Assessing concrete strength by means of small diameter cores

Estimation of concrete strength may be gained from compression tests conducted on cores having a diameter considerably smaller than the recommended one of 100 mm. This paper examines the results of tests applied on the 69 and 46 mm diameter cores. A total number of 2982 cores was tested. The effects of both specimen and aggregate sizes for different length-to-diameter (l/d) ratios on the compressive strength of smaller diameter cores were analysed. The core strengths were compared to those of standard cylinder and cubes. Test results showed that maximum size and type of aggregate significantly affect the strengths of small diameter cores. As the maximum aggregate size increased the strength of core decreased. The strengths of cores removed from crushed aggregate-bearing concrete were somewhat higher than those of ones drilled from natural aggregate-containing concrete. Test results also revealed that the l/d ratio of the specimen is more effective for small diameter cores. The age of concrete was found to be an important factor in the interpretation of the results, the older the concrete the higher the core strength. The coefficient of variation of strength values were noticed to be somewhat higher for 46 mm diameter cores and cores drilled from natural aggregate-containing concrete mixtures. It was indicated that core strength was affected by both specimen and aggregate sizes. Therefore, it was proposed that the size of specimen and aggregate, type of aggregate, age of the specimen should be taken into consideration for the interpretation of the results used to convert the strengths of cores to those of standard cylinders or cubes.