Performance Based Mix Design in Virginia DOT's Future

On August 31, State Materials Engineer Andy Babish formally introduced an effort that will change the mix design process in Virginia. For nearly two decades, Virginia has followed AASHTO’s SUPERPAVE Mix Design process. While never following the national specification, VDOT adopted much of the specification regarding aggregate qualities and gradations. Unlike other states that adjusted design gyrations based on project traffic loading, Virginia held to one design gyration level and adjusted the binder used in the mix. While an improvement in many regards, the SUPERPAVE mixes suffered from lower asphalt contents and premature cracking compared to the Marshall designed mixes.

 

Over the last three years, VDOT and VAA have worked together to improve cracking resistance by increasing the total asphalt content in SUPERPAVE mixes. However, these were just steps in a longer journey. The effort outlined by Babish is a paradigm shift. Instead of solely focusing on the inputs – AC content, gradations and volumetrics, the focus will be on the outcomes – mixture performance. Hence the name Performance-Based Mix Design. Designers will be optimizing mixes for performance in three main areas – cracking resistance, rutting resistance and overall durability.  While this effort begins in the lab, expectations are to link lab results to field performance in order to improve the specification thresholds.

 

More details on the three to five-year effort will be provided in future Asphalt News and Virginia Asphalt magazine articles. At the Virginia Asphalt Conference and Expo in December, Babish and Kevin McGhee, Associate Director-Pavements at VTRC, will be giving a more detailed presentation for attendees. In the Research breakout session, Stacey Diefenderfer will discuss what has been learned so far when using performance mix designs.