Function of Base Course and Subgrade
The base course and subgrade are structural elements of the pavement. In conjunction with the overlying asphalt surface, their purpose is to distribute traffic wheel loads over the whole foundation (fig. 16-4). To perform this function, you build the base course and subgrade with the necessary internal strength properties. In this respect, full-depth asphalt pavements have a special advantage over pavements with granular bases.
Asphalt pavement layers have both tensile and compressive strength to resist internal stresses. For example, figure 16-5 shows how wheel load (W) slightly deflects the pavement structure, causing both tensile and compressive stresses within the pavement.
Untreated granular bases have no tensile strength; therefore, asphalt bases spread the wheel load over broader areas than untreated granular bases. The result of this is that less total pavement structure thickness is required for an asphalt base.
Determining Required Pavement Thickness
A significant advance in highway engineering is the realization and demonstration that structural design of
Figure 16-5.-Pavement deflection results in tensile and compressive stresses in pavement structure.
asphalt pavements is similar to the problem of designing any other complex engineering structure. When asphalt pavement was first being introduced, determining the proper thickness was a matter of guesswork, rule of thumb, and opinion, based on experience. Almost the same situation once prevailed in determining the dimensions of masonry arches and iron and steel structures. However, these early techniques have long since yielded to engineering analysis. Similarly, based on comprehensive analysis of vast volumes of accumulated data, the structural design of asphalt pavements has now been developed into a reliable engineering procedure.
There is no standard thickness for a pavement. Required total thickness is determined by engineering design procedure. Factors considered in the procedure are as follows:
1. Traffic to be served initially and over the design service life of the pavement
2. Strength and other pertinent properties of the prepared subgrade
3. Strength and other influencing characteristics of the materials available or chosen for the layers (or courses) in the total asphalt pavement structure
4. Any special factors peculiar to the road being designed
Because weight and traffic volume normally increase, pavement originally built thick enough to handle immediate traffic volumes may not be thick enough and strong enough to handle future needs. With asphalt pavement, this problem can be met economically by first building the thickness required, then adding, when needed, layers of asphalt to increase total pavement thickness. This procedure is called stage construction. It avoids excessive investment in the beginning; and when a new layer of asphalt is added, the wearing surface is equal to or better than the original.
Several methods for evaluating or estimating the strength and supporting power of a subgrade are in use today, including the following:
1. Loading tests in the field on the subgrade itself. For example, the plate bearing test uses large, circular plates, loaded to produce critical amounts of deformation on the subgrade in place.
2. Loading tests in a laboratory using representative samples of the subgrade soil. A test commonly used by the Seabees is the California bearing ratio (CBR) test, which is sometimes used on the subgrade in place in the field.
3. Evaluations, based on classification of soil by identifying and testing the constituent particles of the soil.
Two well-known classification systems are the American Association of State Highway and Transportation Officials (AASHTO) Classification System and the Unified Soil Classification System, used by the Department of Defense.