Effect of dry deposited particles on the tire/road friction
Résumé
Road accidents increase during the first rain after a long dry period. It is widely accepted that the tire traction loss is due to fine particles accumulated on the road surface. Yet, the involved mechanisms at the tire/road interface are not clearly understood. This paper deals with the particle-induced lubrication on dry road. Tests are performed in laboratory. Sediments are extracted from a catchment area which collects runoff water. The process of extracting particles from sediments in laboratory by drying and sieving is described. Particles are separated into fractions characterized by their chemical composition and size distribution. Protocol to simulate the process of particles' build-up on the road surface is described. The effect of the particle's size and concentration is studied. Surfaces representative of real road surface textures are tested. Friction tests are conducted using the Skid Resistance Tester, which is widely used to assess friction characteristics of road surfaces. Samples are weighed before and after each friction run. On a surface initially covered by a compacted particle layer, successive friction runs are performed, without resupplying particles, to follow the evolution of the friction coefficient with the particle depletion.
Friction drops significantly when a surface is covered by particles. Successive runs induce an increase of the friction coefficient until reaching a stable value which is still lower than that of a clean surface. Particles are considered as third bodies introduced artificially in the interface between the friction slider and the test sample. Analysis of particles' flows (particles trapped and ejected), through particles' mass extracted from weighing, helps to explain the friction variation. Assuming that friction is governed by the surface fraction (X) covered by particles, a model was developed. The calculated friction coefficient is a weighted sum of friction coefficients when the surface is respectively clean (X = 0) and fully covered by particles (X = 1). It was found that the surface fraction can be expressed as a function of the ratio of available particles' mass to initial particles' mass. Comparisons between model and experiment are satisfactory. Discussions are made in terms of interactions between surface textures and particles' size and concentration.
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