Cold & Hot Weather Concreting Flashcards
7 cards from real ACI practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Cold & Hot Weather Concreting flashcards as text
What does ACI 306 define as the minimum duration for protecting concrete from freezing after placement in cold weather?
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.
During hot weather finishing operations, plastic shrinkage cracks are most likely to form when the evaporation rate exceeds:
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.
When heating aggregates or water to raise concrete temperature in cold weather, which approach is generally preferred to avoid localized overheating?
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.
Fogging or misting above a fresh concrete slab in hot weather is used primarily to:
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.
In cold weather concreting, the use of Type III (high-early-strength) cement is advantageous primarily because it:
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.
What is the purpose of the 'gradual cooling' recommendation after the cold-weather protection period ends?
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.
Which of the following conditions would INCREASE the evaporation rate from fresh concrete, worsening hot-weather risks?
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.