Evaluation of Two Compaction Levels for Designing Stone Matrix Asphalt

Abstract

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 the SGC is 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% higher optimum asphalt content and 1.5% higher voids in mineral aggregate (VMA) than those designed by 100 gyrations. All mixtures designed by 65 gyrations met the minimum asphalt content and VMA requirements for SMA, whereas only eight of 15 mixtures designed by 100 gyrations met those two requirements. Compaction at 100 gyrations resulted in an additional 0.62% average aggregate breakdown at the critical sieve as compared with 65 gyrations. SMA mixtures designed by 65 gyrations and 100 gyrations had an average asphalt pavement analyzer rut depth of 3.9 mm and 3.1 mm, respectively. Thirteen of 15 mixtures designed by 65 gyrations performed well if 5.0 mm was set as the maximum allowed rut depth. On the basis of this study, 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.

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