Comparison of Superpave and Marshall Mix Performance in Alabama (05-1980)**
Donald E. Watson, National Center for Asphalt Technology
Elton Ray Brown, U.S. Army Engineer Research and Development Center
Jason Roox Moore, Auburn University

The Alabama Department of Transportation specifies the Superpave design system for the majority of its dense-graded HMA mixes. However, there is concern that the number of design gyrations (NDesign) may be too high for specified traffic levels. Mixes designed with an NDesign level too high would be rut resistant, but may be difficult to compact in the field and may suffer from durability problems such as premature cracking and raveling. The objective of this project is to evaluate the performance of Alabama’s mixes designed using Superpave criteria and compare them to the performance of Alabama’s Marshall designed mixes to determine if current NDesign levels should be adjusted. The following conclusions are based on an evaluation of 25 Marshall and Superpave projects placed about the same time and serving under approximately the same traffic conditions. 1. Both Marshall and Superpave mixtures are generally performing quite well with little rutting and cracking after a period of about four years. 2. It is likely that most Marshall and Superpave mixtures will not reach the design air voids of 4.0 percent during the life of the pavement. After four years, the average air voids measured in the wheelpaths was 5.3 percent for Marshall mixes and 5.9 percent for Superpave mixtures. 3. It appears the durability of Superpave mixtures can be improved by increasing the asphalt content without adversely affecting the rutting resistance. However, practitioners are cautioned to closely monitor rutting indicators during mixture design, production, and construction. Keywords: Marshall, Superpave, rutting, cracking, gyratory