The current Superpave method for designing hot-mix asphalt concrete consists of three phases: (1) materials selection; (2) aggregate blending; and (3) volumetric analysis of specimens compacted using the Superpave gyratory compactor. There is no strength test as has been used in the traditional Hveem mix design. A consensus has developed among paving technicians, engineers, and researchers that a performance-related test should be included in Superpave. Many tests have been proposed for use as a performance test, including Superpave shear tests (SST), wheel tracking tests, triaxial shear tests, and the field shear test.
Gyratory compaction is used for mix design and field control In the mix design process, percent voids should meet certain criteria at different numbers of gyrations in order for the aggregate blend and optimum asphalt cement to be acceptable. Mix verification and field control use the gyratory compactor to prepare specimens of plant-mixed materials to confirm that volumetric properties; i.e., air voids, voids in mineral aggregate and voids filled with asphalt; meet the same criteria used to select the job mix formula during the laboratory phase of mix design (Cominsky 1994). Considering the routine laboratory use of the gyratory compactor, it would be favorable to extract more information from the mixture compaction process such as stability (shear strength) of the mix and its relation to performance.
In the gyratory compactor, the compaction process occurs under a vertical pressure at the top of the specimen and shear displacement induced by the gyratory movement of the compactor. The response of the mix to the compaction effort depends on its shear resistance, which is a function of many factors including aggregate properties (angularity, surface texture, gradation, and percent and types of fines), asphalt content, and asphalt grade. The mix response to compaction can be quantified by different measures. The direct measure is the change of density as a function of number of gyrations. Other measures include the energy required to compact the specimen, the shear resistance to the gyratory movement, and the change in the internal structure of the mix during compaction. These measures are believed to be related to performance but this relationship has not be quantified