Improving Mixture Durability Through Design Gyrations, Air Voids, and Binder Content

About the report:

This report describes a laboratory study that supports a larger and continuing effort to improve durability of Virginia’s dense-graded asphalt mixtures. This component of that larger effort explores the rutting and cracking resistance for a series of dense graded mixtures that represent different design gyration levels and binder contents. The rutting resistance was evaluated using the unconfined Flow Number test (also called unconfined Repeated Load Permanent Deformation test). The cracking resistance was evaluated using the indirect tension strength test. The matrix of trial mixtures was generated from four different Virginia “source” mixtures, all of which conformed to a Virginia Department of Transportation (VDOT) SM-9.5D designation.

The study found that additional binder affects the rutting resistance of the tested mixtures, but that change can be toward both the better and worse. The response appears to be related to VMA and gradation of the mixture. The cracking resistance was nearly universally improved with the addition of binder, although test results were highly variable for the improvement to be deemed statistically significant and results with the coarsest mixture demonstrated that it was possible to weaken a mixture with too much additional binder.

The study developed a series of recommended design criteria and recommends field trials to explore the response from multiple producers. These trials should be well documented and include a similar regimen of laboratory performance tests. As warranted by preliminary results from field trials, additional trials may be tested using the VDOT accelerated loading facility.

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