Surface friction measurements of fine-graded asphalt mixtures : final report, June 2008.
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Surface friction measurements of fine-graded asphalt mixtures : final report, June 2008.

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      Final report; June 2008
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      Skid resistance is generated by the development of friction between the vehicle tire and roadway surface, and is partially dependent upon the characteristics of the pavement texture. Microtexture and macrotexture are the critical components of pavement surface friction. Microtexture is the dominant characteristic at low speeds, and at high speeds, microtexture and macrotexture are both necessary. While these parameters are very important, neither is considered during the design of an asphalt mix. In this project, mixture design parameters were investigated to determine what features of 4.75mm hot-mix asphalt (HMA) mixtures significantly influenced skid resistance. The British Pendulum Test (BPT) was used to quantify microtexture, and a sand patch test (modified for laboratory use) was used to measure macrotexture in terms of texture depth. Mixture properties and aggregate properties were also analyzed in order to develop relationships and guidelines for producing skid resistance during the design phase of an HMA construction project. Additional considerations involved comparisons with traditional surface mixes of larger nominal maximum aggregate size, and the effects of aging and wear on the long-term skid resistance of 4.75mm mixtures. In general, mixture properties did not provide significant relationships to skid resistance (microtexture or macrotexture). Aggregate properties, however, did significantly correlate to measures of macrotexture. Specifically, the most influential variables were fine aggregate angularity, percent passing the #30 sieve, bulk specific gravity of the aggregate blend, and two-dimensional particle shape (as measured by the Aggregate Imaging System). Aggregate gradation was also significant in that a gap-graded aggregate blend appeared to increase both microtexture and macrotexture. When compared to 9.5mm and 12.5mm mixes, the 4.75mm mixes possessed the greatest levels of microtexture. Thus, it was concluded that 4.75mm mixes did provide adequate skid resistance for low-speed roadways. The macrotexture of 4.75mm mixes was significantly less than that for a more traditional 12.5mm surface mix. Thus, if 4.75mm mixes are desired for use on high-speed roadways, the effects of roadway geometry on pavement drainage should be carefully considered.
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