Evaluation of Two Compaction Levels for Designing Stone Matrix Asphalt (05-1655)**
Hongbin Xie, National Center for Asphalt Technology
Donald E. Watson, National Center for Asphalt Technology
Elton Ray Brown, U.S. Army Engineer Research and Development Center

Current Stone Matrix Asphalt (SMA) design guidelines list two compaction options to design SMA, 50 blows Marshall or 100 Gyrations with the Superpave Gyratory Compactor (SGC). However, some states have found that 100 gyrations with SGC are excessive for their materials. In this study, a lower compaction level of 65 gyrations was used to compare with the standard 100 gyrations to design SMA mixtures. Results showed that mixtures designed by 65 gyrations had an average of 0.7 percent higher optimum asphalt content and 1.5 percent higher VMA than those designed by 100 gyrations. All mixtures designed by 65 gyrations met the minimum asphalt content and VMA requirements for SMA, while only 8 of 15 mixtures designed by 100 gyrations met these two requirements. Compaction at 100 gyrations resulted in an additional 0.62 percent average aggregate breakdown at the critical sieve as compared to 65 gyrations. SMA mixtures designed by 65 gyrations had an average Asphalt Pavement Analyzer (APA) rut depth of 3.9 mm. Thirteen of fifteen mixtures performed well if 5.0 mm is set as maximum allowed rut depth. Based on this study, it was concluded that 65 gyrations can be used to design a more durable SMA mixture, while still maintaining the good rutting resistance that SMA mixtures are noted for. The successful design by 65 gyrations for all five aggregates in this study indicates that a lower design compaction level may allow the use of more aggregate sources for SMA mixtures.