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Influence of road surface microtexture on thin water film traction

This paper deals with the contribution of road surface microtexture to the relationship between tire/road friction and water depth. The main objectives are the estimation of local water depths trapped at the tire/road interface and the definition of a critical water depth which can be used for driver assistance and information systems. Tests are performed in laboratory. Specimens are slabs made of asphalt concrete and mosaics composed of coarse aggregates. The aggregate mosaics are sandblasted to simulate different microtexture levels. Friction is measured by the Dynamic Friction Tester (DFT) machine at various speeds ranging between 20km/h and 80km/h. Microtexture profiles are measured by laser sensors. It was found that the friction-water depth curves have an inverse-S shape and present an initial constant-friction part before decreasing to a minimum value. The critical water depth, defined as the water depth above which the friction coefficient collapses significantly, is determined from observed friction-water depth curves. Thickness of the water film trapped between the surface aggregate summits and device measuring pad/tire tread is estimated from contact models using surface texture profiles. The order of magnitude corroborates values found in the literature. Relationship between the local water depth, taking into account the tire/road contact conditions, and the global one, as measured by most sensors, is presented. Influence of surface microtexture is discussed in terms of exerted pressure to break through the water film.