Design of flexible pavement

Topic –
Design of Flexible Pavements
SUBJECT – Pavement Analysis and Design – TE505
FACULTY GUIDE- Prof. Amit .A. Amin
PREPARED BY:-
Bhavik A. Shah (17TS809)
CIVIL ENGG. DEPARTMENT
BIRLA VISHVAKARMA MAHAVIDYALAYA ENGG. COLLEGE
VALLABH VIDYANAGAR-388120
M.TECH - TRANSPORTATION ENGINEERING
1

Table of Contents
 Introduction
 IRC METHOD OF DESIGN OF FLEXIBLE PAVEMENTS (IRC: 37-2012)
 Guidelines for Design by IRC: 37: 2012
2

Introduction
 Flexible pavements are those which on a whole have low or negligible flexural
strength and rather flexible in their structural action under load.
 The wheel load acting on the pavement will be distributed to a wider area, and the
stress decreases with the depth. Flexible pavement layers reflect the deformation
of the lower layers on to the surface layer
3

IRC METHOD OF DESIGN OF FLEXIBLE
PAVEMENTS (IRC: 37-2012)
 1. IRC:37-1970
 based on California Bearing Ratio (CBR) of subgrade
 Traffic in terms of commercial vehicles (more than 3 tonnes laden weight)
 2. IRC:37-1984
 based on California Bearing Ratio (CBR) of subgrade
 Design traffic was considered in terms of cumulative number of equivalent
standard axle load of 80 kN in millions of standard axles (msa)
 Design charts were provided for traffic up to 30 msa using an empirical
approach.
4

Continue …
 3. IRC:37-2001
 based on Mechanistic-Empirical method
 Pavements were required to be designed for traffic as high as 150 msa.
 The limiting rutting is recommended as 20 mm in 20 per cent of the length for
design traffic
 4. IRC:37-2012
 based on Mechanistic-Empirical method
 The limiting rutting is recommended as 20 mm in 20 per cent of the length for
design traffic up to 30 msa and 10 per cent of the length for the design traffic
beyond
5

Guidelines for Design by IRC: 37: 2012
 Design Traffic:
 The recommended method considers design traffic in terms of the
cumulative number of standard axles (80 kN) to be carried by the
pavement during the design life.
 Only the number of commercial vehicles having gross vehicle weight of
30 kN or more and their axle- loading is considered for the purpose of
design of pavement.
 Assessment of the present day average traffic should be based on seven-
day-24-hour count made in accordance with IRC: 9-1972 "Traffic Census
on Non-Urban Roads".
6

Traffic growth rate (r):
 Estimated by Analyzing:
 The past trends of traffic growth,
 Change in demand of Traffic by factors like specific development, Land
use changes etc.
 If the data for the annual growth rate of commercial vehicles is not
available or if it is less than 5 per cent, a growth rate of 5 per cent should
be used (IRC:SP:84-2009).
7

Design life (n)
 The design life is defined in terms of the cumulative number of standard
axles in msa that can be carried before a major strengthening,
rehabilitation or capacity augmentation of the pavement is necessary.
 Depending upon road type, Design traffic is ranges from 10 to 15 years.
8

Vehicle damage factor (VDF)
 It is defined as equivalent number of standard axles per commercial vehicle.
 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 magnitude 80 kN.
9

Lane distribution factor
 Distribution of commercial traffic in each direction and in each lane is required for
determining the total equivalent standard axle load applications to be considered
in the design.
 In the absence of adequate and conclusive data, the following distribution may be
assumed until more reliable data on placement of commercial vehicles on the
carriageway lanes are available:
 1) Single-lane roads
 2) Two-lane single carriageway roads
 3) Four-lane single carriageway roads
 4) Dual carriageway roads
10

Computation of Design traffic
 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:
11

Sub-grade
 Requirements of CBR: Sub grade is made up of in- situ material, select soil
or stabilized soil.
 Compacted to a minimum of 97% of laboratory dry density achieved with
heavy compaction.
 Minimum CBR of 8% for traffic > 450 CVPD
 CBR can also be determined from Dynamic Cone Penetrometer (60º cone)
by ..
 Log10 CBR = 2.465-1.12log10 N
 Where, N = mm/blow
12

Sub-grade (Continue…)
 Where different types of soils are used in sub grade minimum 6 to 8 average value
for each type is required.
 90th percentile for high volume and 80th percentile for other category of road is
adopted as design CBR .
 Maximum permissible variation
 Where variation is more average CBR should be average of 6 samples and not
three.
13

Effective CBR
 Where there is significant difference between the CBRs of the select sub grade and
embankment soils, the design should be based on effective CBR.
 The effective CBR of the subgrade can be determined from Fig.
14

Lab procedure for CBR calculation:
 The test must always be performed on remoulded samples of soils in the
laboratory.
 The pavement thickness should be based on 4-day soaked CBR value of the soil,
remoulded at placement density and moisture content ascertained from the
compaction curve.
 In areas with rainfall less than 1000 mm, four day soaking is too severe a condition
for well protected sub-grade with thick bituminous layer and the strength of the
sub-grade soil may be underestimated.
 If data is available for moisture variation in the existing in-service pavements of a
region in different seasons, molding moisture content for the CBR test can be
based on field data.
 Wherever possible the test specimens should be prepared by static compaction.
Alternatively dynamic compaction may also be used.
15

Resilient Modulus:
 Resilient modulus is the measure of its elastic behavior determined from
recoverable deformation in the laboratory tests.
 The modulus is an important parameter for design and the performance of a
pavement.
 The relation between resilient modulus and the effective CBR is given as:
 The CBR of the sub-grade should be determined as per IS: 2720 (Part 16) (36) at
the most critical moisture conditions likely to occur at site.
16

Various cases in design.
 The flexible pavement with different combinations of traffic loads and material
properties.
 1) Granular base and Granular sub-base.
 2) Cementitious base and sub-base with agg. Interlayer.
 3) Cementitious base and sub-base with SAMI.
 4) RAP agg. Over cemented sub-base
 5) Cemented base and Granular sub-base
17

Problem statement.
 Design the pavement for construction of a new flexible pavement with the
following data:
 Four lanes divided National Highway.
 Design life is 15 years.
 Data collection
 Material properties :
California Bearing Ratio (CBR)
Resilient Modulus (MR)
Modulus of Elasticity (E)
Poisson’s ratio (µ)
18

Material properties
 CBR :The CBR values are calculated after every kilometre on selected stretch
of 10 km having the same type of soil. Suppose the values obtained are: 3.8,
2.8, 4.5, 3.9, 4.2, 2.9, 4.7, 4.3, 4.0 and 4.6%. Based on the collected data the
design CBR (90th percentile CBR) is calculated as below:
 Solution:
 Arrange in ascending order : 2.8, 2.9, 3.8, 3.9, 4.0, 4.2, 4.3, 4.5, 4.6 and 4.7.
 Calculate the percentage greater than equal of the value as follows:
 For CBR of 3.8, percentage of values greater than equal to 3.8 = (8/10) x100
= 80%
 Similarly for 2.8 % is 100%, 4.5% CBR is 80% and so on.
 Now a plot is made between Percentages of values greater than equal to the
CBR values versus the CBR as follows.
19

Continue …
 RESULT : The 90th Percentile CBR value is 2.90%
20

Effective CBR:
 (Figure 5.1, Page 11, IRC: 37: 2012)
21

Poisson’s ratio
 Poisson’s ratio µ is define as the ratio of lateral strain (ɛl) to the axial strain (ɛa),
caused by load parallel to the axis along which ɛa is measured.
 It is found that for most of the pavement structures, the influence of µ value is
normally small.
 For most of cement treated materials (soil cement, cement treated base, lean
concrete and PCC), the value of µ normally lies between 0.10 and 0.25.
 Unbound granular material lie between 0.2 and 0.5 and those for bituminous
mixes range from 0.35 to 0.50
22

Elastic modulus
 Elastic moduli of various pavement materials are obtained either through tests or
through the recommendations available in the guidelines.
 Repeated flexure or indirect tensile tests are carried out to determine the dynamic
modulus Ed of bituminous mixes.
 Resilient modulus
 Resilient modulus is the measure of its elastic behaviour determined from
recoverable deformation in the laboratory tests.
 The behaviour of the subgrade is essentially elastic under the transient traffic
loading with negligible permanent deformation in a single pass.
 This can be determined in the laboratory by conducting tests.
23

Calculation of MR for Sub-grade.
 The resilient modulus is calculated as follow;
MR (Mpa) = 10 x CBR …………. For CBR 5
= 17.6 x CBR0.64 ………For CBR > 5
(From equation 5.2, Page no. 12, IRC: 37: 2012)
24

Calculation of MR for Granular base and
Sub-base
 The resilient modulus is calculated as follow;
MRgsb = 0.20 x h0.45 x MR subgrade
h = Thickness of sub-base layer in mm, …… sub-
base,
= Cumulative thickness of Base layer and Sub- base
layerin mm ... for base
25

Traffic count
 Assessment of average daily traffic should be normally based on 7 day-24hr count
made in accordance with IRC: 9 “Traffic census on non-urban roads”.
 Classify traffic into different categories such as two wheelers, three wheelers,
passenger cars, trucks etc.
 But only commercial vehicle with laden weight > 3 tonne is taken into
consideration of design.
 Commercial vehicles are further categorised as single axle single wheel, single axel
dual wheel, Tandem axle dual wheel and Tridem axle dual wheel.
 Where no traffic count data is available, data from roads of similar classification
and importance may be used to predict the design traffic
26

Calculation of Design factor
 1) Design Traffic,
 2) Axle load survey,
 3) Vehicle Damage Factor
 4) Lane Distribution Factor
27

Design Traffic:
 Initial traffic after construction in terms of number of Commercial Vehicles
per day (CVPD).
 Traffic growth rate during the design life in percentage.
 Design life in number of years.
 Spectrum of axle loads.
 Vehicle Damage Factor (VDF).
 Distribution of commercial traffic over the carriageway.
28

Calculation of Design traffic:
 For our case the number of heavy commercial vehicle per day is taken as 7 day
average for 24 hour count comes to be 2792 vehicle per day as per the last count.
 i. e. P = 2792 cvpd, r = 7 %, and x = 10 years
 A = 2792 (1+0.07)10 = 5000 cvpd.
 RESULT: Traffic in the year of completion of construction is 5000 cvpd in both the
directions.
29

Axle load survey :
 Required for VDF calculation and Fatigue damage analysis of cementitious
base.
 The axle load spectrum is formulated by considering 10 kN, 20 kN and 30
kN intervals for single, tandem and tridem axle respectively.
 RESULT: As per study the percentage of Single, Tandom and Tridom axle
are 45%, 45% and 10% respectively
30

Vehicle damage factor
 The formula to calculate VDF is given as follows:
 W1, W2, ….. are the mean values of the various axle load groups.
 V1, V2, …. are the respective traffic volumes.
 Ws is the standard axle load.
 Standard axle load for Single axle, Tandem axle and Tridem axle is 80 KN, 148 KN
and 224 KN as per IRC: 37:2012 (Page 7)
 RESULT: The VDF for Single axle load, Tandem axle load and Tridem axle load is
4.11, 8.37 and 7.51.
32

Vehicle Damage factor (Continue.)
 Were sufficient information on axle loads are not available or the small size of
project does not warrant an axle load survey the default values of VDF may be
adopted as given in the table given below.
33

Lane distribution factor.
 Distribution of commercial traffic in each direction and in each lane is required for
determining the total equivalent standard axle load applications to be considered
in the design.
 Single lane road : Total vehicle in both direction.
 Two lane single carriage way : 50% of total vehicle in both direction.
 Four lane single carriage way : 40% of total vehicle in both direction.
 Dual carriage way: Two lane 75%, Three lane 60%, Four lane 45% of
number of CV in each direction.
 RESULT: In the present design problem we are given to design a four lane divided
highway, therefore the Lane distribution factor is 75 percent of number of
commercial vehicle in each direction.
34

Million standard axle
 The design traffic is calculated in terms of cumulative number of standard axle of
80 kN carried during the design life of the road.
 r = 7.5 %,
 n = 20 yr. ( Expressway and Urban roads), 15 yr (NH and SH), In this problem we
have to design National highway take n as 15 years,
 A is 5000cvpd in both direction and 2500 in one direction
35

Calculation of Million std. axle.
 Single axle load (N1): 45 percent vehicles are of single axle.
 A : 0.45 x 2500 = 1125, F : 4.11
 N1 = 33.06 x 106 = 33.06 msa
 Tandem axle load (N2): 45 percent vehicles are of tandem axle.
 A : 0.45 x 2500 = 1125, F : 8. 37
 N2 = 67.33 x 106 = 67.33 msa
 Tridem axle load (N3): 10 percent vehicles are of tridem axle.
 A : 0.10 x 2500 = 250, F : 7.51
 Total msa (N1+N2+N3) = 33.06 + 67.33 + 13.42 = 113.81 ̴ 150 msa (Aprox.)
 RESULT: The cumulative million standard axles to be consider for design is 150
msa.
36

Determination pavement thickness
 Case 1 : Bituminous pavement with untreated granular layer
37

Determination of thickness for Case 1
 The thickness of various layers is determined with the help pavement design
catalogue given in IRC: 37: 2012 from page 26 to 28, for various values of effective
CBR.
38

Continue …
 RESULT:
 For design traffic of 150msa and CBR of 7%
 Thickness of subbase (GSB) is 230 mm,
 Thickness of base (G. Base) is 250 mm,
 Thickness of Dense Bitumen macadam (DBM) is 140 mm,
 Thickness of Bituminous concrete (BC) is 50 mm
39

Case 2 : Bituminous pavement with cemented base
and cemented sub-base with aggregate inter layer of
100mm
40

Determination of thickness for case 2.
 RESULT:
 For design traffic of 150msa and CBR of 7%
 Thickness of Cementitious sub-base (CT Subbase) is 250 mm,
 Thickness of Cementitious base (CT Base) is 120 mm, Aggregate
interlayer is 100mm
 Thickness of Dense Bitumen macadam (DBM) is 50 mm
 Thickness of Bituminous concrete (BC) is 50 mm are
 Obtained by interpolating the thickness of CBR 5% and 10%.
42

Calculation of Resilient Modulus (MR) for
case 2
 M R subgrade = 17.6 x CBR0.64 = 17.6 x 70.64 = 61.15 Mpa.
 M R Bituminous layer = 3000 Mpa
 (From table 7.1 Resilienent Modulus of Bituminous Mixes, page 23, IRC:
37: 2012)
 Pavement composition for 90 per cent Reliability is BC + DBM = 100 mm,
 Aggregate interlayer = 100 mm (MR = 450 MPa),
 Cemented base = 120 mm (E = 5000 MPa),
 Cemented subbase = 250 mm (E = 600 Mpa)
43

Case 3 : Bituminous pavement with cemented base and
cemented sub-base with SAMI layer over cemented
base.
44

Continue …
 PAGE 33 AND 34 OF IRC: 37: 2012
45

Determination of thickness for Case 3
 RESULT:
 Design traffic of 150 msa and CBR of 7%
 Thickness of Cementitious base (CT Base) is 165 mm,
 obtained by interpolating the thickness of CBR 5% and 10%.
 SAMI is provided on the top of cemented base.
46

Case 4 : Bituminous pavement with base of fresh
aggregate or RAP treated with foamed bitumen/ Bitumen
emulsion and cemented Sub-base
47

Continue…
 PAGE 36 AND 37 OF IRC: 37: 2012
48

Determination of thickness for case 4
 RESULT:
 Thickness of Treater reclaimed aspalt pavement (Treated RAP) is 180 mm,
 Obtained by interpolating the thickness of CBR 5% and 10%.
 Instead of RAP base of fresh aggregates treated with bitumen emulsion/
foamed bitumen can be used to obtain stronger base.
49

Case 5 : Bituminous pavement with cemented base and
granular sub-base with 100mm WMM layer over
cemented base:
50

Determination of thickness for case 5
 RESULT:
 Thickness of Granulated Subbase (GSB) is 250 mm
 Cementitious sub-base (CT Subbase) is 195 mm,
 Thickness of aggregate layer is 100 mm, Thickness of Dense Bitumen
macadam (DBM) is 50 mm
 Thickness of Bituminous concrete (BC) is 50 mm
 Obtain by interpolating the thickness of CBR 5% and 10%.
 The upper 100 mm of granular sub-base should be open graded so that
its permeability is about 300 mm/day or higher for quick removal of
water entering from surface.
52

Calculation of Resilient Modulus (MR) and
Modulus of Elasticity (E):
 For traffic of 150 msa, Subgrade CBR 7%,
 M R subgrade = 17.6 x CBR0.64 = 17.6 x 70.64 = 61.15 Mpa.
 M R Bituminous layer = 3000 Mpa (From table 7.1 Resilienent Modulus of
Bituminous Mixes, page 23, IRC: 37: 2012)
 M R Aggregate = 450 Mpa and
 E of cemented base is 5000 MPa,
 E Granular subbase = M R subgrade x 0.20 x h0.45
 Where, h = Thickness of GSB = 250 mm
 = 61.15 x 0.20 x 2500.45 = 146.72 Mpa.
53

Design check
 To check the suitability of pavement design discussed above we carry out
checks, which ensure safety against the failure of designed pavement.
 The flexible pavement is checked for two types of failures i.e. Rutting in
pavement and Fatigue in bottom layer of bituminous surfacing.
 The following condition should be satisfied for the design to be
satisfactory
 Design strain < Allowable strain
 Allowable strain = Obtained by fatigue model and rutting model
 Design strain = IITpave software
54

Recommendation
 Specifications should be modified according to local condition. In wet climate
wearing course should be impermeable.
 Long duration and low intensity rainfall causes more damage as compare with
rainfall of small duration and more density.
 If DBM and SDBC/BC are designed properly (4% air voids and protected shoulder)
impermeably can be ensure.
 Adequate provision for sub-surface drainage prevent pavement damage.
 Thickness charts with BC/ SDBC are valid for all rainfall area.
 For pavement carrying heavy traffic wearing course laid over WBM shows better
performance.
 For low traffic (upto 5 msa) bitumen surfacing with two coats is found to be
suitable.
55

Conclusion
 Time to time revisions of code provision are needed keeping in view changes in
traffic pattern and development of new technologies. Further with the gain of
experience in the design as well as construction procedure of flexible pavement
have demanded certain changes.
 Hence by considering the above factors IRC: 37: 2012 includes some conceptual
changes in the design of flexible pavement such as inclusion of Resilience moduli
and consideration of strain in design.
 This code also encourages the use IIT pave software which is newly recommended.
 Since the use of semi-mechanistic approach, the design is not only based on the
experience but it also gives parameters (strain parameter) to check the obtained
design.
 Solution to the above pavement design problem shows that the thickness design
varies with the variation in various factors.
56

References
 IRC: 37: 2012, “Guidelines for Design of Flexible pavement”, second revision.
 IRC: 37: 2001, “Tentative guidelines for Design of Flexible pavement”
57

THANK YOU For Bearing.
Bhavik A. Shah (17TS809)
58

Design of flexible pavement

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