Failing Cylinders, a Trailer in the Heat, and the Next Pour
A 400,000 sq ft warehouse has an 8-inch interior slab specified at 4,000 psi. The specification adopts the standard acceptance criteria: the average of any three consecutive 28-day tests must equal or exceed 4,000 psi, and no single test may fall below 3,500 psi. The first 40,000 sq ft placement produced four cylinder tests: 3,120, 3,650, 3,280, and 3,900 psi. The batch plant records show a consistent mix design and no admixture errors. Cylinders were stored in a job trailer over a weekend when temperatures reached 96°F, and one set was transported loose in a pickup truck. The concrete subcontractor wants to pour the next 40,000 sq ft tomorrow and argues that the slab is 'probably fine' because there are no visible cracks. The owner's engineer proposes cores. The project is 3 weeks behind.
1. Evaluate the test results. Does the slab pass, and should the next pour proceed?
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The slab does not pass on the cylinder results. The first three consecutive tests average (3,120 + 3,650 + 3,280) / 3 = 3,350 psi, below 4,000 psi. The next three average (3,650 + 3,280 + 3,900) / 3 = 3,610 psi, also below 4,000 psi. Two individual tests, 3,120 and 3,280 psi, are below the 3,500 psi minimum. So both acceptance criteria fail.
The next pour should not proceed over the affected area tomorrow. It may proceed elsewhere only if the batch plant, mix, and curing records are verified and the testing error is isolated. The 'no visible cracks' argument is not acceptance evidence; concrete can be below strength without cracking. The superintendent should place the affected area on hold, issue a nonconformance report, notify the owner and engineer, and require a testing plan before any further placement in that area.
2. Design the investigation. Who pays, and what happens if the cores pass or fail?
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The investigation should start with the testing records: cylinder sampling, curing temperature, transport, and break machine calibration. The 96°F trailer storage and loose truck transport are serious irregularities, so the cylinders may not represent the slab. Cores should be taken from the area represented by the failing tests, not from convenient edges. Under the standard acceptance criteria, the average of three cores should be at least 85% of 4,000 psi, or 3,400 psi, and no single core should be below 75%, or 3,000 psi.
If the cores pass, the concrete can be accepted, but the testing agency's handling is still a nonconformance. The contractor should pay for the cores if the original tests were valid; if the cores pass because the cylinders were mishandled, the testing agency or contractor responsible for cylinder handling should bear the cost. If the cores fail, the engineer must evaluate structural adequacy and specify repair or replacement; the concrete subcontractor and testing agency bear their respective responsibility. The owner should not pay for rework caused by the contractor's quality failure.
3. What root causes should be corrected before the next 40,000 sq ft pour?
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The immediate root cause is cylinder mishandling, not necessarily the concrete. Cylinders stored in a trailer at 96°F and transported loose will not give reliable 28-day strengths. The correction is a controlled curing box with temperature monitoring, standard initial curing, protected transport, and a chain-of-custody record from sample to break. The testing agency should be audited and its technician retrained.
Second, the batch plant records should be reviewed for water addition, aggregate moisture, and truck time. Third, the subcontractor's curing method on the slab should be verified, including evaporation control. Fourth, the next pour should have extra cylinders, a hold point before placement, and a pre-pour meeting with the testing agency, subcontractor, and engineer. The project is 3 weeks behind, but pouring more slab on an unverified mix or testing process would multiply the rework. Quality hold points are cheaper than demolition.