Conceptualizing the asphalt film thickness to investigate the Superpave VMA criteria

Abstract

Asphalt binder film thickness (FTb) is the key factor that is responsible for durability of asphalt mixtures. Mixtures with coarse aggregate gradations have difficulty meeting the Superpave minimum voids in mineral aggregate (VMA) criteria even though they tend to have thick asphalt films. In this study, the concept of asphalt binder film thickness (FTb) was used to investigate the Superpave VMA criteria. Superpave aggregate gradations of three nominal maximum aggregate sizes (NMAS): 9.5, 12.5 and 19.0 mm, were used. Aggregate gradations passing above, below, crossover through, humped, and through restricted zone were all considered. Superpave Gyratory Compactor test data of 126 compacted asphalt mixtures were used in the study. The current Superpave VMA criteria relate the mixture durability with VMA and set the same VMA value for mixtures with the same NMAS regardless of other parameters. However, a poor relationship was found between VMA and FTb (a durability measure) with an R2 ≅ 0.01, and yet a relatively good relationship (R2 ≅ 0.38) was found between voids filled with asphalt (VFA) and FTb although the VFA volumetric phase is part of the VMA volumetric phase in the mixture. This result was justified by the poor relationship between VFA and VMA (R2 ≅ 0.13). Despite that the effective binder content (Pbe) as a volumetric phase represents the VFA in the mixture, the relationship between FTb and Pbe was found to be more significant (higher R2) than the relationship between FTb and VFA. In this study, asphalt mixtures that did fail the Superpave VMA criteria in some cases had adequate asphalt FTb, and mixtures that passed the criteria did not necessarily have adequate FTb. In conclusion, although the current Superpave VMA criteria are significant, findings of this study support the tendency to modify the current criteria.