ACI Certification Making and Curing Specimens 2 — Questions and Answers
Question 1: What is the minimum concrete temperature range required for initial field curing of test cylinders per ASTM C31?
- 60°F to 80°F (16°C to 27°C) (Correct answer)
- 40°F to 90°F (4°C to 32°C)
- 50°F to 70°F (10°C to 21°C)
- 70°F to 100°F (21°C to 38°C)
Correct answer: 60°F to 80°F (16°C to 27°C)
ASTM C31 Section 10.1 requires that test cylinders be maintained within 60°F to 80°F (16°C to 27°C) during the initial 24-hour field curing period to ensure proper early strength gain without temperature-induced variability.
The 60–80°F initial curing window balances two competing concerns: too cold slows early hydration and reduces 24-hour strength, risking damage during transport; too warm accelerates hydration, reducing ultimate strength. Temperature during initial curing should be documented, and excursions outside the range noted on the test report.
Question 2: For a 4×8-inch concrete test cylinder made from concrete with a slump between 1 and 3 inches, how many layers and rods per layer are required per ASTM C31?
- Two layers, 25 rods per layer (Correct answer)
- Three layers, 25 rods per layer
- One layer, 50 rods
- Two layers, 10 rods per layer
Correct answer: Two layers, 25 rods per layer
ASTM C31 requires 4×8-inch cylinders to be filled in two approximately equal layers with 25 rods per layer for concrete with a slump greater than 1 inch.
The 4×8-inch cylinder requires two layers (each approximately 4 inches deep) with 25 rods per layer. For 6×12-inch cylinders, three layers of 4 inches each are used, also with 25 rods per layer. For concrete with slump of 1 inch or less, vibration consolidation is required per ASTM C31 Section 9.4.
Question 3: What is the purpose of using a 'curing box' or insulated curing container when making test cylinders in cold-weather field conditions?
- To maintain the cylinders within the 60–80°F (16–27°C) ASTM C31 initial curing temperature range by preventing heat loss to the cold environment (Correct answer)
- To cure the cylinders faster using supplemental heat to achieve early break results at 3 days
- To prevent freeze-thaw cycling of cylinders once they are in the lab
- To seal the cylinders from moisture loss only, without temperature control
Correct answer: To maintain the cylinders within the 60–80°F (16–27°C) ASTM C31 initial curing temperature range by preventing heat loss to the cold environment
In cold weather, cylinders in insulated boxes (sometimes with a small heat source) are kept at the 60–80°F range required by ASTM C31, protecting early-age concrete from temperatures below 60°F that would slow hydration.
Freshly cast cylinders generate very little heat from hydration in the first 24 hours, making them vulnerable to ambient cold temperatures. Below 60°F, hydration slows dramatically, and below 32°F, water in the paste can freeze, permanently damaging the specimen. Insulated curing boxes with a small electric heat pad or propane heater are standard practice on winter pours.
Question 4: When is vibration consolidation required instead of rodding for concrete test cylinder preparation per ASTM C31?
- When the concrete has a slump of 1 inch (25 mm) or less, or when specified by the engineer (Correct answer)
- Only when the concrete has no slump (0 inches) and is a no-slump mix
- Always, because vibration produces more consistent cylinders than rodding
- Only when the concrete contains steel fibers or other reinforcing fibers
Correct answer: When the concrete has a slump of 1 inch (25 mm) or less, or when specified by the engineer
ASTM C31 requires vibration consolidation when concrete slump is 1 inch (25 mm) or less because stiff concrete cannot be adequately consolidated by the standard rodding procedure.
Stiff concrete (slump ≤ 1 inch) does not flow under the force of the tamping rod, so rodding creates channels rather than consolidating the mix. Vibration with an internal vibrator (diameter ≤ 1/3 the cylinder diameter) is inserted at the center and one or two intermediate points per layer and withdrawn slowly to avoid leaving a void.
Question 5: What is the maximum height to which the initial fill should be above the top of the mold when making test cylinders, to allow for rodding without material loss?
- Each layer should overfill the mold by approximately 10% to account for consolidation settling (Correct answer)
- Fill to exactly the top mark with no overfill and strike off immediately
- Overfill by 1 to 2 inches before rodding each layer
- No specific overfill is required; fill until the layer depth is approximate
Correct answer: Each layer should overfill the mold by approximately 10% to account for consolidation settling
ASTM C31 directs technicians to fill each layer so that after consolidation (rodding), the layer is at the intended depth; a slight 10% overfill accounts for the volume reduction from consolidation.
When filling a 4×8-inch cylinder in two layers, each consolidated layer should be about 4 inches deep. Since rodding compacts the concrete slightly, each layer is filled to slightly above the target depth before rodding. After the top layer is rodded and tapped, the concrete should be approximately level with the top of the mold or slightly overfilling, allowing it to be struck off flush.
Question 6: Per ASTM C31, what is the required identification information that must be marked on each test cylinder immediately after casting?
- Project name or number, sample location/structural element, date and time of sampling, and specimen number within the test set (Correct answer)
- Technician's name only
- Batch ticket number only
- Cylinder size (4×8 or 6×12) and break age only
Correct answer: Project name or number, sample location/structural element, date and time of sampling, and specimen number within the test set
ASTM C31 requires immediate positive identification of each cylinder with project, location, sampling time and date, and specimen number to maintain chain of custody and link test results to the correct structural element.
Without proper identification, test results cannot be associated with specific structural elements, making it impossible to evaluate a failing test result. ASTM C31 minimum identification includes: project name or ID, structural element (e.g., 'Column C-4, Level 3'), date and time of sampling, and cylinder number within the set. Additional useful data includes truck ticket number, slump, air content, and temperature.
ACI Certification Making and Curing Specimens 2
What is the minimum concrete temperature range required for initial field curing of test cylinders per ASTM C31?