Slab day! I wasn’t sure exactly when the pour would happen, so it was exciting to pull up to the lot and find two guys under a heated tent already finishing the surface with a power trowel.
Concrete curing is a chemical reaction, and its final strength greatly depends on conditions during the curing process. Most types of concrete reach their highest final strength when curing occurs between 40 and 55 degrees F. The concrete curing reaction is exothermic, meaning that it generates heat. In large public works projects this heat can cause serious problems, but for small projects in cool weather it can be an asset. Additives can speed up curing and generate more heat, and blankets can be laid across the slab to hold the heat in. Wet concrete must be protected from freezing for the first day or two at all costs, or ice crystals can form inside the cement matrix and permanently compromise the concrete’s strength.
In my case, there was already frost on the ground and a brisk Alaska chill in the air. My concrete contractor proposed that we tent the foundation and heat it with a 190,000 BTU propane burner for the first two days and nights. Tenting allows a slab to stay at ideal conditions without the use of excessive additives. After two days, enough water evaporates that freezing is no longer a serious concern and the slab has reached a considerable amount of its final strength. The only downside to tenting a foundation is cost: in my case, tenting added about $2,000 to the foundation process. A definite ‘ouch,’ but necessary to keep the project moving and know that it was done right given current conditions.
Photos taken November 11, 2019. Posted January 1, 2020.
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