EVALUATION OF ALABAMA LIMESTONE AGGREGATES FOR ASPHALT WEARING COURSES

Abstract

The Alabama Highway Department does not permit the use of limestone coarse aggregate in asphalt wearing courses because of potential long-term skid resistance problems. A laboratory study was undertaken to evaluate 32 sources of limestone aggregates in Alabama for possible use in the wearing courses. Twelve approved sources of gravel aggregate were also evaluated for comparison. The frictional properties of all aggregates were determined using the British pendulum tester (ASTM E303) after 9 hr of accelerated polishing on the British wheel (ASTM D3319). Aggregates were also subjected to petrographic analysis. The percentage of noncarbonate material in limestone aggregates was determined by two methods: percent insoluble residue (ASTM D3042) and percent loss on ignition (Tennessee DOT method). The British pendulum number (BPN) values were found to follow a hyperbolic relationship with polishing time. This relationship can possibly be used to predict the limiting BPN value after infinite polish time. There was a general trend that the value of BPN9 (BPN value after 9 hr of polishing) increased as the percentage of insoluble residue increased or the percentage loss by ignition decreased. On the basis of BPN9 values, limestone aggregates were divided into three categories: potentially low, medium, and high skid-resistance levels. If BPN9 is used as an acceptance criterion, the limestone aggregates of the medium and high categories have the potential to provide skid resistance levels comparable with gravel aggregates used at the present time.