Update on Superpave Short Term
The current short term aging protocol in Superpave requires that after mixing, loose mixture samples be aged in a forced draft oven for four hours at 135° C. After short term aging, the mixture sample is compacted in the Superpave gyratory compactor. This aging protocol was developed by SHRP researchers to create a compacted specimen with mechanical properties that simulate an asphalt mixture immediately after construction. If mechanical properties are to be measured on a compacted specimen, then this procedure is very effective at simulating such a condition. However some engineers have asked, "If I am only interested in the volumetric properties of an asphalt blend, can I get by with less oven aging." Recently the short term aging protocol in Superpave and its role in Superpave mix design and analysis has been the subject of research conducted by the Federal Highway Administration (FHWA) Superpave Mixtures Expert Task Group (ETG).
The research was prosecuted as a round robin experiment by ETG members, which included agency and industry laboratories. The data was analyzed by FHWA. Experimental results showed that, except for highly absorptive aggregate, there is little difference in volumetric properties between mixtures aged between two and four hours. The study concluded that two hours of short term aging is sufficient in Superpave mix design where volumetric properties are of primary interest. In Superpave mix design, no mixture mechanical properties are determined.
The FHWA Superpave Mixtures ETG suggested the following changes to AASHTO PP2, Short and Long Term Aging of Hot Mix Asphalt. The short term aging period of Superpave mix design specimens, upon which only volumetric properties are measured, be set at two hours in a forced draft oven at 135° C. However, when dealing with absorptive aggregate (defined as aggregate with water absorption greater than 2.5 percent), four hours of oven aging should be used. The two- hour aging period is not applicable to preparation of specimens to be used for testing in the Superpave shear tester or indirect tensile tester. In other words, Superpave mix analysis should be conducted using specimens prepared by the original short term aging period of four hours at 135° C.
The issue of short term aging of plant produced mixtures also was raised by the FHWA Superpave Mixtures ETG. Short term aging in AASHTO PP2 is currently only applicable to laboratory produced mixtures. Not short term aging is proscribed for mixture produced at a hot mix plant. If mixtures in the field need to be reheated to attain the proper compaction temperature, then the heating time should be limited only for the time necessary to bring the mixture to the proper compaction temperature. The consensus of the ETG was that the issue of field aging of asphalt mixes needed to be addressed, possibly by additional research.
Questions or comments regarding the short term aging experiment and its conclusions and recommendations should be addressed to John Bukowski of FHWA or Bob McGennis of The South Central Superpave Center.