The Effect of Aggregate Type and Mix Design on the Wet Skid Resistance of Bituminous Pavement: Recommendations for Virginia's Wet Accident Reduction Program

About the report:

This report (1) identified and characterized Virginia's nonpolishing aggregates in terms of their wet skid resistance, (2) compared the standard ASTM E 274-90 skid test to the British Accelerated Polishing Machine (BAPM) and British PendulumTester (BPT) system for pre-evaluating the skid resistance of nonpolishing aggregates, (3) compared the effect of different asphalt mixtures on pavement surface macrotexture using the ASTM E 965-87 sand patch method, and (4) briefly considered the effect of texturing on pavement surface friction. Recommendations for Virginia's Wet Accident Reduction Program (WARP) were drawn up from the results of these studies. Fifty-seven sources of nonpolishing aggregates, representing 18 lithologies, were studied. Skid testing of the 18 lithologies was carried out on 1,246 bituminous pavements, using the ASTM E 274-90 procedure. The aggregate groups showing the highest wet friction ranking were Valley Gravel, Fine Grained Granite, Lovingston Granite, Coastal Plain Gravel, and Diabase. Aggregates consistently exhibiting the lowest skid resistance were Aplite, Arch Marble, Slate, and Triassic Hornfels. The BAPM-BPT system showed little correlation with the skid test results. No significant differences were found between the overall skid numbers generated on 1-2 and S-5 asphalt mixes, but two trends were noted: 1) 1-2 pavements showed a faster decrease in skid resistance than S-5 pavements as accumulated traffic increased, and 2) where platy aggregates such as Slate and Arch Marble were used, S-5 pavements exhibited consistently higher long-term skid resistance than 1-2 pavements. One hundred and five sand patch tests were conducted to measure pavement surface macrotexture during the study. Macrotexture depth showed a higher correlation with skid numbers than any other factor investigated. Mixes exhibiting low skid resistance may do so because of rapid aggregate wear or the tendency of platy aggregate to orient horizontally, which both lower macrotexture depth. A brief survey of the effect of texturing suggests that Virginia's WARP should consider including pavement texturing as an alternative to repaving for wet accident hot spots.

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