Pavement Designs &
Investigation

The contribution of a road pavement to the overall road formation is shown in Figure 2.1. The pavement must serve two basic functions, it must perform as an engineering structure and at the same time meet functional requirements. In terms of structural performance, the pavement must be of sufficient thickness, and be composed of materials of sufficient quality, to be able to withstand the various loads that are applied to it by heavy vehicles. In terms of functional performance, the pavement must have a good riding quality to ensure comfortable travel for the road user and, in the case of surfaced pavements, a surface having adequate drainage, skid resistance, reflectivity and line markings to ensure safe travel. The surface must also be capable of resisting both vertical and horizontal surface stresses to maintain its integrity.

If the surface is lost, or cracked, then ride comfort is affected and water can enter the underlying base layers. It must also be capable of withstanding environmental loads, including oxidation of bituminous binders. Inherent in these demands is the need to ensure that construction and maintenance practices are adequate for the demand to be placed upon the pavement. Australia has about 800,000 km of roads, of which about two-thirds are unsealed (Austroads 2000).

The basic function of a pavement is to support the applied traffic loading within acceptable limits of riding quality and deterioration over its design life. To do this, the pavement structure must spread the concentrated wheel loads to the foundation (subgrade material) such that, under peak and accumulated (e.g. fatigue) traffic loads:

• The pavement materials and subgrade do not deform excessively
• The pavement courses do not crack excessively

As mentioned earlier, the load-spreading effect of unbound granular materials is essentially through inter-particle friction and shear strength, which depend on the presence of horizontal confining stresses. On the other hand, bound layers tend to spread the load through slab action, as significant horizontal tensile stresses can be sustained at the bottom of the layer.