CSLB General Building (B)Site Work, Foundations and ConcreteHard
A contractor is placing a concrete footing and overnight temperatures are forecast to drop below freezing before the concrete has cured. According to standard cold-weather concreting practices (ACI 306), what minimum concrete temperature must be maintained, and for how long, to protect the fresh concrete?
- A40°F for at least 48 hours
- B32°F for at least 24 hours
- C50°F for at least 3 days
- D70°F for at least 7 days
Show answer & explanationAnswer & explanation
Correct answer: C. 50°F for at least 3 days
ACI 306 cold-weather concreting guidelines require that concrete be maintained at a minimum temperature of 50°F for at least the first 3 days after placement (longer for high-strength or thin sections) to ensure adequate hydration and prevent freeze damage before the concrete gains sufficient strength.
Why the other options are wrong
- A. 40°F for 48 hours does not meet the minimum temperature or duration required by ACI 306.
- B. 32°F is the freezing point itself and is too low and too short a duration to protect fresh concrete.
- D. 70°F for 7 days exceeds the minimum standard requirement and is not the baseline rule.
Cold Weather Concrete Protection (ACI 306)
Guidelines requiring fresh concrete to be maintained at a minimum temperature of 50°F for at least 3 days after placement in cold weather to prevent freeze damage and ensure proper hydration.
- Minimum protected temperature is 50°F
- Minimum protection period is 3 days for normal structures
- Longer protection needed for thin sections or high early strength requirements
Memory trick: 'Fifty for three' keeps fresh concrete from freezing before it gains strength.