Knowing the in-place characteristics of concrete and how they affect the measured compressive strength of cores can go a long way toward establishing that low-strength cylinders were not the result of bad concrete. Coring is generally deemed necessary by low laboratory-cured or field-cured cylinder strengths. Therefore, core strengths should be obtained from the in-place concrete that represents the low cylinder strengths. Cores do not serve the same purpose as cylinders. Strength of standard cylinders represents the quality of concrete delivered. Strength of cores represents the in-place concrete strength. The relationship between core and cylinder strength varies because of the characteristics that each specimen represents.
Cores obtained by drilling in the direction of concrete casting may provide a higher strength than cores obtained by drilling perpendicular to the direction of casting. This difference is attributed to bleeding in fresh concrete, which creates a weak paste pocket under coarse aggregate particles. A load applied parallel to the weak bond creates a strength-decreasing flaw. However, a load applied perpendicular to the weak bond minimizes the effect of the bleedwater layer.
Concrete strength varies within a single element. Tests indicate two apparent causes of strength variation: strength increase at the bottom attributed to greater static pressure caused by the concrete above and strength decrease at the top attributed to higher water-cement ratios as a result of bleedwater.
The thermal history and curing of cores is quite different than for standard cylinders. Field curing is unlikely to be as good as moist curing. The methods for obtaining and testing a core obscure the effects of curing. Curing dramatically affects the concrete surface, but has less of an effect on the interior concrete.