Effect of particle mobility on aggregate structure formation in asphalt mixtures

During compaction of asphalt mixtures, aggregate structure starts building up by proximity and direct contact of aggregates. In the previous studies, it has been shown that the aggregate structure directly affects the service performance. However, the mechanisms of the aggregate structure formation are not clearly understood. This study is focused on the mechanisms affecting aggregate mobility during compaction and the effect of material properties on the aggregate structure formation. At the initial stages of compaction, there is a relatively thick layer of mastic (i.e. mix of binder and filler) between aggregates, which allows for a shearing mobility in the mix, if the mastic viscosity is sufficiently low. However, as compaction proceeds, the mastic layer at proximity zone of aggregates becomes thinner due to high stress intensity and the higher viscosity of thin mastic film or the aggregates dry contact effect increases the shearing resistance against compaction (i.e. mix becomes locked). In this study, mixes are compacted at different temperatures using one base binder and three different modified binders. The quality of the aggregate structure and packing throughout the compaction is characterised using two-dimensional imaging of mixture sections and the total aggregate on aggregate proximity length is measured as an indication of the aggregate-packing level. It is shown that for mixtures to obtain the maximum packing, the compaction temperature should be picked based on mastic viscosity. The viscosity of mastic should be low enough for lubrication, but high enough to provide sufficient film thickness at proximity zones and prevent locking of mixture at the early stages of compaction.