In the previous chapter we had discussed about the types of pavements and their
failure criteria.
There are many factors that affect pavement design which can be classified
into four categories as traffic and loading, structural models, material
characterization, environment.
They will be discussed in detail in this chapter.
Traffic is the most important factor in the pavement design.
The key factors include contact pressure, wheel load, axle configuration,
moving loads, load, and load repetitions.
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.
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 configuration affect the stress distribution and deflection 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 analysis is made simpler.
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 deflection reduced by 40 per cent.
The influence of traffic 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 effect of number of load repetition is significant.
Therefore, modern design is based on total number of standard axle load
(usually 80 kN single axle).
The structural models are various analysis approaches to determine the pavement
responses (stresses, strains, and deflections) 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.
A layered elastic model can compute stresses, strains, and deflections 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.
The following material properties are important for both flexible and
rigid pavements.
When pavements are considered as linear elastic, the elastic moduli and
poisson ratio of subgrade and each component layer must be specified.
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.
Environmental factors affect the performance of the pavement materials and
cause various damages.
Environmental factors that affect pavement are of two types, temperature and
precipitation and they are discussed below:
The effect of temperature on asphalt pavements is different from that of
concrete pavements.
Temperature affects the resilient modulus of asphalt layers, while it induces
curling of concrete slab.
In rigid pavements, due to difference in temperatures of top and bottom of
slab, temperature stresses or frictional stresses are developed.
While in flexible pavement, dynamic modulus of asphaltic concrete varies with
temperature.
Frost heave causes differential settlements and pavement roughness.
Most detrimental effect of frost penetration occurs during the spring
break up period when the ice melts and subgrade is a saturated condition.
The precipitation from rain and snow affects the quantity of surface water
infiltrating into the subgrade and the depth of ground water table.
Poor drainage may bring lack of shear strength, pumping, loss of support, etc.
Several factors affecting 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 influences pavement design especially the frost action which
is very important in cold countries.
