Cold weather concreting can not be successfully achieved if you don’t understand the process of concrete hydration in low temperature.
The reduction of setting time 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.
• Setting time is more important in flatwork finishing
• Early strength gain is more important for early form removal
Acceleration may be encouraged by using:
•Type III Portland cement
• 20% additional Type I or II cement to provide Type III response
• Set-accelerating admixtures
Calcium chloride is the most cost effective accelerator available, but it causes corrosion of embedded metals in the presence of oxygen and moisture. This is why limits exist on the use of chlorides in concrete. It is important to verify that non-chloride accelerating admixtures are also noncorrosive.
Some accelerating admixtures which are labeled as non-chloride may still contain materials which cause the products to be corrosive to embedded metals. Non-chloride, noncorrosive accelerators are more expensive up-front, but when life-cycle costs and regulations limiting chlorides are considered, they are the most cost effective products.
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 cement at the intended use temperature, prior to placement. Different mixes and materials will exhibit different setting times. It should not be assumed that two different Portland cements will set at similar rates. If pozzolans are to be used in the concrete, they should also be included in trial mixes prior to placement.
If used correctly, accelerator products can be used to add 2°C to 6°C (5°F to 10°F) to the temperature range within which concrete can be used (possibly more). At high enough dosages, concrete can be brought down to temperatures of -7°C (20°F) (depending on materials and mix design) without hydration stopping and without any degradation of long-term strength. In addition, in more massive structures, the heat of hydration can help protect the structure from freezing.