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Road skid resistance decreases under traffic and climate actions. For a surface made of bituminous concrete, the skid resistance evolution involves a removal of the bitumen layer and a polishing of the aggregates. Previous researches have been focused on polishing mechanisms and less is known about bitumen removal phase. This paper presents a laboratory study to better understand the behavior of the stone mastic asphalt under simulated actions of traffic. Due to its formulation, this bituminous concrete contains more bitumen than other materials used for road surfacing. Polishing tests are performed on circular cores using tWehner/Schulze machine, which simulates the traffic-induced polishing by means of rolling rubber cones and measures the tire friction by means of sliding rubber pads. Tests are stopped at predefined numbers of passes for friction measurements, 3D cartographies of surface texture and scanning electron microscopic observations of the cores' surface. Statistical tests are performed to identify relevant texture parameters to explain friction evolution. During bitumen removal phase (i.e. before 20,000 passes of polishing), Ssc (average curvature of peaks) and Sdq (average quadratic slope of asperities) are correlated with friction values. After 50,000 passes, volume parameter Vvc is more adapted due to the abrasion of asphalt mix surface. Then, scanning electron microscopic observations show that a layer composed by bitumen and particles (small aggregates and sand) is a third-body surrounding aggregates, which evolves in thickness and size of aggregates during polishing. The system composed of the aggregates, the bitumen–particles layer and the polishing cones can be assimilated to a tribosystem with a tribological circuit at the interface. The bitumen–particles layer acts as an internal flow, before leaving the contact area. The movement of the layer under the shear stress induced by the polishing cones explains the ejection of big aggregates and the heterogeneity of the layer around aggregates. [ABSTRACT FROM AUTHOR]

Accidents increase during the first rain after a long dry period due to the accumulation of fine particles, which contaminates the road surface and induces a friction loss. This paper presents new experimental evidences that help to understand how the particulate contaminants lubricate the tire–road interface. A laboratory test method is developed to reproduce the deposit of contaminant particles on the road surface and measure the friction coefficient on dry and wet contaminated surfaces. Test protocol is described with respect to the contaminant collection, the spreading of contaminant particles on the road specimen and their compaction to simulate the effect of the traffic, the wetting of the test surface to simulate precipitations, and the friction measurement. Friction tests are conducted in dry and wet conditions (by adding successive known quantities of water). Results show that dry particles can induce friction loss and the friction coefficient of dry contaminated surfaces is always lower than that of a dry and clean surface. The dry lubrication mechanism is attributed to shearing and sliding of the particle layer. Water induces first a significant friction drop, attributed to the viscosity of the particles–water mixture, then a friction increase due to a washing effect. Weight analysis shows a tight relationship between the friction coefficient and the mass of contaminants (particles in dry condition and particles–water mixture in wet conditions). There is a minimum in the friction versus water weight variation. Interpretation is made by considering fine particles as a wet granular material. Discussions are made with respect to the influence of particle concentrations and surface texture. [ABSTRACT FROM AUTHOR]