Section 2: Types of Flexible Pavements
Anchor: #i1003122Definition of Flexible Pavement
A true flexible pavement yields “elastically” to traffic loading and 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 and 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 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 and 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:
- increased thickness in asphalt concrete
- chemical stabilization of the base, subbase, and/or subgrade layers
- asphalt stabilization of the base course.
The higher modulus adds to the structural capacity of the pavement layers. The load is thereby distributed over a wider area of the subgrade.
Anchor: #i1003161Types 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, fly ash, or lime-fly ash combinations.
Perpetual (HMA) Pavements
The idea of a structurally designed deep HMA pavement that would give high assurance of a long pavement life was studied by the Flexible Pavement Design Task Force (FPDTF) in 2001 with the expressed intent of addressing the increased structural demands on heavy truck traffic facilities. The FPDTF was composed of pavement and construction experts from TxDOT and industry. Their study resulted in the establishment of 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. For facilities with a projected 20-yr. one direction cumulative loading of more than 30 million ESALs, “heavy duty” mixes with improved aggregate properties should be used in lieu of conventional QC/QA (or method specification) dense graded asphalt mixtures. In addition, when new or “bottom up” reconstruction is anticipated, the use of a Rich Bottom Layer (RBL) is required. Exceptions to the use of these improved performance mixes for heavy truck traffic routes must be granted by the executive director or designated representative. These mixes are designated as performance-designed or stone matrix asphalt (SMA) mixes under the 2004 Specifications (Items 344 and 346). A pavement structure composed of an RBL with multiple lifts of performance and/or SMA mixes is commonly referred to as a perpetual pavement structure that is not subject to traditional bottom-up fatigue damage but will eventually experience surface distresses and will require surface renewal.
Guidelines for designing a perpetual pavement design can be found in Generalized Perpetual Pavement Design Guidelines referred to in the cited memorandum. Districts desiring to use these improved performance mixes on routes with less than 30 million ESALs must gain approval from the Construction Division, Materials & Pavements Section (CST-M&P).