Development of Surface Friction Guidelines for LADOTD

Abstract

The main objective of this study was to develop a Louisiana pavement surface friction guideline that considers polished stone value (PSV) and mixture type alike in terms of both micro- and macro-surface textures. The polishing and texture properties of aggregates were characterized using the British Pendulum, Micro-Deval and Aggregate Imaging System (AIMS). Asphalt mixture slabs were fabricated with different combinations of two aggregate sources (sandstone and limestone) and four mixture types and polished by a three-wheel accelerated polishing device developed by the National Center for Asphalt Technology (NCAT). The surface frictional characteristics of each slab were measured by Dynamic Friction Tester (DFT) and Circular Texture Meter (CTM) at various pre-determined polishing cycles. In addition, an inventory dataset of field friction number (FN) measurements was obtained from the Louisiana Department of Transportation and Development's (LADOTD’s) Materials Laboratory and analyzed in this study to determine the effects of traffic loading, aggregate and mixture types on the measured FN values. The laboratory results indicated that the accelerated polishing device used in this study performed just as expected, i.e., as the polishing cycle increases, the measured frictional property of testing slab surface decreases. It was found that the DFT measurements were fairly sensitive to the coarse aggregate types (related to micro-texture) used in mix design, but were not very sensitive to different mix types or aggregate gradations (related to macro-texture). The analysis of CTM measured Mean Profile Depth (MPD) results confirmed a strong relationship between MPD and mixture type, indicating MPD does reflect well of surface macro-texture. Because friction resistance of an asphalt mixture should account for both micro- and macro-texture, the International Friction Index (IFI) friction numbers, the F(60), were determined based on an IFI model using measured DF20 (the DFT measurement at a friction speed of 20 mi/hr) and MPD values for each slab tested. Further analysis of F(60) results generally indicated that an open-graded friction coarse (OGFC) mix type considered in this study had the highest friction resistance due to its largest surface macro-texture (or MPD values), followed by the stone matrix asphalt (SMA) mix type, and then by the two Superpave mix types considered (a 19-mm Superpave Level-II mix, a 12.5-mm Superpave Level-II mix). The F(60) results also indicated that a selected sandstone type (AB13) with a high polishing resistance (PSV>37) performed significantly better in terms of mixture friction resistance than a selected limestone (AA50) with a PSV of 31. Mixtures using an aggregate blend of 30 percent of selected sandstone and 70 percent of the limestone tended to have a better surface friction resistance than those with 100 percent of the limestone. This observation demonstrates that blending of low and high friction aggregates together can possibly produce an asphalt mixture with an adequate field friction resistance. The analysis has led to the development of a set of prediction models of mixture frictional properties, and a laboratory mix design procedure that addresses the surface friction resistance of an asphalt mixture in terms of both micro- and macro-surface textures. The developed frictional mix design procedure allows estimating a friction-demand based, design SN value for an asphalt mixture during the mix design stage.