Target and Tolerance Study for the Angle of Gyration Used in the Superpave Gyratory Compactor (sgc)

The Superpave gyratory compactor (SGC) is used to compact asphalt specimens for mixture testing in the Superpave system. Five companies currently manufacture SGCs for use in the United States, offering a total of eight different models. Each model employs a unique method of setting, inducing, and maintaining the specified angle of gyration. Calibration systems are required for each device for the angle of gyration. All angle measurements are made externally relative to the mold and none of the calibration systems can be universally used on all of the models. The specified external angle of gyration under load (α) is 1.25 degrees, with a tolerance of ± 0.02 degrees. The Federal Highway Administration (FHWA), in partnership with the TestQuip Corporation (New Brighton, MN), developed an angle validation kit (AVK) that measures the dynamic internal angle (DIA) of gyration. The DIA accounts for equipment compliance issues, which are not apparent in measuring the external angle. Bending in the upper and lower mold platens during compact reduces the compactive effort imparted to a specimen. Differences in specimen bulk specific gravities produced by different compactors have been attributed to differences in compliance between SGCs measured with the AVK. This necessitates a revision of American Association of State Highway and Transportation Officials (AASHTO) standard method 312-01 to ensure uniformity in compaction effort in the Superpave system. However, it would be inappropriate to assign the external angle target and tolerance to the DIA; compliance issues have existed throughout the development and implementation of the Superpave system. Assignment of the external specifications to the DIA would result in an increase in the compactive effort of all SGCs and in turn would invalidate the Ndesign table. The Transportation Research Board (TRB) Superpave Mixture/Aggregate expert task group (ETG) (the TRB Superpave Mixture/Aggregate ETG replaced the FHWA Superpave