United States Department of Transportation
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Edition:Technical report; Sept. 2008-Aug. 2009.
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Abstract: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.
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