Cold Weather Concreting
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When does cold weather begin to affect the performance of fresh and hardened concrete, and what variables should be considered when planning for a cold weather concrete placement to assure that the concrete will provide good durable service?

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Cold weather concreting is a common and necessary practice, and every cold weather application must be considered carefully to accommodate its unique requirements. The current American Concrete Institute definition of cold-weather concreting, as stated in ACI 306 is, “a period when for more than 3 successive days the average daily air temperature drops below 5°C (40°F) and stays below 10°C (50°F) 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.

Rule number one is that ALL concrete must be protected from freezing until it has reached a minimum strength of 3.5 MPa (500 psi), which typically happens within the first 24 hours. In addition, whenever air temperature at the time of concrete placement is below 5°C (40°F) and freezing temperatures within the first 24 hours after placement are expected, the following general issues should be considered:

1. Initial concrete temperature as delivered. During cold weather it is 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 properties of cement, heating of the cement or using hot cement is not an effective method in raising the initial concrete temperature.



2. 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.



3. Curing temperatures to produce quality concrete. In structures that will carry large loads at an early age, concrete must be maintained at a minimum of 10°C (50°F) to accommodate stripping of forms and shoring and to permit loading of the structure. In many cases, achieving the required durability will require a protection period of more than 24 hours. This may not be an issue with residential applications where applied loads are typically small and may be applied in small increments over several days or weeks. 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.

 
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