Application of fractal analysis for measuring the effects of rubber polishing on the friction of asphalt concrete mixtures

• New laboratory device and data post-processing methodology for asphalt surfaces.
• The new testing device characterizes friction and surface texture during polishing.
• A robust methodology for surface characterization is presented.
• This approach allows highlighting the influence of aggregate type and gradation on friction.

Skid-resistance is one of the most important pavement surface properties with respect to driver safety. For example, the friction of asphalt concrete mixtures continuously evolves because the aggregates are subjected to polishing by rubber tires.Currently available tests for mix design optimization are focused primarily on evaluating the polishing characteristics of aggregate surfaces rather than actual AC mixtures. Factors such as aggregate gradation cannot directly be taken into account in aggregate polishing tests, and hence its effects on the friction of the overall mix are not accounted for when mixes are being designed.Using the Skid Resistance Interface Testing Device (SR-ITD®), developed for laboratory use during the European project SKIDSAFE, the evolving frictional properties of asphalt concrete mixes containing any desired combination of aggregate type, maximum size and gradation can be evaluated. The effects of the number of wheel passes and combinations of contact pressure and speed are documented.During SR-ITD testing, evolution of the surface morphology is recorded using a laser profilometer. Post-processing of the results by means of fractal analysis enables the characterization of the surface texture on the basis of three unique descriptors.By combining the variation of surface descriptors with the friction coefficient, the influence of aggregate type and gradation on the frictional response of asphalt concrete mixes can usefully be determined.