traffic on
design lane.
 The initial daily traffic is in two directions over all traffic
lanes.
 Must be multiplied by direction distribution & Lane
distribution to obtain initial traffic on design lane.
 Traffic to be used in design is the average traffic
during design period (i.e. multiply by growth factor).
Table 16.1 for percentage of total
truck traffic on design lane
 Material Characteristics
Resilient Modulus
Resilient Modulus (Mr) is a fundamental material
property used to characterize unbound pavement
materials. It is a measure of material stiffness and
provides a mean to analyze stiffness of materials
under different conditions, such as moisture, density
and stress level.
 It is also a required input parameter to
mechanistic-empirical pavement design
method. Mr is typically determined through
laboratory tests by measuring stiffness of a
cylinder specimen subject to a cyclic axle load.
Mr is defined as a ratio of applied axial
deviator stress and axial recoverable strain. 
Subgrade Engineering Properties
Materials Evaluation
 The main engineering property required for the
subgrade is its Resilient Modulud (Mr).
 The design subgrade (Mr) should be based on
expected level of traffic expressed in ESALs.
 To ensure more conservative design, lower value of
(Mr) is used for higher volumes of traffic.
 It is recommended that (Mr) is found for (6 to 8)
samples of subgrade.
 Arrange Mr values in descending order.
 Plot as cumulative distribution.
 Chose design subgrade (Mr) from the curve as
follows:
 Design Subgrade Mr???

Mr test Value
(psi) Number Percentile Subgrade Design Limits
13500 1 12.5
Design
11900 2 25 Traffic Level Percentile
11300 3 37.5
ESAL Value
10000 4 50

9500 5 62.5 <= 10,000 60

10000 to
8800 6 75 1000,000 75
7800 7 87.5
> 1000,000 87.5
6200 8 100
 Design Mr

120

100

80
% >=

60

40
Subgrade Design
Limits
20
Design
0 Percentil
0 2000 4000 6000 8000 10000 12000 14000 16000 Traffic Level e
Mr (psi)
ESAL Value

<= 10,000 60
10000 to
1000,000 75

> 1000,000 87.5

 Subbase & Base engineering
Properties
 Certain requirements are needed, which
are given in terms of
 PI
 % passing sieve # 200

 Min. sand equivalent.

Quality Requirements for Base and
Subbase Materials
Select Surface and Base materials

 Designer is free to select either an asphalt concrete

surface or an emulsified asphalt surface along with an
asphalt concrete base, an emulsified asphalt base, or
an untreated aggregate base for the underlying layer.
 The choice will depend on the material that is
economically available.
Pavement Types in The Asphalt
Institute Method
1. Full depth asphalt concrete.
2. Asphalt concrete surface and emulsified asphalt
base.
1. Type I: Emulsified asphalt mixes made with processed
dense-graded aggregates.
2. Type II: Emulsified asphalt mixes made with semi-
processed, crusher-run, pit-run, or bank-run aggregates.
3. Type III: Emulsified asphalt mixes made with sands or silty
sands.
3. Asphalt concrete and untreated aggregate base.
1. Base thickness of 6”.
2. Base thickness of 12”.
Full Depth Asphalt Concrete
Pavement constructed completely from HMA
Full Depth Asphalt Concrete
HMA Over Emulsified Asphalt Base
HMA Over Emulsified Asphalt Base Type-I
HMA Over Emulsified Asphalt Base Type-II
HMA Over Emulsified Asphalt Base Type-III
HMA over Untreated Aggregate Base
HMA over Untreated Aggregate Base 6”
HMA over Untreated Aggregate Base 8”
Feasibility of Planning stage
construction
 Planned Stage Construction involves
successive application of HMA layers
according to a predetermined time schedule.
 Beneficial when:
 Funds are insufficient for constructing a pavement
with long design life.
 Great amount of uncertainty in estimating traffic.

• Concept: Remaining life which implies that the

second stage will be constructed before the
first stage shows serious signs of distresses.
Planned Stage Construction Cont.

 Pavement is designed for initial traffic &

next stage can be designed using traffic
projections based on traffic in service.
 Stage construction allows weak spots
that develop in the first stage to be
detected and repaired in the second
stage.
Planned Stage Construction Cont.
n1: Actual ESAL for stage 1
N1: Allowable ESAL for initial thickness (h1) selected for stage 1.
Then The damage ratio (Dr) at the end of stage 1 is:

Dr = n1/ N1

Dr < 1.0 ………. When Dr =1.0 pavement fails.

 (1-Dr) = Remaining life in the existing pavement at the end of

stage 1.
 h1 is obtained based on Dr =1.0.
 To keep some life, h1 should be determined based on adjusted
ESAL (N1) > ESAL (n1)
N1= n1/Dr
Planned Stage Construction Cont.
n2: Design ESAL for stage 2.
N2: Allowable or adjusted ESAL to permit selection of (h2) that will carry
traffic n2 and use the remaining life in stage 2.

Then The damage incurred in stage 2 should not exceed the remaining life.
n2/ N2 = (1-Dr)

N2 = n2/ (1-Dr)

 hs = h2 –h1 = Additional thickness required in stage 2 (overlay).

 MS-1 recommended (5 – 10 yrs) stage 1 with 60% Dr.

 Planned Stage Construction
Example
Given:
 Full-depth asphalt pavement
 subgrade Mr = 10,000 psi
 Use two stage to construct this pavement
 Stage 1: 5 yrs, ESAL = 150,000 , Dr = 60% at
the end of stage 1.
 Stage 2: 15 yrs, ESAL = 850,000
 Required:
 Determine thickness of HMA required for first 5yrs.
 Thickness of overlay required to accommodate the
additional traffic expected during the next 15 yrs.
 Planned Stage Construction
Example Cont.
Solution:
 n1=150,000 & Dr = 0.6
Find N1 = n1/Dr = 150,000/ 0.6 = 250,000
From design Chart with N1 & Mr find
 h1= x in.
 n2 = 850,000 & 1-Dr = 0.40
Find N2 = n2/ (1-Dr) = 850,000/ 0.4 = 2,100,000
From design chart with N2 & Mr find
h2 = y
 First stage thickness = h1 = x in
 Overlay thickness hs = h2 –h1 = y – x = z in.
 Planned Stage Construction
Example Cont.
Solution:
 If the design was not divided into 2 stages, the
thickness of the pavement using (Mr = 10,000
psi & ESAL = 1000,000) is :
x’ in.
 The use of stage construction decreased the
thickness in first stage by (x’ -x = x’’ in), but
increased the total thickness by
(y – x’ = y’ in).
Step 5: Economic Analysis & Design
Selection
 Find several alternative designs for the
same design ESAL and subgrade Mr.
 Carry out an economic evaluation of
these alternatives.
 Determine best alternative.