Voids in the Mineral Aggregate (Vma) on the Mechanical Properties of Coarse and Fine Asphalt

VOIDS IN THE MINERAL AGGREGATE (VMA) ON THE MECHANICAL PROPERTIES OF COARSE AND FINE ASPHALT MIXTURES
R. Michael Anderson and Ross A. Bentsen

ABSTRACT

The percentage of voids in the mineral aggregate (VMA) has been a property proposed since the late 1950’s for use in hot-mix asphalt (HMA) mix design as an additional tool for evaluating the volumetric properties of asphalt mixtures. Prior to the Strategic Highway Research Program (SHRP), however, VMA was not extensively used as a critical mix design parameter. As late as 1985, less than a third of all state highway agencies were using VMA as a mix design property. Difficulties in achieving VMA in Superpave mixtures have led to several research studies. One such study by Kandhal et.al. argued that the VMA criteria should be different for coarse and fine asphalt mixtures. The belief was that Superpave VMA requirements unfairly penalized coarse mixtures with “low” VMA but adequate film thickness.
The purpose of this experiment was to evaluate the influence of changes in VMA on the performance-related properties of coarse and fine asphalt mixtures. Specifically, does increasing VMA have an effect on mixture properties at intermediate (fatigue cracking resistance) and high (rutting resistance) temperatures? Testing indicated that the majority of properties generated from intermediate temperature stiffness and fatigue tests, and their associated analyses, showed no statistically significant difference between the fatigue properties of a mixture with 13% VMA and a mixture with 15% VMA. There also was no statistically significant difference between the fatigue properties of a Coarse mixture and a Fine mixture with the same VMA.
High temperature testing indicated some statistically significant differences between different VMA mixtures and mixtures with different gradations (Coarse and Fine).
Finally, shear test data suggests that an increase from 13% to 15% VMA significantly improves the shear fatigue characteristics of the Fine mixture by 50%, while reducing the high temperature stiffness and rutting characteristics by no more than 30%. By contrast, increasing the VMA from 13% to 15% in the Coarse mixture appears detrimental to its performance properties. This result can support industry concerns that higher VMA in coarse mixtures may be unnecessary and may lead to poor performance. The Fine mixtures in this study exhibited less sensitivity to changes in VMA.