Flexible Pavement Design

Indian Roads Congress Method

The IRC Guidelines

IRC:37-2001 Guidelines for the Design of Flexible Pavements Result of different research efforts made in India More rational than its previous version IRC:371984

Scope of the Guidelines

Applicable for new pavements Applicable for design of flexible pavements for expressways, National Highways, State Highways, MDR and other categories of roads carrying predominantly motorised vehicles Flexible Pavements bituminous surfacing with granular base and sub-base

Design Criteria
Three main types of distresses are considered be critical Rutting due to permanent deformation in subgrade to

Rutting due to permanent deformation in bituminous layer

Fatigue cracking in bituminous layer

Rutting due to permanent deformation in subgrade

Rut Depth Bituminous Layer

Granular Layer
Subgrade

Rutting due to Permanent Deformation in Bituminous Layer

Rut Depth Bituminous Layer Granular Layer Subgrade

Fatigue Cracking in Bituminous Layer

Crocodile Cracking

Mechanistic Parameters controlling Pavement Performance / distresses

h1
h2

et
ez

E1, m1 Bituminous E2, m2 Granular E3, m3 Subgrade

Mechanistic Parameters as indices for Pavement Performance

vertical strain on top of subgrade (ez) is considered to be causative factor for permanent deformation in subgrade horizontal tensile strain (et) at the bottom of the bituminous bound layer is an indicator for fatigue cracking in bituminous layer

Performance Criteria
Flexible Pavements should be designed to perform satisfactorily without developing unacceptable levels of distresses during the design life period Main distresses

Fatigue cracking in bituminous layer cracking of 20% of paved area considered critical
Rutting Average rut depth of 20mm is taken as critical condition

Performance Criteria
To ensure that unacceptable levels of distresses do not occur during design period, the critical mechanistic parameters identified as indices for performance should be kept within acceptable limits

Fatigue Cracking Horizontal Tensile Strain strain at the bottom of bituminous bound layer (et)
Rutting Vertical strain on top of subgrade (ez)

These parameters are to be computed using a suitable theory

Limiting Strains
The design of the pavement (layer thicknesses and materials) should be selected in such a way that the computed strains will be less than the limiting strain values corresponding to the design traffic selected. The limiting strains correspond to the initial condition of the pavement Limiting strains will be smaller for higher design traffic volumes (or longer design life periods)

Computation of Strains
Indian Roads Congress (IRC:37-2001) adopts Linear Elastic Layered Theory for analysis of flexible pavements Recommends that the pavements be modeled as 3 Layer Systems with Bituminous surface, granular base and subgrade Interfaces between layers are considered to be rough The top two layers are assumed to be infinite in horizontal direction while the subgrade in semiinfinite.

Computation of Critical Strains

h1

E1, m1

h2

E2, m2
E3, m3

Inputs required for analysis thicknesses of the first two layers elastic moduli of the three layers Poisson Ratio values of the three layers

Computation of Critical Strains

Loading Considered Standard axle load (80 kN) One dual wheel set only is considered, tyre pressure 0.56 MPa (80psi) 20kN 20kN

310 mm

Computation of Strains - Loading

Circular contact area assumed
Uniform Vertical stress at Pavement surface No horizontal surface stresses considered for analysis

Performance Criteria adopted in IRC:37-2001

Correlate Performance with Critical Strains Performance Number of equivalent repetitions of standard axle load (80kN) that can be served by the pavement before excessive rutting or fatigue cracking develop The general form of performance criterion is

N = k1 (I / initial strain) k2

IRC Performance Criteria

The criteria developed by IIT, Kharagpur were adopted These criteria were developed on the basis of the vast data collected by a number of institutions in India on the performance of flexible pavements under different loading and climatic conditions Data were collected as part of different research schemes sponsored by the Ministry of Road Transport and Highways (MORTH)

Rutting Criterion NR = 4.1656 * 10-8 (1/ez)4.5337

Where NR = Cumulative std. Axle load repetitions before the pavement develops 20mm average rut depth ez = Initial vertical strain on top of subgrade If the pavement has to serve 50 million standard axle load repetitions without developing excessive rutting the initial vertical strain must be limited to 4.7201*10-04

Fatigue Criterion NF = 2.21 * 10-4 (1/et)3.89(1/Eac)0.854

Where
NF = Cumulative std. Axle load repetitions before the pavement develops 20% fatigue cracking

et = Initial horizontal tensile strain at the bottom of bituminous layer

Eac = Elastic Modulus of bituminous layer, MPa For the pavement to serve 50 msa load repetitions without developing excessive fatigue cracking, and if the modulus value of the bituminous layer is 1000MPa, the initial tensile strain must be limited to 2.6453*10-04

IRC Performance Criteria

The pavement layer thicknesses and materials must be selected in such a way that both the computed strains will be less than the corresponding limiting strains

This will ensure that the pavement will not develop unacceptable levels of fatigue cracking and rutting

Material Characterization for Analysis

Since linear elastic layered theory is used for analysis, elastic moduli and Poisson Ratio values of the three layers are required as inputs for analysis Subgrade Modulus The elastic modulus of subgrade can be determined by conducting repeated triaxial test on representative soil sample The subgrade modulus can also be estimated from the CBR value determined using representative soil sample

Estimation of Subgrade Modulus

E (MPa) = 10 * (CBR) for CBR values less than 5% E(MPa) = 17.6 * (CBR)0.64 for CBR => 5%

Where E is the elastic modulus of subgrade and CBR is the California Bearing Ratio of subgrade soil
For a CBR of 4 % E = 10 * 4 = 40 MPa For a CBR of 7%, E = 17.6 (7)0.64 = 64.8 MPa

Elastic Modulus of Granular Layer

E of Granular material has to be determined by conducting repeated triaxial test on the granular material In the absence of the equipment, the modulus value is to be estimated from the expression E(Granular base) = E (subgrade) * 0.2 * (h)0.45 Where h = thickness of granular layer in mm For a 300mm thick granular layer placed over a subgrade having 40MPa modulus, the granular layer modulus will be 104.2 MPa

Elastic Modulus of Bituminous layer

Different types of mixes are used in India
Bituminous Concrete (BC) 40, 50mm surfacing Semi Dense Bituminous Concrete (SDAC) Dense Bituminous Macadam (DBM) Bituminous Macadam (BM)

Mixes prepared using different binders 30/40, 60/70, 80/100 and modified binders

Elastic Modulus of Bituminous Mix

Pavement temperature is an important parameter in selecting E of Bituminous layer 350C is considered as the average annual pavement temperature for most parts of India Research carried out at Kharagpur and other places in India yielded typical elastic modulus values that can be selected for different average pavement temperatures applicable for different parts of India

Elastic Modulus of Bituminous Mix

Mix Binder Type 80/100 200C 2300 250C 1965 300C 1450 350C 975 400C 800

BC/ DBM BC/ 60/70 DBM BC/ 40/70 DBM BM 80/100 BM 60/70

3600
6000 -----

3125
4930 -----

2580
3810 -----

1700
1945 500 700

1270
2275 -----

Converting DBM into other material

Part of the DBM can be substituted by BM using the equal flexural stiffness principle (E1H13) / (12 (1- m12) = (E2H23) / (12 (1- m22) where E1, H1, m1 and E2, H2, and m2 are the elastic modulus, thickness and Poisson ratio of DBM and BM layers respectively Considering moduli values of 700 and 1700 MPa for BM and DBM respectively, 1 mm of DBM will approximately be equivalent to 1.34mm of BM

Poisson Ratio Values

The Poisson ratio value for bituminous mix for high temperatures (35 and 400C) is taken as 0.50
For temperatures from 20 to 300C the value recommended is 0.35. For granular layer and subgrade, a value of 0.4 is recommended

Design Approach
Select Inputs Climatic Conditions Average pavement temperature No. of layers in the pavement

Materials to be used in each layer

Binder to be used Design subgrade CBR Design traffic (Cumulative standard axle load repetitions)

Design Approach
Select trial designs and evaluate Select trial thicknesses for pavement layers Assign appropriate elastic moduli and Poisson ratio values for each layer

Compute critical responses (tensile strain at the bottom of bituminous layer and vertical strain on top of subgrade) Use linear elastic layered theory for analysis
Consider standard loading

Design Approach
310 mm 20kN
Contact pressure = 0.56MPa

20kN

h1 h2

et
ez

E1, m1 E2, m2 E3, m3

Design Approach
Evaluate the trial design (thicknesses) Compare the computed strains with allowable strains (for rutting and fatigue considerations) Allowable strains to be estimated from design traffic Both allowable strain criteria should be satisfied If criteria are not satisfied, select a new thickness combination and re-analyse

Design Approach
Design Charts
For convenience of the users a number of thickness charts have been developed following the procedure discussed previously Separate thickness charts are available for 1-10msa and 10-150msa traffic levels Subgrade CBR of 2% to 10% are considered

Dense Bituminous Macadam (DBM) prepared with 60/70 bitumen is considered as the bituminous layer.

Pavement Composition
Total thickness obtained (if design charts are used) is split into different layers as per the pavement design catalogues given in IRC:37-2001 Granular sub-base, granular-base and bituminous surfacing (binder course and wearing course)

Typical Thickness Chart

Total Pavement Thickness, mm

CBR

Design Traffic, msa

Estimation of Design Traffic

Cumulative number of standard axle load (80kN) coverages expected during the design life period can be estimated from
Initial traffic intensity after construction in terms of commercial vehicles per day (CVPD) Traffic growth rate during the design life period Design life (years) Vehicle damage factor (VDF) Lateral distribution of commercial vehicles over the carriageway

Estimation of Design Traffic

Design life 15 Years for NH & SH, 20 Years for Expressways and Urban roads. Other categories 10 to 15 years Possibility of stage construction to be examined

Vehicle Damage Factor Multiplier to convert the number of commercial vehicles of different axle loads and axle configurations into equivalent number of standard axle load repetitions
VDF obtained from axle load survey

Estimation of Design Traffic

VDF - in the absence of axle load data, the following values can be adopted
Initial traffic (cvpd) 0 150 150 1500 > 1500 Rolling/plain 1.5 3.5 4.5 Hilly 0.5 1.5 2.5

Lateral distribution
Single - lane road 2-lane single carriageway 4-lane single carriageway 100% of two-way traffic 75% of two-way traffic 40% of two-way traffic

Estimation of Design Traffic

Lateral distribution Dual Carriageway roads
Dual 2-lane carriageways 75% of traffic in each direction Dual 3-lane carriageways 60% of traffic in each direction Dual 4-lane carriageways 45% of traffic in each direction Traffic in each direction assumed to be half the total traffic if directional distribution is not known.

Annual Growth Rate To be projected. Assume 7.5% if no data is available Estimation of Design Traffic N = (365*A*D*F*((1+r)n-1))/(r)

Estimation of Design Traffic

Estimation of Design Traffic N = (365*A*D*F*((1+r)n-1))/(r)
Where, N = Cumulative std. Axle repetitions during design period A = Initial traffic intensity (cvpd) in the year of construction D = Lane distribution factor F = Vehicle damage factor n = design life in years r = annual rate of growth of commercial vehicles (for 7.5% rate of growth, r = 0.075)

Traffic in the year of completion of construction A = P (1 + r) x

where P = traffic intensity (cvpd) at last count x = no. of years between last count and year of completion

Pavement Composition Design catalogue

CBR 4 % (Traffic 1-10) Traffic (msa) Total Pavement Thickness (mm) 480 540 580 620 700 PAVEMENT COMPOSITION Bituminous Surfacing Wearing Course (mm) 20 PC 20 PC 20 PC 25 SDBC 40 BC Binder Course (mm) -50 BM 50 BM 60 DBM 80 DBM Granular Base (mm) 225 225 250 250 250 Granular Sub-base (mm) 255 265 280 285 330

1 2 3 5 10

Pavement Composition Design catalogue

CBR 4 % (Traffic 10-150) Traffic (msa) Total Pavement Thickness (mm) 700 730 750 780 800 PAVEMENT COMPOSITION Bituminous Surfacing BC (mm) 40 40 40 40 50 DBM (mm) 80 110 130 160 170 Granular Base (mm) Granular Sub-base (mm)

10 20 30 50 100

250

330

150

820

50

190

Pavement Composition Subbase

Subbase Min CBR of 20% for traffic upto 2 msa

Min CBR of 30% for traffic > 2msa

For subgrade soils of low permeability, the GSB should be for full width of formation The thickness of the extended portion should not be less than 150 mm for traffic <10 msa and 200 mm for traffic >10 msa If subgrade CBR is < 2%, design of CBR of 2% and provide a capping layer of 150mm thick material having min. CBR of 10% in addition to sub-base

Pavement Composition Base and Surface

Base Min thickness 225mm (traffic upto 2 msa) Min thickness 250mm (traffic > 2 msa)

Material to conform to MORTH and IRC specifications

Bituminous Surfacing wearing course or wearing course + binder course

Wearing courses Surface dressing, open-graded premix carpet, mix seal surfacing, semi-dense bituminous concrete and bituminous concrete Binder course Bituminous Macadam (BM) and Dense Bituminous Macadam (DBM)

Pavement Composition Base and Surface

Use of BM (low bitumen content, high air void) to be restricted for traffic < 5 msa Provide DBM for traffic > 5msa Equivalence of BM in terms of DBM (10BM = 7 DBM)

Selection of binder type and mix type to be made on the basis of traffic and climatic conditions
For snow-bound areas, bus-stops, round-abouts provide Bituminous concrete for water proof, stable surface. Mastic Asphalt also can be used. Open-graded Premix carpet of thickness upto 25mm thickness not considered as a structural layer

IRC:37-2001 Major Limitations

Thickness charts are still available for CBR values of upto 10% only Design charts are available for only a pavement temperature of 350C

Charts available for DBM only

The contribution of individual component layers is still not realized fully with the system of catalog (block) thicknesses. The same can be done through use of an analytical tool for design instead of resorting to thickness charts

Summary
In this lesson, we have

learnt the basis for the IRC method for design of flexible pavements
Understood about the performance criteria adopted in the guidelines Learnt about the model used in the guidelines for analysis of pavements

Understood how different traffic and material inputs are selected for design
Understood the limitations of the method