Gradation effects on hot-mix asphalt performance

The effect of gradation on hot-mix asphalt (HMA) performance has long been a contentious issue. One objective of the National Pooled Fund Study No. 176 was to evaluate the impact of gradation on the rutting performance of HMA. To this end, 21 Superpave mixtures were designed that used a range of materials, nominal maximum size aggregates, and gradations typical of those used throughout the United States. Tests that included both laboratory and prototype-scale loading were used to evaluate the permanent deformation characteristics of the mixtures. Analysis of the data revealed that adequate performance could be obtained with mixture gradations plotting above (ARZ), through (TRZ), and below (BRZ) the restricted zone. Laboratory tests suggested that ARZ and TRZ gradations might provide better deformation resistance than BRZ gradations. However, prototype-scale accelerated pavement testing did not show any clear trends in performance relative to gradation alone with respect to the restricted zone. This means that the restricted zone alone is not adequate to characterize gradation to ensure acceptable rutting performance and should therefore be omitted from Superpave specifications.