Moving-Average Failure, an Anomalous Cylinder Pair, and the Core Decision
Six consecutive 28-day test results (each the average of two 6 by 12 in. cylinders) are reported in test order for a 4,500 psi column mix: 4,720; 4,380; 4,610; 4,290; 4,540; 4,400 psi. All specimens were standard-cured at 73 degrees F and tested after sulfur capping. The break sheet for the 4,290 psi set shows individual cylinder strengths of 4,850 and 3,730 psi; the low cylinder failed through a thick, blistered cap with visible voids. The contractor points out that every result exceeds 4,000 psi, that the average of all six sets is about 4,490 psi, and that no cores should be needed. The structural engineer asks you to determine whether the concrete satisfies ACI 318 acceptance criteria for the column class and, if not, what the next step is.
1. Apply the ACI 318 strength acceptance criteria to these six results in test order and state your conclusion for the column class.
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The criteria are both of the following: (a) every average of three consecutive test results equals or exceeds f'c, and (b) no individual test result falls below f'c minus 500 psi when f'c is 5,000 psi or less.
Criterion (a), moving averages of three: 1-3: (4,720 + 4,380 + 4,610)/3 = 4,570 - passes. 2-4: (4,380 + 4,610 + 4,290)/3 = 4,427 - fails, below 4,500. 3-5: (4,610 + 4,290 + 4,540)/3 = 4,480 - fails. 4-6: (4,290 + 4,540 + 4,400)/3 = 4,410 - fails.
Criterion (b): the lowest individual result is 4,290 psi, and the floor is 4,500 minus 500 = 4,000 psi, so this criterion is satisfied.
Conclusion: the strength level of this class is not acceptable, because three of the four moving averages fall below f'c. The contractor's arguments miss the point: the criterion is not the average of all results and it is not the 500 psi floor alone. The engineer must investigate per ACI 318 and may require core testing. Note also that the results are trending downward, which is itself worth a look at batching, cement source, and curing.
2. The 4,290 psi set is the average of 4,850 and 3,730 psi. How should that pair be handled, and what effect does your decision have on the acceptance evaluation?
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A 4,850 / 3,730 pair is roughly a 26% spread between two cylinders from the same sample. Within-sample scatter is normally only a few percent, so a spread that large points to a specimen or testing defect rather than to variation in the concrete - and the break sheet supports that: the low cylinder failed through a thick, blistered cap with visible voids, which is a classic capping failure that produces a false low.
The rule still stands that a test result is the average of the two cylinders. You do not get to discard the number simply because it is inconvenient, and doing so without documentation would be indefensible in a dispute. But a result that is demonstrably invalid because of a specimen defect may be set aside with documentation and, ideally, replaced by testing a spare cylinder from the same sample; if no spare exists, treat the set as a missing test result and evaluate the remaining valid results. That determination should be reviewed and concurred in by the design professional.
If the 4,290 set is invalidated, the remaining sequence is 4,720; 4,380; 4,610; 4,540; 4,400. Moving averages: 4,570; 4,510; 4,517 - all at or above 4,500, and no individual below 4,000. The class would then be acceptable.
Either way, fix the root cause: review capping technique, cap thickness, and compound condition, and check the machine before the next set is broken.
3. Assume the anomalous set cannot be invalidated and cores are ordered. State the sampling and acceptance criteria for the cores and what happens if they fail.
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Cores are taken and tested per ASTM C42 from the area represented by the questionable results - at least three cores for that area, located to avoid reinforcement, prestressing steel, embedded items, and regions of high stress, and finished and moisture-conditioned as the method requires before compression testing.
Acceptance thresholds: the average of three cores must be at least 0.85 f'c, which here is 0.85 times 4,500 = 3,825 psi, and no single core may fall below 0.75 f'c, which is 3,375 psi. Both conditions must be met.
If the cores meet those thresholds, the concrete in place is considered adequate for the strength requirement even though the cylinder record failed, and the acceptance question is closed on that basis with the core data documented.
If the cores do not meet them, the strength requirement is not satisfied and the design professional must determine structural adequacy: analysis of the affected members using the demonstrated in-place strength, a load test per ACI 318, or remediation such as strengthening or removal and replacement. The testing lab should report core locations, diameters, length-to-diameter ratios, moisture condition, fracture descriptions, and the calculated strengths so the engineer can make that call on real data. Do not let cores be drilled before the core-testing plan is agreed and the locations are marked and photographed.