Flexible Pavements A flexible pavement is constructed with asphaltic cement and aggregates and usually consists of several layers. The lowest layer is referred to as the subgrade (the soil itself). The upper 6 to 8 inches of the subgrade is usually scarified and blended to provide a uniform material before it is compacted to a high density to assist with the overall stability and uniformity of the pavement structure. The next layer up in the structure, the subbase, usually consists of crushed aggregate (rock). This material has better engineering properties (higher modulus values) and more uniformity than the subgrade material in terms of its bearing capacity. The next layer up in the structure is the base layer and it is also often made of crushed aggregates (of a higher strength than those used in the subbase), which can in some cases be stabilized with a cementing material such as Portland cement, lime fly ash, or asphaltic cement.
The top layer of the flexible pavement structure is referred to as the wearing surface and usually consists of asphaltic concrete, which is a mixture of asphalt cement and aggregates. The purpose of the wearing layer is to protect the base layer from wheel abrasion and to waterproof the entire pavement structure. It also provides a skid-resistant surface that is important for safe vehicle stops and cornering. Typical thicknesses of the individual layers. These thicknesses vary with the types and magnitudes of axle loadings, available materials, and expected pavement design life (which is defined as the number of years the pavement is expected to provide adequate service before it must undergo major rehabilitation).