Evaluation of aggregate type, gradation, and other volumetric properties for design and acceptance of durable Superpave mixtures

The volumetric properties of Hot Mix Asphalt (HMA) mixtures including, asphalt content, voids in the mineral aggregates (VMA), and the voids filled with asphalt (VFA) have been identified as important parameters for durability and performance. However, the VMA is considered the most important parameter and is used in the Superpave mixture design specifications to disqualify mixtures from being used.; This research was designed to evaluate the effects of these properties on the performance of HMA. Fine-graded and coarse-graded mixtures were produced at multiple VMA levels by varying the gradations for a common set of aggregates and asphalt cement. Some mixtures were purposely designed not to meet the Superpave VMA requirements, while meeting all other Superpave requirements. The mixtures were tested for durability and performance by measuring their fracture resistance and rutting resistance after both short-term and long-term oven aging, by using the Superpave IDT test and Servopac gyratory testing machine respectively. Permeability tests were also performed on all mixtures to get an indication of void structure. The asphalt binder was extracted and recovered for testing to determine the effect of different mixture characteristics on the rate of binder age-hardening as measured by changes in the binder properties.; Results showed that the rate of binder hardening in mixtures was not related to either the VMA or film thickness as currently determined, but is almost entirely controlled by the percentage of fine aggregates in the mixture. Therefore, mixture gradation appears (coarse-graded and fine-graded) appears to have a stronger influence on durability than either VMA or film thickness as conventionally determined. This was explained by the effect of gradation on aggregate structure. A modified procedure was presented to calculate effective film thickness and effective VMA, which appeared to be more relevant parameters for the purpose of evaluating durability and performance irrespective of the aggregate type used. Other factors, such as gradation and void structure were identified as characteristics that relate more to the mixture performance.