Surface morphology indicators and high-friction design for wet friction evaluation of surface mortar of Portland cement concrete pavement

Surface mortar polishing under repeated vehicular loading is the primary mechanism of wet friction deterioration on Portland cement concrete (PCC) pavements. While long-term friction performance of surface mortar is inherently linked to the lithology of fine aggregates, the evolution of surface topography during polishing and its intrinsic correlation with the wet friction behaviour remain poorly understood. In this study, five fine aggregates with distinct lithologies (river sand, limestone, basalt, granite and steel slag) were selected to fabricate mortar specimens. Accelerated polishing tests were conducted to simulate long-term tire-pavement interaction, while 3D white-light interferometry and British Pendulum Tests (BPT) were employed to characterise the evolution of surface texture and wet friction performance (BPN). The results demonstrate that the root-mean-square height (Sq), skewness (Ssk) and kurtosis (Sku) are the most effective parameters for assessing the wet friction response of the surface mortar. Differential polishing governed by aggregate lithology drives the surface topography toward a ‘platform-valley’ morphology. A texture combination characterised by high Sq, less negative Ssk and low Sku enables significantly improved wet friction performance. These findings establish a quantitative framework linking fine aggregate lithology, three-dimensional morphology, and wet friction performance, providing candidate indicators for PCC pavement design.