Interpretation of pavement skid resistance based on the advanced analysis of surface texture

The World Health Organization estimates that about 1.3 million lives are lost every year worldwide due to road accidents. Pavement skid resistance/friction is one of the most important factors that contribute to road safety, as it ensures the vehicle control during manoeuvres that could give rise to accidents (e.g., turning, emergency braking). It is known that skid resistance changes over time under the action of traffic and weather, and the accident rate increases significantly when the skid resistance falls below a certain threshold. In this regard, this study focuses on the investigation of the friction properties of three types of asphalt mixtures for wearing courses (open-graded, dense-graded and semi-porous) typically employed in Italian motorway pavements. These mixtures were characterized by the same type of coarse aggregates (basalt) from the same quarry. The skid resistance of laboratory-compacted slabs was assessed at increasing polishing levels with the Wehner-Schulze (WS) machine. To interpret the results of the WS tests, an advanced methodology based on photogrammetry was developed to analyze the surface texture of the slabs. It was found that, if the same coarse aggregates are considered, pavement skid resistance mainly depends on the surface texture properties. Specifically, the skid resistance decreases as the contact area between rubber/tyre and sample/pavement decreases, which can be explained by a reduction of the skid resistance contributions due to adhesion. These findings, which underline the importance of texture characteristics on pavement friction, can help shedding light on the skid resistance values typically measured in the field for different types of asphalt mixtures.