Cerchar Abrasive Index (CAI) testing has long been used as a cheap and easy method of determining abrasive wear rates for mechanical excavations by classifying rock masses based on their abrasiveness. However, the relationship between rock abrasivity as expressed by CAI and mineralogical, textural and the engineering properties of rocks is not well established.
The CAI testing process is to scratch a conical hardened tool steel stylet across a rock surface a distance of exactly ten millimetres in one second with a 70kN normal force applied. The needle is then taken away for observation and the wear distance is measured.
The aim of this thesis is to accomplish two tasks. Firstly, as the University of Queensland does not currently have a Cerchar Abrasivity apparatus, one will be fabricated with the help of the Mechanical Engineering workshop. Three rock samples of granite, brown-bedded sandstone and multi-coloured sandstone will then be used on the new machine, controlling variables such as grain orientation to understand how they affect abrasive wear. Results will be compared with other tests done previously to determine how accurate the new system is. The mineralogical properties such as grain size and orientation should have a direct impact on the amount of wear incurred on the steel bit. Also, the hard mineral content should indicate which minerals are most successful at causing wear.
Manufacturing of the CAI test apparatus has been successful. The machine performs all required tasks, is portable and is safe and easy to operate. Twelve CAI trial tests were completed on the rig altogether, with smooth surfaces and rough compared. As predicted, the granite was the hardest rock and had the highest CAI by far, followed by bedded brown and multi-coloured. Rough surfaces always produced higher CAI values than smooth, however no obvious differences were found when testing the X and Y planes of the samples.