What additives in concrete at sub-zero temperatures. Effect of temperature on concrete. Rules for concreting the foundation in the summer months

At the turn of the seasons, the question of at what temperature it is possible to pour concrete under the foundation is of particular relevance so as not to endanger the structural elements of the new building. To get an answer, it is necessary to understand the ongoing processes inside the cement slurry.

Hydration of concrete - thickening and hardening of cement mortar

Hydration is the chemical process of hardening a mixture of cement and water into stone. It begins after mixing the mortar, however, the concrete setting occurs only after the foundation is poured. While the mixture is mixed with a mixer or concrete mixer, it is subjected to vibrations, due to which hardening is delayed, and the quality of the concrete does not suffer at all. After the mortar is removed from the concrete mixer, the material begins to harden and goes through two stages:

1. 1. At the first stage, the mortar sets. This process, depending on the composition of the mixture and external conditions, takes from 1 to 20 hours.
2. 2. At the second stage, the mortar hardens and gains up to 90% of its final strength. This process takes place within 28 days. After that, the concrete continues to harden throughout the entire service life.

The course of hydration directly depends on external factors, in particular, on the ambient temperature. At +5 degrees Celsius, the setting process begins after 2 hours and lasts up to 10 hours. At +20 degrees, 3 hours after pouring the concrete, its hardening begins, while the setting has not yet had time to complete. The hardening of cement is due to the components that make up its composition: tricalcium aluminate, tricalcium silicate, dicalcium silicate, tetracalcium aluminoferrite.

Answering the question at what temperature can the foundation be poured, we can say that ideal conditions for hydration are as follows:

1. 1. The temperature of the mixture is +30 degrees Celsius in normal weather conditions, up to +70 degrees in the cold months due to the heating of the components (water, sand and gravel) warm air or ferry.
2. 2. Temperature environment(air) - from +5 to +30 degrees Celsius.

Due to the fact that the solidification (hardening) temperature of concrete has a framework, when pouring the foundation, it is necessary to take into account external conditions.

Rules for concreting the foundation in the summer months

It is believed that pouring the foundation in winter is the most difficult, because due to sub-zero temperatures, the solution may freeze before the completion of the hydration process. However, the heat is no less dangerous for a fresh foundation than winter frosts. Due to the high outside temperature during the chemical reaction of the cement, the mixture will increase in volume. After the formation of the cement stone, it will inevitably cool down and decrease, but this will not happen due to the formed crystalline structure.

As a result, solidification at high temperature leads to strong internal stress in the foundation before construction begins structural elements a new house, and the owner will receive a foundation in operation, capable of cracking at any time. Usually, their first signs appear already a few hours after laying the solution inside the formwork.

High summer temperatures force builders to use special fast-hardening Portland cements if foundation pouring is planned to be carried out at temperatures above +25 degrees and at air humidity less than 50%.

The brand of cement used should be one and a half times the design parameters of concrete, in addition, to increase operational qualities foundation, plasticizers and all kinds of modifying additives will have to be added to the mixture, due to which hydration will slow down. For concreting, we recommend using the least hot hours of the day (early morning or evening). Another danger to the foundation of a house in the summer is the possible dehydration of the solution. To protect concrete from water evaporation, you must:

• cover the surface with chips, sand or sawdust;
• moisten the foundation from the watering can;
• water wooden formwork from buckets.

Winter work – construction at low temperatures

At low temperatures hydration does not accelerate, but slows down, as a result of which the foundation does not have time to gain the strength it needs. At zero degrees, the process of solidification of the solution in the outer layer stops completely, and inside the structure it continues for several hours until all components have cooled down. In this case, water does not have time to react with cement, after which it freezes, increases in volume and literally breaks the structure of concrete.

Specialists are well aware at what temperature concrete can be poured for the foundation without fear of losing its performance. Frosts are a significant obstacle to concreting, therefore, in winter, specialists are forced to use special technologies and means:

• heating of concrete elements before pouring and formwork heating;
• laying a heated cable inside a slab or;
• feed to reinforcing cage elements electric current for heating;
• installation of heaters around the foundation for the first three days;
• creating a greenhouse by covering the entire perimeter of the foundation with a film;
• introduction to the mixture special reagents, which reduce the t of water crystallization or accelerate the hardening time of the foundation.

The use of any of the described technologies significantly increases the cost of building a foundation. Therefore, it is difficult for private owners to recommend these funds. There are other ways to pour concrete at sub-zero temperatures. One of them is to change the amount of water and cement in the produced solution. It requires the most accurate and competent dosage of the used components of the mixture. Calculate at home required amount water and cement to improve the hardening of the foundation in winter is almost impossible.

• heating two-thirds of the water for the solution to t of the order of 70 degrees Celsius;
• adding sand and gravel to the water;
• adding the remaining third of the water and cement to the mixture.

Mixing the solution should be done twice as long as usual. The time of vibration compaction also increases by 1.5 times. Before pouring the solution, the formwork must be checked for ice, snow removed and the underlying layer warmed up. We cover the poured solution polymer film, which will keep the mixture warm for a while. If the above rules for concreting the foundation cannot be observed at home, it is recommended to wait for the air temperature to rise above +5 degrees. With a favorable forecast for the next three weeks, the owner of the house will be able to fill using standard technology.

The dependence of the cost of work on the weather - how to save on the foundation?

Almost all modern construction companies carry out work on pouring concrete in the summer months, at the end of spring and at the beginning of autumn. All this is explained very simply - at this time it is cheapest to carry out construction in general and build a foundation in particular.

Building a house in winter can lead to additional and extremely significant financial expenses:

• with the most optimistic forecasts, the construction budget in the cold months increases by 25-30% ;
• thaws in winter do not eliminate the need to use expensive reagents in the construction of the foundation;
• in winter it is impossible to prepare the solution on the spot, therefore the owner will have to additionally order the delivery of concrete by mixers;
• a transformer substation is required to heat the foundation, since the home electrical network cannot withstand such high loads.

The only advantage of building a foundation in winter months lies in the possibility in early spring to start erecting the structures of the facility and complete the construction, necessary finishing and other works by the end of autumn. In practice, the owner has to invest colossal funds in winter construction, which will definitely overshadow the joy of building a private house.

Optimal temperature range for the device concrete foundations on traditional technologies– from 15 to 25 °С. Therefore, in the vast majority of cases, these works are carried out in the warm season. But what if you need to continue or start construction in the fall or in the middle of winter, when it's freezing outside? To answer this question, we propose to understand under what minimum temperature you can pour concrete and what it is fraught with for the developer.

Is it possible to concrete in frost

After filling with favorable conditions the hydration process begins in concrete, which takes place in 2 stages:

1. Water enters into a chemical reaction with cement, which causes bonds between the particles and the solution begins to thicken. The setting stage lasts 10-20 hours and is accompanied by heat release.
2. The second stage - solidification (hardening) concrete mix lasting up to 4 weeks. During this period, the monolithic structure gains maximum strength.

The lower the ambient temperature, the longer the hydration takes. For clarity, we give an example in the form of a table reflecting the behavior of standard M250 concrete based on Portland cement grade 400 at different temperatures.

Reference. According to building codes, the formwork is removed after reaching 50% strength, and further work- not earlier than at 70%.

At negative temperatures, water freezes and practically does not react with cement. If ice crystals are formed, then the monolith that has not had time to harden begins to collapse. From the above table it is clearly seen that even at zero temperature your construction will stretch for an indefinite period.

To avoid this, you need to prevent the water from freezing, and ideally, heat it to the required condition in order to accelerate hydration. There are several methods for organizing the concreting of the foundation for a house in the autumn and winter:

• introduction of antifreeze additives into the concrete mixture;
• warming up monolithic construction inside and outside;
• covering and insulating concrete after pouring.

Construction experience shows that correct application of these methods, singly or in combination, allows you to safely build foundations down to -15 ° C on the street. If you live in major city, then they probably noted that in winter, work at large facilities does not stop. AT equally these technologies are also applicable to the construction of small bases for a house or a bath.

Use of additives

These chemicals are divided into groups according to the way they affect the concrete mixture:

1. Additives that prevent water from freezing at sub-zero temperatures and allow cement hydration to proceed. They are best used in combination with heating to shorten setting and curing times.
2. Additives - accelerators with antifreeze properties. Their function is to sharply increase the activity of the cement and thus force the concrete to gain design strength regardless of the ambient temperature. Successfully used without heating structures.
3. Substances that accelerate the setting of the solution and cause maximum heat release, which is why the monolith is heated independently at first.

Reference. From a financial point of view, the introduction of additives is considered the most profitable way, since electric or air heating of concrete is more expensive.

A feature of the use of any additives is the observance of the exact dosage in accordance with the conditions of concreting, especially when combining substances. The fact is that some of the compositions initiate the appearance of rust of the reinforcing cage, but give the water antifreeze properties. Others are corrosion inhibitors, so both groups are used together.

Hence the conclusion: do not try to make a solution for winter construction yourself by adding additives, such material must be ordered at the factory and delivered by a concrete truck (in jargon - a mixer). Contact the specialists of the enterprise and find out at what temperature it will be possible to use their products in order to pour the foundation without consequences. In case of emergency, correctly combined and accurately dosed admixtures can reduce the mortar laying threshold to minus 25 °C.

The following substances most often act as antifreeze and accelerating additives:

• potassium carbonate (potash);
• various calcium and sodium salts;
• urea (otherwise - carbamide);
• sodium formate.

You will learn more details about chemical impurities - modifiers from the following video:

Structure heating

This method is widely used for pouring reinforced concrete foundations small buildings and is implemented in two ways:

1. The entire area of ​​the future house is covered with a film tent mounted on a frame made of wooden beams. Inside are electric or gas heat guns supporting a certain temperature regime throughout the curing time of the concrete.
2. Before pouring, the reinforcement cage of the foundation is wrapped with a heating cable, which is connected to the network after laying the mortar into the formwork. There are other types of electric heaters - heating elements, nichrome spirals and so on.

Note. Both options are usually used without the introduction of antifreeze additives into the concrete mixture. But at too low temperatures (-15 ° C and less), additives are indispensable.

The device of a concrete base, followed by air heating - not too effective technique in severe frosts and frozen soils. The injection of heat into the space under the tent allows you to increase the air temperature by 15-20 ° C, which is not enough. In turn, the heating cable can burn out inside the monolith and then part of the foundation will remain without heating.

Unloading concrete from the machine into the heated shelter

The main disadvantage of these methods is high energy consumption and large initial investments for the construction of a canopy or the purchase of a cable (heat guns can be rented). Again, heaters or cable operation must be monitored and kept on site at all times. To understand the process, we suggest watching the video

Even at the beginning of the last century, work with concrete mixtures was seasonal. In winter, concrete was not laid due to the loss of the strength characteristics of this building material. The builders tried different ways shift the schedule of concrete laying closer to the beginning of persistent frosts. To do this, the surface of the concrete monolith was insulated using various organic materials: sawdust, peat crumbs, reed mats woven for this purpose.

In parallel, scientists were trying to create concrete, the setting temperature of which would be below zero degrees. But since the builders were not satisfied with the gain in time, the search for alternative insulation (heating) of concrete continued with sub-zero temperatures.

Acceptable mixture temperature

During the course of the research, scientists determined what is the temperature of the concrete best for getting quality designs. Its values ​​are in the range between +5 and +15 degrees. The boundary indicators that were worked out by the researchers are minus 20 and plus 45 degrees. At outdoor air values ​​from +5 to -3 degrees. the temperature of the freshly prepared product is not allowed below +5 degrees. These indicators are suitable for a cement mass of 240 kg / cu. m (with brand M200 and more). If less cement is used, the temperature index of the mixture should correspond to +10 degrees. or higher.

Ways to increase the setting temperature of concrete

If it is necessary to pour concrete in winter, the temperature of the mixture can be increased in the following ways:

* through the use of heated water;

* by introducing frost-resistant additives into the mixture;

* with the help of electric heating;

* the method of steaming concrete structures in stationary conditions in special autoclaves up to a curing of 80-85%;

* using electric heating of a concrete monolith, which has reinforcement in its composition. In this case, the electrodes are switched over the entire area of ​​contact between the reinforcement and concrete when a low voltage current is connected;

* by using heat guns with concrete mix enclosure.

Dependence of concrete quality on outdoor air

Does the strength of concrete change with outside temperature? Certainly. When working with building materials winter season chemical reaction accompanying strength gain decays. Therefore, at negative temperatures, hardening will stop. "Save" the mixture of additives in the form of various salts that can stop the formation of ice.

There is a situation when the product began to set, but then froze. In this case, after thawing, it will harden only in the absence of internal damage by freezing water. Experts allow a one-time freeze-thaw cycle, subject to the condition: the temperature of the mixture for three days should not fall below +10 degrees.

If you know certain requirements, then in winter concreting can be done no worse than in the most favorable period. The first condition is the competent delivery of the material. Best Option- use . The second is to build an insulated formwork, it is even better to take care of heating the concreted area.

Speaking about at what temperature to pour concrete into summer period, it should be noted the fact of a decrease in the strength of the product at +30 degrees. A practical way out is to moisten the concrete surface with water. In summer, due to evaporating moisture, concrete is made more liquid. And of course, it should be reported at what temperature to pour concrete in winter - it is recommended to perform all work up to -15C.

Reacting to the effects of temperature, concrete sets more evenly in wet and cool weather in summer. And if the work is carried out during the rainy period, then the resistance of the material to moisture is increased special cement. To prevent the solution from being washed away, the site is covered with polyethylene. However, in heavy rains lead concrete works outdoors is not recommended. If construction begins in a new climate zone, then experts advise testing concrete for strength in laboratory conditions or at a construction site.

Air temperature affecting concrete is not the only factor influencing given material. The quality of the product depends on the humidity of the environment, solar radiation, wind speed and how the mixture is handled.

And now, briefly:

At what temperature can concrete be poured? (on the street / in the foundation / in winter and summer)?
Optimum temperature- 5 to 20 degrees C above zero. With the use of additives and heating of concrete in winter to minus 20 degrees C.

Purpose and types

Hardening of concretes and mortars at low temperature occurs slowly, as the process of cement hydration slows down. Already at a temperature of - 3 ... - 6 ° C, water in concrete freezes, and the processes of hydration of the binder and hardening of concrete practically stop. When thawing, provided that the liquid phase is preserved, these processes are resumed, and the concrete continues to increase its strength. However, for concrete frozen in early age, after thawing and subsequent exposure, a loose structure, low strength and frost resistance are characteristic. This is due to the fact that freshly laid concrete contains a lot of water, which, when frozen, expands, loosens the cement stone and breaks the adhesion of the aggregate to the cement matrix.

you can buy antifreeze additives here: Petrovich, Stroyshopper, Stroylandia

Therefore, to ensure the required set in winter time it is necessary to create such conditions under which the processes of hardening of the binder will actively proceed, i.e. it is necessary to ensure the presence of a liquid phase. This problem can be solved, for example, by keeping the concreted structure at a positive temperature. Such holding can be carried out when concrete is heated in a thermoactive formwork, using heated mixtures, followed by covering the surface of the structure. thermal insulation materials and in other ways.

In cases where such methods cannot be implemented at the construction site for technical or organizational reasons, it is advisable to introduce antifreeze additives- substances that lower the freezing point of water and contribute to the hardening of concrete at low temperatures.

Application of concrete with antifreeze additives carried out during the construction of monolithic concrete and reinforced concrete structures, monolithic parts of prefabricated monolithic structures, monolithic joints of prefabricated structures, in the manufacture of prefabricated concrete and reinforced concrete products and structures in a landfill at a steady average daily temperature of the outside air and soil not lower than 5 ° C and a minimum daily temperature below 0 ° C.

Currently the most effective and field proven antifreeze additives are electrolyte additives: potash P, HH1, XK, NK, NNK, NNHK, their complexes NK + XN, NK + M, NNHK + M and others.

All of the above additives are both additives-accelerators of setting and hardening of concretes and mortars, however, their concentration in "cold" concrete is significantly (2...3 times) higher than that required to accelerate the hardening of concrete at temperatures above 0 °C.

In addition to those listed antifreeze additives also include:

Carbamide (urea) M. Colorless crystals of CO(NH 2) 2 , readily soluble in water.

Compound of calcium nitrate with urea NCM.

Sodium nitrite NN. Product in the form of NaNO 2 crystals white color with a yellowish tint, as well as in the form of aqueous solutions.

Accelerating antifreeze additive UPDM. Balanced by component composition a liquid mixture of waste products from the production of acetoacetic ester, acetylacetone and nitrochloractinide, taken in a ratio of 7:3:1 by volume. The solution is dark brown. The dosage is specified empirically within 0.1. ..0.42 l/kg of cement at ambient temperature from 0°C to -25°C.

Sodium formate alcohol FNS. Waste of petrochemical production, representing 30 ... 40% aqueous solution of sodium salts of formic and sulfuric acids. Straw to dark brown clear liquid. The recommended dosage is 2...6%, the admixture is introduced into the concrete mix with mixing water.

Asol-K. Product from organic and inorganic components: an aqueous solution of potash, corrosion inhibitors and modifiers. The additive provides hardening of concrete at temperatures up to -10 °C. At positive temperatures it causes rapid setting of mixtures (from 5 to 30 minutes).

Hydroconcrete - S-ZM-15. Antifreeze additive for concretes and mortars with plasticizing effect. Dark brown liquid 34...36% concentration. Provides hardening of concrete at temperatures up to -15 °C.

Hydrozyme. Liquid antifreeze for concretes and mortars in the form of a solution of 50% concentration. Provides hardening of concrete at temperatures up to -15 °C. Does not cause corrosion of reinforcement in concrete.

Lignopan-4. Antifreeze additive for concrete and reinforced concrete with plasticizing effect. Aqueous solution of 40% concentration. Provides hardening of concrete at temperatures up to -18 °C. Dosage: 2% at temperatures up to -5 °C, 3% up to -10 °C, 4% up to -15 °C.

WIN-Antimoroz. Antifreeze additive for dry mortars related to accelerators. The recommended dosage is 2...8% by weight of the components of the dry mixture, depending on the application temperature.

ammonia water. Product (NH 4 OH), which is ammonia gas NH 3 dissolved in ordinary water.

Additives of foreign manufacturers:

Betonsan. Dry salt-free antifreeze additive relating to accelerating modifiers for mortars. Provides hardening of concrete at temperatures up to -10 °C. Dosage: 1...2% of cement weight. Producer: CJSC "Company Convention Center".

Cementol B (Cementol B). Antifreeze additive-antifreeze for concrete and solutions. Provides hardening of concrete at temperatures up to + 5 °C. Recommended for concretes based on high-quality cement with increased exotherm. Dosage: 0.2...0.8% by weight of cement. Producer: Firm TKK (Slovenia).

economically rational antifreeze additive Ammonia water is the most important, since compared to aqueous solutions of potash and calcium chloride, it has a significantly lower percentage of volumetric expansion and therefore is the least dangerous in relation to possible deformations from the expansion of the liquid phase with the formation of ice.

Depending on the calculated minimum outdoor temperature, a certain concentration of the solution is assigned ammonia water mixing (Table 1). Unlike others antifreeze additives ammonia water not only does not cause corrosion of reinforcement, but can serve as an anodic steel corrosion inhibitor in reinforced concrete structures containing chloride salts. The additive does not impair the adhesion of reinforcement to concrete, does not reduce the frost resistance of concrete, does not cause efflorescence and the formation of spots on the surfaces of structures. Ammonia water somewhat slows down the setting time of cements, which makes it possible to maintain the workability of the concrete mix from 4 to 7 hours.

Antifreeze additives are allowed for use in heavy and light concrete (class B10 and above) in accordance with the "Guidelines for the use of concrete with antifreeze additives". indicative consumption of antifreeze additives depending on the design temperature of concrete hardening is presented in Table. 2.

The scope of antifreeze additives in concrete, presented in table. 3 is rather limited, which is explained by the following reasons.

Additives during curing of concrete can migrate and accumulate in separate zones structures (ribs, surface layers and other parts) followed by crystallization. These processes are intensified with multiple temperature drops, especially with a periodic transition to the region of positive temperatures, which is typical for the autumn-spring periods, as well as thaws in winter.

Table 2. Dosage of antifreeze additives

Notes.

1. *-ratio of components 1:1 by weight in terms of dry matter.

2. At a concrete temperature above - 5 ° C, instead of CN, it is possible to use HC (up to 3% of the mass fraction of cement).

3. The concentration of the mixing solution (taking into account the moisture content of aggregates) should not exceed,%:

They increase with decreasing relative humidity air, with an increase in the consumption of cement and antifreeze additives in concrete.

The process of crystallization of salts occurs with an increase in volume, so their accumulation in individual zones of structures can lead to defects and destruction of these zones. Dangerous in this respect are additives containing potash and calcium nitrate. Due to the active participation of a number of additives in the processes of cement hydration, their optimal amount for a particular negative temperature, as well as freezing speed of concrete largely depend on the mineralogical and material composition of cement.

Most of the additives used form double salts, which are potentially dangerous components of cement stone during the operation of concretes with such additives in some aggressive aquatic environments. For example, sodium and calcium chlorides sharply intensify the corrosion process of steel in humid conditions with access to atmospheric oxygen.

Table 3. Scope of application of antifreeze additives

Note. "+" - allowed, "-" - not allowed for use, "+*" - allowed in combination with the addition of a setting retarder.

In some cases, the aggressiveness of chloride salts in relation to reinforcement and technological equipment can be reduced by using complex additives, including steel corrosion inhibitors. For example, with the simultaneous presence of nitrite ions in a solution at a mass ratio between NN and HC of at least 1: 1, chloride ions become practically not dangerous in relation to reinforcement. However, in order to exclude the possibility of a corrosion process, scope of antifreeze additives is significantly limited in prestressed structures, where such processes can cause catastrophic consequences associated with rupture or loss of the prestressed state of reinforcement in concrete.

Potash and calcium nitrate are neutral additives in relation to rebar. Sodium and calcium nitrites are inhibitors of reinforcement, but can cause stress corrosion cracking of thermally hardened steel, so their use is strictly limited in reinforced concrete products and structures with prestressed reinforcement.

Features of concreting are that concrete base does not instantly become strong, but gains this indicator gradually. The process of curing is considered optimal if the concrete has reached the design indicator in 28 days. But this time indicator depends on some factors. And the most influential is the temperature and humidity indicators. Moreover, there is an opinion that it is impossible to pour concrete into a minus.

Features of concrete hardening

How does the strength of concrete take place? The simplest explanation is the loss of moisture from the concrete mix. The moisture content must be distributed evenly, in accordance with regulatory requirements. And if the temperature is not optimal, but is approaching minus or negative?

In this case, the concrete will begin to harden within a few hours only due to recoil own heat and then there are two options:

• water in microcracks and pores will freeze and begin to expand, which will lead not to hardening of the monolith, but to its cracking;
• at very low temperatures, the concrete will freeze itself and the curing process will stop until it is completely thawed. Strength after thawing usually decreases, and the life of the entire structure becomes shorter.

Neither the first nor the second version of the builders are satisfied. Therefore, it is worth weighing the pros and cons of pouring concrete at sub-zero temperatures.

The organization of work depends on the grade of concrete being worked with. For different brands there is an indicator of critical strength, which sets the percentage of the design value at which the freezing of concrete will not disrupt its structure so much as to affect the final strength value. For example, for the M300 concrete grade, this percentage is 40%, and for grades lower than M200 - 50%.

The task of the builders is to provide such conditions for concrete solidification so that it has time to reach critical strength before the moment of complete freezing.

There are several options for laying concrete at sub-zero temperatures, when the requirements for critical strength can be met:

• the use of electric heating;
• thermos method;
• the introduction of antifreeze additives;
• creation of shelters with the maintenance of a certain regime or a convective method.

Stage of production and transportation in winter

If you do not comply with all technological requirements at every stage of creation concrete structure in winter, you can lose quality even at the stage of mixture formation or its transportation.

The temperature of the transported mixture must be such that, at the time of its unloading at the construction site, it corresponds to the design temperature for a given brand and installation method. When using heating or additives, the concrete mixture can have a temperature of about 5 0 C, and if the thermos option is used, then it is necessary to provide a temperature of 25 0 C.

Note! High temperature reduces the mobility of concrete.

In order for the final product to have the required temperature, several technologies can be applied:

• use warmed up high temperature water;
• heat the aggregate - sand, crushed stone, gravel;
• steam heating of all components during mixing in a concrete mixer.

Note! In the manufacture of concrete on a construction site, the procedure is as follows: hot water is first combined with aggregate, and then only cement is introduced.

To maintain the temperature of the resulting concrete, the body is insulated in the machines or warm containers are used. But the most profitable way is to use truck mixers, when hot water right on the construction site.

In conditions when the mixture has cooled down during transportation, but has not begun to set, it is heated by electrodes to the required temperature.

Operation at sub-zero temperatures

It is impossible to avoid concreting at sub-zero temperatures, which means that it is necessary to reduce the time to reach the critical strength of the composition. In addition to the methods already listed, they often follow a simpler path and increase the grade of concrete, but this option requires careful calculations and selection of components.

With any method of concreting in winter, it is ensured that the temperature inside the entire structure is above 0 0 C during the entire time of gaining the required percentage of strength.

Creating an insulated protective structure or covering with a thermal film with a flooring of heat-insulating materials is considered the simplest. Sawdust, snow, peat serve as a material for flooring. They provide the necessary temperature, and only humidity control is required.

If necessary, bring the pouring of concrete at minus 5 0 C, insulation alone may not be enough. An effective option may be the use of electric current, which will provide isothermal heating. In this case, the concrete mix itself becomes part of the electrical circuit like resistance. In her Electric Energy will turn into heat. Used as electrodes steel reinforcement. The electrodes themselves can be located inside the mixture and on its surface.

Electric heating is carried out not only with the help of electrodes. To do this, use several more methods and devices, such as:

• thermoactive formwork;
• electric reverberatory furnaces;
• infrared heaters;
• cylindrical resistance devices.

Any electrical appliance, steam generator, induction field can act as a heating device. The choice of instrument and heating method depends on the following factors:

• structure design, its geometrical parameters;
• index of critical strength of the composition;
• the presence of the required electric power.

Antifreeze additives

From the point of view of energy resources, the introduction of additives that increase the frost resistance of the concrete mixture is considered less expensive. Antifreeze additives can be up to 10% of the total volume of cement in the concrete mixture. They do not allow concrete to freeze and contribute to its strength development at temperatures below 0 0 С.

The most common chemical additives in concrete at sub-zero temperatures it is calcium or sodium chloride, sodium nitrate and sulfate, potash. AT reinforced concrete potash is used as an additive, which allows the mixture to harden at a temperature of -25 0 C. Sometimes you can use the so-called cold concrete. In it, the amount of additives reaches 15% by weight of cement. Such concrete is protected by a thermal cushion from all sides to ensure the same temperature throughout the structure.

It is impossible to use antifreeze additives if the structure itself will be operated at a temperature of more than 60 0 C or in an aggressive environment.

It is possible to carry out concreting work in the winter, but this is a rather costly and energy-intensive event. It may be worth waiting a bit to be able to comply with all technological requirements.