Balanced Mix Design Report for Lufkin's Crack Attenuating Mix (CAM) (5-5598-01-1)

Crack Attenuating Mix (CAM) designs are made according to the volumetric procedure defined in Special Specification 3109 (now 3165) where the optimal asphalt content (OAC) is computed as that which achieves 98 percent of the maximum density at 50 gyrations of Superpave Gyratory Compactor. Samples at the proposed OAC are then molded to 93 percent density and then subjected to performance testing in the Hamburg Wheel tracker and Overlay tester. A design was performed for the CAM mix to be placed on BUS 59 in Lufkin and using the specified procedure an optimum asphalt content of 8.3 percent PG76-22 was determined. This material was placed in the summer of 2008. In this study an alternative mix design procedure was evaluated. In the balanced mix design procedure the performance tests are first run at several different asphalt contents, and an acceptable range of asphalt contents is identified where both the rutting and cracking criteria are met. An optimum asphalt content is then defined within the acceptable range, and volumetric checks are then made as the final step. Using this procedure the acceptable range of asphalt contents was defined to be between 7.0 and 8.0 percent (the upper limit tested in the balance mix design procedure). An optimum of 7.5 percent PG76-22 was proposed with the balanced mix design approach, substantially lower than that found with the current volumetric procedure. No acceptable design could be achieved with a PG70-22 binder.