AbstractThis report describes the investigations carried out to determine the performance of a range of diamond impreganted core drills on flint aggregate reinforced concrete. Five core drills of various sizes (10, 16, 25, 32, and 40 mm) containing 6·7 wt-% diamond grit were produced for the initial evaluation. Each drill was subjected to a series of drilling tests to ascertain tool life and free cutting characteristics. The results showed a significant correlation between cutting action and drill diameter. At the lower end of the size range (10–25 mm), acceptable penetration rates were achieved at the expense of tool life. As the drill size was increased, tool life improved. The 32 mm diameter sample displayed the best combination of drilling rate and tool life. Premature failure of the 40 mm bit occurred due to the development of wear flats on the tool surface. Analysis of the initial test results revealed that the tentatively selected diamond content of 6·7 wt-% was not the optimum proportion for all sizes. Further studies involving samples containing different concentrations were therefore undertaken. It was found that the effectiveness of the drills could be markedly improved by varying the diamond content according to tool dimension.