When the air temperature is forecasted to fall below 5 degrees Celsius within 24 hours of placing concrete then special considerations apply for concrete construction. These clauses are in section 7.1.2 of CSA A23.1/A23.2, Canadian Standard for Concrete Materials and Methods of Construction.
If the forecasted temperature is suspected to drop below 5 degrees celsisus (41F) then protection is required. All snow and ice has to be removed from the forms and surface. De-icing salts cannot be used to de-ice the forms. Adequate protection has to be provided to keep the concrete at a minimum of 10 degrees celsius (50F) for the duration of curing, which is typically 3 days. Protection can be heated enclosures, coverings, insulation or any combination of these. Another consideration is that the granular base needs to be preheated before pouring concrete. These prevents such deficiencies as blisters during troweling and delaminations. Using a concrete curing accelerant can also prevent these deficiencies.
Corners, edges and thin sections of concrete are the most vulnerable locations in cold weather and need more protection than plane surfaces. Once the compressive strength reaches 7 MPa it will have sufficient strength to resist frost damage.
Protection should remain in place until the concrete has cooled to the right temperature. This will prevent cracking due to a sudden temperature change.
The enclosures should be constructed to withstand snow and ice build up and being mostly air-tight. The enclosure should have enough space to allow air to circulate over the concrete. Heat can be provided by forced hot air, stationary heaters, hydronic heaters or other approved types. The concrete surface should be protected from any exhaust from the heaters. Carbon Dioxide from direct fire heaters can negatively effect the curing of the concrete.
Protective Covers and Insulation
The cover and insulation should be determined based on the expected temperature differential and wind chill factor. Other factors would include the size and shape of the structure and the amount of cement in the concrete mix.
Additional information can be found in CSA A23.1/A23.2 and is available to be order on their website. Additional information on cold weather concrete can be found in the American Concrete Institute Standard ACI 306R.