Determining NDesign for SMA mixtures in Alabama: final report.
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Determining NDesign for SMA mixtures in Alabama: final report.

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      Final report
    • Abstract:
      Stone Matrix Asphalt (SMA) mixtures have performed well in Alabama and many other states and countries. The mix design procedure used in Alabama has been based on the 50 blow Marshall hammer compactive effort. This design method was adapted from the practices of several European countries that are credited with development of SMA. Earlier research at NCAT attempted to correlate the 50 blow Marshall hammer compactive effort to compaction in the Superpave Gyratory Compactor (SGC). The finding of that study was the 78 gyrations, on average, would provide the same density as the Marshall hammer. However, there was a significant amount of scatter in the correlation. Later, when the SMA mix design procedure was balloted by AASHTO, the experience of one state was influential in setting the standard design gyrations at 100 gyrations. A footnote in the AASHTO procedure permits the use of 75 gyrations for aggregates having Los Angeles Abrasion values greater than 30. According to several mix designers in Alabama and other states, 100 gyrations significantly over compacts their SMA mixes even with high quality aggregates. The mix designers generally find that meeting the VMA requirement is not possible because the gyratory compactor continues to grind the aggregates past the point of stone-on-stone contact. The objective of this study was to determine an equivalent compactive effort with the SGC to match the 50 blow Marshall hammer using aggregates and mix designs common in Alabama. To accomplish this objective, SMA mix designs were prepared with four aggregate types and two maximum aggregate size (MAS) gradations. Optimum asphalt contents and Voids in the Mineral Aggregate (VMA) from the Marshall mix designs were compared to the mix designs performed using 50, 75, and 100 gyrations in the SGC. To evaluate the potential of over compaction in the SGC, comparisons of aggregate breakdown from each of the compactive efforts were analyzed. To assure that the mixtures achieved good stone-on-stone contact, laboratory rutting tests were conducted on each of the mix designs and the Locking Point concept was examined. The results indicate for the small MAS gradation, 88 gyrations in the SGC generally provided the same optimum asphalt content and VMA as the Marshall procedure. For the larger MAS gradation, 58 gyrations, on average, matched the Marshall procedure. Considering all of the data together, 70 gyrations would, on average, match the Marshall hammer compaction. Aggregate breakdown was slightly less with the SGC compared to the Marshall hammer. The laboratory rut tests in the APA indicated that some 50 gyration mixtures may have a problem. Locking Point analysis indicated that stone-on-stone interlock generally occurred around 63 gyrations. Further testing and analysis with several plant produced SMA mixtures provided results which indicated slightly lower gyrations. On average, the field mixtures required 63 gyrations to match the density from the Marshall hammer. Locking Point for the field mixes averaged 57 gyrations. Analysis of the aggregate breakdown for the plant produced mixtures showed that compaction in the SGC caused less breakdown than compaction with the Marshall hammer. As with the laboratory prepared mix designs, slightly more breakdown was evident with increasing gyrations. All of the samples made with the field mixtures performed well in the APA tests. Based on the results and the lab and field mixes, 70 gyrations with the SGC are recommended to replace the 50 blow Marshall hammer for SMA mix design in Alabama.
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