A balanced HMA mix design procedure for overlays
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abstract
The purpose of this paper is to present a balanced hot-mix asphalt (HMA) mix design procedure for overlays, which considers both rutting and cracking requirements. The current Texas Department of Transportation (TxDOT) HMA mix design uses volumetric design method to select the optimum binder content and the Hamburg Wheel Tracking Test (HWTT) to ensure rutting and moisture resistance of the proposed mix. By contrast, the balanced HMA mix design procedure proposed in this paper includes adding the use of the Overlay Tester (OT) to also ensure adequate cracking resistance. To demonstrate the balanced mix design procedure, eleven common Texas HMA mixes were initially designed following the current TxDOT HMA mix design method. Most Dense-Graded mixes and Superpave mixes passed the HWTT rutting criterion, but as anticipated did poorly in the OT cracking test. The SMA mixes on the other hand passed both the HWTT rutting and the OT cracking criteria. Clearly, these results emphasize the need for a balanced mix design procedure particularly for the Dense-Graded and Superpave HMA mixes. Seven out of the 11 HMA mixes, failed to meet the rutting and cracking requirements, and were redesigned according to the proposed balanced mix design procedure. It was found that a balanced HMA mix can be designed provided that good quality aggregates are used. In general, the proposed balanced design required adding more asphalt compared to that determined by the current TxDOT design procedure. Finally, it is recommended that field experimental test sections be constructed to further validate the balanced mix design procedure proposed in the paper.
