Effect of Fine Aggregate Angularity on Compaction and Shearing Resistance of Asphalt Mixtures

The effect of fine aggregate angularity (FAA) on the densification characteristics of asphalt mixtures was evaluated with the Superpave^® gyratory compactor (SGC). Aggregates from three different sources used in production of Superpave mixtures in Wisconsin were studied. From each source, a fine gradation and a coarse-shaped gradation were included. To vary the FAA values of the fine aggregates, the proportions of the manufactured sand to the natural sand from each source were varied and the same overall gradations were maintained. A range of FAA values between 40 and 48 for the sources was achieved. All other mixture variables were kept constant, and the mixtures were compacted in an SGC equipped with a special device called a gyratory load plate assembly to measure the densification and the shear force required for compaction of each mixture. Densification data were analyzed to estimate the effect of the FAA on densification to 92% of theoretical maximum density (G_mm), which is assumed to represent resistance of the mixture to construction compaction, and to densification above 92% G_mm, which is assumed to represent mixture resistance to traffic. Results indicate a consistent trend of higher resistance to compaction with higher FAA values. Mixtures with higher FAA require higher compaction effort, as measured by densification rate and shear force required for gyrating. Resistance to densification above 92% G_mm, which is assumed to represent traffic, shows inconsistent trends. For one source of aggregates, the increase in FAA resulted in less resistance to densification and shear distortion above 92% G_mm. Results indicate that the sensitivity of mixture response variables to change in FAA values is highly dependent on the source of the aggregates and on gradation. Therefore, it is recommended that mixture design be based on limits of densification characteristics instead of a target FAA value for all mixture types.