Concrete work is for the most part outdoor work. The American Concrete Institute (ACI) sets the bar for cold as more than three successive days the average daily air temperature drops below 40 degrees Fahrenheit and stays below 50 degrees Fahrenheit for more than one-half of any 24-hour period.
Effects of Cold Weather on Concrete
Construction doesn’t stop for weather. Concrete must reach its minimum strength of 500 pounds per square inch (psi) within the first 24 hours of being poured. If concrete freezes when freshly poured, it can reduce that strength up to 50 percent. Concrete contractors have to take additional steps pouring and finishing concrete when the temperature is below 40 degrees Fahrenheit.
Cold Weather Pouring Prep
Concrete sets more slowly in cold weather than in hot weather. When cold, the bleed water can take longer to evaporate. When poured, the concrete must be protected from freezing in order to develop the required strength and reduce the risk of cracking.
- Prep by covering the subgrade with insulated blankets a few days prior to pouring the concrete
- Pour the concrete at the warmest time of day
- Heat the water and/or aggregate when pouring concrete in cold weather
Use windbreak enclosures to protect construction crews and the concrete. Contractors use three variations in heaters when installing concrete in cold weather: direct fired, indirect fired and hydronic systems. Direct-fired heaters are preferable when the concrete is not directly exposed to the heater or exhaust. Indirect-fired heaters are used when the goal is to avoid carbonation of fresh concrete surfaces. Hydronic systems feature thawing and preheating options and are used in spaces too large to protect sufficiently with windbreaks.
Post-Pour Tips for Cold Weather
Leave forms in place as long as possible as the concrete cures. Pumping live steam in and around the perimeter of the forms as the concrete cures delivers heat and moisture—the two elements that help concrete cure better and faster. Loosen the forms (but don’t remove them) and cover the work area with heavy duty plastic or insulation blankets. The objective is to control the temperatures around the fresh concrete as the job cools to the temperature of the environment. The plastic and insulation help prevent sudden cooling that can cause thermal cracking.
Massive concrete jobs can take weeks to cool. For every 10 degree difference in concrete temps, the set time the concrete takes to cool increases by 1/3rd value. A 6-hour set at 70 degrees is equal to an 8-hour set at 60 degrees.
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