Target and Tolerance Study for Angle of Gyration Used in Superpave Gyratory Compactor

Abstract

Five companies offer eight models of the Superpave® gyratory compactor (SGC) in the United States. Each model uses a unique method of setting and inducing the specified angle of gyration. However, all angles are set externally relative to the mold and none of the manufacturer’s calibration systems can be universally applied to all models. The specified external angle of gyration (α) is 1.25° FHWA, in partnership with Test Quip, Inc., developed a dynamic angle validation kit that measures the dynamic internal angle (DIA) of gyration during loading. The DIA accounts for equipment compliance issues, such as bending of the platens during compaction, which is not apparent when the angle is measured externally. Differences in specimen density produced by different compactors have been attributed to differences in compliance. Thus, the SGC test method needs to be revised to obtain uniformity. However, it would be inappropriate to assign the external angle of 1.25° to the DIA because the internal angle is always lower than the external angle. Use of a 1.25° internal angle would result in an increase in the compactive effort applied by all SGCs, which would invalidate the N_design table. FHWA investigated the DIA by using the original two SGC models. These models were used to refine the N_design table that is in use today. The study found that the DIAs for these original SGCs were 1.176° and 1.140°, which resulted in a proposed DIA of 1.16° The proposed tolerance is ±0.03°.