Aggregate orientation and segregation in laboratory-compacted asphalt samples

Different laboratory compaction methods can produce volumetrically identical asphalt mixture specimens but with widely varying mechanical properties. Provided that the mixture design is constant, variations in mechanical properties are probably due to differences in the structure of the aggregate-bitumen matrix. The objective of this study is to investigate the structure of the internal aggregate-bitumen matrix created by different laboratory compaction methods and compare it with mechanical performance. Three types of laboratory compaction are considered: gyratory, vibratory, and slab. Image analysis techniques have been used to provide quantitative information on the orientation and distribution of aggregates on horizontal planes within asphalt mixture specimens, and the results indicate that circumferential alignment of aggregate particles occurs in gyratory and vibratory compacted specimens. This behavior is more pronounced for larger aggregate particles and in those with an aspect ratio (maximum length/maximum width) greater than two. Slab-compacted specimens display a more random particle orientation. The distribution and segregation of aggregates has been considered relative to the center of each horizontal asphalt-specimen cross section. While overall levels of aggregate particle density are similar across all compaction methods considered, greater segregation occurs in vibratory- and gyratory-compacted specimens. Repeated load axial testing was done on specimens compacted by each method. The results indicate higher resistance to deformation in the vibratory- and gyratory-compacted samples than in volumetrically identical slab-compacted samples.