AGGREGATE SIZE AND LATERAL DIMENSION EFFECTS ON CORE COMPRESSIVE STRENGTH OF CONCRETE

The compressive test on drilled concrete cores is the most reliable method to determine the concrete strength of existing structure. This paper reports on a recent laboratory investigation devoted to the effect of maximum aggregate size and core diameter on the estimation of the characteristic strength of current concrete (C25/30 class). The compressive strength of concrete cores with different diameters (phi = 100 mm, 75 mm and 50 mm) and with a slenderness ratio of 2 is determined. These lateral and longitudinal core dimensions (respectively phi and h) are the most used in building sites in order to check hardened concrete strength. Concrete mixtures are prepared with different maximum aggregate sizes (D-max = 4 mm, 8 mm, 16 mm and 25 mm). The cores tested are extracted from massive blocks of these concrete formulations. The results show the increase of the compressive strength of cores as the diameter is increased. The strength values of the concrete cores with diameter phi = 100 mm and 75 mm are greater than 80 % of the characteristic strength of the tested concrete. However, the resistance values of the cores drilled with the smallest diameter phi = 50 mm are largely dispersed. The evolution of the core strength values as a function of maximum aggregate size shows an irregular enhancement as the aggregate size is increased. In addition, the result dispersions represented by the coefficient of variation diminishes substantially as the core diameter is increased. Correlation formulae between core diameters and concrete characteristic strengths are also pointed out.