HMA pavements are flexible pavements. Flexible pavements are so named because the total pavement structure deflects, or flexes, under loading. A flexible pavement structure is typically composed of several layers of material. Each layer receives the loads from the above layer, spreads them out, then passes on these loads to the next layer below. Thus, the further down in the pavement structure a particular layer is, the less load (in terms of force per area) it must carry (Figure 1).
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.
The goal of structural design is to determine the number, material composition and thickness of the different layers within a pavement structure required to accommodate a given loading regime. This includes the surface course as well as any underlying base or subbase layers. This section is focused on the structural design of new pavement. Structural design for rehabilitation is covered in Maintenance & Rehabilitation.
Advancements in milling, recycling and HMA technology over the last few decades have created HMA pavements that perform better, longer and with lower life-cycle costs than was previously possible. Today’s HMA pavements can be designed to last in perpetuity.
A Perpetual Pavement is defined as an asphalt pavement designed and built to last longer than 50 years without requiring major structural rehabilitation or reconstruction, and needing only periodic surface renewal in response to distresses confined to the top of the pavement. The concept of Perpetual Pavements, or long-lasting HMA pavements, is not new. Full-depth and deep-strength HMA pavement structures have been constructed since the 1960s, and those that were well-designed and well-built have been very successful in providing long service lives under heavy traffic.