Evaluation of Aggregate Size Characteristics in Stone Matrix Asphalt and Superpave Mixtures

With any hot-mix asphalt (HMA) pavement, the applied loads ultimately must be carried by the aggregate. In addition to meeting minimum aggregate quality requirements, the proper aggregate grading must be used if the HMA is to perform satisfactorily in service. The aggregate gradation is important for ensuring that the proper aggregate structure and mixture volumetric properties are achieved. The findings of an evaluation of aggregate size characteristics—namely, nominal maximum aggregate size (NMAS) and gradation—in stone matrix asphalt (SMA) and Superpave mixtures are described here. The study objectives were twofold. The first objective was to verify that the SMA mix design procedure developed for the NCHRP 9-8 project was applicable to NMASs other than 19 mm. Second, mixture properties of SMA and Superpave mixes with the same NMASs were compared. Mixture volumetric properties, wheel tracking tests, and permeability tests were used to make comparisons. The test data indicated that the NCHRP 9-8 mix design approach for SMA mixes was indeed appropriate for various NMASs. Although differences in the wheel tracking results of the SMA and Superpave mixes were observed, none of the mixes experienced excessive permanent deformation. Permeability results for the SMA mixes indicated that, for a given NMAS, mixes with a smaller breakpoint sieve were less permeable. The SMA mixes were relatively more permeable than the Superpave mixes. Overall, the Superpave mix gradations above the restricted zone exhibited the least amount of rutting and were the most impermeable.