Innovative monitoring of asphalt surface characteristics

This thesis investigates the evolution of friction on flexible asphalt pavements on Italian motorways, with particular attention to open-graded and semi-porous wearing courses, which constitute the majority of the national network. Since road crashes remain a major safety issue worldwide, the control and monitoring of pavement friction play a crucial role in preventing loss of vehicle control during high-risk manoeuvres. In this context, although porous mixtures improve drainage and visibility, their measured friction is often less favourable than that of dense-graded mixtures, making a rigorous and predictive assessment of their long-term behaviour necessary. The Wehner–Schulze (WS) apparatus is employed as the main experimental tool to bridge laboratory testing and in-service performance. A comprehensive laboratory programme on more than thirty asphalt mixtures (open-graded, dense-graded, and semi-porous) combines WS polishing–friction tests with photogrammetric reconstruction of the surface along the WS measurement section, enabling joint analysis of microtexture, macrotexture, and friction decay. The influence of mixture type, aggregate properties (e.g. Polishing Stone Value PSV), and air-void content on contact conditions and friction evolution is quantified. To link laboratory outcomes with field performance, trafficked and companion untrafficked cores were extracted from six motorway branches, together with historical Sideway Force Coefficient (SFC) and traffic data. These data were used to establish a relationship between traffic-induced polishing and WS polishing, as well as a correlation between field skid resistance and WS friction for open-graded and semi-porous mixtures. The resulting WS-based correlations allow laboratory-measured friction-decay curves to be expressed as expected SFC values as a function of traffic, making it possible to forecast in-service friction (potentially even before construction) and to integrate these forecasts into pavement management systems. Overall, the thesis provides a consolidated framework for the use of WS in research and practice, an evidence-based understanding of how current Italian motorway mixtures behave under polishing, and an operational pathway for generating friction predictions that are directly relevant to field monitoring for open-graded wearing courses. In this perspective, the work contributes to improving the safety and sustainability of road transport by providing the research community with tools that can be applied to performance specification, in-service control, and proactive management of skid resistance over the pavement service life.