By R. Michael Anderson, P.E. and Dwight Walker, P.E.
After about ten years of practical experience with Superpave, most engineers and contractors agree that overall, it has been a success. The Performance Graded (PG) Binder system has improved the characterization of the properties of the liquid asphalts. Superpave’s approach for selection of materials components based on local traffic and temperature conditions has provided asphalt binders and aggregate blends that are more likely to perform as expected. Superpave introduced mixture volumetrics to many agencies and suppliers. Other advancements include the mixture aging/conditioning procedure and the ability to estimate pavement compactability.
Even with all of these successes, experience has shown that there is still some work to be done. Work is underway to make improvements in asphalt binder characterization, mix design procedures and performance testing.
PG Binder Refinements
The PG system has been one of the most widely acclaimed parts of the Superpave procedures. However, most binder specialists agree that there is a need to develop a more suitable test for measuring the behavior of modified asphalt binders.
Rather than the PG-Plus tests applied by many agencies, the new Multiple Shear Creep Recovery (MSCR) test shows promise. The MSCR test can be performed using the same sample and Dynamic Shear Rheometer (DSR) as the widely used AASHTO M320 specification test. (Click here for more information on the MSCR procedure.)
Another potential improvement in evaluating modified binders deals with developing a means to accurately simulate the binder aging that occurs in hot mix asphalt (HMA) during mixing and compaction and in service. The Rolling Thin Film Oven Test (RTFOT, AASHTO T240) is the current procedure for simulating binder aging, but experience has shown that the RTFOT may not be appropriate for modified binders.
During the RFTOT, films of some modified asphalt binders do not flow easily within the rotating bottle. This prevents the asphalt from being exposed to heated air in a continuously moving thin film. NCHRP 9-36 is the research effort to develop a laboratory procedure that is suitable for both neat and modified binders. The research is complete and the final report should be available soon.
Mix Design Improvements
Refinements to the Superpave mix design procedures are underway, also. One of the areas of concern is the number of gyrations, Ndesign, applied during laboratory compaction.
Superpave’s mix design procedure uses the gyratory compactor. This device is intended to compact lab specimens to approximately the same density that the in-service pavement achieves. The procedure uses a compaction level that is dependent on the project traffic level.
NCHRP 9-9(1) is the research effort to provide a definitive field verification of the current Ndesign values. The research is complete and the report is expected to be published later this year. It is expected that the Ndesign levels will be reduced.
An effort, NCHRP 9-33, is underway to develop an improved mix design procedure and manual for HMA. The new procedure is intended to apply to dense-graded, open-graded and gap-graded mixes.
The procedure is likely to include a volumetric design method, the use of the simple performance test device, and a means of measuring moisture sensitivity. The report will likely be available in two years.
Fine versus Coarse Mixes
When Superpave was introduced, the use of coarse-graded mixes was actively promoted. Most coarse-graded Superpave mixes have performed well. But starting with the WesTrack test facility in Nevada, there have been questions about the sensitivity of these mixes to small changes in gradation or binder content.
Some coarse-graded mixes have rutted and/or had premature fatigue failures. Others have been permeable, especially if the density is slightly lower than the specification requirements. As a result, many agencies that previously used coarse-graded mixes are now allowing the use of fine-graded mixes on high volume roadways.
Another materials issue is the degree of stone-to-stone contact being achieved in the mixes. In many of the mixes being used, the coarse particles are essentially floating in the mix and are not carrying the load. SMAs’ ability to resist rutting has shown the value of achieving stone-to-stone contact.
One method being used by some asphalt technologists to evaluate a mixture’s sensitivity to small changes in gradation is the Bailey Method. This method, documented in 2002 in the Transportation Research Board’s E-Circular E-C044, is based on understanding aggregate packing characteristics. The way in which aggregate particles orient in a mixture affects the void structure, and, by association, the density of the compacted asphalt mixture. Experience with the Bailey Method has shown that some coarse-graded asphalt mixtures can be so sensitive to small gradation changes (i.e., 2 percent change or less on the coarse sieves) that the density of the mix can change by nearly 2 percent for the same compactive effort. Typically, fine-graded mixtures do not exhibit this level of sensitivity.
Demand for knowledge about the Bailey Method has been increasing in recent years. In response, AI has partnered with Bill Pine (Heritage Research Group and one of the most experienced practitioners of the Bailey Method) to conduct classes in Lexington each winter entitled Achieving Volumetrics and HMA Compactability.
Common Sense Approach
Superpave has established stringent requirements for the component aggregates. These requirements include angularity, cleanliness and shape requirements that are intended to provide a degree of assurance of good performance. However, aggregates that meet these requirements are not always locally available. Then the specifier must balance the cost of importing materials versus the desired performance. Local materials may be suitable for some applications but high volume or high stress locations may justify the extra costs to haul in select aggregate or to use modified binders.
The ultimate purpose behind performing an asphalt mix design is to find the right type and proportion of materials (aggregates and asphalt binders) to provide appropriate performance, such as rutting and cracking, for the project conditions (traffic, speed, and climate). Note that “appropriate performance…for the project conditions” means that cost should be a consideration. It is not necessary to design a strong aggregate skeleton with a modified asphalt binder for a lower volume road. In these cases, local materials may be absolutely acceptable.
Until performance testing becomes widely used, most users will continue to rely on mixture volumetric properties as surrogate performance-related properties. Years of history have shown relationships between the void properties of an asphalt mixture (i.e., density, air voids, and voids in the mineral aggregate) and its performance. However, the relationships are not perfect. It is possible to have acceptable volumetric properties, but a poor-performing mix (once again, WesTrack is an example). Until true performance-based properties of asphalt mixtures are routinely measured, the risk, however low, of potential failure associated with the use of surrogate performance-related properties must be accepted.
Although Superpave is not a perfect system, it does offer numerous advantages over earlier alternatives. With the refinements currently being developed, it should continue to provide asphalt materials that meet and exceed performance expectations.