Precision Estimates of AASHTO T 324, “Hamburg Wheel-Track Testing of Compacted Hot Mix Asphalt (HMA)”

Abstract

The Hamburg wheel tracking test (HWTT) has been extensively used to identify asphalt mixtures prone to rutting or moisture damage. AASHTO T 324, “Hamburg Wheel-Track Testing of Compacted Hot Mix Asphalt (HMA),” describes the procedure for testing asphalt mixture samples using the HWTT device. The method specifies the testing of submerged, compacted asphalt mixture in a reciprocating rolling-wheel device. The test results provide information about the rate of permanent deformation from a moving concentrated load. However, there is no information on the precision of the test method, including the allowable differences between two replicate measurements in one laboratory or measurements in two laboratories. In addition, important aspects of the test are not sufficiently specified in the test method; these include position of the wheel with respect to specimen, verification of the location of the measurements, specimen preparation and assembly, and analysis and reporting of test data. Because these factors could significantly affect HWTT measurements and performance verification of asphalt mixtures, it is important to identify the factors causing variability of measurements and further specify their limits in the test method. The objective of this study was to determine precision estimates for AASHTO T 324. To accomplish this objective, the research: determined the variability of the deformation measurements after specified number of load passes and the creep slope for well-performing mixtures; determined the variability of the number of passes to threshold deformation, creep slope, stripping slope, and number of passes to the stripping inflection point for poorly performing mixtures; compared the mean and variance of the measured properties of gyratory and slab specimens; compared the mean and variance of properties measured using all measurement locations with those measured using all except the three middle measurement locations and all except two measurement locations at each end; identified causes of variability of the test results; and proposed modifications to the test method for optimum use of the deformation measurements, improvement to the specimen preparation and assembly, and necessary adjustments to the machine components.