Temperature during concreting: conditions for strength

The temperature regime during concreting is one of the main criteria for the quality performance of work according to the norms of SNiP and GOST. Given the variability of climatic conditions and the need for work in the offseason, as well as during the cold season, temperature control is of particular relevance. We consider the temperature at which it is best to concrete, as well as the ways of artificially ensuring optimal conditions.

Temperatures during concrete work

Physical and chemical processes in concrete

Concrete is a mixture of four main components that provide it with the necessary characteristics. The purpose and participation of each of these components in various processes are discussed in the table:

As you can see, the main components that ensure the curing (hydration) reaction are water and cement.

The basis of the formation of a concrete stone is the cement hydration reaction, in which cement milk is first formed, which then quickly sets and forms a monolithic stone-like structure. Sand and crushed stone are needed here to ensure the physical processes within the mixture associated with the redistribution of loads and internal stresses.

Note! One of the conditions for the normal course of any chemical reaction is the compliance of the conditions with the temperature regime under which the ingredients may interact.

Optimal mode

Cement hydration reaction is unpretentious and allows you to determine at what temperature you can concrete, in a fairly wide range - from 0 to 90 degrees Celsius. However, in practice, to ensure adequate hardening conditions and the ability to perform work, this range is reduced from 4 to 30 degrees.

Note! The optimum temperature of concreting is considered to be the mode in which it does not fall below +15 degrees and does not rise above +25.

Obviously, such conditions can only be achieved with stable summer weather or indoors. That is why, when carrying out outdoor work, the concrete is always laid in the summer in dry and not very hot weather.

Normal mode

The practice of construction is such that in real conditions it is far from always possible to organize the work in such a way that the optimum concreting conditions are always observed. Therefore, most often we have to talk about the normal mode, in which work is possible both in the warm and in the cold season.

Cold is considered to be such a time at which the air temperature drops below +10 degrees. You should immediately indicate at what temperature it is impossible to concrete: without additional heating, the lower limit of temperature is +4 degrees, with a stretch we can say that the extreme value is 0 degrees, although the instruction excludes such stretches.

Note! The offseason is a dangerous time, as it is characterized by frequent and unpredictable temperature changes. This is fraught with repeated freezing and thawing, in which the material is dealt particularly noticeable damage.

It should also be borne in mind that for a set of grade strength a concrete product must stand for at least 28 days at a normal temperature, and if the temperature is lowered, this period increases noticeably. Therefore, if you plan to lay the walls immediately after the foundation has been poured, we recommend you to concrete to concrete no later than the end of August - mid-September.

Work in hot weather also imposes certain conditions: too active evaporation of moisture leads to its lack and reduction in the quality of the product, and also leads to the appearance of surface cracks.

In order to avoid such consequences, it is necessary to care for the concrete after laying:

• moisturize it;
• protect from dry wind;
• direct sunlight.

During rain it is necessary to ensure that fresh water does not get a lot of water, otherwise the water / cement ratio will be disturbed and the material will lose strength. For this, the formwork after pouring is covered with a waterproof film or other material that prevents the ingress of water.

Note! It should be remembered that overheating is no less dangerous than overcooling, as it leads to too fast hardening, loss of ductility, cracking and, as a result, loss of quality of the finished structure.

Winter concreting

Concreting at low temperatures is a special practice that requires the use of special technologies and additives. As we have said, the temperature should not fall below +4 degrees, but often the production process eliminates the possibility of a winter break in work.

In this case, apply the heating of concrete.

This can be done in several ways:

• Thermo method. Here, the heat generated by the cement hydration reaction is used, to keep it in, the mixture is poured into insulated formworks, which are additionally covered with hot water and other protective agents. Suitable for massive large objects;
• Heating method with steam or hot air. A wooden or canvas greenhouse is built around the object, in which steam is supplied, warm air or a heater is installed;
• The method of heating the mixture with electrodes or special heating cables (PNSV). According to a pre-calculated scheme, special heating elements are introduced into the concrete mass, which allow the electrical energy to be converted into heat;
• Warming up method using electric mats or infrared emitters. On the surface of the screed stack heating mats or IR emitters, which do not allow the mixture to freeze.

The most important thing is to prevent the concrete from freezing during the first three days, during which it gains up to 70% strength. Then the frost is no longer dangerous, it only slows down the process of building strength, which will continue after thawing.

Warming up to produce temperatures from +10 to +30 degrees. To go beyond this framework is undesirable.

Note! If the mixture has not gained 50% strength, then after thawing it decompresses and does not gain brand strength.

In addition to heating, you can use chemical additives that do not allow water to freeze and accelerate the hardening process.

It may be:

• sodium chloride;
• calcium chloride;
• sweat;
• and sodium nitrate.

It is also advisable to use high grade cement.

It should be borne in mind that cutting of reinforced concrete with diamond circles and diamond drilling of holes in concrete should be done after curing, and not over the frozen material. Also keep in mind that the price of works increases by 30 - 40%.

Note! For a successful winter concreting, it is better to use a set of measures, including heating, the use of chemical additives and highly active cement.

Conclusion

Proper temperature is a decisive condition for the normal hardening of the concrete mix and set the design of the expected strength. There are many methods to maintain this mode, you can learn more about them using the video in this article.