The middle layer relies on the stone-on-stone contact and a high-modulus asphalt cement to provide resistance to bending and rutting, in turn reducing the magnitude of the tensile strain at the bottom of the pavement. The surface layer typically employs a long-life mix such as stone-matrix asphalt (SMA), Superpave, or open-graded friction course (OGFC) to resist rutting, weathering, thermal cracking and wear.

In considering the options for each layer, the variables of material and thickness must also be evaluated in terms of how they will respond to the environmental conditions and traffic load of the particular location. The foundation beneath the layers is critical as well, and must meet minimum requirements for support throughout construction as well as for the life of the pavement.

The Fatigue Factor

Many agencies have traditionally approached their pavement designs by putting a thin asphalt layer on top of a thick rock base, believing this would produce the most cost-effective design.

This approach can be an outcome of traditional empirical design methodologies that are unable to model or predict fatigue development in the asphalt layers.

Huddleston recommends using only enough aggregate or improved sub-base material to support construction equipment and properly grade the site. The remaining structural requirements should be focused on the asphalt layers.

Experience and studies show that the fatigue life of the asphalt pavement is not highly influenced by the thickness of the aggregate base course, but is very sensitive to the thickness and properties of the asphalt layers themselves. Beyond optimized performance, there are economic advantages to trading base rock thickness for asphalt thickness.

Layer on Layer:
Adding asphalt layers as thick as a quarter can be a sound way to manage secondary road. By adding one inch of asphalt, you can reduce the aggregate base requirement by about four inches or more, offsetting the increased cost of the additional asphalt. Furthermore, thinner aggregate bases require less excavation, yielding even more savings.

A New Approach

Research confirms that properly designed asphalt pavements, with thicknesses of five to six inches or more, tend to produce top-down cracking rather than bottom-up cracking. Rutting in thicker asphalt pavements is confined to the surface layers, and does not develop in base or subgrade layers. These are some of the primary characteristics qualifying a pavement as perpetual.

This evidence indicates that thinner county roads and city streets that were designed for low-volume traffic (and are still in good condition) can be made perpetual by adding asphalt thickness to the surface. It is not necessary to build these structures to high-volume highway or interstate standards, nor is it necessary for the structure to have been originally designed and built as a perpetual pavement in order to reap the longevity and cost-saving benefits that perpetual pavements provide. Rather, in the case of existing pavements, Huddleston promotes the concept of achieving perpetual pavement “one inch at a time.”

This gradual approach to achieving perpetual pavement includes a few key factors:

• Determine the ultimate thickness needed to become perpetual for your specific site and loading conditions. Generally this would require the addition of one to two inches of asphalt to an existing 20-year-design pavement that has not yet failed from the bottom up.

• Schedule overlays to achieve optimum thickness (typically a minimum of five to six inches of asphalt) before full-depth structural distress occurs. Applying a surface treatment (like a chip or slurry seal) prior to achieving optimum thickness is a risky decision, and by not addressing the structural integrity of the road, can lead to full-depth failures.

• Manage the surface – i.e., mill and fill as necessary – to ensure confinement of distress to the top layer of the pavement.