Quantifying aggregate microtexture with respect to wear—Case of New Zealand aggregates

• Parameters characterizing asperity height, shape and density are investigated.
• A method to select appropriate depth of aggregate profile for analysis is proposed.
• Aggregate polishing is a result of both smoothing and abrasion actions.
• Smoothing decreases the curvature of asperity tips.
• Abrasion creates new asperities, explaining the fluctuation of height and density.

Quantifying road surface microtexture (wavelengths shorter than 0.5 mm) and relating it to surface friction help to better understand mechanisms involved in the evolution of skid resistance. In this study, a three-step methodology was developed to systematically measure and analyse aggregate profiles and compare the evolution of surface friction and roughness parameters. Criteria used for the three steps (selection of the areas to be measured, filtering of the profiles to keep only the relevant part for analysis, selection of roughness parameters) are presented and justified by considering on the one hand polishing and abrasion mechanisms and, on the other hand, the contact between a viscoelastic solid and a rough surface. The methodology is applied to three different New Zealand geologically sourced aggregates and polished by using the Wehner/Schulze device. Three-dimensional images of the samples were taken periodically during the test to record the microtexture changes due to polishing. Results show that roughness parameters expressing local shape, such as curvature of asperity tips, can reasonably explain the surface friction variation; height parameters such as the standardised profile root-mean-square are also explanatory but to a lesser extent. Relevance of the studied roughness parameters is discussed and interpretation in terms of physical change of surface texture is provided.