Evaluation of Superpave Gyratory Compaction of Hot Mix Asphalt

The effect of restricted zone on volumetric properties of mixes consisting of all crushed, and all crushed and partially uncrushed, materials was evaluated. For a given aggregate blend, gradations below or above the restricted zone provided higher voids in mineral aggregate (VMA) than mixes through the restricted zone. Mixes with crushed aggregate provided higher VMA than mixes with partially crushed aggregate. It is recommended that further work be conducted to evaluate the effect of different types and shapes of aggregates on the volumetric properties of specimens compacted with the Superpave gyratory compactor. According to the current Superpave mix design system, the bulk specific gravity of a compacted specimen at any gyration is back calculated from the bulk specific gravity determined at N_maximum and a correction factor determined at N_maximum. This procedure assumes that the correction factor is constant at all gyrations. A part of this study was carried out to compare the correction factors obtained at different gyration levels during compaction of hot mix asphalt and to evaluate the change in correction factor with gyration levels. Specimens were compacted at different gyration levels, and the bulk specific gravity of each was determined at each gyration level. Bulk specific gravities at each of the gyration levels were also obtained by back calculation from bulk specific gravity at N_maximum. The correction factor was found to decrease and become close to constant at higher gyration levels. At lower gyrations, densities were found to be greater than those obtained by back calculation. It is recommended that mixes be compacted to N_design for determination of design asphalt content.