The American Concrete Institute's (ACI) "Building Code Requirements for Reinforced Concrete" state that the average strength of a mix must always exceed the specified value of the 28-day compressive strength, f'c. ACI 318 indicates the amount by which the average strength of a concrete mix, f'cr, should exceed f'c. ACI requires the average compressive strength, f'cr, used as the basis for selection of concrete proportions, to be the larger of one of the following: (1)f'cr=f'c+1.34s; or (2)f'cr=f'c+2.33s-500. The standard deviation for each mix must be determined from strength tests.
When choosing a mix and evaluating test records, carefully select a mix similar to that for the proposed work. Consider changes in type and brand of cement, admixtures, aggregate source, mix proportions, batching, mixing, delivery, or testing to significantly alter the mix so that test records from the new mix can't be combined with the previous test records. In general changes in materials and procedures have a larger effect on the average strength level than on the standard deviation.
As more data becomes available during construction, ACI allows f'cr to change as the quality control improves. This allows the concrete supplier to save money by reducing cement content to achieve a lower strength.