Effect of the core sample in low aspect ratio on concrete compressive strength

Cores with diameters of 100, 75, and 50 mm and with varied length-to-diameter (l/d) ratios of 1.0, 0.9, 0.8, 0.7, and 0.6 were extracted from concrete panels of 11 different concrete mixtures and prepared in the lab. Compressive strength tests of concrete cores and cubic samples were performed at 7, 28, 60, and 120 days, respectively. Strengths of cores with l/d ratios ranging from 0.6 to 1.0 could be converted by the strength correction coefficient into that of the core with the l/d ratio of 1.0. The evidence for the reliability of compressive strength of cores with low l/d ratios is very seldom in literatures. The test results indicated that correction factors gradually decreased with the decrease in l/d ratio, the strength of the core increased and the effect was more pronounced for cores with smaller diameter and the maximum aggregate size. Correction factors of cores drilled from river gravel aggregate concrete are slightly higher than that from crushed stone aggregate concrete. The longer the age of concrete is, the lower the correction coefficient becomes. The proposed correction factor values for core test samples with the l/d ratios ranging from 0.6 to 1.0 differ from the prior researches. For compliance with the requirement of the test data variability, there is no reason to limit the ratio of aggregate sizes (d(a)) to the diameter of the core (d) to a minimum of 3.0 (d/d(a)). The results show that the concrete core with a diameter less than the standard size and l/d less than 1.0 can effectively reflect the variation of concrete strength with acceptable reliability. Drilling-core with low l/d ratios will facilitate the reduction of the damage to the structures, so the members with thickness less than 150 mm (e.g., slab and shearwall) can be drilled and tested.