Evolution of the road bitumen/aggregate interface under traffic-induced polishing

Abstract

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.

Multiscale analyses of pavement texture during polishing

“…y 0 is the non-normalized wavelet. Equation (7) gives the formula for calculating the angular frequency:…”

“…For a better understanding of friction evolution, it is essential to know the evolution of the texture of aggregates at each scale, which is not the case at this moment. Some attempts were done to connect standard texture parameters to skid resistance evolution but it remains rough [5][6][7]. More research is needed to tackle this issue.…”

Contribution of multiscale analysis to the understanding of friction evolution of aggregates surfaces

“…-Slope parameter: root mean square gradient (Sdq, used in [35] to explain the decrease of pavement surfaces friction due to aggregates polishing) calculated using equation (2).…”

“…-Curvature parameter: mean summit curvature (Ssc, used in [12,35] to explain the decrease of to explain the decrease of aggregates surfaces friction) calculated using equation (3):…”

“…Volume parameter: peak material volume (Vmp, used in[35] to explain the decrease of pavement surfaces friction due to aggregates polishing) calculated using Abott-Firestone curve. It represents the cumulative statistical distribution of surface heights.…”