1. Introduction

One of the more important aspects of the work of the Road Research Laboratory is to develop better materials for surfacing roads. By 'better materials' is meant materials having greater resistance to cracking and deformation, greater durability against the effects of weather and greater ability to maintain an even and non-skid surface for traffic.

The surfacing materials in common use are concrete, asphalt and bituminous macadams; these consist of a mixture of mineral aggregates bound together with either Portland cement, bitumen or tar. The Laboratory is therefore concerned with obtaining an understanding of the properties of crushed rock and gravel aggregates that influence the behaviour of these surfacing materials.

This paper describes briefly the research into one important requirement of aggregates used in bituminous road surfacings. This requirement is that the exposed aggregate should not become polished by the action of traffic, since the degree of slipperiness of many types of bituminous surfacing is directly related to the extent to which the aggregate becomes polished.

2. Laboratory test

The first step was to develop a laboratory apparatus to determine how the polishing occurred. The aim was to reproduce as closely as possible the conditions of wear to which stone is subjected in the road surface. Thus, the apparatus (Pl. 1a) comprises a pneumatic-tyred wheel which rotates in contact with another wheel on the flat periphery of which are mounted small specimens of ⅜-in sized chipping bedded in a cement mortar (Pl. 1b). The wheel carrying the specimens is

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