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# FACTORS-AFFECTING-PAVEMENT-DESIGN Notes | EduRev

## : FACTORS-AFFECTING-PAVEMENT-DESIGN Notes | EduRev

``` Page 1

CHAPTER 20. FACTORS AFFECTING PAVEMENT DESIGN NPTEL May 3, 2007
Chapter 20
Factors aecting pavement design
20.1 Overview
In the previous chapter we had discussed about the types of pavements and their failure criteria. There are
many factors that aect pavement design which can be classied into four categories as trac and loading,
structural models, material characterization, environment. They will be discussed in detail in this chapter.
Trac is the most important factor in the pavement design. The key factors include contact pressure, wheel
Contact pressure: The tyre pressure is an important factor, as it determine the contact area and the contact
pressure between the wheel and the pavement surface. Even though the shape of the contact area is elliptical,
for sake of simplicity in analysis, a circular area is often considered.
Wheel load: The next important factor is the wheel load which determines the depth of the pavement required
to ensure that the subgrade soil is not failed. Wheel conguration aect the stress distribution and de
ection
within a pavemnet. Many commercial vehicles have dual rear wheels which ensure that the contact pressure is
within the limits. The normal practice is to convert dual wheel into an equivalent single wheel load so that the
Axle conguration: The load carrying capacity of the commercial vehicle is further enhanced by the intro-
duction of multiple axles.
Moving loads: The damage to the pavement is much higher if the vehicle is moving at creep speed. Many
studies show that when the speed is increased from 2 km/hr to 24 km/hr, the stresses and de
ection reduced
by 40 per cent.
uence of trac on pavement not only depend on the magnitude of the wheel
load, but also on the frequency of the load applications. Each load application causes some deformation and the
total deformation is the summation of all these. Although the pavement deformation due to single axle load is
very small, the cumulative eect of number of load repetition is signicant. Therefore, modern design is based
on total number of standard axle load (usually 80 kN single axle).
Introduction to Transportation Engineering 20.1 Tom V. Mathew and K V Krishna Rao
Page 2

CHAPTER 20. FACTORS AFFECTING PAVEMENT DESIGN NPTEL May 3, 2007
Chapter 20
Factors aecting pavement design
20.1 Overview
In the previous chapter we had discussed about the types of pavements and their failure criteria. There are
many factors that aect pavement design which can be classied into four categories as trac and loading,
structural models, material characterization, environment. They will be discussed in detail in this chapter.
Trac is the most important factor in the pavement design. The key factors include contact pressure, wheel
Contact pressure: The tyre pressure is an important factor, as it determine the contact area and the contact
pressure between the wheel and the pavement surface. Even though the shape of the contact area is elliptical,
for sake of simplicity in analysis, a circular area is often considered.
Wheel load: The next important factor is the wheel load which determines the depth of the pavement required
to ensure that the subgrade soil is not failed. Wheel conguration aect the stress distribution and de
ection
within a pavemnet. Many commercial vehicles have dual rear wheels which ensure that the contact pressure is
within the limits. The normal practice is to convert dual wheel into an equivalent single wheel load so that the
Axle conguration: The load carrying capacity of the commercial vehicle is further enhanced by the intro-
duction of multiple axles.
Moving loads: The damage to the pavement is much higher if the vehicle is moving at creep speed. Many
studies show that when the speed is increased from 2 km/hr to 24 km/hr, the stresses and de
ection reduced
by 40 per cent.
uence of trac on pavement not only depend on the magnitude of the wheel
load, but also on the frequency of the load applications. Each load application causes some deformation and the
total deformation is the summation of all these. Although the pavement deformation due to single axle load is
very small, the cumulative eect of number of load repetition is signicant. Therefore, modern design is based
on total number of standard axle load (usually 80 kN single axle).
Introduction to Transportation Engineering 20.1 Tom V. Mathew and K V Krishna Rao
CHAPTER 20. FACTORS AFFECTING PAVEMENT DESIGN NPTEL May 3, 2007
20.3 Structural models
The structural models are various analysis approaches to determine the pavement responses (stresses, strains,
and de
ections) at various locations in a pavement due to the application of wheel load. The most common
structural models are layered elastic model and visco-elastic models.
Layered elastic model: A layered elastic model can compute stresses, strains, and de
ections at any point
in a pavement structure resulting from the application of a surface load. Layered elastic models assume that
each pavement structural layer is homogeneous, isotropic, and linearly elastic. In other words, the material
properties are same at every point in a given layer and the layer will rebound to its original form once the load
is removed. The layered elastic approach works with relatively simple mathematical models that relates stress,
strain, and deformation with wheel loading and material properties like modulus of elasticity and poissons ratio.
20.4 Material characterization
The following material properties are important for both
exible and rigid pavements.
 When pavements are considered as linear elastic, the elastic moduli and poisson ratio of subgrade and
each component layer must be specied.
 If the elastic modulus of a material varies with the time of loading, then the resilient modulus, which is
elastic modulus under repeated loads, must be selected in accordance with a load duration corresponding
to the vehicle speed.
 When a material is considered non-linear elastic, the constitutive equation relating the resilient modulus
to the state of the stress must be provided.
However, many of these material properties are used in visco-elastic models which are very complex and in
the development stage. This book covers the layered elastic model which require the modulus of elasticity and
poisson ratio only.
20.5 Environmental factors
Environmental factors aect the performance of the pavement materials and cause various damages. Environ-
mental factors that aect pavement are of two types, temperature and precipitation and they are discussed
below:
20.5.1 Temperature
The eect of temperature on asphalt pavements is dierent from that of concrete pavements. Temperature
aects the resilient modulus of asphalt layers, while it induces curling of concrete slab. In rigid pavements, due
to dierence in temperatures of top and bottom of slab, temperature stresses or frictional stresses are developed.
While in
exible pavement, dynamic modulus of asphaltic concrete varies with temperature. Frost heave causes
dierential settlements and pavement roughness. Most detrimental eect of frost penetration occurs during the
spring break up period when the ice melts and subgrade is a saturated condition.
Introduction to Transportation Engineering 20.2 Tom V. Mathew and K V Krishna Rao
Page 3

CHAPTER 20. FACTORS AFFECTING PAVEMENT DESIGN NPTEL May 3, 2007
Chapter 20
Factors aecting pavement design
20.1 Overview
In the previous chapter we had discussed about the types of pavements and their failure criteria. There are
many factors that aect pavement design which can be classied into four categories as trac and loading,
structural models, material characterization, environment. They will be discussed in detail in this chapter.
Trac is the most important factor in the pavement design. The key factors include contact pressure, wheel
Contact pressure: The tyre pressure is an important factor, as it determine the contact area and the contact
pressure between the wheel and the pavement surface. Even though the shape of the contact area is elliptical,
for sake of simplicity in analysis, a circular area is often considered.
Wheel load: The next important factor is the wheel load which determines the depth of the pavement required
to ensure that the subgrade soil is not failed. Wheel conguration aect the stress distribution and de
ection
within a pavemnet. Many commercial vehicles have dual rear wheels which ensure that the contact pressure is
within the limits. The normal practice is to convert dual wheel into an equivalent single wheel load so that the
Axle conguration: The load carrying capacity of the commercial vehicle is further enhanced by the intro-
duction of multiple axles.
Moving loads: The damage to the pavement is much higher if the vehicle is moving at creep speed. Many
studies show that when the speed is increased from 2 km/hr to 24 km/hr, the stresses and de
ection reduced
by 40 per cent.
uence of trac on pavement not only depend on the magnitude of the wheel
load, but also on the frequency of the load applications. Each load application causes some deformation and the
total deformation is the summation of all these. Although the pavement deformation due to single axle load is
very small, the cumulative eect of number of load repetition is signicant. Therefore, modern design is based
on total number of standard axle load (usually 80 kN single axle).
Introduction to Transportation Engineering 20.1 Tom V. Mathew and K V Krishna Rao
CHAPTER 20. FACTORS AFFECTING PAVEMENT DESIGN NPTEL May 3, 2007
20.3 Structural models
The structural models are various analysis approaches to determine the pavement responses (stresses, strains,
and de
ections) at various locations in a pavement due to the application of wheel load. The most common
structural models are layered elastic model and visco-elastic models.
Layered elastic model: A layered elastic model can compute stresses, strains, and de
ections at any point
in a pavement structure resulting from the application of a surface load. Layered elastic models assume that
each pavement structural layer is homogeneous, isotropic, and linearly elastic. In other words, the material
properties are same at every point in a given layer and the layer will rebound to its original form once the load
is removed. The layered elastic approach works with relatively simple mathematical models that relates stress,
strain, and deformation with wheel loading and material properties like modulus of elasticity and poissons ratio.
20.4 Material characterization
The following material properties are important for both
exible and rigid pavements.
 When pavements are considered as linear elastic, the elastic moduli and poisson ratio of subgrade and
each component layer must be specied.
 If the elastic modulus of a material varies with the time of loading, then the resilient modulus, which is
elastic modulus under repeated loads, must be selected in accordance with a load duration corresponding
to the vehicle speed.
 When a material is considered non-linear elastic, the constitutive equation relating the resilient modulus
to the state of the stress must be provided.
However, many of these material properties are used in visco-elastic models which are very complex and in
the development stage. This book covers the layered elastic model which require the modulus of elasticity and
poisson ratio only.
20.5 Environmental factors
Environmental factors aect the performance of the pavement materials and cause various damages. Environ-
mental factors that aect pavement are of two types, temperature and precipitation and they are discussed
below:
20.5.1 Temperature
The eect of temperature on asphalt pavements is dierent from that of concrete pavements. Temperature
aects the resilient modulus of asphalt layers, while it induces curling of concrete slab. In rigid pavements, due
to dierence in temperatures of top and bottom of slab, temperature stresses or frictional stresses are developed.
While in
exible pavement, dynamic modulus of asphaltic concrete varies with temperature. Frost heave causes
dierential settlements and pavement roughness. Most detrimental eect of frost penetration occurs during the
spring break up period when the ice melts and subgrade is a saturated condition.
Introduction to Transportation Engineering 20.2 Tom V. Mathew and K V Krishna Rao
CHAPTER 20. FACTORS AFFECTING PAVEMENT DESIGN NPTEL May 3, 2007
20.5.2 Precipitation
The precipitation from rain and snow aects the quantity of surface water inltrating into the subgrade and
the depth of ground water table. Poor drainage may bring lack of shear strength, pumping, loss of support, etc.
20.6 Summary
Several factors aecting pavement design were discussed, the most important being wheel load. Since pavements
are designed to take moving loads, slow moving loads and static loads can be detrimental to the pavement.
Temperature also in
uences pavement design especially the frost action which is very important in cold countries.
20.7 Problems
1. Factor that least aect the pavement is
(a) Speed of vehicles
(c) Axle conguration
(a) 40kN
(b) 60kN
(c) 80kN
(d) 10kN
20.8 Solutions
1. Factor that least aect the pavement is
(a) Speed of vehicles
p