The effects of varying L/D ratios and core size on the strength of cores from large elements of high strength concrete (HSC) made both in the laboratory and field were examined. 154 cores representing 5 laboratory mixtures having moist-cured cylinder compressive strengths from 114 to 142 MPa at 5-years age and 101 cores representing actual field placement of 28,150 sq. m of concrete in an offshore concrete structure having continuously moist-cured compressive strengths at 1-year from 88 to 92 MPa were evaluated.