Section 2: Types of Flexible Pavements

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Definition of Flexible Pavement

A true flexible pavement yields “elastically” to traffic loading. It is constructed with a bituminous-treated surface 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 surface layer must also be resistant to fatigue damage and stable under traffic loads when pavement temperatures are in excess of 150ºF.

In this guide, hot-mix asphalt-surfaced pavements is a more generalized term used to describe any “black-topped” structure outside of HMA-overlaid concrete. These pavements are categorized as being flexible or semi-rigid (to include the full-depth or perpetual design). This chapter addresses 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 of elasticity than a 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:

  1. increased thickness in asphalt concrete
  2. chemical stabilization of the base, subbase, and/or subgrade layers
  3. 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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Types of Hot Mix Asphalt-Surfaced Pavements

These pavements may generally be placed into one of the following categories:

  • surface-treatment on a granular base
  • thin hot mix asphalt concrete (< 2 in.) on a granular base
  • intermediate hot mix asphalt concrete (2 - 5 in.) on a granular base
  • thick hot mix asphalt concrete (> 5 in.) (semi-rigid)
  • thin hot mix asphalt concrete on a chemically stabilized base or subbase (semi-rigid)
  • thin hot mix asphalt on an asphaltic bound base (semi-rigid).

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

Perpetual (HMA) Pavements

In 2001, the Flexible Pavement Design Task Force (FPDTF) studied structurally designed deep HMA pavement; a type of pavement typically associated with high assurance of long pavement life. The intent of the FPDTF study was to address the increased structural demands on heavy truck traffic facilities. The task force was composed of pavement and construction experts from TxDOT and industry.

As a result of the 2001 study, the following guidelines were established:

  • department guidelines for materials to be used
  • the general (“conceptual”) structural design format and
  • the locations where these structures should be considered (2001).

Current guidelines (2006) have taken into consideration design and constructability issues experienced in the structures designed under the original guidelines. Recommended structural layer composition (see Figure 2-2) for facilities with a projected 20-yr. one direction cumulative loading of at least 30 million ESALs was clarified.

When 30 million ESALs are exceeded, use Stone Matrix Asphalt (SMA), Item 346, and Performance-Designed Mixtures, Item 344, in lieu of conventional quality control/quality assurance dense-graded HMA specifications. When Items 346 and 344 are used with stiffer modified performance graded binders, these mixtures provide excellent resistance to permanent deformation. In addition, when Items 346 and 344 are used in a structure of at least 12” combined HMA thickness, or in conjunction with a high density rich bottom layer, the structure will be resistant to conventional bottom up fatigue cracking.

For the fatigue resistance characteristic to be effective, all HMA layers must be fully bonded to allow the system to act as a composite mass. The SMA and performance-designed mixes are specifically engineered to offer exceptional performance under heavy traffic loads. Dense-graded mixes under Item 341 may be used as an alternative to the stiff, rut-resistant HMA base (layer ‘C’). Prior approval by the Construction Division Director or designee is required for this substitution.

Special attention is required in designing a durable foundation by investigating the underlying soils to determine the appropriate type and level of stabilization needed. In lieu of subgrade stabilization, a high quality granular base, cement-treated base, or other engineered foundation should be used.

See Perpetual Pavement Design and Mechanistic Design Guidelines for using FPS-19W.

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