Selection of optimum gravel aggregate size to resist permanent deformation in hot-mix asphalt

Siliceous river gravels form a significant fraction of the different types of aggregates used in the construction of hot-mix asphalt (HMA) pavements. Even after gravel particles are crushed into smaller-size fractions, a significant proportion of the larger-size fractions retain their original shape and surface texture. As a result, HMA with a larger nominal maximum aggregate size (NMAS) has a higher percentage of rounded and low-angularity particles, which might make it more susceptible to rutting. Using a smaller NMAS may decrease the percentage of rounded and angular aggregates in the mix, but potentially at the expense of loss in shear strength of the mix. This study evaluated the effect of increasing aggregate angularity by decreasing NMAS on the rutting potential of HMA mixes containing siliceous gravels. Nine mix designs composed of three different NMASs and siliceous aggregates from three different sources were included in this study. The percentage of rounded and angular aggregates in each mix was quantified using image analysis and crushed-face count The rutting potential of HMA mixes was assessed on the basis of Hamburg, dynamic modulus, flow time, and flow number tests. Results verify that a decrease in NMAS results in a decrease in the percentage of rounded and angular aggregates. However, rutting performance improves initially with a decrease in NMAS but then decreases; this indicates an optimum size that maximizes the rut resistance of HMA with siliceous aggregates. Results from this study are especially useful in selecting an optimum aggregate size for HMA mixes in locations where river gravels are the main source of locally available aggregates.