University of California Pavement Research Center

A round robin testing program was undertaken between 20 participating laboratories in California to assess the reproducibility of Hamburg Wheel-Track (HWT) test results as part of a Superpave implementation initiative. Each laboratory conducted four HWT tests. Two of the tests were conducted on gyratory-compacted specimens prepared by the University of California Pavement Research Center (UCPRC), and the other two were conducted on gyratory-compacted specimens prepared by each of the participating laboratories using loose mix supplied by the UCPRC. A three-quarter inch mix with five percent PG 64-16 asphalt binder content was sampled from a northern California asphalt plant for the study. Laboratories reported test results in terms of rut depth after 5,000, 10,000, 15,000, and 20,000 wheel passes; number of passes to 0.5 in. (12.5 mm) rut depth; creep slope; stripping slope; and stripping inflection point. Some laboratories also submitted raw test data for further analysis by the UCPRC. An analysis of variance was used to determine which factors had a significant influence on the test results and to determine the indices of precision of the different result variables. Indices of precision could not be determined for all variables due the relatively good performance of the mix. Single-operator variability was relatively high for all variables. Between-laboratory variability was shown to be strongly related to several measurement and result-interpretation aspects that are not fully defined in the AASHTO T 324 test method. This variability was clearly reduced when unique criteria were used in the data analysis.