Cylinder Break Pattern Below Design Strength
A 28-day set of three 6x12 in. cylinders from a ready-mix pour (design f'c = 4000 psi) breaks at 3420, 3560, and 3710 psi. The technician notes that the cylinders were cast per ASTM C31, stored in the field under a plastic cover for 48 hours, then transported to the lab. During transport the ambient temperature reached 95°F. The lab received them at 50 hours after casting. The technician also reports that the cylinder ends were capped with a sulfur compound and tested at a loading rate of 35 psi/s. The project engineer asks whether the concrete failed to meet strength requirements and whether retesting is warranted.
1. Does this set meet the acceptance criteria for compressive strength, and what is the controlling issue?
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Compute the average of the three cylinders: (3420 + 3560 + 3710)/3 = 3563 psi. The acceptance criteria require every average of three consecutive tests to equal or exceed f'c, and no single test below f'c by more than 500 psi. Here 3563 < 4000 psi, so the average fails. The lowest single value, 3420 psi, is 580 psi below f'c, which also exceeds the 500 psi single-cylinder tolerance. Both criteria fail, so the set does not meet acceptance as tested. However, the controlling issue is procedural: field curing under plastic for 48 hours plus a 95°F transport excursion likely depressed the 28-day strength. Before declaring the concrete deficient, the curing and transport record must be evaluated against ASTM C31 requirements (initial curing 60-80°F, protection from moisture loss, transport after 8-48 hours).
2. What retest or investigation path should the engineer pursue, and what data are needed?
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Because the low breaks may be attributable to non-compliant initial curing, the engineer should first verify the curing history (temperature log, time in molds, transport conditions). If curing was non-compliant, the results may be invalid and a new set of cylinders should be cast from the same concrete or core samples taken per ASTM C42. Cores should be at least 2 in. diameter, with a length-to-diameter ratio of 1.5 to 2.0, and tested per ASTM C42. Acceptance of cores typically requires an average of three cores equal to at least 85% of f'c and no single core below 75% of f'c. Also confirm the loading rate: 35 psi/s is within the ASTM C39 range for a 4000 psi concrete (20-50 psi/s), so rate is not the culprit. The engineer should obtain the batch ticket, water-cement ratio, and any field test records before deciding.