Accidents increase during the first rain after a long dry period. This trend is due to the accumulation of fine particles originated from different sources such as atmosphere, road and tires debris, fuel emissions, etc. These particles accumulate on the road surface during a long dry period and induce a friction loss between the tire and the road surface. In the road field, the tire/road skid resistance on dry conditions is considered satisfactory (considering the road surface as clean) and the loss of skid resistance is attributed to the sole action of water. Thus, there is an unexplored field concerning the particles' effect, alone in dry period and mixed with water in wet period, on the skid resistance.

In this paper, we investigate the action of the particles during a dry period using two approaches developed in tribology: the third body approach, particularly the mass analysis inspired from Fillot et al. [1], to understand the particles flows at the tire/road interface; and the dry lubrication theories developed for powders (as molybdenum disulfide MoS2 ) [2] [3] to understand and model the friction. 

Experiments are conducted in laboratory to understand and model this phenomenon. Particles are collected, by drying and sieving, from sediments sampled from a catchment area which collects runoff water. Analyses are performed to determine the particles' chemical composition and size distribution. Experimental protocol allows simulating the particles' build up process on the road surface. The specimen surface, representative of a road surface, includes a microtexture scale, representing the asperities of the aggregates, and a macrotexture scale, representing the space between the aggregates. Friction measurements are realized by means of the so-called Skid Resistance Tester Pendulum, widely used in the road field, which simulates the friction between a rubber pad sliding at 3 m/s on the specimen surface. On a surface initially covered with particles, successive friction runs are performed, without resupplying particles between two consecutive runs. Specimen's weight is recorded before and after each friction run.

Results show that friction drops significantly, compared to a clean state, when the surface is covered by particles. Successive runs induce an increase of friction coefficient until reaching a stable value which is below that of a clean surface. Three particles' flows are calculated: particles ejected from the contact area between the friction slider and the test surface; particles trapped by the surface microtexture; particles stored by the surface macrotexture. Close relationship was found between the friction coefficient and the flow of particles trapped by the microtexture. Similarities are found, in terms of lubrication mechanisms, between the behavior of studied particles and powder [3]. 

Discussions allow understanding the relationship between particles' characteristics and Stribeck curves' parameters such as viscosity, lubricant film thickness and pressure. A first attempt of modeling allows to calculate the friction coefficient from the fraction of surface covered by particles. This study has allowed to see the contribution of tribology to the understanding of complex phenomena in the road field such as skid resistance on contaminated road surfaces. Perspectives, especially in terms of study of water and particles mixing, are presented.

[1] Fillot, N., Iordanoff, I., and Berthier, Y. Wear modeling and the third body concept. Wear, 262, 949-957, 2007.

[2] Higgs, C.F., Wornyoh, E.Y.A. An in situ mechanism for self-replenishing powder transfer films: Experiments and modeling. Wear, 2008, 264, 131-138.

[3] Heshmat, H. Wear reduction systems for coal-fueled diesel engines – Experimental results and hydrodynamic model of powder lubrication. Wear, 1993, 162-164, 518-528.