A true flexible pavement yields “elastically” to traffic loading. It is constructed with a bituminous surface treatment or a relatively thin surface of hot-mix asphalt (HMA) over one or more unbound base courses resting on a subgrade. Its strength is derived from the load-distributing characteristics of a layered system designed to ultimately protect each underlying layer including the subgrade from compressive shear failure.

Progressively better materials are used in the upper structure to resist higher near-surface stress conditions caused by traffic wheel loads. These materials include an all-weather surface that is resistant to erosion by the environment and traffic action. The bituminous/ HMA surface layer must also be resistant to fatigue damage and remain stable under traffic loads when pavement surface temperatures are in excess of 150ºF.

In this manual, the term “flexible pavements” is used in a more generalized way to describe any Asphaltic Surfaced structure (other than HMA-overlaid concrete). These pavements range in composition from true flexible pavement to semi-rigid systems (including the full-depth or

perpetual

design). This chapter is applicable to the design of these types of structures.

The fundamental difference between a flexible, semi-rigid, and rigid pavement is the load distribution over the subgrade. The semi-rigid pavement has a higher composite modulus than a true flexible pavement and begins to resemble the rigid structure in terms of how the traffic loads are distributed over the subgrade. The elements contributing to the higher modulus may be:

The higher modulus adds to the structural capacity of the pavement layers. As a result, the load is distributed over a wider area of the subgrade.