Basically, all hard surfaced pavement types can be categorized into two groups, flexible and rigid. Flexible pavements are those which are surfaced with bituminous (or asphalt) materials. These can be either in the form of pavement surface treatments (such as a bituminous surface treatment (BST) generally found on lower volume roads) or, HMA surface courses (generally used on higher volume roads such as the Interstate highway network). 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”. On the other hand, rigid pavements are composed of a PCC surface course. Such pavements are substantially “stiffer” than flexible pavements due to the high modulus of elasticity of the PCC material. Further, these pavements can have reinforcing steel, which is generally used to reduce or eliminate joints.
Each of these pavement types distributes load over the subgrade in a different fashion. Rigid pavement, because of PCC’s high elastic modulus (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.
Figure 1. Rigid and Flexible Pavement Load DistributionOverall, it may be somewhat confusing as to why one pavement is used versus another. Basically, state highway agencies generally select pavement type either by policy, economics or both. Flexible pavements generally require some sort of maintenance or rehabilitation every 10 to 15 years. Rigid pavements, on the other hand, can often serve 20 to 40 years with little or no maintenance or rehabilitation. Thus, it should come as no surprise that rigid pavements are often used in urban, high traffic areas. But, naturally, there are trade-offs. For example, when a flexible pavement requires major rehabilitation, the options are generally less expensive and quicker to perform than for rigid pavements.
This section will discuss flexible and rigid pavements and the basic characteristics and types of each.