Concrete Strength Acceptance and Early Form Removal
A cast-in-place retaining wall stem is specified as f'c = 4,000 psi at 28 days. The mix design is a 6-bag mix with w/c = 0.45 and no admixtures other than a retarder. The contractor wants to strip the wall forms at 3 days to reuse them. Two 6×12 in. cylinders were broken at 3 days and gave 2,050 and 2,250 psi. The specification requires 70% of f'c before form removal and requires the 28-day acceptance to be based on pairs of cylinders with no single test more than 500 psi below f'c. The engineer observes that the cylinders were stored in the field next to the wall, uncovered, in 40°F weather.
1. Evaluate whether the 3-day strengths satisfy the form-removal requirement.
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Required 3-day strength = 0.70 × 4,000 = 2,800 psi. The measured strengths are 2,050 and 2,250 psi, averaging 2,150 psi - about 650 psi (23%) short of the requirement. So the form-removal criterion is not met. Even the higher cylinder (2,250 psi) fails. The contractor must either wait longer, provide additional curing heat, or use a different mix. Note that the cylinders were stored uncovered in 40°F weather: at that temperature, cement hydration is severely retarded, so the cylinders (and the wall) are likely colder than the assumed curing condition. The measured strengths may therefore be an optimistic or at least unrepresentative indicator of in-place strength, and the field concrete could be even weaker.
2. Assess the validity of the cylinder results as a measure of in-place strength.
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The results are questionable as an in-place indicator for three reasons. (1) Curing temperature: cylinders stored uncovered at 40°F experience retarded hydration; if the wall itself is warmer (e.g., from hydration heat or enclosure), the cylinders underestimate in-place strength; if the wall is colder, they overestimate. Either way, the cylinders do not represent the wall unless cured per the standard (moist, 60–80°F, or field-cured alongside the member). (2) Moisture: uncovered cylinders lose moisture, which reduces strength gain. (3) Sampling: two cylinders from one batch is a small sample; a single low pair does not establish the population strength. The defensible approach is to field-cure a set alongside the wall under the same conditions, or to use maturity/NDT methods calibrated to the mix, rather than relying on lab-cured cylinders stored in the open.
3. The 28-day results come back as 3,400 and 3,600 psi. Apply the acceptance rule and state the outcome.
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Average of the pair = 3,500 psi, which is below f'c = 4,000 psi. The rule also says no single test may be more than 500 psi below f'c: the low cylinder is 4,000 − 3,400 = 600 psi below, which exceeds the 500 psi limit. Both the average criterion and the single-test criterion are violated, so the lot fails acceptance. The next step is not automatic rejection: the engineer should require core testing (three cores per the standard, with a specified core-strength acceptance based on 85% of f'c), and evaluate structural adequacy by analysis. If cores pass and the structure is adequate, the concrete may be accepted; otherwise, remediation or removal is required. The 3-day shortfall is consistent with the 28-day shortfall, suggesting the mix or curing was systematically deficient.