Concreting in winter — what is the difficulty?

Concrete setting(hydration) is a chemical reaction between cement and water. This process is highly dependent on air temperature. The optimum temperature for hydration is +15-20 degrees.

At normal temperatures, concrete gains up to 30% of its strength during the first three days. After two weeks, it reaches 60-80% of its strength. After four weeks, concrete gains 100% strength and 120% after three months.

What happens when the temperature is below normal? At temperatures below 15 degrees, the rate of strength gaining is greatly reduced. If the temperature is 5 degreesCelsius, the rate of strength gaining drops from hours to days. And at 0, its tops altogether due to the freezing of water. Accordingly, in construction, the transition to "winter time" is necessary when the average daily temperature is 5 degrees Celsius.

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How to pour concrete in cold weather?

The main mission of all modern winter concreting technologies is to keep water in the mixture from crystallising. The objective of winter concreting is to ensure that the concrete reaches 50% of its original strength before freezing. This is the critical indicator, after which freezing of concrete will not have a significant impact on the structure parameters.

To achieve this, we use the following methods:

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-   We use anti-freeze additives (AFA). This is an effective and the most common way to increase the frost resistance of concrete. In addition to the fact that such additives can be used independently, today it is possible to purchase winter concrete that already contains them. Antifreeze additives are added to the concrete mixture in strict accordance with the percentage of cement in different grades.In addition, the amount of AFA depends on the air temperature during concrete laying. Some additives allow concreting at air temperatures as low as -25degrees. The process of strength gaining can be accelerated by additional steam or electric heating.
-   Concrete covering. Another common method of winter concreting. The main issue is that concrete covering will not help in extreme cold conditions. The air temperature should remain within a range of +3-3. This method is also quite risky, because it is difficult to predict the temperature regime and the air temperature can drop sharply. What is used to cover concrete? Most often, insulation or PVC film is used, which is laid on the surface of the concrete.
-   Heating with guns. This additional method is used in conjunction with the previous one. It is used when the temperature drops just below the permissible temperature for simple concrete covering. To do this, the PVC film is not laid on the surface of the concrete; rather, something akin to a concrete tent is created - a temporary frame is constructed from boards, on which the film is laid. Gas or electric guns are installed under the film. It takes up to 3 days of heating with heat guns for the concrete to gain initial strength.
-   Component heating. This method also cannot be considered independently. It helps to comply with the temperature regime, but does not protect concrete from freezing.Heated water can be used for mixing. In addition, the filler (crushed stone, for example) is also heated, but the cement itself and fine additives cannot be heated.
-   Electric heating. The electric heating method is only used on large construction sites due to the fact that it is expensive and requires special equipment. The technology of electric heating of concrete is similar to the principle of underfloor heating. Before pouring concrete, a special electric wire is installed on the reinforcing structure.
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The amount of wire depends on how much concrete needs to be heated - on average, its consumption is about 60 p/m per 1 m3.