1. DESIGN OF FLEXIBLE
PAVEMENT
G . R AV I K U M A R

2. IRC METHOD
Design approach:
The objective of this method of design is, to design the pavement structure to control rut.
The design of pavement involves in two steps,
1. Provision of thickness of pavement
2. Deciding the Design life/ Life time

3. THICKNESS OF PAVEMENT
The thickness of highway pavement (H), is the combination of
three layers.
Layer-1: Contains surface course and binding layer. Take
thickness as h1 and modulus of elasticity as E1.
Layer-2: Contains granular course or drainage layer. Take
thickness as h2 and modulus of elasticity as E2.
Layer-3: Contains prepared subgrade soil. Depth is not
specified and take modulus of elasticity as E3.
The total thickness of flexible pavement (H) = h1+h2+h3

4. THICKNESS OF PAVEMENT
The total thickness of highway
pavement, H is decided based on,
• CBR Value of subgrade soil
• Design traffic

5. CBR VALUE
• The CBR test should be carried out at
appropriate moisture content.
• The CBR test should be conducted on
a number of test specimens, and the
average of at least 3 consistent values
taken as the design CBR value.
• A weaker soil subgrade with lower
CBR value, requires a flexible
pavement of higher thickness.

6. DESIGN TRAFFIC/ TRAFFIC LOADS
• The design traffic is a function of initial traffic of different classes of heavy vehicles,
their axle loads, growth rate, design period and lane distribution factor.
• The study involved in the following factors:
• Magnitude of wheel loads
• Wheel load repetitions
• Equivalent Wheel Load Factors (EWLF)
• Cumulative Standard Axle (CSA) values

7. MAGNITUDE OF WHEEL LOADS
• If the magnitude of wheel load will be high, then the thickness of the
pavement should be high.
• So, while designing the pavement, it is necessary to consider various
wheel load factors, such as
 Maximum wheel load
 Contact pressure
 Wheel load configuration such as dual or multiple wheel load
assembly
 The repetition of these loads during the design life of pavement
• It is also essential to estimate the total traffic volume consisting of all the
categories of vehicles expected to flow on the road.

8. MAGNITUDE OF WHEEL LOADS
Maximum Wheel Load:
• Generally the wheel load is assumed to be distributed over a circular
area. But by measurement of the imprints of tyres with different load and
inflation pressures.
• Three terms in use with reference to tyre pressure are:
 Tyre pressure
 Contact pressure
• Generally these terms should mean same thing. The contact pressure is
found to be more than the tyre pressure when tyre pressure is less than
7 kg/cm2

9. MAGNITUDE OF WHEEL LOAD
Contact Pressure:
Contact Pressure, p =
𝐿𝑜𝑎𝑑 𝑜𝑓 𝑤ℎ𝑒𝑒𝑙
𝐶𝑜𝑛𝑡𝑎𝑐𝑡 𝐴𝑟𝑒𝑎
=
𝑃
𝐴
• The concept of contact pressure is important for the analysis
of stresses and the stress distribution within the pavement.
• If the loaded are by wheel is assumed to be circular in shape,
then the load P = 𝐴𝑝 = 𝜋𝑎2
𝑝

10. MAGNITUDE OF WHEEL LOAD
Wheel Load Configurations:
The wheel load configurations are important to know the way in which the loads of a
vehicle are applied on the pavement surface.

11. MAGNITUDE OF WHEEL LOAD
(IRC: 37-2001)
• For highways maximum legal axle load as specified
by IRC is 8170kg with a maximum equivalent single
wheel load of 4085kg.
• The design load axle load of two axle heavy
commercial vehicles by IRC taken as 1020kg and the
design wheel load on each dual wheel assembly is
taken as 5100kg.
• The maximum total legal load on the tandem axles of
HCR is 19000kg and thus the legal load on each axle
is 9500kg.

12. WHEEL LOAD REPETITIONS
Effect of Repeated Application of Wheel Loads:
• The effect of load repetitions during the design life of flexible
pavement are to be taken in to account.
• Higher number of load repetitions during the design life of the
pavement will require higher thickness of flexible pavement.
• The deformation of pavement of subgrade due to single
application of wheel load may be small.
• But due to repeated application of the heavy loads, there
would be increased magnitude of both plastic and elastic
deformations.

13. WHEEL LOAD REPETITIONS
Effect of Number of Repetitions of Different Magnitudes of
Loads:
• Traffic composition in India is of mixed type and it is essential to
evaluate the effects of number of repetitions of different
magnitudes of loads.
• It essential to convert the various wheel loads to one single
standard wheel load for the structural design of flexible
pavement.
• For this purpose, it is required to carry out the traffic surveys.
• From this objective, the concept EWLF (Equivalent Wheel Load
Factors) has been developed.

14. EQUIVALENT WHEEL LOAD FACTORS
If the pavement structure fails with
N1 number of repetitions of P1 kg load and
similarly if N2 number of repetitions of P2
kg load can also cause failure of the same
pavement structure, then P1N1 and P2N2 are
considered equivalent.
• The concept is developed by American
‘Association of State Highways
Officials’ (AASHO).
• Mc1eod had given a procedure for
evolving equivalent load factors for
designing flexible pavements.
• AASHO conducted a survey and then
derived AASHO Road Test equations,
were widely accepted for the
determination of Equivalent Wheel
Load Factors.

15. Typical outcome results of these studies are
represented in following table:
Wheel
Load,
Kg
Repetitions to Failure,
Number
Equivalent to
2268 kg
Equivalent
Wheel Load
factor (EWLF)
2268 1,05,000 1.0 1
2722 50,000 2.0 2
3175 22,500 4.7 4
3629 13,000 8.2 8
4082 6,500 16.3 16
4536 3,300 32.0 32
4990 1,700 62.0 64
5443 1,000 105.0 128

16. EQUIVALENT WHEEL LOAD FACTOR
• Generally accepted approach for the conversion of axle loads of different magnitudes in terms
of a standard axle is by Fourth Power Law.
Equivalent Wheel Load Factor = (given wheel load/standard wheel load)4
= ( given axle load/standard axle load)4
i.e. EWLF = (P1/P)4
• McLeod assumes that the pavement thickness which are designed for a given wheel load would
support one million repetitions of such load during the life of pavement. For one load
application, the pavement thickness so required is only one fourth the pavement thickness
designed for 10,00,000 load repetitions.

17. VEHICLE DAMAGE FACTORS
• The EWLF values are termed as ‘ Equivalency factors and Damaging power of
different vehicles’ or vehicle damage factors of VDF values in India.
• VDF Values, IRC: 37-2001

18. CUMULATIVE STANDARD AXLE VALUES
The EWDL or VDF values of these loads are calculated in terms of standard axle loads
and denoted as F1, F2, F3, …….. F2. The cumulative effect of all the vehicle classes that affect
the design of flexible pavement structure is taken into account by the following methodology:
 Let the initial number of heavy vehicle classes 1, 2, 3, ….., n be N1, N2, N3, … Nn per day
determined during the preparation of project report.
 The EWDL of VDF values can be calculated from tables or by using Fourth Law as F1, F2,
F3, ….., Fn.
 The period of delay (‘m’ years) between the date of traffic studies and the date of completing
construction and opening to traffic is estimated.
 The desired design life (‘n’ years) of the flexible pavement structure is decided.

19. CUMULATIVE STANDARD AXLE VALUES
 The effect of load repetitions of each vehicle class during the desired design period of ‘n’
years is represented in terms of Cumulative axle load of the selected vehicle class, CSA1
using the relation given below:
CSA1 =
365{𝑁1𝐹1 1+𝑟1 𝑚+𝑛−1}
𝑟1
 The total CSA value for design of flexible pavement structure is the sum of the values
determined for each vehicle class, given by:
CSA = sum of (CSA1+CSA2+CSA3+……..+CSAn)

20. DESIGN LIFE
• Design life recommendations as per IRC: 37-2001
S. No. Type of Road Design Life
1 National Highways and
State Highways
15 years
2 Express ways 20 years
3 Urban arterial and sub-
arterial roads
20 years
4 All other roads 10 to 15 years

21. DESIGN CRITERIA
• The total thickness of the pavement, H is decided based on the design factors such as,
(i) CBR value of the soil subgrade and (ii) Design traffic.
• The combined effects of the traffic factors are represented in terms of ‘cumulative
standard axles’ (CSA) in terms of million standard axles (msa).
• The strain on the subgrade will depend on the total thickness, H of the flexible
pavement and also type and thickness of pavement materials in different layers.
• The rut depth depends on the vertical subgrade strain and the CSA. The number of
repetitions for causing rut depth can be obtained by the following formula:
𝑁𝑟 = 4.1656(1/𝐸𝑧)4.5337
𝑋10−8
Where, 𝑁𝑟 = CSA to produce rut depth of 20mm
𝐸𝑧 = Vertical strain on subgrade

22. DESIGN CRITERIA
• The thickness of layer-1, ℎ1, decided by the fatigue criterion. The no. of repetitions of
standard axles or the CSA causing the fatigue failure can be calculated by the
following equation:
𝑁𝑓 = 2.21 (1/𝐸𝑓)3.89(1/ 𝐸1)0.854× 10−4
Where, 𝑁𝑓 = CSA to produce 20% of cracked area on the bituminous surface
𝐸𝑓 = tensile strain at the bottom of stiff bituminous layer
𝐸1 = modulus of elasticity of first layer or bituminous layer
• The thickness of layer-2, ℎ2, is obtained by subtracting the following equation:
ℎ2 = (H−ℎ1)
• As per IRC guidelines, the minimum thickness of granular base course is 225mm when
the design traffic is up to 2.0msa and 250mm up to when the traffic exceeds this value.