Calculating Air Voids at Specified Number of Gyrations in Superpave Gyratory Compactor

Abstract

The Superpave volumetric mix design is based on compaction of mixtures to Nmax revolutions, yielding approximately 2 percent air voids, and then back-calculating the properties of the mixture at N design revolutions, which is the specified number of revolutions at which the sample should achieve 4 percent air voids. It has been shown, both in mixture design and in field quality control testing, that this back-calculation procedure is not always accurate for determining the number of gyrations that give 4 percent air voids in the compacted sample. The current back-calculation procedure is examined in this paper, and the observed inaccuracy in the current method is shown in both quality control/quality assurance (QC/QA) and the mixture design procedures. Also examined is an alternative method that has been shown to be more accurate in predicting the number of gyrations at which a mixture reaches a given air void level. This study recommends a method that gives higher accuracy in determining the number of gyrations at which a mixture reaches 4 percent air voids. A diverse group of mixtures, each having different properties, was examined, and an improved method for predicting the number of gyrations to accurately produce 4 percent air voids in the compacted mixture was determined.