Short Notes for Pavements Design Core | Short Notes for Civil Engineering - Civil Engineering (CE) PDF Download

 Page 1


 ? Introduction
 ? Requirements of highway pavements
 ? Difference between ? exible and rigid 
pavements
 ? Components of pavements
 ? Flexible pavements
 ? Design methods
 ? Equivalency factor
 ? Fatigue and rutting criteria
 ? Rigid pavements
CHAPTER HIGHLIGHTS
Chapter 3
Introduction
The road surface should be stable and non-yielding and allow 
the heavy wheel loads to move with least possible rolling
resistance. The road surface should be even along the 
longitudinal profi le to enable the fast moving vehicles to 
move safely and comfortably.
Based on structural behaviour, pavements are generally 
classifi ed into two categories:
 1. Flexible pavements
 2. Rigid pavements
Requirements of Highway 
Pavements
 1. Functional requirements from the view of road users.
 • Road should be fi rm and non-yielding under wheel 
load.
 • Have good riding quality.
 • Should be less slippery.
 2. Structural requirements from the view of highway 
engineer.
 • It has to sustain heavy wheel loads and their 
repeated applications due to the moving traffi  c.
Difference between Flexible
and Rigid Pavements
Flexible Pavements
 • It has low fl exural strength.
 • It has series of layers with quality of materials reducing 
from top to bottom.
 • Its stability depends on aggregate interlock, particle
friction and cohesion.
 • It refl ects the deformations of subgrade and subsequent 
layers on the surface.
 • Load transfer is by grain to grain to the lower layers.
 • Its design is greatly infl uenced by subgrade strength.
 • IRC: 37–2012 is used for design.
 • Designed for a life of 15 years.
Rigid Pavements
 • It has good fl exural strength or fl exural rigidity which is 
the major factor for design.
 • It has concrete layer on the top with base course and soil 
subgrade under it.
 • Distributes load over a wide area, because of its rigidity.
 • Load transfer is by slab action.
Pavements Design
Part III_Unit 11_Chapter 03.indd   960 5/31/2017   4:08:44 PM
Page 2


 ? Introduction
 ? Requirements of highway pavements
 ? Difference between ? exible and rigid 
pavements
 ? Components of pavements
 ? Flexible pavements
 ? Design methods
 ? Equivalency factor
 ? Fatigue and rutting criteria
 ? Rigid pavements
CHAPTER HIGHLIGHTS
Chapter 3
Introduction
The road surface should be stable and non-yielding and allow 
the heavy wheel loads to move with least possible rolling
resistance. The road surface should be even along the 
longitudinal profi le to enable the fast moving vehicles to 
move safely and comfortably.
Based on structural behaviour, pavements are generally 
classifi ed into two categories:
 1. Flexible pavements
 2. Rigid pavements
Requirements of Highway 
Pavements
 1. Functional requirements from the view of road users.
 • Road should be fi rm and non-yielding under wheel 
load.
 • Have good riding quality.
 • Should be less slippery.
 2. Structural requirements from the view of highway 
engineer.
 • It has to sustain heavy wheel loads and their 
repeated applications due to the moving traffi  c.
Difference between Flexible
and Rigid Pavements
Flexible Pavements
 • It has low fl exural strength.
 • It has series of layers with quality of materials reducing 
from top to bottom.
 • Its stability depends on aggregate interlock, particle
friction and cohesion.
 • It refl ects the deformations of subgrade and subsequent 
layers on the surface.
 • Load transfer is by grain to grain to the lower layers.
 • Its design is greatly infl uenced by subgrade strength.
 • IRC: 37–2012 is used for design.
 • Designed for a life of 15 years.
Rigid Pavements
 • It has good fl exural strength or fl exural rigidity which is 
the major factor for design.
 • It has concrete layer on the top with base course and soil 
subgrade under it.
 • Distributes load over a wide area, because of its rigidity.
 • Load transfer is by slab action.
Pavements Design
Part III_Unit 11_Chapter 03.indd   960 5/31/2017   4:08:44 PM
Chapter 3 
¦
 Pavements Design | 3.961
 • Total thickness of pavement and quality of aggregates are 
lower than in flexible pavements.
 • IRC: 58–2011 is used in design of pavement.
 • Design life of pavement is 30 years.
Components of Pavements
 1. Soil subgrade: This is the lowest layer of pavement 
made of natural soil available at site and compacted. 
As the soil should never be over-stressed, its strength 
is evaluated using CBR (California bearing ratio) test, 
plate bearing test, dynamic cone test, direct shear test.
 2. Subbase: It is a stabilized layer of soil, gravel, broken 
stone which acts as a drainage layer. It takes loads 
from base course.
 3. Base course: This is the important layer for flexible 
pavement. It enhances the load bearing capacity of 
the pavement which is laid between wearing course 
and subbase. It is made of either graded stone, WBM 
or bituminous layer. 
 • Under rigid pavements 
(a) It prevents mud pumping.
(b) Protects the subgrade against frost action.
 4. Wearing course: This is to give a smooth riding 
surface and made of dense materials. This resists 
pressure exerted by tyres and takes up wear and tear 
due to traffic. Generally made of bitumen or asphalt.
Flexible Pavements
Factors Considered for Design 
of Pavement
 1. Design traffic: It is based on 7 day 24 hour traffic 
count as per IRC-9.
 2. Design life:
 • Flexible pavement:
 Expressways—20 years.
 NH and SH—15 years
 Other roads—10–15 years
 • Rigid Pavements:
 High volume roads—30 years
 Low volume roads—20 years
 3. Anticipated traffic: 
  To find the increased traffic at the end of design life of 
project.
A = P[1 + r]
n
  Where 
     A = Traffic intensity, i.e., no. of commercial 
vehicles per day at the end of ‘n’ years.
     P = Number of commercial vehicles per day at last 
count.
    r = Rate of growth of traffic (7.5%)
     n = Number of years between the last count and till 
the end of life of pavement.
 4. Other factors:
(a) Variation in moisture content: Stability of 
 subgrade is reduced under adverse moisture 
 conditions. Because of variation in moisture con-
tent between centre and edge of pavement, dif-
ferential settlement occurs.
(b) Frost action
(c) Variation in temperature: 
 Bituminous binders of flexible pavement become 
soft due to hot weather and brittle in very cold 
weather. These continuous softening and harden-
ing of the pavement affect the performance and 
life of pavement.
 5. Design wheel load:
(a) Maximum wheel load:
 • Design of pavement is based on 98th percen-
tile of axle load.
 • Tyre pressure influences the quality of surface 
course. 
 • Total load influences the thickness require-
ments of pavements.
Type of Load
Flexible 
Pavements
Rigid 
Pavements
Maximum legal axle load 8,200 kg 10.2 t
Maximum equivalent single 
wheel load
4,100 kg 5.1 t
Maximum tandom axle load 14,500 kg 19 t
Maximum tridem axle load 24 t
(b) Contact pressure:
 Contact pressure 
 = 
Load on wheel
Contact area (or) area of imprint
 = 
P
A
 • Contact area is assumed as circle.
 • At greater depth, the effect of tyre pressure 
diminishes as the load starts dispersing (dis-
tributing) with depth.
 6. Rigidity factor:
7 kg/cm
2
7 kg/cm
2
Tyre pressure
Contact pressure
Part III_Unit 11_Chapter 03.indd   961 5/31/2017   4:08:45 PM
Page 3


 ? Introduction
 ? Requirements of highway pavements
 ? Difference between ? exible and rigid 
pavements
 ? Components of pavements
 ? Flexible pavements
 ? Design methods
 ? Equivalency factor
 ? Fatigue and rutting criteria
 ? Rigid pavements
CHAPTER HIGHLIGHTS
Chapter 3
Introduction
The road surface should be stable and non-yielding and allow 
the heavy wheel loads to move with least possible rolling
resistance. The road surface should be even along the 
longitudinal profi le to enable the fast moving vehicles to 
move safely and comfortably.
Based on structural behaviour, pavements are generally 
classifi ed into two categories:
 1. Flexible pavements
 2. Rigid pavements
Requirements of Highway 
Pavements
 1. Functional requirements from the view of road users.
 • Road should be fi rm and non-yielding under wheel 
load.
 • Have good riding quality.
 • Should be less slippery.
 2. Structural requirements from the view of highway 
engineer.
 • It has to sustain heavy wheel loads and their 
repeated applications due to the moving traffi  c.
Difference between Flexible
and Rigid Pavements
Flexible Pavements
 • It has low fl exural strength.
 • It has series of layers with quality of materials reducing 
from top to bottom.
 • Its stability depends on aggregate interlock, particle
friction and cohesion.
 • It refl ects the deformations of subgrade and subsequent 
layers on the surface.
 • Load transfer is by grain to grain to the lower layers.
 • Its design is greatly infl uenced by subgrade strength.
 • IRC: 37–2012 is used for design.
 • Designed for a life of 15 years.
Rigid Pavements
 • It has good fl exural strength or fl exural rigidity which is 
the major factor for design.
 • It has concrete layer on the top with base course and soil 
subgrade under it.
 • Distributes load over a wide area, because of its rigidity.
 • Load transfer is by slab action.
Pavements Design
Part III_Unit 11_Chapter 03.indd   960 5/31/2017   4:08:44 PM
Chapter 3 
¦
 Pavements Design | 3.961
 • Total thickness of pavement and quality of aggregates are 
lower than in flexible pavements.
 • IRC: 58–2011 is used in design of pavement.
 • Design life of pavement is 30 years.
Components of Pavements
 1. Soil subgrade: This is the lowest layer of pavement 
made of natural soil available at site and compacted. 
As the soil should never be over-stressed, its strength 
is evaluated using CBR (California bearing ratio) test, 
plate bearing test, dynamic cone test, direct shear test.
 2. Subbase: It is a stabilized layer of soil, gravel, broken 
stone which acts as a drainage layer. It takes loads 
from base course.
 3. Base course: This is the important layer for flexible 
pavement. It enhances the load bearing capacity of 
the pavement which is laid between wearing course 
and subbase. It is made of either graded stone, WBM 
or bituminous layer. 
 • Under rigid pavements 
(a) It prevents mud pumping.
(b) Protects the subgrade against frost action.
 4. Wearing course: This is to give a smooth riding 
surface and made of dense materials. This resists 
pressure exerted by tyres and takes up wear and tear 
due to traffic. Generally made of bitumen or asphalt.
Flexible Pavements
Factors Considered for Design 
of Pavement
 1. Design traffic: It is based on 7 day 24 hour traffic 
count as per IRC-9.
 2. Design life:
 • Flexible pavement:
 Expressways—20 years.
 NH and SH—15 years
 Other roads—10–15 years
 • Rigid Pavements:
 High volume roads—30 years
 Low volume roads—20 years
 3. Anticipated traffic: 
  To find the increased traffic at the end of design life of 
project.
A = P[1 + r]
n
  Where 
     A = Traffic intensity, i.e., no. of commercial 
vehicles per day at the end of ‘n’ years.
     P = Number of commercial vehicles per day at last 
count.
    r = Rate of growth of traffic (7.5%)
     n = Number of years between the last count and till 
the end of life of pavement.
 4. Other factors:
(a) Variation in moisture content: Stability of 
 subgrade is reduced under adverse moisture 
 conditions. Because of variation in moisture con-
tent between centre and edge of pavement, dif-
ferential settlement occurs.
(b) Frost action
(c) Variation in temperature: 
 Bituminous binders of flexible pavement become 
soft due to hot weather and brittle in very cold 
weather. These continuous softening and harden-
ing of the pavement affect the performance and 
life of pavement.
 5. Design wheel load:
(a) Maximum wheel load:
 • Design of pavement is based on 98th percen-
tile of axle load.
 • Tyre pressure influences the quality of surface 
course. 
 • Total load influences the thickness require-
ments of pavements.
Type of Load
Flexible 
Pavements
Rigid 
Pavements
Maximum legal axle load 8,200 kg 10.2 t
Maximum equivalent single 
wheel load
4,100 kg 5.1 t
Maximum tandom axle load 14,500 kg 19 t
Maximum tridem axle load 24 t
(b) Contact pressure:
 Contact pressure 
 = 
Load on wheel
Contact area (or) area of imprint
 = 
P
A
 • Contact area is assumed as circle.
 • At greater depth, the effect of tyre pressure 
diminishes as the load starts dispersing (dis-
tributing) with depth.
 6. Rigidity factor:
7 kg/cm
2
7 kg/cm
2
Tyre pressure
Contact pressure
Part III_Unit 11_Chapter 03.indd   961 5/31/2017   4:08:45 PM
3.962 | Part III 
¦
 Unit 11 
¦
 Transportation Engineering
  Rigidity factor 
	 	= 
Contact pressure
Tyre pressure(or)Inflationpressure
	 	= 1, for tyre pressure = 7 kg/cm
2
  > 1, for low tyre pressure < 7 kg/cm
2
  < 1, for high tyre pressure > 7 kg/cm
2
 • Tyre pressure for the design is 0.8 MPa (8 kg/cm
2
) 
in the design of rigid pavements as per IRC 58.
(c) Equivalent single wheel load (ESWL):
 • To carry greater load and to reduce the inten-
sity on road, it is necessary to provide dual 
wheel assembly to rear axles of road vehicles.
 • The pressure at any depth lies between single 
load and two lines load carried by any one 
wheel.
2S
45°
d
S
d
2
 • ESWL may be calculated either by equivalent 
deflection or equivalent stress criterion.
 • Equivalent deflection criteria is more reliable.
Depth Z (log scale)
ESWL (log scale)
2P
P
1
Z
1
P
Z = d/2 Z = 2S
 7. Repetition of loads:
  P
1
N
1
 = P
2
N
2
  P
1
, P
2
 = Corresponding loads of vehicles.
  N
1
, N
2
 = Number of repetitions
 • Load method is based on 1 million repetitions 
= 10
6
 load repetitions.
Design Methods
 1. Empirical:
 • These are based on physical properties or strength 
parameters of soil subgrade and experience or per-
formance studies of the flexible pavements.
 • GI method, CBR, stabilitometer and MC load 
methods.
 2. Semi-empirical or semi-theoretical: When the 
design is based on stress-strain function and modified 
based on experience, it may be called semi-empirical 
or semi-theoretical.
   Tri-axial test method is modified by Kansas state 
highway department.
 3. Theoretical method: Burmister method
CBR Method
 • It is based on design curves and is a simple method.
 • Higher the load, larger will be the thickness of pavement.
 • The curves are drawn for CBR value versus depth of con-
struction with number of commercial vehicles varying for 
each curve.
 • For certain load values (or vehicles per day) and material 
CBR value, the thickness of pavement is found.
IRC Guidelines
 • CBR test is performed based on OMC (Optimum mois-
ture content) for new roads and FMC (field moisture con-
tent) for existing roads.
 • Specimen is soaked in water for 4 days (minimum 3 spec-
imens) and tested.
 For subgrade CBR = 8% and cumulative standard axle 
= 100 msa
(a) 200 mm Granular subbase.
(b) 250 mm Granular base.
(c) 140 mm dense bituminous macadam.
(d) 50 mm bituminous surface course.
Limitations of CBR Method
 1. This gives the total thickness of pavement as a whole 
and it is based on the CBR value of subgrade alone. 
Thickness of individual layers is not specified.
 2. Damaging effect of heavier loads and their frequency 
are not taken into consideration.
 3. The test conditions of CBR and the pavement may 
not be same throughout the life of the pavement.
 4. The load-penetration curves do not vary if the road is 
single lane or multi-lane.
Modified CBR (IRC–37:2012)
 • Design is based on cumulative number of standard axles 
in the traffic lane.
N = 
365 11 () +- ?
?
?
?
r
r
n
× A × D × F
Part III_Unit 11_Chapter 03.indd   962 5/31/2017   4:08:45 PM
Page 4


 ? Introduction
 ? Requirements of highway pavements
 ? Difference between ? exible and rigid 
pavements
 ? Components of pavements
 ? Flexible pavements
 ? Design methods
 ? Equivalency factor
 ? Fatigue and rutting criteria
 ? Rigid pavements
CHAPTER HIGHLIGHTS
Chapter 3
Introduction
The road surface should be stable and non-yielding and allow 
the heavy wheel loads to move with least possible rolling
resistance. The road surface should be even along the 
longitudinal profi le to enable the fast moving vehicles to 
move safely and comfortably.
Based on structural behaviour, pavements are generally 
classifi ed into two categories:
 1. Flexible pavements
 2. Rigid pavements
Requirements of Highway 
Pavements
 1. Functional requirements from the view of road users.
 • Road should be fi rm and non-yielding under wheel 
load.
 • Have good riding quality.
 • Should be less slippery.
 2. Structural requirements from the view of highway 
engineer.
 • It has to sustain heavy wheel loads and their 
repeated applications due to the moving traffi  c.
Difference between Flexible
and Rigid Pavements
Flexible Pavements
 • It has low fl exural strength.
 • It has series of layers with quality of materials reducing 
from top to bottom.
 • Its stability depends on aggregate interlock, particle
friction and cohesion.
 • It refl ects the deformations of subgrade and subsequent 
layers on the surface.
 • Load transfer is by grain to grain to the lower layers.
 • Its design is greatly infl uenced by subgrade strength.
 • IRC: 37–2012 is used for design.
 • Designed for a life of 15 years.
Rigid Pavements
 • It has good fl exural strength or fl exural rigidity which is 
the major factor for design.
 • It has concrete layer on the top with base course and soil 
subgrade under it.
 • Distributes load over a wide area, because of its rigidity.
 • Load transfer is by slab action.
Pavements Design
Part III_Unit 11_Chapter 03.indd   960 5/31/2017   4:08:44 PM
Chapter 3 
¦
 Pavements Design | 3.961
 • Total thickness of pavement and quality of aggregates are 
lower than in flexible pavements.
 • IRC: 58–2011 is used in design of pavement.
 • Design life of pavement is 30 years.
Components of Pavements
 1. Soil subgrade: This is the lowest layer of pavement 
made of natural soil available at site and compacted. 
As the soil should never be over-stressed, its strength 
is evaluated using CBR (California bearing ratio) test, 
plate bearing test, dynamic cone test, direct shear test.
 2. Subbase: It is a stabilized layer of soil, gravel, broken 
stone which acts as a drainage layer. It takes loads 
from base course.
 3. Base course: This is the important layer for flexible 
pavement. It enhances the load bearing capacity of 
the pavement which is laid between wearing course 
and subbase. It is made of either graded stone, WBM 
or bituminous layer. 
 • Under rigid pavements 
(a) It prevents mud pumping.
(b) Protects the subgrade against frost action.
 4. Wearing course: This is to give a smooth riding 
surface and made of dense materials. This resists 
pressure exerted by tyres and takes up wear and tear 
due to traffic. Generally made of bitumen or asphalt.
Flexible Pavements
Factors Considered for Design 
of Pavement
 1. Design traffic: It is based on 7 day 24 hour traffic 
count as per IRC-9.
 2. Design life:
 • Flexible pavement:
 Expressways—20 years.
 NH and SH—15 years
 Other roads—10–15 years
 • Rigid Pavements:
 High volume roads—30 years
 Low volume roads—20 years
 3. Anticipated traffic: 
  To find the increased traffic at the end of design life of 
project.
A = P[1 + r]
n
  Where 
     A = Traffic intensity, i.e., no. of commercial 
vehicles per day at the end of ‘n’ years.
     P = Number of commercial vehicles per day at last 
count.
    r = Rate of growth of traffic (7.5%)
     n = Number of years between the last count and till 
the end of life of pavement.
 4. Other factors:
(a) Variation in moisture content: Stability of 
 subgrade is reduced under adverse moisture 
 conditions. Because of variation in moisture con-
tent between centre and edge of pavement, dif-
ferential settlement occurs.
(b) Frost action
(c) Variation in temperature: 
 Bituminous binders of flexible pavement become 
soft due to hot weather and brittle in very cold 
weather. These continuous softening and harden-
ing of the pavement affect the performance and 
life of pavement.
 5. Design wheel load:
(a) Maximum wheel load:
 • Design of pavement is based on 98th percen-
tile of axle load.
 • Tyre pressure influences the quality of surface 
course. 
 • Total load influences the thickness require-
ments of pavements.
Type of Load
Flexible 
Pavements
Rigid 
Pavements
Maximum legal axle load 8,200 kg 10.2 t
Maximum equivalent single 
wheel load
4,100 kg 5.1 t
Maximum tandom axle load 14,500 kg 19 t
Maximum tridem axle load 24 t
(b) Contact pressure:
 Contact pressure 
 = 
Load on wheel
Contact area (or) area of imprint
 = 
P
A
 • Contact area is assumed as circle.
 • At greater depth, the effect of tyre pressure 
diminishes as the load starts dispersing (dis-
tributing) with depth.
 6. Rigidity factor:
7 kg/cm
2
7 kg/cm
2
Tyre pressure
Contact pressure
Part III_Unit 11_Chapter 03.indd   961 5/31/2017   4:08:45 PM
3.962 | Part III 
¦
 Unit 11 
¦
 Transportation Engineering
  Rigidity factor 
	 	= 
Contact pressure
Tyre pressure(or)Inflationpressure
	 	= 1, for tyre pressure = 7 kg/cm
2
  > 1, for low tyre pressure < 7 kg/cm
2
  < 1, for high tyre pressure > 7 kg/cm
2
 • Tyre pressure for the design is 0.8 MPa (8 kg/cm
2
) 
in the design of rigid pavements as per IRC 58.
(c) Equivalent single wheel load (ESWL):
 • To carry greater load and to reduce the inten-
sity on road, it is necessary to provide dual 
wheel assembly to rear axles of road vehicles.
 • The pressure at any depth lies between single 
load and two lines load carried by any one 
wheel.
2S
45°
d
S
d
2
 • ESWL may be calculated either by equivalent 
deflection or equivalent stress criterion.
 • Equivalent deflection criteria is more reliable.
Depth Z (log scale)
ESWL (log scale)
2P
P
1
Z
1
P
Z = d/2 Z = 2S
 7. Repetition of loads:
  P
1
N
1
 = P
2
N
2
  P
1
, P
2
 = Corresponding loads of vehicles.
  N
1
, N
2
 = Number of repetitions
 • Load method is based on 1 million repetitions 
= 10
6
 load repetitions.
Design Methods
 1. Empirical:
 • These are based on physical properties or strength 
parameters of soil subgrade and experience or per-
formance studies of the flexible pavements.
 • GI method, CBR, stabilitometer and MC load 
methods.
 2. Semi-empirical or semi-theoretical: When the 
design is based on stress-strain function and modified 
based on experience, it may be called semi-empirical 
or semi-theoretical.
   Tri-axial test method is modified by Kansas state 
highway department.
 3. Theoretical method: Burmister method
CBR Method
 • It is based on design curves and is a simple method.
 • Higher the load, larger will be the thickness of pavement.
 • The curves are drawn for CBR value versus depth of con-
struction with number of commercial vehicles varying for 
each curve.
 • For certain load values (or vehicles per day) and material 
CBR value, the thickness of pavement is found.
IRC Guidelines
 • CBR test is performed based on OMC (Optimum mois-
ture content) for new roads and FMC (field moisture con-
tent) for existing roads.
 • Specimen is soaked in water for 4 days (minimum 3 spec-
imens) and tested.
 For subgrade CBR = 8% and cumulative standard axle 
= 100 msa
(a) 200 mm Granular subbase.
(b) 250 mm Granular base.
(c) 140 mm dense bituminous macadam.
(d) 50 mm bituminous surface course.
Limitations of CBR Method
 1. This gives the total thickness of pavement as a whole 
and it is based on the CBR value of subgrade alone. 
Thickness of individual layers is not specified.
 2. Damaging effect of heavier loads and their frequency 
are not taken into consideration.
 3. The test conditions of CBR and the pavement may 
not be same throughout the life of the pavement.
 4. The load-penetration curves do not vary if the road is 
single lane or multi-lane.
Modified CBR (IRC–37:2012)
 • Design is based on cumulative number of standard axles 
in the traffic lane.
N = 
365 11 () +- ?
?
?
?
r
r
n
× A × D × F
Part III_Unit 11_Chapter 03.indd   962 5/31/2017   4:08:45 PM
Chapter 3 
¦
 Pavements Design | 3.963
 Where
  N = Million standard axles (msa)
  r = Rate of traffic growth per year
  n = Design life in years
   A = Traffic at the time of completion of construction (cv/
day)
  F = Vehicle damage factor (VDF)
  D = Lane distribution factor (LDF)
LDF for Various Roads
Type of Traffic LDF
Single lane road (cv in both directions are 
considered)
1.0
Two lane single carriage way roads  
(cv in both directions are considered)
0.75
Four-lane single carriage way roads  
(cv in both directions are considered)
0.4
Dual two lane carriage way roads  
(cv in one direction is considered)
0.75
Dual 3 or 4 lane carriage way roads  
(cv in one direction is considered)
0.6/0.45
cv – Commercial vehicles
VDF Values
Realistic value of VDF should be taken after conducting 
axle load surveys.
Initial Traffic Volume in Terms 
of Number of cv/day Terrain
Rolling/plain Hilly
0–150 1.5 0.5
150–1500 3.5 1.5
> 1500 4.5 2.5
Traffic in one direction is equal to half of the total traffic in 
both the directions. If significant difference between two 
streams occur then maximum traffic should be considered 
for the design.
NOTE
SOLVED EXAMPLES
Example 1
A two-lane undivided carriage way whose CBR = 6%
Initial traffic A = 400 cv per day
Traffic growth rate r = 5% per year
Design life = 15 years
Vehicle damage factor = 2.5
Find the cumulative standard axle on the road (in msa)
(A) 3 (B) 4
(C) 5 (D) 6
Solution
CSA 
ADF
=
+- ?
?
?
?
365 11 () r
r
n
=
+- ?
?
?
?
×× × 365 10 05 1 400 0752 5
005
15
(. ). .
.
As per IRC, for 2 lane undivided road lane distribution 
factor D = 0.75
? CSA = 5.9 msa
? 6 msa 
Hence, the correct answer is option (D).
For express ways, NH and SH, subgrade dry density g 
< 1.75 g/cc
NOTE
CBR% Maximum Variation
5 ±1
5–10 ±2
11–30 ±3
> 31 ±5
 • Minimum 3 samples are to be tested, with maximum vari-
ation as in above table.
 • If variations are more than specified values, 6 samples are 
to be tested.
Equivalency Factor
To find the damaging effect of any load with respect to 
standard load
 • Single axle load = 
Axle load in kg
8200
4
?
?
?
?
?
?
 • Tandem axle load = 
Axle load in kg
14500
4
?
?
?
?
?
?
Fatigue and Rutting Criteria
The total cumulative standard axles to be used for the design 
of the pavement should include fatigue and rutting criteria 
also.
Fatigue Criteria
The number of cumulative standard axles to produce 20% 
cracked surface area of bitumen is:
N
F
 = 2.21 × 
1
389
?
?
?
?
?
?
?
.
1
0 854
E
?
?
?
?
?
?
.
× 10
–4
Where 
	 ? = Tensile strain at bottom of stiff bituminous layer
 E = Modulus of elasticity (MPa) of bituminous layer
Rutting Criteria
Number of cumulative standard axles to produce rutting of 
20 mm is.
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Page 5


 ? Introduction
 ? Requirements of highway pavements
 ? Difference between ? exible and rigid 
pavements
 ? Components of pavements
 ? Flexible pavements
 ? Design methods
 ? Equivalency factor
 ? Fatigue and rutting criteria
 ? Rigid pavements
CHAPTER HIGHLIGHTS
Chapter 3
Introduction
The road surface should be stable and non-yielding and allow 
the heavy wheel loads to move with least possible rolling
resistance. The road surface should be even along the 
longitudinal profi le to enable the fast moving vehicles to 
move safely and comfortably.
Based on structural behaviour, pavements are generally 
classifi ed into two categories:
 1. Flexible pavements
 2. Rigid pavements
Requirements of Highway 
Pavements
 1. Functional requirements from the view of road users.
 • Road should be fi rm and non-yielding under wheel 
load.
 • Have good riding quality.
 • Should be less slippery.
 2. Structural requirements from the view of highway 
engineer.
 • It has to sustain heavy wheel loads and their 
repeated applications due to the moving traffi  c.
Difference between Flexible
and Rigid Pavements
Flexible Pavements
 • It has low fl exural strength.
 • It has series of layers with quality of materials reducing 
from top to bottom.
 • Its stability depends on aggregate interlock, particle
friction and cohesion.
 • It refl ects the deformations of subgrade and subsequent 
layers on the surface.
 • Load transfer is by grain to grain to the lower layers.
 • Its design is greatly infl uenced by subgrade strength.
 • IRC: 37–2012 is used for design.
 • Designed for a life of 15 years.
Rigid Pavements
 • It has good fl exural strength or fl exural rigidity which is 
the major factor for design.
 • It has concrete layer on the top with base course and soil 
subgrade under it.
 • Distributes load over a wide area, because of its rigidity.
 • Load transfer is by slab action.
Pavements Design
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Chapter 3 
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 • Total thickness of pavement and quality of aggregates are 
lower than in flexible pavements.
 • IRC: 58–2011 is used in design of pavement.
 • Design life of pavement is 30 years.
Components of Pavements
 1. Soil subgrade: This is the lowest layer of pavement 
made of natural soil available at site and compacted. 
As the soil should never be over-stressed, its strength 
is evaluated using CBR (California bearing ratio) test, 
plate bearing test, dynamic cone test, direct shear test.
 2. Subbase: It is a stabilized layer of soil, gravel, broken 
stone which acts as a drainage layer. It takes loads 
from base course.
 3. Base course: This is the important layer for flexible 
pavement. It enhances the load bearing capacity of 
the pavement which is laid between wearing course 
and subbase. It is made of either graded stone, WBM 
or bituminous layer. 
 • Under rigid pavements 
(a) It prevents mud pumping.
(b) Protects the subgrade against frost action.
 4. Wearing course: This is to give a smooth riding 
surface and made of dense materials. This resists 
pressure exerted by tyres and takes up wear and tear 
due to traffic. Generally made of bitumen or asphalt.
Flexible Pavements
Factors Considered for Design 
of Pavement
 1. Design traffic: It is based on 7 day 24 hour traffic 
count as per IRC-9.
 2. Design life:
 • Flexible pavement:
 Expressways—20 years.
 NH and SH—15 years
 Other roads—10–15 years
 • Rigid Pavements:
 High volume roads—30 years
 Low volume roads—20 years
 3. Anticipated traffic: 
  To find the increased traffic at the end of design life of 
project.
A = P[1 + r]
n
  Where 
     A = Traffic intensity, i.e., no. of commercial 
vehicles per day at the end of ‘n’ years.
     P = Number of commercial vehicles per day at last 
count.
    r = Rate of growth of traffic (7.5%)
     n = Number of years between the last count and till 
the end of life of pavement.
 4. Other factors:
(a) Variation in moisture content: Stability of 
 subgrade is reduced under adverse moisture 
 conditions. Because of variation in moisture con-
tent between centre and edge of pavement, dif-
ferential settlement occurs.
(b) Frost action
(c) Variation in temperature: 
 Bituminous binders of flexible pavement become 
soft due to hot weather and brittle in very cold 
weather. These continuous softening and harden-
ing of the pavement affect the performance and 
life of pavement.
 5. Design wheel load:
(a) Maximum wheel load:
 • Design of pavement is based on 98th percen-
tile of axle load.
 • Tyre pressure influences the quality of surface 
course. 
 • Total load influences the thickness require-
ments of pavements.
Type of Load
Flexible 
Pavements
Rigid 
Pavements
Maximum legal axle load 8,200 kg 10.2 t
Maximum equivalent single 
wheel load
4,100 kg 5.1 t
Maximum tandom axle load 14,500 kg 19 t
Maximum tridem axle load 24 t
(b) Contact pressure:
 Contact pressure 
 = 
Load on wheel
Contact area (or) area of imprint
 = 
P
A
 • Contact area is assumed as circle.
 • At greater depth, the effect of tyre pressure 
diminishes as the load starts dispersing (dis-
tributing) with depth.
 6. Rigidity factor:
7 kg/cm
2
7 kg/cm
2
Tyre pressure
Contact pressure
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  Rigidity factor 
	 	= 
Contact pressure
Tyre pressure(or)Inflationpressure
	 	= 1, for tyre pressure = 7 kg/cm
2
  > 1, for low tyre pressure < 7 kg/cm
2
  < 1, for high tyre pressure > 7 kg/cm
2
 • Tyre pressure for the design is 0.8 MPa (8 kg/cm
2
) 
in the design of rigid pavements as per IRC 58.
(c) Equivalent single wheel load (ESWL):
 • To carry greater load and to reduce the inten-
sity on road, it is necessary to provide dual 
wheel assembly to rear axles of road vehicles.
 • The pressure at any depth lies between single 
load and two lines load carried by any one 
wheel.
2S
45°
d
S
d
2
 • ESWL may be calculated either by equivalent 
deflection or equivalent stress criterion.
 • Equivalent deflection criteria is more reliable.
Depth Z (log scale)
ESWL (log scale)
2P
P
1
Z
1
P
Z = d/2 Z = 2S
 7. Repetition of loads:
  P
1
N
1
 = P
2
N
2
  P
1
, P
2
 = Corresponding loads of vehicles.
  N
1
, N
2
 = Number of repetitions
 • Load method is based on 1 million repetitions 
= 10
6
 load repetitions.
Design Methods
 1. Empirical:
 • These are based on physical properties or strength 
parameters of soil subgrade and experience or per-
formance studies of the flexible pavements.
 • GI method, CBR, stabilitometer and MC load 
methods.
 2. Semi-empirical or semi-theoretical: When the 
design is based on stress-strain function and modified 
based on experience, it may be called semi-empirical 
or semi-theoretical.
   Tri-axial test method is modified by Kansas state 
highway department.
 3. Theoretical method: Burmister method
CBR Method
 • It is based on design curves and is a simple method.
 • Higher the load, larger will be the thickness of pavement.
 • The curves are drawn for CBR value versus depth of con-
struction with number of commercial vehicles varying for 
each curve.
 • For certain load values (or vehicles per day) and material 
CBR value, the thickness of pavement is found.
IRC Guidelines
 • CBR test is performed based on OMC (Optimum mois-
ture content) for new roads and FMC (field moisture con-
tent) for existing roads.
 • Specimen is soaked in water for 4 days (minimum 3 spec-
imens) and tested.
 For subgrade CBR = 8% and cumulative standard axle 
= 100 msa
(a) 200 mm Granular subbase.
(b) 250 mm Granular base.
(c) 140 mm dense bituminous macadam.
(d) 50 mm bituminous surface course.
Limitations of CBR Method
 1. This gives the total thickness of pavement as a whole 
and it is based on the CBR value of subgrade alone. 
Thickness of individual layers is not specified.
 2. Damaging effect of heavier loads and their frequency 
are not taken into consideration.
 3. The test conditions of CBR and the pavement may 
not be same throughout the life of the pavement.
 4. The load-penetration curves do not vary if the road is 
single lane or multi-lane.
Modified CBR (IRC–37:2012)
 • Design is based on cumulative number of standard axles 
in the traffic lane.
N = 
365 11 () +- ?
?
?
?
r
r
n
× A × D × F
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 Where
  N = Million standard axles (msa)
  r = Rate of traffic growth per year
  n = Design life in years
   A = Traffic at the time of completion of construction (cv/
day)
  F = Vehicle damage factor (VDF)
  D = Lane distribution factor (LDF)
LDF for Various Roads
Type of Traffic LDF
Single lane road (cv in both directions are 
considered)
1.0
Two lane single carriage way roads  
(cv in both directions are considered)
0.75
Four-lane single carriage way roads  
(cv in both directions are considered)
0.4
Dual two lane carriage way roads  
(cv in one direction is considered)
0.75
Dual 3 or 4 lane carriage way roads  
(cv in one direction is considered)
0.6/0.45
cv – Commercial vehicles
VDF Values
Realistic value of VDF should be taken after conducting 
axle load surveys.
Initial Traffic Volume in Terms 
of Number of cv/day Terrain
Rolling/plain Hilly
0–150 1.5 0.5
150–1500 3.5 1.5
> 1500 4.5 2.5
Traffic in one direction is equal to half of the total traffic in 
both the directions. If significant difference between two 
streams occur then maximum traffic should be considered 
for the design.
NOTE
SOLVED EXAMPLES
Example 1
A two-lane undivided carriage way whose CBR = 6%
Initial traffic A = 400 cv per day
Traffic growth rate r = 5% per year
Design life = 15 years
Vehicle damage factor = 2.5
Find the cumulative standard axle on the road (in msa)
(A) 3 (B) 4
(C) 5 (D) 6
Solution
CSA 
ADF
=
+- ?
?
?
?
365 11 () r
r
n
=
+- ?
?
?
?
×× × 365 10 05 1 400 0752 5
005
15
(. ). .
.
As per IRC, for 2 lane undivided road lane distribution 
factor D = 0.75
? CSA = 5.9 msa
? 6 msa 
Hence, the correct answer is option (D).
For express ways, NH and SH, subgrade dry density g 
< 1.75 g/cc
NOTE
CBR% Maximum Variation
5 ±1
5–10 ±2
11–30 ±3
> 31 ±5
 • Minimum 3 samples are to be tested, with maximum vari-
ation as in above table.
 • If variations are more than specified values, 6 samples are 
to be tested.
Equivalency Factor
To find the damaging effect of any load with respect to 
standard load
 • Single axle load = 
Axle load in kg
8200
4
?
?
?
?
?
?
 • Tandem axle load = 
Axle load in kg
14500
4
?
?
?
?
?
?
Fatigue and Rutting Criteria
The total cumulative standard axles to be used for the design 
of the pavement should include fatigue and rutting criteria 
also.
Fatigue Criteria
The number of cumulative standard axles to produce 20% 
cracked surface area of bitumen is:
N
F
 = 2.21 × 
1
389
?
?
?
?
?
?
?
.
1
0 854
E
?
?
?
?
?
?
.
× 10
–4
Where 
	 ? = Tensile strain at bottom of stiff bituminous layer
 E = Modulus of elasticity (MPa) of bituminous layer
Rutting Criteria
Number of cumulative standard axles to produce rutting of 
20 mm is.
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¦
 Unit 11 
¦
 Transportation Engineering
N
r
 = 4.1656 
1
4 5337
?
?
?
?
?
?
?
.
× 10
–8
Where, ? = Vertical subgrade strain, (micro strain).
Rigid Pavements
Load carrying capacity of rigid pavements is mainly due to 
rigidity and high modulus of elasticity of the slab itself, i.e., 
slab action.
a
a
P
b
l
l
a = Radius of contact between road and tyre 
b = Radius of resisting section
l = Radius of relative stiffness. 
 • Load transfer is by bending/slab action/flexual action.
Factors Affecting Design and Performance 
of CC Pavements
 1. Design axle load (wheel load).
 2. Temperature variations at locations on the road.
 3. Types of joints and their spacing.
 4. Subgrade and the other supporting layers below the 
CC pavement slab.
 5. Drainage characteristics.
Design Parameters of Subgrade
 • Strength: CBR
 • Stiffness: Modulus of subgrade reaction (K)
Radius of Relative Stiffness (l)
 • A certain degree of resistance to slab action is offered 
by the subgrade. The relative stiffness of the slab with 
respect to subgrade support depends upon properties of 
the slab and pressure–deformation characteristics of the 
subgrade material.
  Westergaard defined this term as:
l = 
Eh
K
3
2
14
12 1 ()
/
-
?
?
?
?
?
?
µ
 Where 
  l = Radius of relative stiffness, cm
  h = Slab thickness, cm
  E = Modulus of elasticity of cement concrete (kg/cm
2
)
  m = Poissons ratio concrete = 0.15
   K = Subgrade modulus or modulus of subgrade 
reaction, kg/cm
3
.
Example 2
Compute the radius of relative stiffness of 15 cm thick 
cement concrete slab using following data:
Modulus of elasticity of cement concrete = 2.1 × 10
5
 kg/cm
2
Poisons ratio for concrete = 0.15
Modulus of subgrade reaction, K = 3 kg/cm
3
(A) 67 cm (B) 53 cm
(C) 47 cm (D) 32 cm
Solution
For K = 3.0
 L = 
Eh
k
3
2
14
12 1 ()
/
-
?
?
?
?
?
?
µ
	= 
21 10 15
12 31 015
53
2
14
.
(. )
/
××
×-
?
?
?
?
?
?
	= 67 cm. 
Hence, the correct answer is option (A).
Critical Positions of Loading
 • Interior
 • Edge
 • Corner
Equivalent Radius of Resisting Section (b)
With the load concentrated on a small area of the pavement, 
Westergaard designed for the equivalent radius of resisting 
section as
 
b =
 16
22
. ah + – 0.675 h, if a < 1.724 h 
  a – otherwise.
Where
 b = Equivalent radius of resisting section, cm
 a = Radius of wheel load distribution, cm
 h = Slab thickness, cm
Example 3
Find equivalent radius of resisting section of 20 cm thick 
slab if radius of contact area of wheel load is 15 cm
(A) 12 cm (B) 14 cm
(C) 16 cm (D) 18 cm
{
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