ASTM C 94 requires a strength test to be the average of at least two standard test cylinders and taken from each 150 cubic yards of concrete. If a strength test is more than 500 psi below f'c, ACI 318 allows the average of three core strength tests to be used for acceptance. ACI uses three cores because the average core result is known with the same degree of confidence as the average cylinder strength.
ACI 228 provides an equation for evaluating tests so that in-place tests are measured to the same degree of certainty as a compressive test. Using this equation with the within-test coefficient of variations (COV), the number of tests for each in-place test can be calculated to be equal to the average of two standard cylinders. For instance, it takes 12 rebound hammer tests (note that one test is equal to the average of 10 readings) to provide the same degree of testing certainty as a standard cylinder test.
Using the number of in-place tests recommended by the equation assures that the average test result is known with the same degree of certainty as the average core strength or cylinder strength. This, however, does not mean that the compressive strength of the concrete will be known with the same degree of certainty. ACI 228 recommends procedures for determining an appropriate correlation between in-place tests and compressive strength.