A Superpave Primer

Dec. 28, 2000
While virtually everyone in the industry is familiar with the term "Superpave," there still appears to be some confusion over just what Superpave is. To begin with, let's clarify what Superpave isn't: a tangible product. Superpave is an asphalt mix-design process.
While virtually everyone in the industry is familiar with the term "Superpave," there still appears to be some confusion over just what Superpave is. To begin with, let's clarify what Superpave isn't: a tangible product. Superpave is an asphalt mix-design process. The resulting hot-mix asphalt is often called "Superpave," but it would be more accurate to call it a "Superpave mix." The term "Superpave" stands for Superior performing asphalt pavements; the acronym is obviously constructed to connote a mix that is superior to conventional HMA.

The Superpave design process is a product of the Strategic Highway Research Program (SHRP); the goal of the system is to overcome many of the problems associated with asphalt pavements-in particular, rutting, low-temperature cracking and fatigue cracking.

Since local conditions-such as temperature ranges, moisture levels and aggregate type- can greatly influence HMA performance, SHRP focused on developing a system that would provide performance-based specifications for asphalt binders and asphalt-aggregate mixes. These specifications allow asphalt-pavement developers to tailor mixes to their unique needs. In this way, contractors in Florida can follow the same mix guidelines as contractors in Alaska and both can obtain optimal results. The actually properties of the mixes will, of course, vary greatly, but each mix will be best for the given conditions.

Mike Anderson, associate director of research at the Asphalt Institute, Lexington, Ky., was involved with Superpave research during SHRP. He says the design process for a Superpave mix is a three-part operation: volumetric mix design, Superpave mix analysis and field control. Each of these processes contains a formatted-yet flexible-quality control/quality assurance program designed to help produce a superior mix.

"Within volumetric mix design, materials selection and development of aggregate structure are vital," Anderson explains. "Once these are set, the determination of binder content and moisture-sensitivity just flow from the other conditions."

Aggregate selection is critical: Aggregate should be clean and angular, and the gradation Superpave employs is designed to control the amount of rounded, natural sand in a mix that may create instability. For the most part, a Superpave mix is coarser than most conventional mixes, not unlike stone-matrix asphalts.

Binder selection is based on a number of factors, according to Anderson. "We use local weather data, physical-property tests, AASHTO MP-1 guidelines and Superpave guidelines."

Once the materials have been selected, the design of the aggregate structure begins. "We evaluate multiple blends using the Superpave gyratory compactor," Anderson says. "There are no strength tests, such as the Marshall stability and flow test, with Superpave mixes. We then do a short-term aging of the mix; the final mix choice will-by this point-come down to necessary properties and economics."

A mix designer should be "very close" to the actual asphalt content by this point, Anderson says, adding that Superpave uses AASHTO T-283 for moisture-sensitivity determination.

At this point, a Superpave Level 1 mix should be ready to go. SHRP has divided highway traffic into three levels, with Level 1 designated as "light to moderate" traffic; levels 2 and 3 are for increasingly heavy traffic loads. All Superpave mixes undergo the Level 1 analysis outlined by Anderson; the magnitude of compaction in the gyratory compactor depends on the anticipated traffic and environmental conditions at the site.

For pavements that will carry heavier traffic loads, additional analysis-using the Superpave shear tester and the indirect tensile tester-is performed. Level 3 pavements, defined by SHRP as those carrying over 10 million equivalent single-axle loads (ESALs), are subjected to the same tests as Level 2 loads (1 to 10 million ESALs), but over a greater range of temperatures and pressures.

While SHRP has published guidelines for Superpave mixes, there may be some adjustments in the near future. Gerry Eller, chief of the FHWA's construction and maintenance division, comments, "As we are implementing our [Superpave] knowledge, we are still testing." For example, SHRP expects to publish guidelines for incorporating reclaimed asphalt pavement (RAP) into Superpave mixes sometime early this year.

Although there may be some subtle changes to mix specifications, don't expect any radical departures from current norms. Both laboratory analysis and field tests (see accompanying story) have demonstrated the quality of the process that creates Superpave mixes; as Eller notes, "Superpave is the future of asphalt pavement."