Investigation into Locking Point of Asphalt Mixtures Utilizing Superpave and Marshall Compactors

Locking point is an important concept in selecting a proper aggregate gradation in the asphalt mixture design. The locking point can be defined as the moment during asphalt mixture compaction at which an aggregate structure becomes stable. Beyond this point, further compaction does not contribute much to the increase in mixture density and can even damage aggregate particles. This study employed two compaction methods, a Superpave Gyratory Compactor and a Marshall Compactor, to investigate the locking points of different asphalt mixtures. For the Marshall compaction method, an accelerometer was attached to the Marshall Hammer to record the response of the asphalt mixtures. For the Superpave gyratory compaction, changes in the height and density of the compacted samples were utilized to determine locking points. A total of ten mixtures was designed and evaluated. All the mixtures were prepared with the same asphalt binder PG 64-22 and aggregates. The aggregates were prescreened by different sieves to control variability in gradation. The results of this study show that it is feasible to determine the locking point for most of the mixtures with an accelerometer. The one exception was the largest aggregate, which experienced particle breakage during compaction. The locking point was found to be dependent on the full range of aggregate sizes of a mixture. Stone mastic asphalt (SMA) mixtures with similar gradation in fine particles showed less variability in locking point. Dense-graded mixtures with smaller maximum aggregate size exhibited lower locking points. These results can be attributed to their higher asphalt content, which served as a lubricant. The 101.6-mm (4-in.) Marshall mold was not adequate for determining the locking point for the mixtures with particles larger than 25.4 mm (1 in.).