ACI defines required average compressive strength (fc’r) as the larger of the following equations:
- fc’r = fc’ + 1.34 x S
- fc’r = fc’ + 2.33 x S – 500 (<= 5000psi)
- fc’r = 0.9 x fc’ + 2.33 x S (> 5000psi)
Where:
fc’ is design or specified strength
S is the sample standard deviation
The required strength represents the strength at which there is a 1-in-100 chance or probability that each equation is not met. Equation 1 represents the 1-in-100 chance that the average of three consecutive tests may fall below the fc’. Equation 2 represents the 1-in-100 chance that a single test may fall below fc’-500psi when evaluating fc’<=5000psi.. Equation 3 represents the 1-in-100 chance that a single test may fall below 0.9xfc’ when evaluating fc’>5000psi. Therefore, the acceptance of concrete uses a similar criteria, which is the average of three consecutive strength test results don’t fall below fc’ and that no individual strength test falls below fc’-500psi (<=5000psi) or 0.9xfc’ (>5000psi). If a strength test result fails either of these criteria then steps should be taken to increase the concrete strength. For more information on low concrete cylinder strength we recommend this Concrete in Practice brief from NRMCA.
StonemontQC includes a Low Strength Analysis tool that can be run for a detailed review of concrete strength results, including the detection and identification of low compressive strengths. However, the real power is when this analysis is automated and not only for a single mix but for all mixes at all plants. That is the power of StonemontQC’s automatic reporting, available in the enterprise and hosted editions. Furthermore, there is an option to only report low strength results so the report is only generated if low strength results have occurred.
For more information on how easily you can evaluate low concrete strength results or use our automatic reporting features contact Stonemont Solutions, Inc.
Adrian Field
Stonemont Solutions, Inc.
