Paper presented at the 26th Annual Southern African Transport Conference 9 - 12 July 2007 "The challenges of implementing policy?", CSIR International Convention Centre, Pretoria, South Africa. ABSTRACT:The aggregate structure of the compacted mix is a determining factor for the performance of Hot-Mix Asphalt (HMA). In this paper, the grading characteristics of good and poor performing HMA mixes are explored using the concepts of the Bailey method and related techniques for determining the porosity of the Dominant Aggregate Size Range (DASR) and the permeability characteristics of the mix. The aim is to assess the potential benefits of these new gradation analysis techniques for HMA performance in South Africa. The aggregate gradation of past mix designs, for which the field performance is known, are analysed and discussed in the paper. Typical properties of the aggregate structures of good and poor performing medium continuously graded asphalt (ACM) mixes are identified. The primary differences between the grading of the studied good and poor performing mixes lie in the grading of the coarser aggregate. While the poor performing mixes are coarser according to the classical definition, the good performers have a more voluminous DASR. The paper concludes that the grading analysis concepts allow the design engineer new insights into the structure of aggregates. The principles can be used to develop more specific guidelines for aggregate structure selection.
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