Use of Wehner-Schulze machine to evaluate pavement skid resistance: A review

Pavement skid resistance plays a crucial role in ensuring road safety and avoiding accidents. In the past, the laboratory evaluation of the skid resistance was carried out by studying only the coarse aggregates of the wearing course. To overcome this drawback, the Wehner-Schulze (WS) machine was developed in Germany in the 1960s. This equipment, composed of a polishing unit and a measuring unit, has great potential in predicting pavement skid resistance and its evolution over time, but is still little known in the pavement community (especially outside Europe). For these reasons, there is a need of a comprehensive review of the existing technical-scientific literature concerning the use of the WS machine. Specifically, this paper focuses on the main factors affecting the skid resistance in WS tests, the correlation of WS data with other laboratory test methods and with field skid resistance/polishing, and the available prediction models that have been validated through WS measurements. The critical analysis of the existing literature highlights that it is possible to correlate WS data with typical skid resistance field measurements as well as WS polishing with traffic polishing, but further efforts are needed in this regard. Future work should focus especially on open-graded mixtures and innovative asphalt mixtures (e.g., containing recycled materials and additives). From the perspective of pavement management, based on a theoretical background, the WS test results could be used as starting point for simplified prediction models of the in-situ skid resistance.