Temperature Measurement and Initial Cylinder Curing
On a 98°F afternoon, a technician checks the fresh concrete temperature before placing a slab. The specification limits concrete to 95°F. The technician fills a steel wheelbarrow from the chute, sets it in direct sunlight, inserts the thermometer about 1 in. into the concrete, and reads 94.0°F after 45 seconds. The concrete is accepted. Later, six 6 by 12 in. cylinders are molded and left uncovered beside the forms in the sun. At 4 p.m. they are moved in an open pickup bed and stored in a jobsite office at 82°F overnight. They are demolded at 30 hours and placed in a curing box at 70°F.
1. Evaluate the temperature measurement. Was 94.0°F a valid basis for acceptance? Give the correct procedure.
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No. The measurement does not meet ASTM C1064 requirements. The thermometer should be inserted at least 3 in. (75 mm) into a representative concrete sample, left in place for at least 2 minutes, and read to the nearest 0.5°C (1°F). The sample should be shaded and protected from wind and direct sun, and the thermometer must have a current accuracy check.
A steel wheelbarrow in direct sunlight can heat the concrete, a 1 in. embedment may not reach the concrete temperature, and 45 seconds is too short for equilibration. The 94.0°F reading could therefore be biased either way and is not a reliable acceptance basis. The technician should obtain a fresh sample, place it in a suitable container in the shade, insert the thermometer at least 3 in., wait at least 2 minutes, and read again. If the corrected reading exceeds 95°F, the load should be handled according to the specification, typically rejection or supplier notification.
2. Identify the curing and handling errors and explain how each could affect the 28-day strength result.
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Initial curing: Cylinders left uncovered in 98°F sun lose moisture and become too hot. Initial curing should be at 60 to 80°F (16 to 27°C), in a moist environment, and protected from vibration and damage. High early temperature and drying can reduce later strength and increase variability.
Transport: Moving cylinders in an open pickup bed can subject them to impact, vibration, and temperature extremes. They should be secured and protected.
Storage: An office at 82°F is above the recommended initial-curing range. Demolding at 30 hours is within the usual 24 ± 8 hour window, but only if the cylinders have gained enough strength and have been protected. After demolding, they should be standard-cured in moisture at 73.5 ± 3.5°F (23 ± 2°C). The 70°F curing box is at the low edge of that range and must be moist. Overall, these errors can lower the measured 28-day strength and make the results unrepresentative of properly cured concrete.
3. If the specification also requires field-cured cylinders to determine when the slab can be opened, how should those cylinders differ from acceptance cylinders?
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Acceptance cylinders are standard-cured to evaluate the concrete mixture and must follow the standard moisture and temperature requirements. Field-cured cylinders are used to estimate strength in the structure and should be cured as near as practicable to the actual structure, including its temperature and moisture exposure.
The technician should not use one set for both purposes. Field-cured cylinders need to experience the same early-age conditions as the slab, while acceptance cylinders need protected initial curing and standard moist curing. Both sets should be molded and handled properly, identified clearly, and tested at the specified ages. If field-cured cylinders are mishandled in the same way as described here, they will not reliably indicate when the slab can be opened.