Many factors affect pavement design, the most important being wheel load. Since pavement is designed to take moving loads, slow moving loads and static loads can be detrimental to pavement.
Temperature also affects the design of pavement, especially the action of frost which is very important in cold countries.
There are many factors affecting pavement design which can be classified into four categories:
- traffic and loading,
- structural model,
- material characterization,
They will be discussed in detail in this chapter.
1.traffic and loading
Traffic is the most important factor in sidewalk design. Major factors include contact pressure, wheel load, axle configuration, moving load, load and load repeatability.
Tire pressure is an important factor, as it determines the contact area and the contact pressure between the wheel and the sidewall surface. Although the shape of the contact region is elliptical, for simplicity in analysis, a circular region is often assumed.
The next important factor is wheel load which determines the required pavement depth to ensure that the subgrade soil does not fail. Wheel configuration affects stress distribution and deflection within a sidewall. Many commercial vehicles have dual wheels that ensure that the contact pressure is within limits. Common practice is to convert a dual wheel load to a uniform single wheel load to simplify analysis.
The load carrying capacity of a commercial vehicle is further enhanced by the introduction of multiple axles.
If the vehicle is moving at a crawling speed then the damage to the pavement is very high. Several studies show that when the speed is increased from 2 km/h to 24 km/h, the tension and deflection are reduced by 40 percent.
The impact of traffic on the pavement depends not only on the magnitude of the wheel load, but also on the frequency of load applications. Each load application causes some distortion and the total distortion is the sum of all these. Although the pavement deformation due to single axle load is very small, the cumulative effect of the number of load repetitions is significant. Therefore, the modern design standard is based on a total number of axle loads (typically 80 kN single axle).
Structural models for determining pavement responses (stress, tension, and deflection) at different locations in the pavement are due to the use of wheel loads. The most common structural models are the layered elastic model and the visco-elastic model.
Layered elastic model:
A layered elastic model can calculate the stress, strain, and deflection at any point in the pavement structure as a result of the application of surface loads. Layered elastic models assume that each pavement structural layer is homogeneous, isotropic and linearly elastic. In other words, the physical properties at each point in a given layer are the same and the layer will return to its original form once the weights are removed. The layered elastic approach works with a relatively simple mathematical model that deals with wheel loading and physical properties such as the modulus of elasticity and Poisson’s ratio of stress, strain, and deformation.
The following physical properties are important for both flexible and rigid pavements.
When pavements are assumed to be linearly elastic, the elastic moduli and Poisson ratios of the subgrade and each component layer must be specified.
If the elastic modulus of a material varies with time of loading, the elastic modulus, which is the elastic modulus under repeated load, should be selected according to the load duration corresponding to the speed of the vehicle.
When a material is assumed to be non-linear elastic, the constitutive equation relating the elastic modulus to the stress state must be provided.
However, many of these physical properties used in visco-elastic models are very complex and in the development stage. This book covers layered elastic models which only require modulus of elasticity and Poisson’s ratio.
Environmental factors affect the performance of pavement materials and cause various damages. There are two types of environmental factors affecting pavement, temperature and rainfall and they are discussed below:
The effect of temperature on asphalt pavement is different from that of concrete pavement. Temperature affects the ductility modulus of asphalt layers, while it induces curling of concrete slabs. In hard pavements, temperature stresses or frictional stresses develop due to the difference in temperature at the top and bottom of the slab.
Whereas in flexible pavement, the dynamic modulus of asphalt concrete varies with temperature. Frost heave causes gap settlements and pavement roughness. The most damaging effects of frost penetration occur during the spring break period when the snow melts and the subgrade is a saturated state.
Rainfall and snowfall affect the amount of surface water to infiltrate the subgrade and the depth of the groundwater table. Poor drainage can cause shear strength, lack of pumping, loss of support, etc.
Dr. Tom V Matthews (IIT Bombay)