Laboratory Optimization of Asphalt Concrete Intermediate Course Mixes To Improve Flexible Pavement Performance

It has long been known that the use of carefully designed asphalt concrete mixes can increase the rutting resistance and structural capacity of flexible pavements. Unfortunately, optimization of asphalt mixes for intermediate pavement courses has been inhibited in the past by the use of wide limits in the aggregate gradation. This has sometimes led to insufficient amounts of top-size and fine aggregate, suboptimal packing density, unnecessarily high asphalt cement contents, and a resultant lack of stone-on-stone contact and mix stability. In addition, accurate laboratory evaluation of intermediate course mixes has been hampered by the use of traditional 10-cm (4-in.) specimen molds, which are too small to yield representative test results for some mixes with 2.5-cm (1-in.) aggregate. This study consisted of laboratory evaluation of 10 asphalt concrete intermediate course trial mixes, with five different dense gradations for two aggregate types: natural gravel (rounded) and crushed limestone (angular). The five gradations (nominal maximum aggregate size, 2.5 cm) were developed following an overall volumetric concept as used in the design of portland cement concretes. Test specimens were prepared with 15-cm (6-in.) molds. Specimens prepared at the determined optimum asphalt cement contents were tested for modulus of resilience, indirect tensile strength, indirect tensile creep, unconfined compressive strength, and resistance to moisture-induced damage. Mixes based on three of the evaluated gradations consistently showed superior performance to traditional mixes. Recommended mix design criteria are provided to assist in optimization of intermediate course mixes.