Volumetric Mix Design Adjustments for Improving Asphalt Mixture Durability

State highway agencies (SHAs) are looking for guidance on how they can make changes to their Superpave volumetric mix design procedures to obtain higher design binder contents to improve the cracking resistance and durability of asphalt mixtures. To assist SHAs, this paper was prepared to: (a) synthesize most common adjustments made by SHAs to their Superpave volumetric mix design procedures to obtain higher design binder contents; and (b) verify the effectiveness of these adjustments through laboratory experiments. Based on a survey of SHAs, the three most common adjustments to the Superpave mix design system to obtain higher design binder contents include: lowered design gyrations (Ndesign), increased minimum requirements for voids in mineral aggregate (VMA), and lowered design air voids (including air voids regression). These adjustments were verified in a laboratory testing program which included a virgin fine-graded mixture and a 20% reclaimed asphalt pavement (RAP) coarse-graded mixture. The results suggest that lowering Ndesign may help increase the design binder content initially but mix designers can select design gradations to reduce VMA and binder content back to levels achieved with the higher Ndesign levels. The other common adjustments (i.e., increased minimum VMA and lowered design air voids) can yield higher design binder contents. However, these approaches may be ineffective if Gsb is not accurately measured during mix design or if VMA is not controlled during mix production.