United States Department of Transportation
i
Target and Tolerance Study for The Angle of Gyration Used in The Superpave Gyratory Compactor
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2002-01-01
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[PDF-893.18 KB]
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Abstract:Five companies offer eight models of the Superpave (Registered trademark) 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 is 1.25 deg. The Federal Highway Administration (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 deg to the DIA because the internal angle is always lower than the external angle. Use of a 1.25 deg internal angle would result in an increase in the compactive effort applied by all SGCs, which would invalidate the Ndesign table. FHWA investigated the DIA by using the original two SGC models. These models were used to refine the Ndesign table that is in use today. The study found that the DIAs for these original SGCs were 1.176 deg and 1.140 deg, which resulted in a proposed DIA of 1.16 deg. The proposed tolerance is +/-0.03 deg.
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