Cylinder Break Pattern Below Design Strength
A set of three 6x12 in. cylinders for a 4000 psi mix is tested at 28 days. Results: 3620, 3710, and 4180 psi. The specification requires that no individual test fall below 3500 psi and that the average of any three consecutive tests equal or exceed 4000 psi. This is the first set of the placement. The field technician notes that the cylinders were stored in a job-site shed for the first 24 hours with daytime temperatures reaching 100 F, then moved to a moist-curing room at 73 F. The shed had no temperature control.
1. Does this set pass acceptance, and what is the most defensible next step?
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Check both criteria. Individual minimum: 3620 and 3710 both clear 3500 psi, and 4180 clears it. Average of the set: (3620+3710+4180)/3 = 3837 psi, which is below 4000 psi. Under ACI 318 acceptance rules, the average of any three consecutive tests must meet or exceed f'c, so this set fails the average criterion. Judgment: the set is nonconforming. The next step is not automatic rejection of the structure - it is to investigate. The 100 F shed storage is a plausible cause of depressed early strength (elevated curing temperature reduces later strength gain). The defensible action is to require an investigation: review batching records, curing history, and consider cores per ACI 318 Chapter 26 provisions before any structural decision.
2. The contractor argues the cylinders were mis-cured and should be discarded. How do you respond?
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Discarding inconvenient results is not a valid response. Field-cured cylinders stored at 100 F are not standard-cured and their results are not directly comparable to the 73 F moist-cured baseline - but they are still evidence of what the concrete in place experienced, and they are the record. The correct handling is to document the storage deviation, note it as a probable contributing factor, and let the investigation proceed on the record. If the engineer determines the test is invalid due to a documented curing deviation, the remedy is a new evaluation (cores or in-place testing), not erasure of the data. Judgment: you protect the integrity of the test record; you do not suppress results because they are inconvenient.