Cold weather concreting poses significant difficulties in the areas of installation and durability. We recommend that you consider the following in your cold weather site planning:
Granular bases must be protected from freezing prior to concreting to avoid settlement and finishing problems. CSA stipulates that concrete shall not be placed on, or against, any surface that will lower the temperature of the concrete in place below 10°C for concrete less than 1.0 M in thickness.
Low temperatures during the curing period can place concrete in a near dormant state (concrete requires heat in order to maintain its strength gain). If you are pouring in cold weather, consider adding site cured test specimens to your inspection plan. This will assist you in understanding that actual strength of the concrete in place.
Early freezing of exterior pavements can result in weak surfaces and permanent strength impairment. CSA A23.1 stipulates that interior concrete floors shall be cured at a minimum temperature of 10°C for 7 days AND the time necessary to attain 70% of the specified 28-day compressive strength of the concrete. CSA recommends that a period of at least one month of air drying should elapse before the application of de-icing chemicals on new exterior concrete (to permit the mix water within the concrete to evaporate).
Direct fired (open flame) heaters emit both carbon monoxide and carbon dioxide fumes into the work area. Carbon monoxide can be deadly to workers and needs to be monitored at all times. Carbon dioxide reacts with the surface of fresh concrete to create a soft, chalky surface. Heat sources must be of an indirect fired type (no open flame type heaters) which are the only safe means of heating where fresh concrete is being placed.
The set time and evaporation rate of water from the surface of a slab increases as the temperature decreases. This may lead to greater crazing or plastic shrinkage cracking on the concrete surface.
Freezing of concrete surfaces caused by snow and/or freezing wind and rain exposure will lead to permanent damage to the concrete surface which may require grinding or re-topping to repair.
We recommend that owners consider the use of high early strength concrete mixes, non-chloride accelerating admixtures, indirect fired vented heaters (indirect fired), and even the deferral of exterior work until warm weather to avoid the problems associated with cold weather conditions. Further information is available from the Canadian Standards Association CSA A23.1 mandatory standard for Concrete Materials and Methods of Concrete Construction (Tel: 416-747-4307).
You may also find the information on heater and exhaust fumes helpful.