Austroads latest technical report concludes that the Extra Large Wheel Tracking Device (XL-WT) test is a more useful test than the Repeated Load Triaxial (RLT) test in predicting the deformation resistance and in-service performance of granular materials.
In 2010, Austroads investigated how RLT testing could be used to predict and rank the deformation properties of granular materials. The investigation included a comparison of the RLT test results to full-scale pavement accelerated loading data. As a result, doubts were raised about the usefulness of the test to evaluate the rut‑resistance of granular materials. Subsequently, some exploratory research using an XL‑WT device was undertaken, which indicated that it may have greater potential than the RLT test to predict deformation resistance and in-service performance of granular materials.
As a result, Austroads undertook the development and commissioning of new XL‑WT test equipment and test methods aimed at assessing the performance ranking of granular materials through the measurement of deformation and moisture sensitivity. This led to the need to assess how well the XL‑WT laboratory test method was able to predict in-service performance of granular materials.
Austroads commissioned the Australian Road Research Board to undertake research project TT1819 ‘Characterisation and Performance Evaluation of Granular Bases’.
Full-scale accelerated pavement testing was applied to five crushed rocks using the accelerated loading facility (ALF). The materials were prepared from two source rocks (hornfels and granite). For each source rock, a standard crushed base product was used and one or two other materials were prepared which had an increased fine content and plasticity index. ALF trafficking was undertaken under the 40 kN dual-wheel load which represents half of a standard axle. The materials were tested under two moisture content regimes to enable an assessment to be made of the sensitivity of the performance of the pavement to changes in moisture content.
The same materials were tested in the laboratory under wheel-tracking and RLT testing. For each of the tests, the moisture conditions were varied to assess the moisture sensitivity under laboratory conditions.
The laboratory results were compared to the full-scale accelerated loading results. It was concluded that the deformation resistance measured using the XL-WT was more closely aligned with the performance under accelerated loading than the RLT test. Additionally, the laboratory‑field comparison showed that the XL-WT better reflected moisture sensitivity impacts on the relative performance rather than the RLT test.
Unbound granular pavements are a cost-effective way of providing road networks across large areas with low population densities. Granular pavements represent around 85% of the Australian and New Zealand road network.