Making and Curing Concrete Test Specimens in the Field
Molds and filling
Field specimens must represent the concrete and be made promptly. Use clean, properly sized molds: 6 by 12 in. (150 by 300 mm) or 4 by 8 in. (100 by 200 mm). For 6 by 12 in. cylinders, fill in three layers and rod each layer 25 times; for 4 by 8 in. cylinders, fill in two layers and rod each layer 25 times. Use a rounded rod, tap the outside of the mold after each layer to close voids, and strike off the top.
Curing and handling
Initial curing must protect specimens from moisture loss and keep them at 60 to 80°F (16 to 27°C). Remove molds after about 24 hours, then cure in moist conditions at 73.4 ± 3.5°F (23 ± 2°C) for standard curing. Mark specimens for identification, keep them moist, and transport them without shock or freezing. Field-cured specimens are cured at the job site, not in the moist room.
Traps and worked example
Traps include using three layers for a 4 by 8 in. cylinder, too few rod strokes, letting the top dry, and storing cylinders in a hot truck. Another trap is confusing field curing with standard curing.
Rule of thumb: 6 by 12 in. = three layers; 4 by 8 in. = two layers; 25 strokes in every layer. Worked example: a 6 by 12 in. cylinder has a volume of about 0.196 ft³, so three cylinders need about 0.59 ft³ of concrete.