Fourth-Floor Concrete Strength Failure and Core Evaluation
On a six-story cast-in-place office project, the specified 28-day compressive strength is 4,000 psi. The testing agency reports that three 6 by 12 in. cylinders from fourth-floor columns, representing 90 cy, broke at 3,150, 3,420, and 3,680 psi. The average is 3,417 psi, and two results fall below f'c minus 500 psi. The contractor's superintendent says the cylinders sat in the sun for two days before standard curing and asks the CA to wait for the 56-day breaks. The concrete subcontractor has stripped forms and is scheduled to place the fifth floor in five days. The structural engineer is available by phone. The owner's QA plan requires a written nonconformance report within 24 hours, requires the testing agency to be retained by the owner, and does not authorize the CA to direct the testing agency or stop the contractor's work. The lab is accredited, and the mix design was previously approved.
1. What immediate actions should the CA take, and what acceptance criteria govern the failed cylinder results?
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The CA should treat this as a potential nonconformance, not as a numbers dispute. First, verify the report on its face: cylinder size and sampling frequency, lab accreditation, standard curing records, and whether the cylinders represented the fourth-floor columns. The reported values fail the ACI 318/301 strength-test criterion because the average of three tests is below f'c and two results are below f'c minus 500 psi.
Within 24 hours, issue a written nonconformance report to the contractor, copy the owner and structural engineer, and state the facts and the required evaluation without directing the testing agency or the contractor's means and methods. Require the contractor to propose evaluation by cores per ASTM C42, with core locations selected or approved by the structural engineer.
Do not treat the fifth-floor pour as automatically cleared; ask the structural engineer whether interim shoring or load restrictions are needed. The CA's authority is to report, reject nonconforming work where the contract gives that authority, and document; it is not to unilaterally accept, retest, or stop work.
2. The contractor alleges curing error and wants to wait for 56-day breaks. How should the CA evaluate that request?
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The allegation about sun exposure matters only if it affected standard curing. The CA should obtain the testing agency's cylinder curing and transportation records and the lab's receiving records. Cylinders for acceptance must be standard-cured; field-cured cylinders are for form removal or early loading, not acceptance.
If there is credible evidence of lab or curing error, the CA should report it to the owner and structural engineer and ask for a written evaluation, but the CA should not waive the failed result or unilaterally authorize 56-day breaks. A 56-day evaluation is acceptable only if the specifications or the structural engineer authorize it and the delay does not compromise the schedule or safety.
The contractor's request to wait is not a substitute for the required low-strength evaluation; the CA should require a written proposal with dates and consequences.
3. If cores meet the core criteria, who bears the cost of coring, and what happens if they do not?
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If cores meet the ACI 318 core criteria-average of three cores at least 85% of f'c and no single core below 75%-the structural engineer may accept the concrete. The nonconformance report should then be closed with the engineer's written acceptance and the core report attached.
Cost allocation depends on the contract and the cause: if the original test failure was caused by the contractor's concrete, the contractor typically bears coring and evaluation costs; if the failure was caused by the owner-retained testing agency's error, the owner may bear them. The CA should not decide this informally; document the cause and let the contract terms govern.
If cores fail, the structural engineer must decide whether load tests, strengthening, or removal and replacement are required. The CA should not certify the affected work for payment until the disposition is documented, and remediation should be handled by change order or contract enforcement, not by informal direction.