This paper presents the results of a laboratory study concerning the effects of the particle shape characteristics and gradation of aggregates on the skid resistance of asphalt surface mixtures, as determined by the British portable skid tester. Four coarse aggregates and two fine aggregates, representing both natural and crushed materials, were used. The particle shape characteristics of these materials were determined by the particle index test for positive, quantitative identification.
The aggregate materials were combined to form three different gradations, and mixed with the optimum content of an 85 to 100 penetration grade asphalt cement, as determined by the Marshall method of mixture design. Circular specimens 6 in. in diameter and 2 in. in thickness were used for the laboratory skid test. An accelerated polishing device similar to the one previously used by the investigators at Purdue University was employed to polish the surface of the test specimens. Skid resistance values were determined both before the specimens were polished and after different stages of polishing.
The results of this investigation show that, within the range of the variables studied, the antiskid properties of the experimental surface mixtures improved with increasing values of the particle index of the fine aggregate. The skid resistance was also affected significantly by the gradation, as represented by the composition index, of the aggregates. For the unpolished specimens, the British portable number (BPN) is shown to increase with increasing values of the composition index. The trend was reversed for the polished specimens in that the BPN was increased as the values of the composition index were decreased.