Influence of Aggregate Gradation on the Compactability of Asphalt Mixtures Utilizing Locking Point

Aggregate gradation is crucial for the performance of asphalt mixtures. However, the adequate mix design must include the compromise between performance and workability. This study used two compaction methods: the impact (Marshall Hammer, Humboldt, Elgin, Illinois) and gyratory [Superpave Gyratory Compactor (SGC), Pine Instrument Company, Grove City, Pennsylania], to investigate the influence of aggregate gradation on the compaction of asphalt mixture utilizing the concept of the locking point. For the impact compaction, an accelerometer was placed on the falling mass of the Marshall Hammer to record the asphalt mix response, with which the impact locking point was defined. For the gyratory compaction, the densification curves were utilized to define the locking point. Twelve mixes, three for each mixture type (base, intermediate, and surface) were designed and prepared in the laboratory. The asphalt mixes were tested for their locking points in relation to their type and gradation. Results indicated that the locking point could be determined for the majority of the mixtures compacted by both the impact and gyration methods, except for the mixture composed of the bigger stone and coarser gradation. The locking point of the asphalt mixture was strongly dependent on its gradation. A coarser mix resulted in a higher locking point. Thus, the maximum aggregate size was not the only factor that determined the locking point but the whole gradation.