Evaluating the Sensitivity of the Auckland Pavement Polishing Device and Analysis of Aggregate Microtextural Wear
Skid resistance is a key factor in road safety. However, over time, the pavement surface is polished by vehicle tyres, reducing the micro-texture of aggregates and macro-texture of the road surface, resulting in a deterioration of skid resistance. Consequently, it is necessary to be able to accurately predict the skid resistance performance of aggregate as part of the design process. Laboratory tests are the main tool used to obtain such predictions. One such test, developed at the University of Auckland, utilises the Auckland Pavement Polishing Device (APPD) for accelerated polishing of the samples followed by skid resistance measurement via the Dynamic Friction Tester (DFT). However, the APPD device has never been standardised and little is known about the sensitivity of the device to changes in setup variables such as tyre type, tyre pressure, applied load and chip sizes. Even less is known about the micro-textural wear of aggregates after polishing using the APPD device or the evolution of microtextural wear during the polishing process. Consequently, this research: Evaluates the sensitivity of the APPD polishing mechanism to changes in the aforementioned setup conditions, as measured by corresponding changes in the coefficient of friction. Quantifies the microtextural wear at Equilibrium Skid Resistance (ESR) by undertaking microtextural analysis of height, shape and volume microtexture parameters, using a 3D micro coordinate measurement device. Quantifies the evolution of microtextural wear during the polishing process by undertaking microtextural analysis, using a 3D micro coordinate measurement device. The sensitivity results showed that the coefficient of friction significantly changed when the pressure of the pneumatic tyres changed. It can, therefore, be concluded that the results from the APPD-DFT are sensitive to a ±4 N/cm2 (±5 psi) variation from the typical 14 N/cm2 (20 psi) used in previous research. However, the tyre pressure with 14 N/cm2 (20 psi) and the solid tyre was not significantly different. Also, the sensitivity results show there is a ii significant difference between the coefficient of friction results obtained using different applied loads on the APPD device. It can, therefore, be concluded that under the reported conditions, the results from the APPD-DFT are sensitive to a ±10 kg variation from the typical 58 kg load used in previous research. The final sensitivity results showed that the coefficient of friction was significantly different for specimens made using different aggregate sizes. It can, therefore, be concluded that the results obtained from grade 2 (with ALD between 9.5mm to 12mm), grade 4 (With ALD between 5.5mm to 8mm) and grade 6 With ALD less than 5.5mm) were significantly different when all specimens were polished with 14 N/cm2 (20 psi) pneumatic tyre pressure and 570 Newton (58 kg) applied load. The microtextural analysis samples polished by the APPD to ESR shows that the height of microtexture parameters (such as Root Mean Square Height – Sq and Arithmetic Mean Height – Sa) are not suitable for analysing the microtextural wear. In contrast, the Root Mean Square Slope (Sdq) and Arithmetic Mean Summit Curvature (Ssc), which measure the shape of texture, are potentially suitable for analysing the microtextural wear of aggregates polished using the APPD. The results show that the effect of polishing on the coefficient of friction follows a similar trend to the shape of texture parameter variation for each specimen. It can, therefore, be concluded that the shape of texture is a key factor in determining the Coefficient of Friction (CoF), in particular, the Sdq and Ssc parameters. Analysis of the evolution of microtextural wear was undertaken using samples polished using the Wehner/Schulze (WS) device. The results show that the height of microtexture parameters (Sq and Sa) better represent the evolution of microtextural wear variation during the WS polishing process. In addition, the results show that the core material volume (Vmc) is a suitable candidate to evaluate the effect of the WS device on the microtextural wear during the polishing process. The variation of the Sq, Sa, and Vmc follow a similar trend as the coefficient of friction value for specimens polished using the WS device. This research expectedly found that different accelerated polishing devices affected microtextural wavelength parameters differently. The most significant parameters from the WS device were microtextural height (Sq and Sa) and core material volume (Vmc) parameters whereas when polishing was undertaken with the APPD device these parameters were less significant and in contrast, the shape parameters are more significant (Sdq and Ssc). In terms of further research, it is recognised that while a greater understanding of the sensitivity of the APPD device has been achieved, smaller variations to the setup tyre pressure and applied load are needed to determine tolerance levels in order to fully standardise the APPD-DFT device. Similarly, further sensitivity tests incorporating a wider range of aggregates, with varying geological properties, should be undertaken.
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