Comparison of open-graded, dense-graded and semi-porous asphalt mixtures for motorway wearing courses based on Wehner-Schulze tests and photogrammetry

Achieving sustainable mobility necessarily involves improving road safety. When open-graded (or porous) asphalt mixtures are used as wearing course in road pavements, they can drastically reduce the number of accidents thanks to their water drainage capability and improved visibility for drivers during rainy weather. However, in practice, the field skid resistance of such pavements measured in wet conditions is often lower than that of dense-graded wearing courses, which seems counterintuitive and needs further investigation. Within this context, this study compared the surface properties of over 30 asphalt mixtures typically employed for wearing courses in Italian motorway pavements, which were collected for quality control during the construction works. The mixtures differed in terms of type (open-graded, dense-graded, semi-porous), composition (i.e. mix design, aggregate origin and polymer modified bitumen supplier) as well as production date. The skid resistance of laboratory-compacted slabs was evaluated at increasing polishing levels with the Wehner-Schulze (WS) machine. The surface texture of selected slabs was assessed through a photogrammetric reconstruction, thus obtaining the surface profile characteristics exactly along the trajectory investigated in the WS tests. On average, the friction values of open-graded mixtures were found to be lower than dense-graded mixtures by more than 20% due to the lower effective contact area between the WS measurement sliding blocks and the sample surface (percentage of contact = 15÷25% vs. 65÷80%), which can determine a reduction of the skid resistance contributions due to adhesion. For the same type of mixture, higher friction values were observed in the presence of coarse aggregates with high Polished Stone Value (PSV) and/or in the case of lower air void content.