ACI Certification Fresh Concrete Field Testing 2 — Questions and Answers
Question 1: When performing a slump test per ASTM C143, the tamping rod should penetrate the preceding layer by approximately how much?
- Approximately 1 inch (25 mm) into the underlying layer (Correct answer)
- 6 inches above the top of the concrete layer
- The rod should never penetrate the previous layer
- The rod should be submerged to the full depth of each layer
Correct answer: Approximately 1 inch (25 mm) into the underlying layer
ASTM C143 requires that the tamping rod penetrate approximately 1 inch (25 mm) into the underlying layer during filling to knit the layers together without excessively disturbing the lower portion.
The slump cone is filled in three equal layers, rodded 25 times each. The rod must penetrate just into the previous layer (about 1 inch) to achieve good integration. If the rod does not penetrate the lower layer, the three layers may not bond properly; if it penetrates too deeply, the previous layer is disturbed and the test results become erratic.
Question 2: What does a 'shear slump' vs. a 'true slump' indicate when performing ASTM C143?
- A shear slump occurs when the concrete shears off to one side rather than subsiding evenly; it indicates an unrepresentative sample or harsh mix, and the test must be repeated (Correct answer)
- A shear slump occurs only in high-slump concrete above 8 inches and is always acceptable
- A shear slump indicates high air content and must be reported as zero
- A shear slump is caused by too much vibration during consolidation of the sample
Correct answer: A shear slump occurs when the concrete shears off to one side rather than subsiding evenly; it indicates an unrepresentative sample or harsh mix, and the test must be repeated
In a shear slump, the upper portion slides off to one side instead of slumping uniformly; ASTM C143 requires that the test be repeated on a fresh sample because the result is not representative.
A true slump shows the concrete subsiding symmetrically from the center, maintaining general cone shape. A shear slump involves diagonal shearing, with part of the mass sliding off one side. ASTM C143 specifies that if a shear slump occurs, the test result is discarded and another test is immediately run on a new portion of the same composite sample.
Question 3: At what precision must concrete temperature be recorded when performing field testing per ASTM C1064?
- To the nearest 1°F (0.5°C) (Correct answer)
- To the nearest 5°F (2.5°C)
- To the nearest 0.1°F (0.05°C)
- To the nearest 10°F (5°C)
Correct answer: To the nearest 1°F (0.5°C)
ASTM C1064 requires that concrete temperature be reported to the nearest 1°F (0.5°C), using a thermometer with a resolution of at least 1°F and calibrated to ±1°F accuracy.
Temperature recording precision is important because temperature affects setting time, strength gain rate, air content, and admixture effectiveness. ASTM C1064 requires a thermometer accurate to ±1°F (±0.5°C). The probe must be immersed at least 3 inches (75 mm) into the freshly sampled concrete and left until the reading stabilizes (typically 2 minutes).
Question 4: When making concrete test cylinders per ASTM C31, how long must the molds remain undisturbed in the field after casting?
- A minimum of 24 hours and a maximum of 48 hours, stored at 60–80°F (16–27°C) (Correct answer)
- Exactly 12 hours, then immediately transported to the lab
- 72 hours at ambient temperature regardless of conditions
- Until the concrete reaches 1,000 psi, measured by penetration resistance
Correct answer: A minimum of 24 hours and a maximum of 48 hours, stored at 60–80°F (16–27°C)
ASTM C31 requires cylinders to remain in their molds in the field for a minimum of 24 hours and a maximum of 48 hours, protected from vibration and temperature extremes (60–80°F), before transport to the lab.
During the first 24 hours, concrete gains enough initial strength to resist disturbance during transport. Removing cylinders too early risks cracking the weak early concrete; keeping them too long (more than 48 hours) delays lab curing and affects test results. Field temperature during initial curing must be 60–80°F; temperatures outside this range require documentation.
Question 5: Per ASTM C31, how many blows with a rubber mallet are required to close rod holes on each layer when making a 4×8-inch cylinder?
- 10–15 blows around the outside of the mold after each layer is rodded (Correct answer)
- 5 blows only after the final layer
- 25 blows in the same pattern used for rodding
- No mallet tapping is required for 4×8-inch cylinders
Correct answer: 10–15 blows around the outside of the mold after each layer is rodded
ASTM C31 requires 10–15 blows with a rubber mallet around the outside circumference of the mold after each layer is rodded, to close rod holes and eliminate surface voids regardless of cylinder size.
After rodding each layer of a 4×8-inch cylinder (two layers, rodded 25 times each), the outside of the mold is tapped 10–15 times with a rubber mallet. The tapping closes rod holes and drives out surface bubbles that would otherwise leave voids in the hardened cylinder, reducing the measured compressive strength.
Question 6: What is the required composite sample volume when obtaining fresh concrete for a full set of field tests including two 4×8-inch cylinders per ASTM C172?
- At least 1.0 ft³ (28 L) to ensure sufficient material for all tests and cylinder fabrication (Correct answer)
- 0.25 ft³ (7 L), one wheelbarrow scoop is sufficient
- Exactly 0.5 ft³ (14 L) per ASTM C172
- 2.0 ft³ (57 L) is the mandatory minimum regardless of tests performed
Correct answer: At least 1.0 ft³ (28 L) to ensure sufficient material for all tests and cylinder fabrication
A full set of tests including two 4×8-inch cylinders requires approximately 0.5–1.0 ft³; ASTM C172 recommends obtaining at least 1 ft³ when strength specimens will be made to allow for remixing and handling losses.
ASTM C172 states the sample should not be less than 1 ft³ (28 L) when strength specimens will be made. Each 4×8-inch cylinder uses approximately 0.06 ft³, a slump cone uses about 0.2 ft³, and the pressure meter bowl uses about 0.25 ft³. Total for two cylinders plus three tests equals approximately 0.6 ft³, plus waste in handling and remixing.
ACI Certification Fresh Concrete Field Testing 2
When performing a slump test per ASTM C143, the tamping rod should penetrate the preceding layer by approximately how much?