Cylinder Break Results Below fc' on a Column Lift
A 4,000 psi specified compressive strength mix is used for a 12 ft column lift. Six standard 6x12 in. cylinders were cast from the truck and cured in the field in a curing box; the lab reports 28-day breaks of 3,650, 3,720, 3,780, 3,600, 3,690, and 3,750 psi. The batch ticket shows the mix met the approved design, slump was 5 in. within tolerance, and air was 5.5%. The contractor points out that field-cured cylinders are required for evaluating the structure, and argues the average is close to 4,000 psi and the structure is fine. The engineer is considering requiring cores.
1. Apply the acceptance criteria and determine whether the lot passes.
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Use the standard acceptance criteria for compressive strength (ACI 318 / ASTM C94 framework):
- No single test result falls below f'c by more than 500 psi. Here the lowest is 3,600 psi; f'c − 500 = 3,500 psi. So 3,600 > 3,500 - this criterion is met.
- The average of any three consecutive tests equals or exceeds f'c. The results average (3,650+3,720+3,780+3,600+3,690+3,750)/6 = 3,698 psi, which is below 4,000 psi. Depending on how the tests are grouped, the three-test averages (roughly 3,717, 3,700, 3,690, 3,680) are all below 4,000 psi. This criterion fails.
So the lot does not satisfy the average criterion. The contractor's 'close to 4,000' argument is not the standard; the average of three consecutive tests must meet or exceed f'c. The lot is nonconforming on the strength criterion and the engineer is justified in requiring further evaluation.
2. The contractor argues field-cured cylinders govern. Assess that argument and the role of standard-cured cylinders.
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The argument is partly right and partly wrong. Standard-cured cylinders (moist-cured at 73 °F) are the basis for acceptance of the mix and for the strength acceptance criteria. Field-cured cylinders are used to evaluate whether the structure has reached the strength assumed for form removal, post-tensioning, or opening to traffic - they reflect in-place curing and are typically lower.
The record here does not say whether the cylinders were standard-cured or field-cured. The setup says 'cured in the field in a curing box,' which is field curing. If these are field-cured, they are not the acceptance specimens; the engineer should first confirm whether standard-cured companion cylinders exist and what they show. If only field-cured cylinders exist, the acceptance criteria cannot be applied cleanly, and the engineer should require standard-cured cylinders or cores.
The key judgment: do not accept or reject based on the wrong specimen type; establish which specimens govern, then apply the criteria.
3. If cores are required, describe the investigation and the decision logic.
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Core investigation per ACI 318 / ASTM C42:
- Take at least three cores from representative locations in the questionable lift, avoiding reinforcement and edges; core diameter at least 2 in., preferably 3–4 in., with length at least 1× diameter (preferably 2×).
- Test cores in compression; the average of three cores must be at least 85% of f'c, and no single core below 75% of f'c, for the area to be considered adequate. (Confirm the exact criteria in the governing code edition.)
- Cores reflect in-place strength and curing, so they are the appropriate basis when cylinder results are questioned.
Decision logic:
- If cores meet the criteria, the structure may be accepted; document and proceed.
- If cores fail, the engineer evaluates structural adequacy (perhaps with a structural analysis) and chooses remediation: reduce loads, strengthen, or remove and replace.
- In all cases, investigate the cause (batching, curing, sampling, testing error) so the next lift is corrected.
Do not let the contractor's 'average is close' argument substitute for the code criteria.