ACI Cold & Hot Weather Concreting 2 — Questions and Answers
Question 1: When concreting in cold weather, what is the minimum concrete temperature recommended by ACI 306 for sections greater than 12 inches thick?
- 45°F (7°C) (Correct answer)
- 50°F (10°C)
- 55°F (13°C)
- 60°F (16°C)
Correct answer: 45°F (7°C)
ACI 306 specifies a minimum placed concrete temperature of 45°F for sections greater than 12 inches thick, as thicker sections retain heat better.
Question 2: During hot weather concreting, which admixture type is most commonly used to counteract accelerated setting?
- Type A water-reducing retarder
- Type C accelerating admixture
- Type E water-reducing accelerator
- Type B retarding admixture (Correct answer)
Correct answer: Type B retarding admixture
Type B retarding admixtures slow the hydration reaction, offsetting the accelerated setting caused by high temperatures.
Question 3: What is the primary risk of using calcium chloride as an accelerator in cold weather concrete containing embedded steel?
- It causes excessive bleeding
- It promotes corrosion of the reinforcement (Correct answer)
- It reduces compressive strength
- It increases air entrainment
Correct answer: It promotes corrosion of the reinforcement
Calcium chloride is a chloride-based accelerator that can initiate and accelerate corrosion of embedded steel reinforcement.
Question 4: In hot weather, why should concrete be placed as quickly as possible after mixing?
- To prevent excessive bleed water
- To avoid loss of slump and premature stiffening (Correct answer)
- To reduce the water-cement ratio
- To improve air entrainment stability
Correct answer: To avoid loss of slump and premature stiffening
High temperatures accelerate hydration, causing rapid slump loss and premature stiffening if placement is delayed.
Question 5: Which curing method is particularly effective in cold weather because it provides both insulation and moisture retention?
- Liquid membrane-forming compound
- Insulating blankets with moisture underneath (Correct answer)
- White-pigmented plastic sheeting only
- Steam curing at ambient pressure
Correct answer: Insulating blankets with moisture underneath
Insulating blankets placed over wet burlap or plastic sheeting trap both heat and moisture, satisfying cold-weather curing requirements simultaneously.
Question 6: ACI 305 defines hot weather concreting conditions as any combination of factors that causes what primary concern?
- Excessive bleeding and segregation
- Rapid evaporation of mixing water and accelerated setting (Correct answer)
- Reduced air entrainment
- Increased shrinkage cracking only after hardening
Correct answer: Rapid evaporation of mixing water and accelerated setting
ACI 305 focuses on rapid moisture evaporation and accelerated setting as the defining hot-weather risks that impair concrete quality.
Question 7: What effect does high concrete temperature have on the required air-entraining admixture dosage?
- Less admixture is needed at higher temperatures
- Temperature has no effect on admixture dosage
- More admixture is needed to achieve the same air content (Correct answer)
- Air entrainment is not used in hot weather
Correct answer: More admixture is needed to achieve the same air content
Higher concrete temperatures reduce the effectiveness of air-entraining admixtures, requiring increased dosage to achieve the specified air content.
ACI Cold & Hot Weather Concreting 2
When concreting in cold weather, what is the minimum concrete temperature recommended by ACI 306 for sections greater than 12 inches thick?