Cylinder Molding, Curing, and a Cap Failure
A technician molds three 4 by 8 in. cylinders for a 7-day check. The cylinders are filled in three equal layers, each rodded 25 times, and struck off. They are left outdoors at 95°F, demolded at 24 hours, transported in a loose bucket, and capped with sulfur mortar about 1/2 in. thick. At 7 days, one cylinder breaks at 2,800 psi and the fracture runs through the cap. The other two break at 3,050 and 3,100 psi with proper fractures. The specification requires 4,000 psi at 28 days. The technician wants to report the average of 3,050 and 3,100 psi, ignoring the low break.
1. List the procedural errors in molding, curing, transporting, and capping, and explain how each can affect the measured strength.
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Molding: A 4 by 8 in. cylinder should be filled in two layers, not three, with 25 rod strokes per layer. Three layers can increase segregation and is not the standard procedure.
Initial curing: Cylinders left outdoors at 95°F are too hot and may dry out. Initial curing should be at 60 to 80°F (16 to 27°C), in a moist environment, and protected from vibration and impact.
Transport: A loose bucket allows cylinders to shift, strike each other, and vibrate, which can damage the specimen and lower strength.
Capping: A sulfur mortar cap about 1/2 in. thick is too thick; sulfur caps should be about 1/8 in. (3 mm) and must be plane and perpendicular to the cylinder axis. A thick or poorly bonded cap can crush or fracture before the concrete fails, producing a low or invalid result. The 2,800 psi break through the cap is a clear example of a cap-related invalid failure.
2. How should the 2,800 psi result be treated, and what is the appropriate 7-day result?
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The 2,800 psi result should not be treated as a valid concrete strength. The fracture through the cap shows that the failure occurred in the capping material or at the cap-concrete interface, not in the concrete. The result should be marked invalid and excluded from the strength average.
However, the technician should not simply ignore a low break because it is inconvenient. The reason for exclusion must be documented. With two valid specimens remaining, the 7-day informational average is (3,050 + 3,100) / 2 = 3,075 psi. That value is only an indication of early strength; it is not a 28-day acceptance result. If the specification requires a certain number of valid specimens for acceptance, the missing specimen cannot be replaced after the fact without additional sampling or other approved action.
3. What changes are needed for the 28-day acceptance cylinders?
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Use the correct molds and the correct number of layers: two layers for 4 by 8 in. cylinders or three layers for 6 by 12 in. cylinders, with 25 rod strokes per layer. Initially cure in a moist environment at 60 to 80°F, protected from sun, drying, vibration, and impact. Transport cylinders securely in a padded container.
Demold at 24 ± 8 hours only when the cylinders are strong enough, then standard-cure in moisture at 73.5 ± 3.5°F (23 ± 2°C) until testing. Cap with a thin, plane, perpendicular cap, using sulfur mortar about 1/8 in. thick or another approved capping system. Test at 28 days with a calibrated machine and the proper loading rate. Use standard-cured cylinders for acceptance and keep field-cured cylinders separate if they are needed for service-opening decisions.