A flexible pavement structure is typically composed of several layers of material with better quality materials usually placed on top where the intensity of stress from traffic loads is high and lower quality materials at the bottom where the stress intensity is low. Flexible pavements can be analyzed as a multilayer system under loading.

A typical flexible pavement structure consists of the surface course and underlying base and subbase courses. Each of these layers contributes to structural support and, ideally, maintains proper drainage.

When hot-mix asphalt (HMA) is used as the surface course, it typically is the stiffest (as measured by elastic modulus) layer and may contribute the most (depending upon thickness) to pavement strength. The underlying layers are less stiff but are still important to pavement strength as well as drainage and frost protection.

Thicker HMA sections and or sections with stabilized bases behave as a semi-rigid system under traffic loading, whereby loads are spread to a greater degree over the natural subgrade than conventional flexible pavements. See “Rigid and Flexible Pavement Characteristics” above.

When a seal coat is used as the surface course, the base generally is the layer that contributes most to the structural stiffness. A typical structural design results in a series of layers that gradually decrease in material quality with depth. Figure 2-2 shows a typical section for a flexible pavement.