Influence of aggregate type and gradation on critical voids in the mineral aggregate in asphalt paving mixtures

Abstract

The implementation of Superpave has led to concerns with volumetric mix design, in particular, the minimum voids in the mineral aggregate (VMA) requirements which are based exclusively on nominal maximum aggregate size (NMAS). Achieving the minimum VMA requirement is one of the most dif^cult tasks in Superpave volumetric mix design. Under current specifications, many otherwise sound mixtures are subject to rejection solely on the basis of failing to meet the VMA requirement. The goal of this research was to validate the existing VMA criterion and to see if including additional aggregate factors would improve it. The work was accomplished in three phases: a literature review; extensive laboratory testing; and statistical analysis of test results. The available literature on the development of the minimum VMA criterion is sketchy; the relationship was originally presented without supporting research or data and the suggestion that it would be modified with experience and test data. The literature review also suggested that the tria.xial test was the preferred laboratory test for identifying when a mixture transitions from sound to unsound behavior, i.e., becomes plastic. The laboratory testing involved triaxial testing with the Nottingham Asphalt Tester of 36 mixes with different aggregate properties. ANOVA and linear regression was used to examine the effects of identified aggregate factors on critical state transitions in asphalt paving mixtures and to develop predictive equations. The results clearly demonstrate that the volumetric conditions of an asphalt mixture at the stable/unstable threshold are influenced by a composite measure of the maximum aggregate size and gradation and by aggregate shape and texture. The currently defined VMA criterion, while significant, is seen to be insufficient, by itself, to correctly differentiate sound from unsound mixtures. Based on the laboratory data and statistical analysis, a new paradigm to volumetric mix design is proposed that explicitly accounts for several aggregate factors (gradation, shape, and texture) in predicting the critical VMA of an asphalt paving mixture.