Cylinder Mold Filling and Rodding Dispute
A technician is casting 6x12 in. cylinders from a slump of 1.5 in. concrete. The technician fills the mold in two layers and rods each layer 25 times with a 5/8 in. rod, then taps the mold with a mallet. A second technician argues that three layers and 25 strokes per layer are required, and that the rod should be 3/4 in. The first technician also leaves the cylinders outdoors at 45°F overnight without a cover. The next morning, the cylinders show a 0.5 in. settlement at the top and some surface drying. The lab supervisor must decide whether the cylinders are acceptable for 28-day testing.
1. Identify the procedural errors in casting and initial curing.
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For a 6x12 in. cylinder, ASTM C31 requires filling in three layers of approximately equal volume, each rodded 25 times with a 5/8 in. diameter rod (for 6-in. cylinders). The first technician used two layers, which is non-compliant, and the second technician's claim of a 3/4 in. rod is incorrect for 6-in. molds (3/4 in. is for larger molds). Tapping the mold with a mallet is allowed to close voids but should not substitute for rodding. Initial curing must be at 60-80°F and protected from moisture loss; leaving cylinders uncovered at 45°F overnight violates both temperature and moisture requirements, causing the 0.5 in. settlement and surface drying. These errors compromise the 28-day strength and make the cylinders unsuitable for acceptance testing.
2. What should the supervisor do with these cylinders, and what is the correct procedure going forward?
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The supervisor should reject the cylinders for acceptance testing because curing temperature was below 60°F and moisture was not maintained, both of which can reduce strength. The cylinders may be kept for informational purposes only, clearly labeled. Going forward: fill 6x12 in. molds in three equal layers, rod each layer 25 times with a 5/8 in. rod, tap the sides to release air, strike off the top, and cover immediately. Initial curing must be 60-80°F with moisture retention (plastic cover or wet burlap) for 24-48 hours, then transport to the lab. Record curing temperatures. If the concrete is still being placed, cast a new set of cylinders per ASTM C31 and ensure the field curing box maintains temperature.