Effect of Aggregate Structure on Rutting Potential of Dense-Graded Asphalt Mixtures

Superpave ® mix design, as the name suggests, was developed to provide superior-performing pavements for the transportation industry. However, researchers and designers in their quest to provide such mixtures have encountered problems with some design specifications, such as the restricted zone. Superpave recommends that designers avoid grading through the restricted zone to eliminate tender mixes and gradations that may be too close to the maximum density line. But studies of the performance of the restricted zone have indicated that dense-graded mixtures that violate this requirement have provided good performance. It has also been reported that mixtures that are graded below the restricted zone (BRZ) have poorer rutting performance than those that are graded above the restricted zone (ARZ) or through the restricted zone. It is also known that the minimum voids in mineral aggregates (VMA) requirement, which was established to ensure durability, is very difficult to achieve. It also leads to overasphalting in BRZ mixtures and compromises their resistance to rutting. The effect of aggregate structures on rutting performance of mixtures is examined. Seven limestone and three granite mixtures were used. Six were BRZ mixtures and four were ARZ mixtures. After the behavior of the mixtures was investigated, it was concluded that BRZ mixtures developed different aggregate structures than ARZ mixtures and that the performance of mixtures depended on their aggregate structures. It was observed that the high VMA requirement caused overasphalting in BRZ mixtures, hence their dismal rutting performance compared with ARZ mixtures.