Kercher Engineering, Inc.

This article is the first part in a three part series on SUperior PERforming asphalt PAVEment (Superpave). Superpave is a new way of specifying hot mix asphalt component materials, mixture design and analysis, and flexible pavement performance. Part 1 will explain the need for a superior pavement system. Additionally, a brief introduction of asphalt concrete is provided in the article entitled “Asphalt Concrete – A Primer” to provide background information for the HMA novice.

Most of us are very familiar with common problems found in hot mix asphalt (HMA) pavements including such distresses as rutting, low temperature thermal cracking, fatigue (alligator) cracking and durability (raveling, etc). These distresses are the cause of a majority of costly repairs to HMA pavements. Millions of dollars are spent every year in the United States to repair damage caused by these distresses. In many cases, better HMA materials and mix design could help to prevent or delay the onset of these distresses.

The term rutting refers to channel-like depressions in the wheel path. The depressions are created by the compaction and/or movement of unstable material in one or more of the pavement layers and/or the supporting subgrade. Rutting in the HMA layers may be due either to inadequate compaction during construction or an unstable HMA (plastic movement) that cannot adequately support traffic loads. High temperature and/or slow moving vehicles can aggravate unstable HMA.

Low temperature cracking, more commonly referred to as transverse cracking, occurs when the asphalt binder becomes brittle and cracks due to tensile stresses created by shrinkage of the pavement in cold weather. Over time, as the asphalt cement is exposed to oxygen, it oxidizes and becomes brittle. In cold weather when the pavement shrinks, the shrinking action creates tensile stresses in the pavement (small “localized” sections of the pavement try to pull away from the adjacent sections).

When the tensile stress exceeds the strength of the asphalt binder, transverse cracks that run perpendicular to the roadway centerline form in the pavement.

Fatigue cracking, more commonly referred to as alligator cracking, occurs when the applied loads from vehicles overstress the HMA layers in the pavement. As vehicular loads are applied to the pavement, the HMA layers flex. Large amounts of flexing (known too as deflection) can develop as a result of inadequate pavement thickness or weak supporting subgrade (soil).

At some point in time, the HMA reaches its fatigue limit as a result of too much flexing and cracking starts to develop in the wheel path.

Over time, the cracks become interconnected to form a pattern that looks similar to the skin of an alligator.

Although fatigue cracking is considered a structural failure resulting from repetitive heavy loads, higher quality HMA materials will provide a pavement that is more resilient, and as such, will help to slow down the fatigue process in many cases.

Note: Improved materials are not a cure-all for fatigue cracking. All pavements still require a proper pavement design that takes into account vehicular traffic loads (especially heavy trucks), soil strength, drainage, design life, etc.

Another common distress that everyone is familiar with is potholes. They are usually a direct descendent of fatigue cracking, forming in areas of severe fatigue cracking that were left untreated. As such, potholes will be considered as fatigue cracking for the purposes of this article.

Raveling is the progressive loss of material from the pavement surface as a result of a loss of adhesion between the asphalt binder and the aggregate. Initially, fine aggregate breaks loose and leaves small, rough patches in the surface of the pavement. As the disintegration continues, larger aggregates break loose resulting in rough surfaces. Causes of raveling include hardening of the asphalt binder due to aging, lack of compaction - especially during cold weather construction, and/or insufficient asphalt content in the mix.

Before HMA can be produced, the composition of the mixture must be determined. In other words, a materials engineer must select the type and gradation of aggregate, the type and amount of asphalt binder and if any modifiers are to be used. This process of developing a recipe for making HMA is referred to as a mix design. The recipe is called a job mix formula.

Unfortunately, previous solutions to the above-mentioned problems were usually in conflict. In the past, materials engineers usually tried to develop a solution to the above-mentioned pavement distresses by changing the mix design. Methods used to determine the mix design were empirical in nature and far from an exact science, and as such, did not provide a perfect solution. For example, if an agency increased the amount of asphalt binder or used a softer grade of asphalt binder to reduce raveling or thermal cracking, rutting problems would occur because the change created a tender mixture.

If the agency then experienced rutting, it would reduce the asphalt binder content or use a harder grade of asphalt binder that in turn caused raveling, fatigue cracking or thermal cracking.

Also, other problems existed such as the grading systems (viscosity and penetration) for asphalt binder. For example, two samples of AC-20 (a commonly used grade of asphalt binder in this region) could have the same characteristics at elevated temperatures but could behave quite differently at lower temperatures. In order to deal with this frustrating situation, a better method of grading asphalt binder and developing mix designs were needed.

One of the main goals of the Strategic Highway Research Program (SHRP), which was authorized by Congress in 1987, was to increase the life span of asphalt pavements. The results of this research were the creation of the Superpave system for the design and analysis of asphalt concrete mixtures. Superpave is short for SUperioir PERforming asphalt PAVEment.