How a pavement responds to applied stresses is the combined result of loading, environment, subgrade, and pavement material characteristics.  This section presents the typical stresses and deformations experienced by an HMA pavement structure under load.

Stress

The stresses that occur in an HMA pavement under load are quite complex; routine calculation of these stresses is a recent development.  Using a basic two-dimensional layered elastic model, the basic relationships between layer stiffness and stress for a two-layer flexible pavement structure is shown in Figure 1.

Figure 1: Typical Two-Layer Flexible Pavement Stresses as Calculated by a Two-Dimensional Linear Elastic Model.  Click the yellow boxes to view different stresses.  Note that “E” refers to a layer’s stiffness.

Deflection

HMA pavements are often described as “flexible” because they deflect under load.  Figure 2 shows schematically how pavements deflect under load.  FWDs can be used to accurately determine deflection characteristics of in-service pavements.

Figure 2: Schematic Showing Deflections for Different Pavement Thicknesses.  The same HMA material characteristics are assumed for each graph – only the thickness varies.