Low Cylinder Breaks on a 4,000 psi Deck
The jurisdiction requires special inspection and testing for structural concrete. The project is a six-story cast-in-place frame, f'c = 4,000 psi at 28 days, and the approved plans require the testing agency to sample per ACI 318 and to report mix design, truck time, slump, and placement location. For the twelfth-floor deck pour, the lab's 28-day report shows: Set A, three cylinders, averaging 3,420 psi with a low cylinder of 3,150 psi; Set B averaging 3,610 psi; Set C averaging 3,880 psi. The report does not identify which truck or which portion of the deck each set represents, and the slump and concrete-temperature columns are blank. Column forms for the next tier are already set, and the contractor argues that "the average of all nine cylinders is 3,630, so it's fine."
1. Apply the ACI 318 acceptance criteria to these results and rule on the deck.
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The two criteria (ACI 318, acceptance of concrete - 26.12.3 in recent editions):
- The average of any three consecutive strength test results must equal or exceed f'c.
- No individual strength test result may fall below f'c by more than 500 psi.
A "strength test result" is the average of the cylinders in one set - not a single cylinder, and not an average of every cylinder on the job.
Criterion 2: the floor is 4,000 − 500 = 3,500 psi. Set A at 3,420 psi is below 3,500. Fails.
Criterion 1: the average of the three consecutive results is (3,420 + 3,610 + 3,880) ÷ 3 = 3,637 psi, below 4,000. Fails.
The contractor's method is wrong twice over. Averaging all nine individual cylinders is not a code criterion, and even that number, about 3,637 psi, is still below f'c. The 3,150 psi cylinder is not itself a separate failure - the set average is the test result - but it is a signal worth noting.
Ruling: the concrete represented by Set A does not meet the acceptance criteria. The deck cannot be accepted on these results, and the fact that the forms are set for the next tier does not change that. Nothing here authorizes the inspector to waive or split the difference; the path forward runs through the design professional and a defined investigation.
2. What options exist to resolve the nonconformance, and what must happen before you accept the affected deck?
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Option 1 - 56-day results, if the project documents allow it. Some specifications and ACI provisions permit evaluation at 56 days when 28-day results fail. This is not automatic and not the inspector's call; it requires the design professional's approval and must be established in the project documents before or contemporaneously with the failure, not invented afterward.
Option 2 - core tests (ACI 318, 26.12.6). If the design professional directs it, at least three cores are taken from the affected region, and the concrete is acceptable if the average of three cores is at least 0.85 f'c (0.85 × 4,000 = 3,400 psi) and no single core is below 0.75 f'c (3,000 psi). Cores must come from the actual questionable concrete, which is impossible to identify until the missing location data is reconstructed.
Option 3 - strength evaluation. If cores fail, the design professional performs a strength evaluation of the structure (ACI 318 Chapter 27), which may be analytical or may involve load testing.
Option 4 - removal and replacement of the affected concrete, which is rarely the first choice but is legitimate.
What you require before accepting: a written evaluation and recommendation from the design professional of record, the test report for any cores, and a documented disposition of the affected area that includes location. Until then, withhold concrete inspection sign-off for the affected element and do not let the next tier proceed on the assumption the deck is fine. Formalize the nonconformance in writing to the contractor and the design professional so the clock and the responsibility are both recorded.
3. The test report itself is incomplete. Why does that matter, and what do you require?
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Why it matters: a strength result is only usable if it can be tied to a location in the structure. Without truck identification, placement location, slump, and temperature, you cannot tell which part of the twelfth-floor deck Set A represents - which means you cannot tell the design professional where to core, and you cannot rule out that a different, untested portion of the deck is the weak concrete. The report as written is not adequate evidence of compliance; it is a number without an address.
What to require from the testing agency:
- Reconstruction of placement locations from batch tickets, delivery tickets, and the contractor's placement records, correlated to cylinder set identification.
- Confirmation that sampling and specimen preparation followed ASTM C172 and C31 and were performed by a qualified technician (ACI certification is the usual benchmark).
- Curing records: standard-cured cylinders for acceptance, field-cured only where the project documents call for them for scheduling purposes.
- Slump, air content where specified, concrete temperature, and mix identification for each set.
Also check the special inspection side: confirm the special inspector was on site during placement and that daily reports exist for the pour date. Gaps there compound the testing gap.
If locations cannot be reconstructed, say so in writing and let the design professional choose the core locations conservatively - typically broader coverage across the deck rather than a single set at one convenient point.