Precision of the Hamburg Wheel-Track Test

This paper presents the results of an interlaboratory study (ILS) to determine precision estimates for AASHTO T 324, Hamburg wheel-track testing (HWTT) of compacted hot-mix asphalt. The ILS included preparing and sending four replicates of Superpave® gyratory and two replicates of linearly kneaded compacted slab specimens each of a rut and moisture resistant (well-performing) and a rut and moisture susceptible (poorly performing) mixture to participating laboratories to be tested according to AASHTO T 324. The reported results were used to prepare the precision estimates for the properties of the two mixtures. The precision estimates included the within- and between-laboratory precisions for deformation, the number of passes to threshold rut depths, the number of passes to the inflection point, and the creep and strip slopes. The effects of the measurement locations used in the analysis and the specimen type on mean and variance of HWTT properties were determined. The properties of the mixtures measured with all measurement locations were statistically compared with those measured with all except three middle measurement locations and those measured with all except the two measurement locations at each end. Moreover, the mean and the variance of the properties of the gyratory and slab specimens were statistically compared. In addition to developing precision estimates, the data from the ILS and in-house research were used to gain insight into the causes of the variability of the test results. The effects of various components of HWTT and the effect of specimen assembly on the test measurements were investigated. This paper discusses several issues with the test method and device and suggests solutions to improve the test.