ACI Curing, Protection & Quality Control 3 — Questions and Answers
Question 1: Which chemical admixture is commonly used in hot weather concreting to offset the accelerating effect of high temperatures?
- Calcium chloride
- Fly ash accelerator
- Set retarder (Correct answer)
- Air-entraining agent
Correct answer: Set retarder
Set retarders slow hydration, compensating for the accelerated set caused by elevated concrete temperatures in hot weather.
Question 2: The water-to-cementitious-materials ratio (w/cm) primarily affects which property of hardened concrete?
- Color uniformity
- Compressive strength and durability (Correct answer)
- Surface texture
- Workability only
Correct answer: Compressive strength and durability
The w/cm ratio is the single most important factor controlling concrete compressive strength and long-term durability.
Question 3: During cold weather concreting, the concrete temperature at the point of placement should be maintained at a minimum of:
- 25°F (-4°C)
- 40°F (5°C)
- 50°F (10°C) (Correct answer)
- 55°F (13°C)
Correct answer: 50°F (10°C)
ACI 306 requires concrete to be placed and maintained at a minimum of 50°F (10°C) to ensure adequate hydration in cold weather.
Question 4: What is the primary purpose of fogging or misting water over fresh concrete in hot, dry, or windy conditions?
- To add mix water to restore slump
- To reduce plastic shrinkage cracking by slowing evaporation (Correct answer)
- To cool the subgrade before placement
- To wash excess bleed water off the surface
Correct answer: To reduce plastic shrinkage cracking by slowing evaporation
Fogging raises humidity immediately above the slab surface, reducing evaporation rate and thus preventing plastic shrinkage cracking.
Question 5: A cylinder break at 28 days shows strength below the specified f'c. What is the first step in ACI-compliant quality control response?
- Immediately remove the suspect concrete
- Conduct additional testing such as cores per ACI 318 (Correct answer)
- Stop all concrete placement on the project
- Double the curing period for all future placements
Correct answer: Conduct additional testing such as cores per ACI 318
ACI 318 requires taking cores from the suspect area as the next investigative step before any remedial action is decided.
Question 6: Which test is performed in the field to determine the air content of fresh concrete?
- Kelly ball test
- Pressure meter (Type B) test (Correct answer)
- Vebe consistometer test
- Proctor compaction test
Correct answer: Pressure meter (Type B) test
The pressure meter (ASTM C231) is the most common field method for measuring air content in fresh concrete.
Question 7: Plastic shrinkage cracking typically occurs during which phase of concrete construction?
- During demolding after 24 hours
- Before the concrete has set while still in a plastic state (Correct answer)
- During the 7-day curing period
- During drying shrinkage after 28 days
Correct answer: Before the concrete has set while still in a plastic state
Plastic shrinkage cracks form when the evaporation rate from the surface exceeds the rate of bleed water supply, while concrete is still plastic.
ACI Curing, Protection & Quality Control 3
Which chemical admixture is commonly used in hot weather concreting to offset the accelerating effect of high temperatures?