Mineral aggregates make up between 80% and 90% of the total volume or 94% to 95% of the mass of hot mix asphalt (HMA). For this reason, it is important to maximizethe quality of the mineral aggregates to ensure the proper performance of our nation’s roadways. The quality of mineral aggregates for road-paving materials has generally been specified by the toughness, soundness (durability), cleanliness, particle shape, angularity, surface texture, and absorption. The Superpave® mix design method is a product of the Strategic Highway Research Program (SHRP). Research to investigate aggregate’s contribution to pavement performance was intentionally excluded from the SHRP Asphalt Research Program. Instead, the aggregate gradations and physical properties included in the Superpave mix design http://gulliver.trb.org/publications/nchrp/nchrp_rpt_539.pdfmethod were developed through the use of a modified Delphi approach (1).* The Delphi process is designed to ascertain the consensus of a group of experts while avoiding some of the negative aspects of group dynamics. The final results of the modified Delphi process used by SHRP included aggregate properties, test methods to determine those properties, and specification criteria. Gradation limits were included as part of the aggregate properties. The new gradation limits included definitions for nominal maximum and maximum aggregate size, control points for various nominal maximum aggregate sizes, and the restricted zone. The remaining aggregate physical properties were divided into two categories: consensus and source. The consensus properties—including coarse aggregate angularity, flat and elongated particles, fine aggregate angularity, and sand equivalent—were chosen to ensure the aggregate quality was sufficient to provide This report may be accessed by Internet users at satisfactory HMA performance for the design traffic level. Specification values were to be uniform throughout the United States without regard for locally available materials. The specification values for the source properties, including LA abrasion, sulfate soundness, and deleterious materials, were to be set by the agency. This was done to allow for variances in locally available materials. Finally, the modified Delphi process identified volumetric properties of the resulting HMA mix including air voids, voids in mineral aggregate (VMA), voids filled with asphalt, and dust-to-asphalt proportion. The aggregate bulk specific gravity is required to calculate VMA, and the aggregate fines content is required to calculate dust-to-asphalt proportion. (Author/publisher)
Samenvatting