Critical Review of Voids in Mineral Aggregate Requirements in Superpave

Reports of increased difficulties in meeting the minimum voids in mineral aggregate (VMA) requirements have surfaced with the recent use of Superpave volumetric mix design. The low VMA of Superpave mixes generally can be contributed to the increased compactive effort by the Superpave gyratory compactor. This has led to the increased use of coarser asphalt mixes (gradations near the lower control points). However, the minimum VMA requirements in Superpave volumetric mix design for these coarse mixes are the same as those developed for the dense mixes designed by the Marshall method. Literature review has indicated that the rationale behind the minimum VMA requirement was to incorporate at least a minimum permissible asphalt content into the mix to ensure its durability. Studies have shown that asphalt mix durability is directly related to asphalt film thickness. Therefore, the minimum VMA should be based on the minimum desirable asphalt film thickness instead of on a minimum asphalt content because the latter will be different for mixes with different gradations. Mixes with coarse gradation (and, therefore, a low surface area) have difficulty meeting the minimum VMA requirement based on minimum asphalt content despite thick asphalt films. A rational approach based on a minimum asphalt film thickness has been proposed and validated. The film thickness approach represents a more direct, equitable, and appropriate method of ensuring asphalt mix durability, and it encompasses various mix gradations.