Q.: When the testing laboratory broke some cores they calculated the strength in psi by dividing the load by the average cross-sectional area. Then they multiplied the result by a "strength correction factor" that gave a lower value. Where did this correction factor come from and how is it justified?

A.: The correction factor should have come from Section 5.7 of ASTM C 42, "Standard Method of Obtaining and Testing Drilled Cores and Sawed Beams of Concrete." The reason for applying a correction factor is that tall specimens tend to give lower test values than short specimens of the same cross-sectional area. In order to convert all core specimens to approximately the same strength basis as the standard 6x12-inch test cylinder, a correction factor is applied to the calculated strength. This factor, based on the ratio of length to diameter (l/d), has been given as 0.98 for l/d = 1.75, 0.96 for l/d = 1.50, 0.93 for l/d = 1.25 and 0.87 for l/d = 1.00.