When concrete strength is consistent from batch to batch, suppliers can reduce their cement costs because they can meet the code requirements with a lower average strength concrete. But to take advantage of this opportunity, the suppliers have to document mix performance with records that show average strength and variations in strength. A computer program from the American Concrete Institute (ACI) makes the job easier.

PERFORMANCE HISTORY REQUIREMENTS

The average strength of a concrete mix must exceed the design strength to limit the amount of low-strength concrete in a structure. The required over-design depends on the producer's standard deviation for strength test results. The higher the standard deviation, the larger the overdesign has to be.

ADVANTAGES OF USING THE ACI COMPUTER PROGRAM

It's possible to collect cylinder test results and compute standard deviations using a calculator. But using a computer program developed for analyzing concrete strength test data makes the job easier. The ACI program we use calculates average strength, standard deviation, and required average strength based on the standard deviation. It also calculates within-test coefficient of variation, a measure of testing laboratory performance.