Hard surfaced pavements, which make up about 67 percent of U.S. roads and 70 percent of Washington State roads are typically categorized into flexible and rigid pavements:
- Flexible pavements. Those which are surfaced with bituminous (or asphalt) materials. These types of pavements are called “flexible” since the total pavement structure “bends” or “deflects” due to traffic loads. A flexible pavement structure is generally composed of several layers of materials which can accommodate this “flexing”.
- Rigid pavements. Those which are surfaced with portland cement concrete (PCC). These types of pavements are called “rigid” because they are substantially stiffer than flexible pavements due to PCC's high stiffness.
Each of these pavement types distributes load over the subgrade in a different fashion. Rigid pavement, because of PCC's high stiffness, tends to distribute the load over a relatively wide area of subgrade (Figure 1). The concrete slab itself supplies most of a rigid pavement's structural capacity. Flexible pavement uses more flexible surface course and distributes loads over a smaller area. It relies on a combination of layers for transmitting load to the subgrade (Figure 1). This Guide focuses on flexible pavements.
Figure 1: Rigid and Flexible Pavement Load Distribution
In general, both flexible and rigid pavements can be designed for long life (e.g., in excess of 35 years) with only minimal maintenance. Both types have been used for just about every classification of road. Certainly there are many different reasons for choosing one type of pavement or the other, some practical, some economical, and some political. As a point of fact, 95 percent of U.S. paved roads are surfaced with bituminous (asphalt) materials (FHWA 2009).