A proposal for the prediction of the characteristic cube strength of concrete from tests on small cores of various diameters

A method is proposed for the assessment of characteristic cube strength, f_ck, based on tests performed on 28 mm diameter microcores, small 45 mm diameter cores, and 70 mm diameter cores. In particular, on the basis of the tests performed on the specimens (1270 between cubes and cores of the three diameters being considered, manufactured from 16 concrete types), relationships are defined which make it possible to estimate mean cube strength, $bar f_c$ , with, a desired confidence level (1?α)%, starting from a mean core strength, $bar f_{28} ,bar f_{45,} bar f_{70} $ . Subsequently, given a number of microcores, n₂₈, n₄₅, n₇₀, subject to compressive tests the proposed method makes it possible to determine the number of cubes n_c necessary to evaluate the mean cube strength with the same confidence level, and characteristic strength with a good approximation. Finally, having worked out the one-side tolerance factor, k, f_ck is evaluated with the aid of the estimated value of $bar f_c$ , the calculated value of k, and an estimate of the mean standard deviation on cubes, s _c , as determined from the core tests.