Comparison of Superpave and Marshall Mix Performance in Alabama

The Alabama Department of Transportation specifies the Superpave® design system for most 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 that is 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 according to Superpave criteria and compare them to the performance of Alabama's Marshall-designed mixes to determine whether 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: ( a) Generally, both Marshall and Superpave mixtures are performing quite well with little rutting and cracking after a period of about 4 years. ( b) It is likely that most Marshall and Superpave mixtures will not reach the design air voids of 4.0% during the life of the pavement. After 4 years, the average air voids measured in the wheelpaths was 5.3% for Marshall mixes and 5.9% for Superpave mixtures. ( c) It appears that the durability of Superpave mixtures can be improved, without adversely affecting the rutting resistance, by increasing the asphalt content. Practitioners, however, are cautioned to monitor rutting indicators closely during mixture design, production, and construction.