Most departments of transportation, including Indiana, currently use the Superpave mixture design method to design asphalt mixtures. This method specifies that the optimum asphalt content for a given gradation be selected at 4 percent air voids. During construction, these mixtures are typically compacted to 7-8 percent air voids. If mixtures were designed to be more compactable in the field they could be compacted to the same density as the laboratory mixture design, which would increase pavement durability by decreasing the in-place air voids. The objective of this research was to optimize the asphalt mixture design in order to increase in-place asphalt pavement durability without sacrificing the permanent deformation characteristics of the mixture.

Three asphalt mixtures were designed using the standard Superpave design method at 100 gyrations of the Superpave Gyratory Compactor, suitable for traffic levels of 3 to 30 million Equivalent Single Axle Loads. Each mixture was then used as a starting point to design three additional mixtures using 70, 50, and 30 gyrations, with optimum binder content chosen at 5 percent air voids, rather than the currently specified 4 percent. The effective asphalt content was held constant for the original and redesigned mixtures. Permanent deformation characteristics of the sets of four mixtures were determined by measuring the dynamic modulus and flow number. The results suggest that the mixture designs produced using 70, 50, and 30 gyrations had permanent deformation characteristics equal to or better than the original 100-gyration mixtures.

Based on the laboratory test results, two field trials were placed evaluate the design method, ease of construction and to compare the construction results of the re-designed and original mixtures. Samples from both projects were collected during construction, test specimens compacted, and additional physical testing completed. The field trial results suggest that it is possible to place a mixture at 5 percent air voids and that mixtures designed at 5 percent air voids should have equivalent performance to those designed at the conventional 4 percent air voids.

Report Number



hot-mix asphalt, mixture design, Superpave

SPR Number


Performing Organization

Joint Transportation Research Program

Publisher Place

West Lafayette, Indiana

Date of this Version