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Section 2: Types of Flexible Pavements

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2.1 Definition of Flexible Pavement

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 “black-topped” 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:

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  1. increased thickness in asphalt concrete pavement,
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  3. chemical stabilization of the base, subbase, and/or subgrade layers,
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  5. asphalt stabilization of the base course.

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.

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2.2 Types of Flexible Pavements

These pavements may fall into one of the following categories:

Stabilization of the subgrade layer can be used with any of the above pavement types. Typical stabilizers include asphalt cement (for base only), lime, cement, fly ash, or lime-fly ash combinations.

2.2.1 Perpetual (HMA) Pavements

Perpetual pavements take into account the increased structural demands due to heavy truck traffic, where cumulative one-direction traffic loading of more than 30 million ESALs over a 20-30 year design life is projected. By limiting the strain level at critical locations, these pavements are designed to have a virtually infinite fatigue and full-depth rut life, requiring only periodic surface renewal.

For designing perpetual pavements using FPS 21, see Section 6, Perpetual Pavement Design and Mechanistic Design Guidelines.

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