Gradation Chart for Asphalt Mixes: Development

Abstract

The gradation chart approach is based on the two parameters of Fuller’s model (i.e., maximum size diameter and shape factor) which are obtained by fitting well-graded structures like those of Superpave or Covenin. ASTM mathematical definitions for gravel and sand, and Fuller’s model are used to develop an analytical expression for a gravel-to-sand ratio used here to transform gradation into a numerical that may be correlated to HMA performance parameters. The gradation chart is a Cartesian plot conformed by shape factor and maximum size diameter and thus gradations, specification bands, gravel-to-sand ratios, and response parameters can be represented. Experimental results from recent research works performed by the National Center for Asphalt Technology on the mechanical and hydraulic performance of Superpave mixes are analyzed here. Based on these analyses, the writer resolved the controversy between below the restricted zone and above the restricted zone behavior, and found evidence that gradation specification bands (based on structure and maximum size diameter) would make no sense. Instead, the writer promotes “free design” in which a designer uses available aggregate, supported by a gradation-chart approach, to meet HMA performance parameters required by pavement design.