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