Cylinder Break Results Below Design Strength at 28 Days
A set of six 6x12 in. cylinders from a column pour (4000 psi design) is tested at 28 days. Results: 3620, 3710, 3550, 3800, 3650, and 3900 psi. The cylinders were field-cured in a protected box and transported to the lab at 7 days, then standard-cured. The lab report notes that two cylinders had capped ends that appeared slightly uneven. The batch plant records show the mix was within water-cement tolerance. The contractor wants to accept the concrete because 'the average is over 3600.'
1. Evaluate the results against the standard 28-day acceptance criteria and state whether the concrete passes.
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Apply the standard ACI 318 strength acceptance criteria for a 4000 psi design:
- Every individual average of three consecutive tests must equal or exceed f'c (4000 psi).
- No single test result (average of two cylinders) may fall below f'c minus 500 psi (i.e., below 3500 psi).
Group the six cylinders into three test results (pairs):
- Test 1: (3620 + 3710)/2 = 3665 psi
- Test 2: (3550 + 3800)/2 = 3675 psi
- Test 3: (3650 + 3900)/2 = 3775 psi
All three test averages are below 4000 psi. Criterion 1 fails. Criterion 2 is satisfied (all above 3500 psi), but both criteria must be met.
Conclusion: the concrete does not meet the 28-day acceptance criteria. The contractor's 'average over 3600' argument is irrelevant; the criterion is f'c, not 3600. The concrete is nonconforming pending further evaluation.
2. What are the appropriate follow-up steps, including core testing, and what conditions must cores meet?
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Follow-up sequence:
- Verify test validity first: review cylinder handling, curing, capping, and the two suspect caps. Improper capping can lower results by several percent. If testing errors are documented, retest or obtain new cores.
- If results stand, the EOR may require cores per ACI 318. Cores are taken from the actual member (here the column), typically 3 cores for the affected area, and tested.
- Core acceptance: the average of three cores must be at least 85% of f'c (3400 psi), and no single core may be below 75% of f'c (3000 psi).
- If cores fail, the EOR evaluates structural adequacy and may require remedial action or a reduced acceptance.
- Document all steps, notify EOR and contractor, and do not allow the contractor to self-accept.
Model answer should stress that cores test the in-place concrete, not the lab cylinders, and that the 85%/75% thresholds are the standard benchmarks.
3. The cores average 3550 psi with a low of 3100 psi. Is the concrete acceptable, and what is your recommendation?
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Check core criteria:
- Average of three cores: 3550 psi vs. 85% of 4000 = 3400 psi. Passes (3550 > 3400).
- Lowest core: 3100 psi vs. 75% of 4000 = 3000 psi. Passes (3100 > 3000).
So the in-place concrete meets the core acceptance criteria. The 28-day cylinder failure is resolved by the core results, provided the cores were properly extracted, conditioned, and tested.
Recommendation: document that the member satisfies core criteria, notify the EOR for final acceptance, and note the discrepancy between cylinder and core results for the record. The inspector's role is to report the data and the criteria check, not to unilaterally declare structural adequacy; the EOR makes the final call. If the EOR requires, additional evaluation may still apply.