Superpave Comes Of Age Refining A Mature System Posted July 10th, 2006 By R. Michael Anderson, P.E. and Dwight Walker, P.E. By R. Michael Anderson, P.E. and Dwight Walker, P.E. After about ten years of practical experience with Superpave, m
Superpave Comes Of Age Refining A Mature System
Posted July 10th, 2006 By R. Michael Anderson, P.E. and Dwight Walker, P.E.
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 Ndesignvalues. 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 sma.