Impact of gradation relative to Superpave restricted zone on hot-mix asphalt performance

Significant controversy has revolved around the Superpave gradation specifications, in particular the restricted zone, since the completion of the Strategic Highway Research Program (SHRP). Historically, dense-graded mixtures that encroach on the restricted zone had provided good performance prior to SHRP. However, current Superpave guidelines recommend that gradations passing through the restricted zone not be used. A synopsis of recent research related specifically to the impact of the Superpave restricted zone on performance of hot-mix asphalt (HMA) is provided. The evolution and purpose of the Superpave restricted zone are presented, along with findings of both recently completed and ongoing research. Studies involving laboratory and full-scale accelerated performance tests of mixtures with gradations plotting above (ARZ), through (TRZ), and below the restricted zone were considered. The research reviewed clearly suggests that good performance can be achieved with fine-graded (ARZ and TRZ) mixtures and that no relationship exists between the Superpave restricted zone and HMA rutting or fatigue performance. It is suggested on the basis of this research that the restricted zone recommendation be eliminated from the Superpave volumetric mixture design specifications.