Superpave Gyratory Compactor Internal Angle of Gyration Study

This study on the angle of gyration for Superpave compactors was done to determine if there is a difference in the bulk specific gravity (Gmb) – and ultimately the volumetric properties – when calibrating the compactor’s angle of gyration internally and externally. The Washington State Department of Transportation (WSDOT) currently has Troxler, Pine, Pine-Brovold, and Interlaken compactors across the state, so a representative sample (two to four compactors per region) was tested in this study. Triplicate samples of hot-mix asphalt (HMA) were used for each compactor. It was found that 41 percent of the compactors tested were not in specification when using the internal angle to calibrate the compactor. Adjustments were made to those compactors and the bulk specific gravity results were compared. The results of the bulk specific gravity affect the volumetric properties of HMA (Va, VMA, and VFA) and therefore can have an affect on the design and acceptance of HMA. When calibrating the compactors via external angle (current AASHTO standard), the average air voids (target of 4.0) were 3.81 with a standard deviation of 0.54. When calibrating via internal angle, the average air voids were 3.98 with a standard deviation of 0.50. Based on the results of this study, WSDOT is working to: (1) evaluate additional gyratory compactors (both WSDOT- and Contractor-owned), (2) make changes to current verification/calibration procedures, and (3) implement the use of the internal angle of calibration for 2008.