ACI Cold & Hot Weather Concreting 3 — Questions and Answers
Question 1: What does ACI 306 define as the minimum duration for protecting concrete from freezing after placement in cold weather?
- Until compressive strength reaches 500 psi
- Until compressive strength reaches 3,500 psi
- Until compressive strength reaches at least 500 psi, typically not less than 2 days (Correct answer)
- Until the concrete has cured for 28 days
Correct answer: Until compressive strength reaches at least 500 psi, typically not less than 2 days
ACI 306 requires concrete to be protected until it achieves at least 500 psi compressive strength, generally a minimum of 2 days, to resist freeze damage.
Question 2: During hot weather finishing operations, plastic shrinkage cracks are most likely to form when the evaporation rate exceeds:
- 0.10 lb/ft²/hr
- 0.15 lb/ft²/hr
- 0.20 lb/ft²/hr (Correct answer)
- 0.25 lb/ft²/hr
Correct answer: 0.20 lb/ft²/hr
ACI 305 identifies an evaporation rate exceeding 0.20 lb/ft²/hr as the threshold above which plastic shrinkage cracking becomes a significant risk.
Question 3: When heating aggregates or water to raise concrete temperature in cold weather, which approach is generally preferred to avoid localized overheating?
- Heat only the mixing water
- Heat only the coarse aggregate
- Heat the mixing water rather than aggregates when possible (Correct answer)
- Heat cement directly in silos
Correct answer: Heat the mixing water rather than aggregates when possible
Heating mixing water is preferred because it is easier to control and avoids the risk of flash set that can occur when hot water contacts cement directly.
Question 4: Fogging or misting above a fresh concrete slab in hot weather is used primarily to:
- Accelerate curing by adding moisture to the concrete
- Reduce the ambient temperature and evaporation rate above the surface (Correct answer)
- Replace the use of curing compounds
- Add water to increase the water-cement ratio
Correct answer: Reduce the ambient temperature and evaporation rate above the surface
Fogging cools the air above the slab and raises humidity, reducing the evaporation rate from the concrete surface without adding water to the mix.
Question 5: In cold weather concreting, the use of Type III (high-early-strength) cement is advantageous primarily because it:
- Generates less heat of hydration
- Reduces the water demand significantly
- Reaches protective strength faster, reducing required heating duration (Correct answer)
- Improves workability in cold temperatures
Correct answer: Reaches protective strength faster, reducing required heating duration
Type III cement's faster strength gain allows concrete to reach the 500 psi freeze-resistance threshold sooner, shortening the required protection period.
Question 6: What is the purpose of the 'gradual cooling' recommendation after the cold-weather protection period ends?
- To prevent sudden thermal shock and cracking (Correct answer)
- To improve the final compressive strength
- To allow bleed water to evaporate slowly
- To enhance the bond of curing compounds
Correct answer: To prevent sudden thermal shock and cracking
Gradually reducing heat prevents rapid temperature differentials between the concrete surface and interior that could cause thermal shock cracking.
Question 7: Which of the following conditions would INCREASE the evaporation rate from fresh concrete, worsening hot-weather risks?
- High relative humidity and calm winds
- Low relative humidity and high wind speed (Correct answer)
- Low air temperature and high humidity
- Overcast skies with moderate humidity
Correct answer: Low relative humidity and high wind speed
Low relative humidity combined with high wind speed dramatically increases the evaporation rate from the concrete surface, raising the risk of plastic shrinkage cracking.
ACI Cold & Hot Weather Concreting 3
What does ACI 306 define as the minimum duration for protecting concrete from freezing after placement in cold weather?