Friction on road surfaces contaminated by fine particles: Some new experimental evidences

Abstract

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.