This document discusses the design of flexible pavements according to the IRC method. It covers types of pavements, pavement composition, factors considered in pavement design such as subgrade strength, traffic loading, and climatic conditions. It describes methods to characterize traffic loading, including defining a standard axle load, conducting axle load surveys to determine vehicle damage factors, and distributing traffic loads across lanes. Design life and traffic volumes are estimated using growth formulas. The evolution of the IRC flexible pavement design code is summarized, from the initial empirical method to current mechanistic-empirical approaches.

1. Flexible Pavement- Aspects of
Basic Design as per IRC method
Rajib Chattaraj
Superintending Engineer
Public Works Directorate
Govt. of West Bengal

2. What is Pavement ?
• Pavement is a structure engineered to carry
vehicular loading.
• It is built over a foundation : embankment and
subgrade and is a layered structure.
Fig from Prof. S K Rao’s Ppt

3. Types of Pavement
Highway Pavements, service wise
may be:
• Low Volume Roads
• Main carriageways of Major
Highways
• Shoulders of Major Highways
• Service Roads
• Intersection areas
Ref: Ppt of Prof. K S Reddy

4. Types of Pavement
Structurally pavement may be :
• Flexible pavement : Asphalt topping
• Rigid Pavement : Cement Concrete
Topping
• Composite Pavement: Either Asphalt or
Cement concrete topping

6. Pavement composition
Courtesy: Nick Thom text Book

7. On the basis of funding, connectivity, importance and
control, highways may also be classified as:
• National Highway
• State Highway
• Major District Road
• Other District Road
• Village Road
• Municipal Road(Town Roads)
• Arterial & Sub arterial Road(City Roads)

8. What for a Pavement is designed or
constructed?
• The Pavement should perform satisfactorily for a
specified period of time both:
• Structurally : No rutting
No Cracking: Bottom Up
Top Down
• Serviceability wise : Skid resistant means safe
Noise resistant
have enough riding comfort
• And Economical too.

9. a) Bottom up cracks b) Top down cracks
Ref: Ppt of Prof. B B Pandey

10. Bottom
up
cracking
of a
sample
cored
from a
section of
NH .

11. Surface cracking : top down nature, alligator cracking,
caused due to fatigue.

12. Rutting

13. Theories involved in the design of
Flexible Pavement
• Hooke’s Law of Elasticity
• Boussinesq’s Method
• Burmister’s Method for Two-Layer Systems
• Odemark’s Method of Equivalent Layers
• Fox and Acum and Fox’s solutions
Ref: Text book by Prof.Rajib Basu
Mallick

14. Methods of design of Flexible
Pavement
• Empirical Method
• Limiting Shear Failure Methods
• Limiting Deflection method
• Regression Method based on
Pavement Performance or Road
Test
• Mechanistic-Empirical Method: It
is based on the mechanics of
materials that relates an input
such as a wheel load to an output
or pavement response such as
stress or strain.
• Other developments: This include
the application of computer
programs, the incorporation of
serviceability and reliability and
consideration of dynamic loading
Text ref: Huang Text Book, Fig ref:
Prof Pandey’s Ppt.
The concept of use of vertical compressive
strain on the surface of the subgrade to
minimize rutting and the use of horizontal
tensile strain at the bottom of bituminous
layer to minimize the fatigue cracking was
first presented by Dormon & Metcalf in
1965.

15. Evolution of IRC:37
• 1970: First Empirical design guidelines for flexible
pavements (IRC:37-1970), were based on (i)
subgrade (foundation) strength (California
Bearing Ratio) and (ii) traffic, in terms of number
of commercial vehicles (having a laden weight of
3 tonnes or more) per day.
• 1984: First Revision of IRC:37, considering the
design traffic in terms of cumulative number of
equivalent standard axle load of 80 kN and
design charts were provided for design traffic
volumes up to 30 million standard axle (msa)
repetitions, it was also empirical approach(past
experience and judgement)

16. Evolution of IRC:37
• 2001 Second revision: It was done using semi-
mechanistic (or mechanistic -empirical) approach
based on the results available from R-6 ,R-56 and other
research schemes of the Ministry of Road Transport
and Highways (MoRTH). The mechanistic-empirical
performance models for subgrade rutting and
bottom-up cracking in the bottom bituminous layer,
developed using the results of these research
schemes, were used for the design of flexible
pavements. FPAVE software, developed for R-56
research scheme for the analysis of linear elastic
layered pavement systems, was used for the analysis
of pavements and for the development of thickness
design charts. Thickness charts were provided for
design traffic levels up to 150 msa.

17. Evolution of IRC:37
• 2012 Third revision: For increasing traffic load and in view of
newer construction technique and materials, this revision was done
to facilitate (i) design of bituminous pavements for traffic volumes
more than 150 msa (ii) utilization of new types of pavement
materials such as bituminous mixes with modified binders,
foam/emulsion treated granular or recycled asphalt pavement
(RAP) material bases and sub-bases and cement treated sub-bases
and bases and stabilized subgrades and (iii) utilization of new
construction techniques/practices. Recommendations were made
for the use of harder grade binders to resist rutting and top-down
cracking in the upper bituminous layer and for fatigue resistant
bituminous mixes for the bottom bituminous layer.
• Mechanistic-empirical performance models were given for rutting
in subgrade and bottom-up cracking in bituminous layers for two
different levels (80% and 90%) of reliability. Fatigue criteria were
also included for cement treated bases.
• IIT-Pave software were introduced to validate the pavement
composition and lift thickness which are obtained from the charts.

18. Evolution of IRC:37
• 2018 Fourth revision : salient features of the fourth
revision are: (a) recommendation of better performing
bituminous mixes and binders for surface and
base/binder courses (b) guidelines for selection of
appropriate elastic moduli for bituminous mixes used
in the surface and other courses (c) recommendation
of minimum thicknesses of granular and cement
treated sub-bases and bases and bituminous layers
from functional requirements (d) generalization of the
procedure for the estimation of the effective resilient
modulus/CBR of subgrade (e) provision for the use of
geo-synthetics and (f) rationalization of the design
approach for stage construction.

19. Selection of design Parameters
The three main external parameters for design are:
• Subgrade Strength
• Traffic Loading
• Climatic Conditions
Some non-load associated factors are :
• Temperature
• Moisture
• Pavement drainage
• Materials properties specially binder
• Construction practices
• Maintenance Ref: Ppt of Prof. K S Reddy

20. Definition of Pavement Performance
• Functional Performance: The function of a
pavement is to provide comfortable, safe and
economical Ride. Road user is concerned
about the functional performance of the
pavement. Depends mostly on the surface
characteristics which are:
• Variation in Longitudinal Profile
• Variation in Transverse Profile
• Pot Holes, Cracks, Patching, Rutting, Loss of
Aggregates, etc.

21. Definition of Pavement Performance
• Structural Failure/ Performance:
demonstration of structural adequacy
• If a pavement shows load associated distress
(fracture, permanent deformation, etc.), then
it is considered to have failed structurally
• Common forms of Structural Failures –
Cracking, Rutting, permanent deformation

22. What to Design Pavements for ?
For Acceptable Functional Performance ?
OR
Acceptable Structural Performance ?
•For a Road User–Functional Performance
•For a Designer–Structural Performance
•A road designed to give adequate structural
performance can be expected to give
satisfactory functional performance as well

23. Design Frame work
• Traffic Loading Consideration
• Environmental Factors
• Material Characterization
• Analysis of Pavements
• Design based on selected performance criteria

24. Traffic Load considerations in Design
• Pavements of highways and airports carry
different types of vehicles
• Vehicles carry different magnitudes of loads
• Loads occur repeatedly
• How to take these variations into account in
designing pavements ?
• Which vehicle, how many repetitions, to
design the pavement for ?
Ref: Prof K S Reddy’s Ppt

25. Load consideration in Design
Three different approaches
• Fixed Traffic :Heaviest anticipated vehicle is the main
concern for design, Number of repetitions not
considered, Used for design of airport pavements
• Fixed Vehicle: Design is governed by the number of
repetitions of a standard vehicle or axle , 80 kN single axle is
considered to be the standard axle load ,Axles that are not
either single or not equal to 80kN are converted into
equivalent standard (80kN) axle load using Equivalent Axle
load Factor, Sum of the equivalent repetitions obtained for all
the axle loads anticipated (during the design period) is used as
design parameter
• Variable Traffic and Vehicle: Variations in loads and
repetitions of each individual load are considered important
for design , normally used with procedures adopting
cumulative damage approach.
Ref: Prof K S Reddy’s Ppt

26. Equivalent Single Wheel Load (ESWL)
• Defined as the load on a single tire that will
cause an equal magnitude of a pre selected
parameter(stress, strain, deflection, or
distress) at a given location within a specific
pavement system to that resulting from a
multiple –wheel load at the same location
within the pavement structure.
• Equivalence in terms of a selected parameter
(for a selected pavement and selected
location)

27. ESWL –Parameters for Equivalence
• Equal vertical stress
• Equal vertical deflection
• Equal tensile strain
• Equal tensile stress
• Equal contact pressure
• Equivalent contact radius
(these parameters can be theoretically
calculated or experimentally determined as
specified in the design methodology)
Ref: Prof K S Reddy’s Ppt

28. Boyd & Foster 1950

29. ESWL –Equal Vertical Stress Concept

30. Equivalent Axle Load Factors (EALF)
• Pavements are designed for a selected
number of repetitions of a standard load
(Standard axle load –80kN)
• EALFs are used to covert different axle loads
into equivalent repetitions of standard axle.
• EALF defines the damage caused to the
pavement by one application of the axle load
under consideration relative to the damage
caused by a single application of a standard
axle
Ref: Prof K S Reddy’s Ppt

31. Equivalent Axle Load Factors (EALF)
• EALFs are usually obtained from field
observation of performance of pavements
carrying different types of axle loads
• The EALFs obtained from the AASHO road test
are more commonly used
• EALFs can also be obtained from theoretical
exercise using appropriate mechanistic criteria
• EALFs are different for different types of
pavements and for different performance
criteria

32. • US faced a huge need for an adequate
pavement design procedure in late 1950’s.
• AASHO primary purpose was to
determine the relationship between axle
loading and pavement structure on
pavement performance .
• Use to design pavements to provide an
engineering basis for establishing
maximum axle load limits, and to provide
a basis for the allocation of highway user
taxation.
AASHO Road Test 1958-60

33. Vehicle Damage Factor (VDF)
• Used for converting a given traffic volume into
equivalent number of standard axles
• VDF is a typical value of the factor
representing the loads carried by the
commercial vehicles plying on the road
converted to standard axle load.
• Determined by conducting axle load surveys

34. The Vehicle Damage Factor (VDF) is a multiplier to convert the
number of commercial vehicles of different axle loads and axle
configuration into the number of repetitions of standard axle load of 80 kN
The equations for computing
equivalent single axle load factors
for single, tandem and tridem axles
are given below

35. Formula of VDF

36. Different types of commercial vehicles

37. Axle load survey

39. Sample Size for Axle Load Survey
Total number of
Commercial Vehicles
per day
Minimum percentage
of Commercial Traffic
to be surveyed
<3000 20 per cent
3000 to 6000 15 per cent
>6000 10 per cent

40. Sample VDF study

41. Sample VDF study

42. Summary of Axle load Survey
On some sections, there may be significant difference in axle loading in two directions of
traffic. In such situations, the VDF should be evaluated direction wise. Each direction can
have different pavement thickness for divided highways depending upon the loading
pattern. If vehicle damage factor in one direction is higher, the traffic in the direction of
higher VDF is recommended for design.

43. Where sufficient information on axle loads is not available and the
small size of the project does not warrant an axle load survey, the
default values of vehicle damage factor as given in Table below may
be used. The changes from 2012 version to 2018 version is shown.
Initial traffic
volume in
terms of
commercial
vehicles per day
Rolling/Plain
terrain
Hilly terrain
0-150 1.5/1.7 0.5/0.6
150-1500 3.5/3.9 1.5/1.7
More than 1500 4.5/5.0 2.5/2.8

44. The gist of the notification dated 16th July, 2018, as
amended by notification dated 6th August, 2018

45. Type of Axle Standard Axle Load Legal Axle Load
(Prerevised)
Legal Axle Load
(Revised after
August ,2018)
Single Axle-with
single tyre on
either side
65 kN= 6.63 Ton 6.0 Ton 7.0 Ton
Single Axle with
dual tyres on either
side
80 kN = 8.16 Ton 10.2 Ton
(100kN)
11.5 Ton
Tandem Axle with
dual tyres on either
side
148 kN = 15.09 Ton 19 Ton
(186kN)
21 Ton
Tridem Axle with
dual tyres on either
side
224 kN = 22.84 Ton 24 Ton
(235kN)
27 Ton
Table showing the standard axle load and legal axle load

46. Lateral/Lane Distribution of wheel loads
• All the commercial vehicles do not take the same
lateral position of the highway
• Depending on the type of facility (two-way,
multilane), number of lanes, etc. the paths that
the wheels of commercial vehicles tread differ
• As a result all the wheels of all the commercial
vehicles utilizing the pavement during the design
period do not stress the same point on the
pavement
• Each part of the pavement gets different
repetitions of loads

47. Lateral Distribution
Four
Lane
divided
Carriage
way
Critical portion of flexible pavement

48. Distribution of Commercial Traffic over the Carriageway
• (i) Single-lane roads
• Traffic tends to be more channelized on single-lane roads than two-lane roads and
to allow for this concentration of wheel load repetitions, the design should be
based on total number of commercial vehicles in both directions.
• (ii)Intermediate lane of width 5.5 mts ( New Introduction in 2018 version)
• The design traffic should be based on 75per cent of the two way commercial traffic
• (iii) Two-lane single carriageway roads
• The design should be based on 50 per cent of the total number of commercial
vehicles in both directions.
• (iv) Four-lane single carriageway roads
• The design should be based on 40 per cent of the total number of commercial
vehicles in both directions.
• (v) Dual carriageway roads
• The design of dual two-lane carriageway roads should be based on 75 per cent of
the number of commercial vehicles in each direction. For dual three-lane
carriageway and dual four-lane carriageway, the distribution factor will be 60 per
cent and 45 per cent respectively.

49. Computation of Design Traffic
• Traffic (in terms of CVPD) in the year of
completion is estimated using the following
formula:
• Where, P = Number of commercial vehicles as
per last count.
• x = Number of years between the last count
and the year of completion of construction

50. The design traffic in terms of the cumulative number of standard
axles to be carried during the design life of the road should be
computed using the following equation

51. The three main external parameters for design are
• Subgrade Strength
Traffic Loading
• Climatic Conditions
Extent of moisture level and temperature changes are the main
two variables which cause significant variation in material
properties both in bound and unbound layers. Freezing &
Thawing is another important climatic condition, may not be
that significant in countries like us but have substantial effect in
cooler regions. The effect of climatic conditions have been taken
care of in material characterisation. Like the effect of temperature
change in bituminous layer( bitumen being a thermo-viscous
material) has been listed in terms of MR value in table 9.2 of
IRC:37(2018) .

52. Resilient Modulus of bituminous mix
IRC :37(2018)

53. MR & Poisson’s ratio values
IRC: 37(2018)

54. The three main external parameters for design are:
• Subgrade Strength
Traffic Loading
Climatic Conditions
Requirements of CBR for Subgrade:
• The subgrade is the top 500 mm of the embankment
immediately below the bottom of the pavement, and is made
up of in-situ material, select soil, or stabilized soil that forms
the foundation of a pavement.
• The select soil forming the subgrade should have a minimum
CBR of 5 per cent (as per cl. 6.4.3 of 2018 version which was
8% as per 2012 version) for roads having traffic of 450
commercial vehicles per day or higher.

55. Calculation of Design CBR (As per ANNEX-IV of IRC:37-2012)
Also as per provision of cl.6.2.2 of IRC 37 (2018)

56. Calculation of Design CBR (As per ANNEX-IV of IRC:37-2012)
The 90th percentile CBR value = 5,
Hence the Design CBR adopted is 5.0%

57. Effective or Equivalent CBR
• The subgrade forms the foundation of a pavement. The
usual practice in India is to compact upper 500 mm of
the embankment to a higher density than that of the
rest of the embankment and the compacted layer is
defined as the subgrade.
• In case the embankment soil is weak, borrow material
of higher strength is used as a subgrade and the CBR of
this layer was used to be taken as design CBR for design
of flexible pavements irrespective of the CBR of the
embankment material below the 500 mm subgrade.
• This does not appear to be sound practice since it is
the composite strength of the subgrade and the
embankment soil below it that should enter into the
design rather than the strength of 500 mm thick
subgrade alone. Ref:Design CBR of Subgrade for Flexible Pavements by
M. Amaranatha Reddy ,K. Sudhakar Reddy, B. B.
Pandey

58. Design Subgrade CBR values for borrow material of
compacted thickness of 500 mm
Ref:Design CBR of Subgrade for Flexible Pavements by
M. Amaranatha Reddy ,K. Sudhakar Reddy, B. B. Pandey

59. Effective CBR
Ref: IRC 37(2012)

60. Worked out example in Annex II
IRC 37(2018)

61. Determination of Resilient Modulus
• What is Resilient Modulus ?
Ref: Yoder & Witczak
• Resilient modulus is the measure of its elastic behaviour determined
from recoverable deformation in the laboratory tests.
• It is an important parameter for design and the performance of a
pavement.
• This can be determined in the laboratory by conducting repeated tri-
axial tests as per procedure specified in AASHTO T307-99 (2003)

62. Determination of Resilient Modulus of the subgrade &
subbase

63. Mechanistic Parameters as indices for Pavement Performance vertical
strain:
On top of subgrade (εv or εz)is considered to be causative factor for permanent
deformation in subgrade
horizontal tensile strain (εt)at the bottom of the bituminous bound layer is an
indicator for fatigue cracking in bituminous layer

64. 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 (εt)
• Rutting –Vertical strain on top of subgrade (εz or
εv)
• These parameters are to be computed using a
suitable theory

65. What does reliability mean ?
• Fatigue :Only ten percent of the area may have 20 per cent cracks
or more(interconnected cracks which is considered to be the
critical condition i.e, failure condition) if 90 per cent reliability is
considered which is used for high volume highways. To avoid
frequent maintenance, a reliability level of 90 per cent is
recommended for highways having a design traffic exceeding 20
msa in 2018 version which has been modified from 30 msa
according to 2012 version. 80% reliability means there are chances
of 20% of the above critical condition to occur.
• Rutting :A relationship between limiting surface rutting of 20mm
and the vertical elastic subgrade strain was developed for different
repetitions of standard axle loads and it was subsequently adopted
in IRC-37:2001. The bituminous layers were not very thick in India in
eighties and nineties when rutting data was collected and most of
the rutting took place in the subgrade and the granular layers only.
90 per cent reliability corresponds to 10 per cent of road length
may be expected to have 20mm rut depth or more in subgrade.
80% reliability corresponds to 20% of the road length may be
expected to have 20mm rut depth or more in subgrade.

66. Fatigue in Bottom Layer of Bituminous
Pavement and Fatigue Life
• With load repetitions, the tensile strain induced at the bottom
of the bituminous layer causes micro cracks, which go on
widening and expanding till the load repetitions are large
enough for the cracks to propagate to the surface over an
area that is unacceptable from the point of view of long term
serviceability of the pavement. This phenomenon is called
fatigue fracture of the bituminous layer and the number of
load repetitions in terms of standard axles that cause fatigue
failure denotes the fatigue life of the pavement.
• In IRC 37(2012), 80per cent reliability has been considered
for traffic up to 30 msa and 90 per cent reliability for traffic
beyond that. In 2018 version, this threshold limit has been
brought down to 20 msa.

67. Fatigue Model

68. As per clause no 12.3 of IRC 37(2018), the following values of the
volumetric parameters have been considered. These values are
considered for determination of the catalogues only and have
got not bearing with the actual field data which will differ from
project to project . During preparation of DPR, some practical
values of the volumetric parameters may be assumed. On the
basis of that, “C”, the adjustment factor and in turn, the allowable
strain at the bottom of bituminous layer may be determined from
the fatigue equation . But these values need to be validated as
per the actual field values and the pavement design should be
fine tuned accordingly. This is very important to understand and
not to be omitted.
Part of Clause no 12.3 of IRC 37(2018) :

69. Rutting in Pavement
• Rutting is the permanent deformation in pavement
usually occurring longitudinally along the wheel path. The
rutting may partly be caused by deformation in the
subgrade and other non-bituminous layers which would
reflect to the overlying layers to take a deformed shape.
• The bituminous mixes also may undergo rutting due to
secondary compaction and shear deformation under
heavy traffic load and higher temperature. Rutting of
bituminous layer is taken care of by appropriate mix design
and grade of bitumen
• Limiting rutting is recommended as 20 mm in subgrade,
may be expected in 20 per cent of the length for 80%
reliability (for design traffic up to 20 msa) and 10 per cent
of the length for 90% reliability ( in case of design traffic
more than 20 msa).
Ref: IRC 37(2018)

70. Rutting model
The rutting performance model developed initially based
on the MoRTH R-6 Research scheme performance data
was subsequently developed into two separate models for
two different reliability levels based on the additional
performance data collected for MoRTH R-56 Research
scheme

71. To Sum up the failures in pavements
The salient modes of Pavement failure:
• Fatigue cracking of bituminous layers, due to high
horizontal tensile strains occurring at the bottom of
bituminous layers( Bottom up cracking)
• Permanent deformation or rutting, caused by high
vertical compressive strains on subgrade.(Rutting of
subgrade)
• Permanent deformation within the bituminous
layers (viscous deformation of bituminous mix)
• Surface cracking of the wearing course , the
bituminous layer (Top down cracking)

72. To Sum up the failures in pavements
• The first two modes of failure are taken care
of by the equations Fatigue model and
Rutting Model respectively.
• The rest two modes of failure are taken care
of by appropriate asphalt mix design which is
also very important.

73. • Selection of appropriate grade of binder i.e., bitumen is very important.
In IRC 37(2018) Table 9.1 has categorically laid down the provision of the
grade as per the traffic volume.

74. A design example of flexible pavement
• N= 50 msa ( projected traffic)
• Design subgrade CBR= 5%
• Let us design a conventional flexible pavement and check with IITPAVE
software.
• Pavement composition as per IRC 37 (2018) :

75. Pavement composition as per 2012 version

76. Design example
So, as per 2018 version, from plate 1, we are getting,
• GSB=200mm
• WMM=250mm
• NB: As the MR value of GSB and WMM are rated same,
advantage can be taken by replacing costlier WMM
layer by GSB layer. As per Cl.8.1, minimum Base layer
thickness is 150mm. It should be used judiciously, if
rainfall is high(Annual rainfall 3000mm or more) and
traffic is less to moderate, GSB thickness should be
more, if the rainfall is not that high but traffic is more(
More than 50 msa), Base layer thickness should not be
compromised. If both are high, the functional
requirement should prevail, even if it may not be
structurally required, specially the drainage.

77. Design Example continued
• DBM=140mm, bottom layer=70 mm with air void
3.5% and Effective bitumen content by volume =
11.5%, top layer =70mm with air void 4% and
bitumen content= 4.5%, by weight of total mix.
• Bituminous surfacing course=40mm
• Check with IITPAVE :
• MR value of subgrade with 5% CBR=
10.0 * CBR for CBR=5: 50 MPa
• MR value of subbase and base (GSB+WMM)=
MR(granular)= 0.2* h^0.45 MR subgrade =

78. Design Example contd.
• 0.2(200+250)^ 0.45* 50.0= 156.3 Mpa
• For more than 20 Msa traffic we have to use
VG-40 grade bitumen. For 35 deg. C average
temp,
• MR value of bituminous layers= 3000MPa.
• Poisson’s ratio µ = 0.35 at 35 deg. C
• Wheel load for a single wheel= 80/4=20kN
• Tyre pressure =5.6kg/cm = 0.56 MPa
The above are the input data for IITPAVE

79. Design Example contd.
• To calculate the allowable tensile strain at the bottom of
bituminous layer and allowable compressive strain at the top
of subgrade:

80. Design Example contd.
• Step-1 : Calculation of allowable tensile strain at the
bottom of bituminous layer.
• Step -2 : Calculation of allowable compressive strain
at the top of subgrade
Here, Nf = 50 msa, Mrm= 3000 Mpa, Va=3.5%, Vbe=11.5%, C=2.35
Putting the above values, we get, εt = 1.78130*10 ^ -4
Here, Nr = 50 msa, putting this value in the above equation, we get,
= 3.71694^10 -4

81. N B : The values of the volumetric properties which are considered here to determine the
adjustment factor “C”, are as used in the guideline to determine the catalogues. In
practical cases, these values should be as per actual mix design data.

82. Step-3:Check by IITPAVE-- Input data

83. Step-4: IIT Pave --Output

84. IITPAVE results in Notepad

85. Step-5:Validity
• So, horizontal tensile strain at the bottom of
bituminous layer coming is less than the allowable.
• 1.723* 10^-4< 1.7813*10^-4, hence ok
• And the vertical compressive strain on the top of
subgrade coming is less than the allowable.
• 3.202*10^-4< 3.71694*10^-4, hence ok.
• NB :1. The GSB layer should be checked for
constructional vehicles with estimated no of
repetitions of standard axles with minimum of 10000
SA.(Design example 11.2 of IRC 37(2018)
2. The volumetric properties of bottom DBM
layer(bituminous base layer, in case of one DBM
layer) has to be checked with the actual field values
to validate the “ C “ factor of Fatigue equation.

86. Step:6: Drainage
• Though in 2018 version, the drainage chapter of
the pavement has not been included, it is advised
not to ignore this very important aspect of
pavement.
• Better, drainage requirement should be checked
as per provision of the earlier version of the code
i.e., 2012 version of IRC:37.
• In clause 7.2.1 of IRC 37(2018), it has been
mentioned that the drainage and filter layer
should be designed as per IRC:SP:42(Guideline for
road drainage) and IRC:SP:50(Guideline for Urban
Drainage)

87. Stage Construction
• As per clause no 4.3.2, the provision for stage
construction has been allowed for the bituminous layer.
• For the provision of the stage construction, situation
may occur in such a way that the design traffic in terms
of MSA for non-bituminous layers( for full design life)
may be more than 20 MSA whereas the same for stage-
1 traffic for bituminous layer may be less than 20 MSA.
• In that case, there is a confusion regarding what
reliability factor would be considered for bituminous
layer design, whether it would be 80% or 90%.
• It is prudent to consider the reliability factor as per the
actual design life and not for the partial design life for
stage construction, in case of the bituminous layer.

88. Different combinations of base and
surfacing course for flexible pavement
For the purpose of the guideline, suggested in IRC:37 (2012),
flexible pavements include pavements with Bituminous surfacing
over:
• (i) Granular base and sub-base ( Conventional)
• (ii) Cementitious bases and sub-bases with a crack relief
granular interlayer below the bituminous surfacing
• (iii) Cementitious bases and sub-bases with SAMI in-between
bituminous surfacing and the cementitious base layer for
retarding the reflection cracks into the bituminous layer
• (iv) Reclaimed Asphalt Pavement (RAP) with or without
addition of fresh aggregates treated with foamed
bitumen/bitumen emulsion
• (v) Use of deep strength long life bituminous pavement
(Perpetual Pavement)

89. Different combinations as per 2018
version
• six categories of pavements:
• (a) bituminous surface course with granular base and
sub-base : conventional
• (b) bituminous surface course with CTSB, CTB and
granular crack relief layer
• (c) bituminous surface course with CTSB, CTB and SAMI
• (d) bituminous surface course with CTSB and
emulsion/foam bitumen stabilised RAP/virgin aggregate
• (e) bituminous surface course with GSB, CTB and
granular crack relief layer and
• (f) bituminous surface course with CTSB and granular
base course.

90. Use of Geosynthetics in Pavement
• IRC 37(2018) has also provided another option of
incorporating Geosynthetics in different layers of
pavement.
• By using LCR( Layer Co-efficient Ratio) or MIF(Modulus
Improvement factor), the details of which are given in
IRC:SP:59(2019), subgrade strength can be improved a
lot and thereby conservation of natural resources can be
achieved.
• In Table:III-1of IRC:SP:59, various design considerations
for different asphalt reinforcements has also been
mentioned which can take care of many problems like
rutting, cracking specially in the overloaded sections and
other problematic sections of the road.

91. Scope of further study and work
• Take different combinations of base and sub-base courses as
suggested in IRC:37(2012 &2018) and design the pavement
with same traffic volume and same subgrade strength and
check the economy of different combinations.
• Design of Bituminous layer is an integral part of the design of
pavement. In the scenario of increasing traffic volume and
loading, newer developments of bituminous layer like Stone
Matrix Asphalt –SMA (IRC:SP:79) with Polymer Modified
Bitumen , Gap graded rubberized bitumen mix – GGRB (
IRC:SP:107) etc. should be studied and adopted in the roads
with higher traffic volume.
• To achieve the goal of sustainability, engineering fraternity
should come forward to take the challenge of designing and
constructing the pavements with alternative technologies
rather than by doing it in the conventional way.

92. References
• Principles of Pavement Design by Yoder &
Witczak
• Pavement Analysis and Design by Yang H Huang
• Pavement Engineering-Principles & Practice by
Rajib B. Mallick and Tahar El-Korchi
• Principles of Pavement Engineering by Nick Thom
• Presentations by Prof B B Pandey , Prof K S Reddy
& Prof S K Rao in different workshops in IIT
Kharagpur, M.Tech & PhD Course work PPTs.
• IRC 37(1984,2001,2012,2018)
• Flexible Pavement design in India: Past, Present
and Future by Sanjoy Garg.(Indian Highways, IRC,
August, 2014)