The effect of fine aggregate angularity, asphalt content and performance graded asphalts on hot mix asphalt performance

Abstract

The Wisconsin Department of Transportation (WisDOT) has implemented the Superpave recommendations in the 2000 Supplemental Specifications. This research was directed towards better understanding of the influence of angularity of fine aggregates, asphalt content, and performance grade of asphalts on hot mix asphalt (HMA) performance. Aggregates from four sources across the state of Wisconsin were used in this study. Each source provided two blends that vary in gradation: (1) a fine blend and (2) an S-shaped blend. Testing was accomplished using the Superpave gyratory compactor (SGC) and a device that fits inside of the SGC known as the gyratory load plate assembly (GLPA). Mixture densification and performance indices were calculated based on the data obtained from both the SGC and GLPA. Two indices reported data relating to the construction compaction phase of the gyratory compaction (initial stage) and two indices related to the traffic densification stage (later stage). Volumetric properties such as %Gmm @ Ninit, %Gmm @ Ndes, %Gmm @ Nmax, and VMA were also reported. The results of this study do not support the assumption that higher values of FAA would always result in better performing mixtures. The findings show the effect of FAA to be highly dependent on the source of the aggregates and their gradation. For certain blends of aggregates the laboratory compaction data show adverse effect of angularity on mixture resistance to shearing. It is therefore difficult to suggest a limit on FAA independent of the source or gradation that would improve quality of asphalt mixtures. The results of this study confirm earlier findings that varying the asphalt content has an important influence on critical properties of HMA mixtures, which include volumetric and frictional resistance of mixtures. The effect is found to be highly dependent on the source of the aggregate. The effect of asphalt content is found to be more important during the initial stages of compaction compared to the final stages for the majority of the blends tested in this study. The results indicate that the new parameters selected (CEI and TEI) are sensitive to asphalt content and follow the known trend of changes. Because of this sensitivity, they show the potential of complimenting the volumetric properties in selecting a mixture design that would result in good performance. The results of this study indicate that varying the performance grade of the asphalt has an important influence on certain critical properties of HMA mixtures. The effect is found to be highly dependent on the source of the aggregate. The effect of the performance grade is found to be more important during the final stages compared to the initial stages of compaction for the majority of the blends tested in this study. Because of the lack of trends between FAA values and the indices measured in this study, it is recommended that actual testing of the mixture with the selected asphalt be conducted and that the selection be based on the results of CEI, TEI, and volumetrics.