Difficult Nature of Minimum Voids in the Mineral Aggregate: Historical Perspective

Several researchers have recently discussed problems in achieving the minimum voids in the mineral aggregate (VMA) requirements specified in Superpave volumetric mix design. The minimum VMA requirements are often difficult to achieve; they can “fail” mixes that have acceptable performance records and they may require a higher asphalt content, leading to higher project costs. To address these issues, the background and logic behind the minimum VMA requirement are reexamined. A review of asphalt mix design methods indicates that a minimum VMA requirement was not used until the 1950s, when Norman W. McLeod almost single-handedly advanced VMA to a critical design parameter. In 1962, the Asphalt Institute specified VMA as a Marshall mix design requirement, but as late as 1985 it was still misused by many highway agencies. Only through the Strategic Highway Research Program and the implementation of Superpave has the minimum VMA requirement become a design standard. The literature review uncovered several problems with specifying minimum VMA: (a) the field data from which the minimum VMA requirements were established could not be found in the literature, (b) the original VMA requirements were based on Marshall compaction, and (c) the precision of the testing necessary for measuring VMA is not good enough for rigid enforcement of VMA specifications. In light of the information presented, it appears clear that the minimum VMA requirements need to be reexamined in light of Superpave and validated with field data.