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Concrete placed during cold weather will develop sufficient strength and durability to satisfy intended service requirements only if it is properly produced, placed, and protected.
ACI 306 “Cold Weather Concreting” defines cold weather concreting as a period when for more than three (3) consecutive days, the following conditions exist:
The average daily air temperature is less than 5 C (40 F) and
The air temperature is not greater than 10 C (50 F) for more than one-half of any 24 hour period.
Even though not defined as cold weather, protection during Spring & Fall is required during the first 24 hours.
The objectives of cold weather concreting are to:
- Prevent damage to concrete due to freezing at early ages
- Assure that concrete develops the required strength for the safe removal of forms
- Maintain curing conditions that foster normal strength development without using excessive heat
- Limit rapid temperature changes in the concrete to prevent thermal cracking
- Provide protection consistent with the intended serviceability of the structure
Concrete that is protected from freezing until it has attained a compressive strength of at least 3.45 MPA (500 psi) will not be damaged by exposure to a single freezing cycle. Concrete that is protected properly cured will mature to its potential strength despite subsequent exposure to cold weather.
Except in heated, protective enclosures, little or no external supply of moisture is required for curing during cold weather.
While hot water improves setting time of cold weather concrete, after the first few batches of concrete hot water heaters may not be able to maintain hot water temperature. Later in the pour, concrete may be cooler than at the beginning of the pour.
Additional Accelerator Toward the End of the Pour:
Adding more accelerating admixture at the end of the pour will offset the effects of cooler mixing water and provide a more consistent set. Another reason to add more accelerating admixture as the pour progresses is to speed up the setting of the latter mixes and have all the concrete in the pour reach set at about the same time in order to facilitate the finishing operation.
Acceleration of Concrete Hydration in Cold Weather:
The reduction of setting and the acceleration of strength gain often result in substantial savings due to shorter protection periods, faster form reuse, earlier removal of shores, and less labor in finishing flatwork.
Accelerators have been introduced successfully into concrete both before and after the addition of cement to the mix, but it may be best policy to add the accelerator to the mix after the cement has been wetted. On rare occasions, when accelerators are added to the mix prior to the batching of “under-sulfated” cements, there may be adverse reactions with the tricalcium aluminate (C3A) in the cement which may result in retardation. Therefore, we recommend that if the accelerator is to be added up-front, before the cement, it should be tested with the intended use temperature, prior to placement.