WHY DO HIGH PSV CHIPPINGS POLISH

Aggregate makes up more than 90% by mass of bituminous surfacing materials. The resulting level of performance of the wearing course, particularly skid resistance, is largely dependent on the type of aggregate used. In most bituminous wearing courses, it is generally the coarse aggregate that comes in touch with the vehicle tyres. Its role is to maintain both a level of macro-texture and micro-texture. Macro-texture or texture depth is required to induce hysteresis (energy loss) to facilitate drainage and so enhance high-speed skid resistance. Therefore, macro-texture is the texture of the finished road surface. Micro-texture is the small scale texture of the aggregate used and is again necessary for skid-resistance. An aggregate may be termed polished when it loses its micro-texture (less than 0.5mm). In-service this leads to a loss of friction and an overall reduction in the skidding resistance of the road surface. This paper considers some of the factors which affect the polishing of surfacing aggregate. This has included applied stressing at the tyre/exposed aggregate interface due to loading, tyre pressure, speed etc., aggregate mineralogy to the effects of applied in-service factors. (A) For the covering abstract see IRRD 877985.