A quarry's aggregate has been successfully used in pavements since processing facilities were significantly upgraded in 1999. However, despite laboratory test results of aggregate properties exceeding specification requirements, performance in service was below expectations in some applications. This was originally attributed to weathering that was thought to produce expansive clays. A series of investigations, trials of modified production processes and other remedial actions did not resolve the problem. Aggregate in all stages of production process is washed, chemical added to the wash water to settle the suspensions, and the water reused, to minimise discharge into local streams and impact on the environment of the quarrys operations. Although processed water was guaranteed to be chemical-free by suppliers, it was found to contain large amounts of coagulant, which have a high surface charge, and attract any material in suspension. After primary crushing, fine material produced was mostly rock dust that was attracted and strongly held to chip surface, creating a coating. Residual positive surface charge of coating repelled bitumen, resulting in a weak bond and eventually unravelling. Reducing and controlling coagulants corrected problem, producing a clean, durable chip. The paper describes the investigation and solution, in detail. (a) For the covering entry of this conference, please see ITRD abstract no. E217099.
Samenvatting