Achieving 4% Air Voids in Real Pavements with Superpave

Abstract

The Superpave® mix design method has been used by the Colorado Department of Transportation since 1995. Initially, the number of design gyrations used was as recommended by SHRP research. However, the asphalt contents of these preliminary paving mixtures were lower than what had been historically successful for similar dense graded mixtures under similar conditions. This lower asphalt content began leading to premature moisture damage failures. Therefore, a study was initiated to determine how the pavements constructed according to the original Superpave compaction criteria densified under traffic. An experiment was designed to evaluate the in situ voids in 22 full-scale pavements over a 6-year period. Projects were selected on the basis of variation in traffic, environment, temperature, and elevation. Results of this research indicate that the original number of design gyrations recommended by SHRP for field-mixed–laboratory-compacted materials results in pavements that did not compact sufficiently under traffic, resulting in higher air voids than desired after 3 years service. An adjustment to the original SHRP design gyration levels was done in a subsequent study based on NCHRP research. The result of this revision has been a decrease in voids levels after 3 years of service with a corresponding improvement in performance. Further fine-tuning is needed and will be implemented in the future.