Core Testing After Low Cylinder Breaks
A set of three 6x12 in. cylinders from a 4500 psi mix breaks at 3900, 4100, and 4200 psi at 28 days. The average is 4067 psi, below 4500 psi. The cylinders were cast and cured per ASTM C31, and the lab's records show compliant curing. The project engineer orders cores per ASTM C42. Three cores are taken: 4 in. diameter, length-to-diameter ratio 1.8, tested at 28 days plus 7 days after coring. Core results are 3800, 4000, and 4100 psi. The engineer must decide whether the concrete meets the acceptance criteria.
1. Apply the ASTM C42 acceptance criteria to the core results.
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The core average is (3800 + 4000 + 4100)/3 = 3967 psi. ASTM C42 acceptance typically requires the average of three cores to be at least 85% of f'c and no single core below 75% of f'c. 85% of 4500 = 3825 psi; 3967 > 3825, so the average passes. 75% of 4500 = 3375 psi; the lowest core is 3800 psi, which is above 3375 psi. Therefore, the cores satisfy the acceptance criteria, and the concrete can be accepted despite the low cylinder average. Note that core strength is generally lower than cylinder strength, so this outcome is consistent.
2. What factors could explain the difference between cylinder and core results, and what should be documented?
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Cores are typically 4 in. diameter with an L/D of 1.8, which requires a correction factor per ASTM C42 (e.g., for L/D 1.8, a factor near 0.98). The core strength is affected by drilling damage, moisture condition, and the fact that cores are taken from the actual structure, which may have different consolidation and curing than the cylinders. The cylinders may have been over-vibrated or the lab may have had a systematic bias. Documentation should include core location, diameter, L/D, correction factors applied, moisture conditioning, and test dates. The engineer should also verify that the core test was performed at the correct age and that the cores were not damaged during extraction. The acceptance decision should be recorded with the core average and the 85%/75% criteria.