DEVELOPMENT OF MIXTURE CREEP PERFORMANCE TESTS USING A DYNAMIC SHEAR RHEOMETER

Abstract

A hot-mix asphalt (HMA) mixture performance test has been developed using a constant stress dynamic shear rheometer. The test is conducted using rectangular bars of mix, which are cut from either field cores or gyratory-compacted HMA specimens with a diamond-tipped saw. The bars are nominally 50-mm long, 12-mm wide, and 10-mm thick. Specimens are tested using the solids testing fixture provided by the rheometer manufacturer. Specimen test temperature is maintained by air circulation through a temperature-controlled oven that surrounds the specimen. Two types of creep tests were developed: a static creep test and a repeated creep recovery test. The static creep test uses the application of a rotational stress to one end of the long dimension of the specimen while the other end is held rigidly in place. As the stress is maintained at a constant value, the rectangular specimen undergoes a strain, which is monitored to failure. The repeated creep and recovery test uses the same equipment and specimen shape. A stress is applied to the specimen for 1 s followed by a 9-s rest period of zero stress during which time the specimen recovers some of the strain developed during the 1-s stress period. Repeated stress and relaxation cycles are applied to the test specimen up to 2,000 complete cycles or until specimen failure occurs. From these two creep test procedures, several types of results have been obtained, including time to 5% strain, time to failure, strain at failure, strain at 100 cycles, and mix viscosity at 100 cycles. Test data generated on specimens cut from field cores have been shown to correlate well (R-squared > 0.9) to the field rutting behavior of test track mixes placed at Minnesota Test Road at the Minnesota Department of Transportation research site and at the FHWA Accelerated Loading Facility test track in Sterling, Va. The time required to perform the creep tests varies with mix quality, stress level applied, and test temperature. It is believed, on the basis of work to date, that failure criteria data can be established that would enable this procedure to be used as a mix design tool. In addition, the test is easy and rapid enough to perform that quality control monitoring of HMA mixes being placed in the field could be performed and the results can be available within 24 h.