The Prediction of Pavement Surface Aggregate Wear and Microtextural Polishing

Abstract

Skid resistance of road surfaces generally decreases over time. Since there is a direct relationship between the inadequacy of skid resistance and the increasing number of crashes caused by wet pavements and loss of control vehicles, it is necessary to ensure the skid resistance of road surfaces is always present at an adequate level. Thus, it is desirable to be able to predict the long term in-field skid resistance performance of road materials (aggregates) before the road is constructed. The most popular method to assess the skid resistance performance of aggregates is the Polished Stone Value (PSV) test. However, the PSV test has been acknowledged to have certain limitations, and hence there are alternative laboratory tests that have been developed to replace the PSV test. In this research, two alternative laboratory tests were analysed, namely the Wehner/Schulze (WS) test and the Auckland Pavement Polishing Device (APPD) test (which is used in conjunction with the Dynamic Friction Tester (DFT)) to produce a more accurate assessment of skid resistance of aggregates or road surfaces. There are eight different New Zealand aggregates used in this research, which consist of three Greywacke, two Basalt, one Andesite and two artificial aggregates. This research explored four main areas, namely the mineralogy of aggregates, the laboratory tests, the historical infield skid resistance evolution and the microtexture evolution due to polishing. The research findings suggest that the APPD-DFT or the WS tests are better than the PSV test method in predicting the aggregates’ resistance to polishing, as they are more sensitive to polishing differences and that they are less reliant on the operator. A methodology has also been developed to convert the skid resistance test results generated by either the APPD-DFT test or the WS test to the in-field skid resistance as measured by the SCRIM+. Even though the APPD-DFT and the WS tests cannot predict the exact long term in-field skid resistance value of aggregates at different stage of polishing, but those tests can provide a global prediction of how the in-field skid resistance of an aggregate will perform under certain traffic conditions. The mineralogy of aggregates were also assessed by using the X-Ray Diffraction and Thin Section methods and two mineralogy parameters, Cd and dmp, were calculated to characterise the effect of mineral hardness on skid resistance. The Cd parameter represents the difference in mineral hardness in the aggregate, while the dmp parameter represents the average hardness of the minerals in the aggregate. It was found that there is a strong relationship between the skid resistance slope and the Cd parameter, which suggests that aggregates containing minerals with various hardness levels are less susceptible to polishing action, which is shown by the small skid resistance slope. The thesis also presents the early stages of research on how the skid resistance can be predicted through a contactless method, i.e. by quantifying microtexture changes throughout the polishing process. Five parameters characterising the microtexture were explored, namely the height root-mean-square Rq, the asperity-peak curvature, two angular parameters characterising the asperity sharpness and relief, and the peak density. It was found that the curvature parameter can be well correlated to skid resistance. The Rq parameter was also found to have a relationship with skid resistance evolution, but to a lesser extent than the curvature, while the sharpness, relief and peak density parameters cannot be used to explain the surface friction variation because of newly created asperities during the polishing process due to aggregate abrasion. In summary, the work presented herein has added some improvements to the research in skid resistance area, especially in the attempt to predict skid resistance. The improvements include:  Giving a recommendation on alternative laboratory test methodologies that can be used to provide in-field skid resistance performance prediction;  Introducing a new methodology to predict the global trend of the long term in-field skid resistance performance based on the laboratory test results; and  Developing a new methodology in characterizing microtexture of aggregates to be related to skid resistance.