Hard surfaced pavements 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". Flexible pavements comprise about 94 percent of U.S. paved roads (about 98 percent of Hawai'i roads).
- 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. Rigid pavements comprise 6 percent of U.S. paved roads (about 2 percent of Hawai'i roads).
Figure 1: Rigid and flexible pavement load distribution.
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 (see 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 (see Figure 1).
This Guide addresses flexible pavements. In general, both flexible and rigid pavements can be designed for long life (e.g., in excess of 30 years) with only minimal maintenance. Both types have been used for just about every classification of road. There are many different reasons for choosing one type of pavement or the other, some practical, some economical, and some political. Many road-owning agencies have documented pavement type selection protocols they use for determining pavement types for new or reconstructed roads.