Weather conditions at a jobsite – hot or cold, windy or calm, dry or humid – may be vastly different from the optimum conditions assumed at the time a concrete mix is specified, designed, or selected – or from laboratory conditions in which concrete specimens are stored and tested. Concrete can be placed in cold weather conditions provided adequate precautions are taken to alleviate the negative impacts of low ambient temperatures. The current American Concrete Institute (ACI) definition of cold-weather concreting, as stated in ACI 306 is, “a period when for 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.” This definition can potentially lead to problems with freezing of the concrete at an early age.
All concrete must be protected from freezing until it has reached a minimum strength of 500 pounds per square inch (psi), which typically happens within the first 24 hours. If concrete freezes while it is still fresh or before it has developed sufficient strength to resist the expansive forces associated with the freezing water, ice formation results in the disruption of the cement paste matrix causing an irreparable loss in strength. Early freezing can result in a reduction of up to 50 percent in the ultimate strength. Once concrete has attained a compressive strength of around 500 psi, it is generally considered to have sufficient strength to resist significant expansion and damage if frozen. Whenever air temperature at the time of concrete placement is below 40 degrees Fahrenheit and freezing temperatures within the first 24 hours after placement are expected, the following general issues should be considered:
Initial concrete temperature as delivered
During cold weather, it may be necessary to heat one or more of the concrete materials (water and/or aggregates) to provide the proper concrete temperature as delivered. Due to the quantities and heat capacity of cement, using hot cement is not an effective method in raising the initial concrete temperature.
Protection while the concrete is placed, consolidated, and finished
The exposure of concrete to cold weather will extend the time required for it to reach initial set, which may require finishing crews to be available for a longer period. Depending on the actual ambient temperature, protection of a concrete placement may require the use of windbreaks, enclosures, or supplementary heat. It may also be appropriate to adjust the concrete mixture constituents for the effect of ambient temperature on setting time. This may require an increase in cement content, the use of an accelerating chemical admixture, or both.
Windbreaks protect the concrete and construction personnel from biting winds that cause temperature drops and excessive evaporation. Typically, a height of six feet is sufficient. Windbreaks could be taller or shorter depending on anticipated wind velocities, ambient temperatures, relative humidity, and concrete placement temperatures.
Heated enclosures are very effective for protecting concrete in cold weather, but are probably the most expensive option. Enclosures can be made of wood, canvas tarpaulins, or polyethylene. Prefabricated rigid-plastic enclosures are also available.
Three types of heaters are used in cold-weather concrete construction: direct fired, indirect fired, and hydronic systems. To avoid carbonation of fresh concrete surfaces, indirect-fired heaters should be used. If the concrete is not exposed to the heater or exhaust directly, then a direct-fired heater is suitable. Caution should be taken to ensure that workers are not overexposed to carbon monoxide anytime a heater is used inside an enclosure. Hydronic systems transfer heat by circulating a glycol/water solution in a closed system of pipes or hoses. Typical applications for hydronic systems include thawing and preheating subgrades and heating areas that are too large to be practical for an enclosure.
Curing to produce quality concrete
Curing not only requires adequate moisture, but also appropriate temperature. The temperature of the concrete as placed should be above 40 degrees Fahrenheit using methods described above, however the duration of heating is dependent on the type of service for the concrete, ranging from one day for high-early strength concrete that is not exposed to freeze-thaw events during service to 20 days or more for a concrete element that would carry large loads at an early age. In structures that will carry large loads at an early age, concrete must be maintained at a minimum of 50 degrees Fahrenheit to accommodate stripping of forms and shoring and to permit loading of the structure.
In no case should concrete be allowed to freeze during the first 24 hours after it has been placed. Since cement hydration is an exothermic reaction, the concrete mixture produces some heat on its own. Protecting that heat from escaping the system using polyethylene sheeting or insulating blankets may be all that is required for good concrete quality. More severe temperatures may require supplemental heat.
Concrete retained in forms or covered with insulation seldom loses enough moisture at 40 to 55 degrees Fahrenheit) to impair curing. However, drying from low wintertime humidities and heaters used in enclosures is a concern. It is good practice to leave forms in place as long as possible, because they help distribute heat more evenly and help prevent drying of the concrete. Live steam exhausted into an enclosure around the concrete is an excellent method of curing because it provides both heat and moisture. Liquid membrane-forming compounds can also be used within heated enclosures for early curing of concrete surfaces.
It is also important to prevent rapid cooling of the concrete upon termination of the heating period. Sudden cooling of the concrete surface while the interior is warm may cause thermal cracking. Methods for gradual cooling of concrete include loosening the forms while maintaining cover with plastic sheeting or insulation, gradual decrease in heating inside an enclosure, or turning off the heat and allowing the enclosure to slowly equilibrate to ambient temperature. Massive structures may require several days or even weeks of gradual cooling to mitigate the probability of thermal cracking.