Influence of aggregates angularity on the locking point of asphalt mixtures

Abstract

Aggregate angularity plays a critical role in the performance of asphalt mixtures. Current asphalt mix design methods do not specify compaction effort according to aggregate angularity or locking points. This study utilised two methods of compaction, gyratory (Superpave Gyratory Compactor) and impact (Marshall Hammer), to investigate the influence of aggregate angularity on the locking point of asphalt mixtures. For gyratory compaction, densification curves were used to determine the gyratory locking point. For impact compaction, an accelerometer was connected to the Marshall Hammer to register asphalt response, by which the impact locking point was determined. Five mixtures were designed and produced in the laboratory with different aggregate angularities. The mixtures were tested for their locking points and then related to aggregate angularities. Results show that locking point could be determined for most of the mixtures in the study by the gyration and impact methods, except for the mixture composed of uncrushed, round aggregates compacted with the Marshall Hammer. The locking point of asphalt mixtures was highly related to aggregate angularity. A higher aggregate angularity resulted in a lower locking point. Mixtures produced with only crushed aggregates showed locking points much lower than mixtures made with both crushed and uncrushed aggregates, while mixtures made with only uncrushed aggregates had much higher locking points than those made with both aggregates. The findings from this study indicate that compaction effort in asphalt mix design should be specified according to the locking point of the mixtures.

Keywords:

• locking point
• angularity
• asphalt mix

Additional information

Funding

This work was supported by Tennessee Department of Transportation.