Additives for concrete in cold weather. Pouring concrete at sub-zero temperatures: features and process of work. Is it possible to pour concrete in frost without heating

Even at the beginning of the last century, work with concrete mixtures was seasonal. AT winter period 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 at sub-zero temperatures continued.

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 a fence concrete mix.

Dependence of concrete quality on outdoor air

Does the strength of concrete change with outside temperature? Certainly. When working with building materials winter season the chemical reaction that accompanies the curing decays. Therefore, when 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. The best option is to 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 in the summer, it should be noted that the strength of the product decreases 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.

Responding 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 outdoor concrete work is not recommended. If construction begins in a new climatic 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)?
The optimum temperature is from 5 to 20 degrees C above zero. With the use of additives and heating of concrete in winter to minus 20 degrees C.

The reliability and durability of the entire structure directly depends on the strength of the foundation. When planning such a “zero” construction cycle, many factors must be taken into account. Of particular importance is the information at what temperature the foundation can be poured.

If you do not take into account weather during the process, the quality and grade of the mortar, the use of additives that can reduce the temperature crystallization of water, measures to maintain the necessary conditions for the maturation of concrete, then the work can be done in vain, and the erected foundation of the building will begin to crumble immediately after hardening.

Some owners of suburban areas are in a hurry to build capital structures on their newfound territory, not paying attention to the season. In some, rather rare cases, this is justified, however, there are many difficulties with this approach, and they begin already at the stage of preparatory work.

Regardless of the time of year, preparatory activities will include a whole list of mandatory work:

• The place where the foundation for the construction of the structure will be arranged must be cleared of the topsoil and marked accordingly. Removing the top layers of soil in frosty weather is a rather time-consuming task.
• When common place it will be determined, the marking of the internal boundaries of the trench is made, which must be dug under the foundation. Its depth should be from 500 to 800 mm - this value will depend on the type of soil of the area where construction is being carried out, the depth of their freezing, the features of the building being erected (its number of storeys, wall and roof material, etc.) Earth-moving equipment for extracting narrow and deep enough trenches with even walls can not always be used. Manual digging of frozen soil is another difficulty during winter work.
• A waterproofing and reinforcing pillow is laid at the bottom of the trench pit. Sand is laid first and compacted well, the layer thickness can be from 100 to 150 mm. Gravel is poured on it and is also well compacted. Very often in winter, both sand and gravel are in a state “stuck” by frost. There is a high probability that with a rise in temperature, the pad may lose the necessary density, even with the highest quality tamping.

• In addition, there is no complete certainty that during the general thawing of the soil in the spring and its possible movements, the entire structure of the foundation under construction will not “lead”, and this can lead to the formation of internal stresses and cracks.
• The next step is to install or wooden shields, which waterproofed dense plastic wrap. In severe frost, polyethylene often loses elasticity, becomes brittle, and waterproofing can be damaged.

• Can also be used fixed formwork from extruded polystyrene foam, which, in addition to its direct function, also performs the role of a heater.

• Further, it is necessary to install a reinforcing structure in the formwork, which is welded or twisted with steel wire. Reinforcement for this design is taken from 10 to 15 mm thick. We must not forget that reinforcing steel has a fairly significant coefficient of linear thermal expansion. A reinforcing cage welded in severe frost will certainly tend to change dimensions with increasing temperatures. This is another significant "additive" to the unnecessary internal stresses of the foundation structure.

Nevertheless, as already mentioned, there are situations when, for one reason or another, building a foundation in the winter season is justified:

• This may be due to the nature of the soil. If sandy loose soils predominate in the area where the construction is being carried out, then it is better to build the foundation in frozen solid soil, which retains the shape necessary for the foundation pit.
• Should not be dropped from accounts the impossibility of construction in the summer due to special climatic conditions region.
• In a number of areas, due to the poor development of road networks, the delivery of large volumes building materials or movement of heavy special equipment possible only on frozen ground.
• Sometimes winter construction is resorted to in order to save money, since prices for necessary materials. This will be beneficial if the work is carried out independently.
• Often there is an opportunity to save money by reducing the cost of services by construction companies, due to a sharp decrease in demand for their activities in the cold season.

When all the preparatory processes are completed, it is possible to calculate the density and composition of the solution, the parameters of which will depend on the temperature at which it will be poured into the formwork.

Pouring the foundation with concrete

• Whenever pouring is carried out, the foundation mortar should not be too thin, so it is most often made from cement and medium-sized gravel.
• Often, plasticizers are added to the mortar, which improve the condition and strength, increase its adhesion to reinforcing structures, and increase the moisture resistance of the foundation. In addition, according to manufacturers, when pouring, plasticizers reduce the consumption of cement mortar by 20%.

Due to the fact that plasticizers have a positive effect on the frost resistance of the solution, they are often added in cases where it is necessary to fill the foundation at and below freezing air temperatures.

Optimal conditions for pouring concrete - temperature from 15 to 25 degrees

• According to all recommendations, concrete pouring should be carried out at a temperature of at least 5 degrees - this, in fact, is a critical indicator for normal maturation. However, the summer heat is also not very suitable for these construction processes. The optimal temperature regime for pouring the solution into the pit is +15 ÷ 25 degrees. Such conditions will allow you to get extra costs and technological methods as much as possible solid foundation for the construction of walls and in the shortest possible time.

• In the case when the solution is made independently right at the construction site, all materials used for it should not be in a frozen state and should not contain snow or ice crystals. Therefore, it is better to purchase them in companies that are guaranteed to provide them with proper storage.
• The solution must be poured and distributed over the formwork quickly so that the frost does not have time to grab the moisture in the solution. Therefore, the filling of the entire volume is carried out in such conditions only at a time. If the foundation has a large volume and area, then it is better to use offers of specialized companies, which are engaged in the manufacture, delivery and unloading of the necessary solution into the prepared formwork.

• It is not recommended to pour concrete in layers, since gaps can form between them due to low temperatures, which will make the foundation less durable.

If circumstances have developed in such a way that do work under conditions of critical temperatures, you need to know that the processes of setting and hardening will be increased several times. Therefore, pouring the foundation in the winter is carried out only when absolutely necessary.

Below is a table that clearly shows how the ambient temperature affects the maturation time and the full set of the required strength of a conventional concrete mortar grade M200 - M300, made on the basis of Portland cement M-400 or M-500.

Notes:

- percentages are calculated in relation to the reference strength of mature concrete of a given grade.

- under the icons (٭) the so-called conditional normative-safe terms for the stripping of the poured concrete structure are indicated.

- badges (٭٭) - these are the maturation dates for the start of completely safe further work.

To ensure the required minimum allowable temperature regime, several technologies are used:

• The process is carried out with heating of the reinforcing structure or with the installation of special heating cables. In this case, the appropriate electrical voltage must be supplied to the foundation. There are several technologies, both with the use of currents high voltage(before 380 volts) and low-current (12 volts). Having warmed up, the reinforcing structure or the heating cable will not let the wet, not seized mortar freeze.

However, such methods are justified only for large-scale industrial construction - such technologies are very dangerous and require the highest qualifications of specialists. In addition, the cost of a large amount of electricity will be important, which means you will have to pay a hefty amount. As a result, the total budget for such an arrangement of the foundation is hardly acceptable for average Russian private developer.

• You can use another technology - this is the use of fixed insulated formwork for the foundation.

For this method, hollow blocks of extruded polystyrene foam are used, which are convenient to install on top of each other, thanks to the existing teeth on the side and upper surfaces. They are ideally combined with each other, leaving no gaps (a kind of analogue children's constructor Lego). Reinforcing structures are installed in the interior space, which give the foundation the required overall rigidity.

They do not remove it from the foundation, and after the concrete has hardened, the surface is covered with a reinforcing sickle mesh, plastered and waterproofed.

However, this approach only reduces Negative influence negative temperatures during the maturation of concrete, but will not completely eliminate the problem.

• In any case, for a reliable setting of the poured foundation at sub-zero temperatures, it must be closed from above with a dense polyethylene film. To do this, erect a temporary structure made of wood or reinforcing bars over it, which is also tightened with polyethylene. In the resulting closed room, outwardly resembling a greenhouse, the temperature necessary for high-quality solidification is maintained with the help of heat guns. It is clear that this will require very significant material costs.

As can be seen, even with the economic from the point of view of carrying out work on pouring the foundation in winter time causes great concern. Therefore, before starting such work during the cold weather, you need to carefully weigh all the financial risks and the justification for such a rush.

Video: work on the "winter" laying of concrete in the foundation

Are antifreeze supplements a "panacea"?

There is a very widespread opinion that the problem of winter filling concrete foundation it is quite possible to solve the use of special salt additives in the prepared solution. Judging by the numerous advertisements, it is worth adding this component when mixing, and then everything will go on as usual. This should be dealt with in more detail.

Video: one of the options for antifreeze additives in concrete

First of all, it is necessary to understand how, in fact, the process of hardening and maturation of concrete stone takes place.

When the mortar is poured into the formwork, it goes through two stages until it is ready - this is setting and hardening.

• The poured concrete sets within 24 ÷ 30 hours. During this time, the liquid consistency turns into a solid state, however, it still does not have sufficient strength. It is during this period that crystalline bonds begin to form, the binding of free water and the cement components of the solution occurs.
• Then the second stage begins - this is the final hardening, maturation and hardening of the concrete structure - it passes within much longer period. This period depends on several factors, such as the brand of the prepared solution, the level of humidity and temperature, as well as the presence of special hardening additives.

As stated above, optimum temperature for chemical reactions of both processes varies from 15 to 25 degrees. The higher it is in this range, the faster the final crystallization will take place, the transition of water to a gel-like state. But even in optimal conditions you can talk about the readiness of the foundation no earlier than after 4 weeks - the base is completely ready for the construction of walls.

The danger of negative temperatures lies in several reasons:

• First, the expansion of frozen, uncemented water causes internal pressure on the porous structure of the concrete, which leads to its destruction. This becomes especially noticeable after the temperature rises to positive levels - such concrete is neither strong nor goes nothing compared to "normal".
• Secondly, the complex chemical process of cement hydration itself requires a certain temperature regime. Already at temperatures below +5° With the activity of these processes decreases to critical levels, and the colder it is, the more sluggish the maturation of concrete will be. And even in this case, such an "intermittent" process s ssignificantly degrades the quality finished concrete structure.

Potash is a very common additive for increasing the frost resistance of freshly poured concrete.

In order to minimize the negative impact of frost to some extent, special additives have been developed. Technologists are trying to solve both problems in this way. The addition of saline solutions increases the density of water, sharply reducing the temperature of its crystallization. Special additives, in addition, are intended for a kind of catalyzation and additional involvement of air in the processes of chemical aging of concrete stone under conditions of negative temperatures. Typically, such compounds are made on the basis of potash, lignosulfonate, calcium hydrochloride, nitrite or sodium formate.

However, if with the freezing of water the issue is more or less resolved positively, then it is much more difficult to “deceive” the laws of chemistry. The ripening process is still not fast, and takes much longer than under optimal conditions.

The table below, for example, shows the approximate maturation of concrete in conditions of negative temperatures using antifreeze additives:

It can be seen that even at -5 °, the complete readiness of the concrete structure can only be discussed after 3 months. In a colder atmosphere, the period increases even more.

A quite reasonable question arises: does it make sense for the owner of an individual site to “tie up” with the winter laying of the foundation (if this is not caused by some special circumstances), to spend significant material resources, physical effort on this, if the real gain in time is practically unattainable. Moreover, with antifreeze additives, too, everything is not so simple:

• Some types of additives cause active corrosion processes on the metal reinforcing cage. Not all steel grades are suitable for these purposes.
• Mixing "winter" concrete mortar is much more difficult than usual. It is simply impossible to carry out such work manually, since particularly thorough mixing of the components to an absolutely homogeneous consistency is required. The mixing time increases significantly, the components must undergo certain preparation (warming up to the desired temperature), a strictly verified order of supplying materials to the mixer is necessary, etc. a very precise dosage of additional additives is needed, depending on both the brand of concrete and the air temperature - here it is required professional approach. As an option - the delivery of a ready-made modified solution made in the factory - but this again rests on the question of the profitability of such construction.
• Even with the use of additives the temperature is below -20° With everything early is critical, and such a sharp fall in winter, for example, at night, is by no means uncommon.
• It is impossible to fill in such solutions even if a thaw suddenly began, it started to rain, or the relative humidity of the air jumped above the 60% mark.
• The use of such additives does not exempt from necessary activities on the device of the "greenhouse" after pouring, and maintaining the desired temperature with the help of heat guns. If this is not possible, then the foundation etc it is necessary to cover with heaters (for example, with a layer of dry sawdust, grass or turf, about 300 mm thick, and then cover with a film until ripe. in the spring, at the end of stable cold weather.

Care behind the cast foundation

It would be a serious mistake to believe that even under ideal conditions one can only pour the formwork with concrete and then curing. No matter how strange it may sound, but a freshly poured foundation always, under any circumstances, needs some care. The main objectives of such technological events are:

• Minimizing shrinkage of the poured concrete structure.
• Provide the most optimal modes for the aging process.
• Protect the hardening foundation as much as possible from temperature extremes, including daily ones.
• Prevent the poured solution from drying out, the rapid evaporation of unbound water - this will most likely end in cracking the surface.
• To create protection for a structure that has not hardened and has not gained strength against mechanical damage.

Measures for such care should begin literally from the moment the foundation is poured, and can be considered fully completed only when the concrete has gained at least 70% of brand strength, that is, with the optimal time for formwork removal (as indicated in the first table).

• Immediately after pouring, you should make sure that you have not lost the set geometric shapes– before the initial setting (the first 1 ÷ 2 hours) there is still an opportunity to make adjustments.

• No one is immune from sudden rains. In order to prevent erosion of a fragile surface or its mechanical damage, immediately after pouring, cover the surface with plastic wrap, burlap or tarpaulin.
• Do not forget that too high temperatures are also quite detrimental to the normal course of maturation processes. First of all, this is due to the active evaporation of water, the drying of the surface layer and the appearance of cracks. Necessarily steps are being taken surface wetting and moisture retention. In addition to covering with a vapor barrier film, it is sometimes necessary to resort to more drastic measures, for example, after the initial setting, cover the surface with a layer of material that absorbs moisture well. It can be wet sawdust or a rough cloth - it creates a semblance of a constant wet compress under a plastic cover.

In a word, each temperature regime requires its own concrete care measures. For ease of selection of the necessary, you can give the following table:

A few more important notes:

• Even if shallow strip foundation poured in ideal conditions for this, you should not leave it “naked” and unloaded for the winter. This will inevitably lead to the onset of surface erosion processes, and the structure will lose its strength, begin to crack and crumble. Thus, it is necessary to plan the construction work in such a way that during the season we can move on to the construction of walls on created basis.
• Immediately after the concrete structure has set the required 100% strength, it is recommended to carry out and without delay. More about these technological processes described in the relevant articles of our portal.

What can be the general conclusion?

Despite the fact that modern building technologies, in principle, allow pouring the foundation at temperatures that are quite far from the optimal 15-25 degrees, the best option nevertheless, there will be planning of such works in the most favorable conditions. So the foundation will turn out to be guaranteed reliable and of high quality. If, nevertheless, circumstances force a “zero cycle” to be carried out in other conditions, all technological recommendations for pouring and caring for concrete, in relation to the actual temperature regime, should be strictly observed.

Video: the complexity of the foundation pouring processes in winter

The construction of prefabricated concrete and reinforced concrete structures, as well as the construction of monolithic structures, does not cease to increase its pace, but often the craftsmen have to face the rush due to the approaching end of the construction season. This is explained operational characteristics cement slurry, one of which is the presence of a liquid phase that contributes to the continuous process of hydration and maturation of the composition. If the temperature drops below 5 degrees, the concrete maturation phase slows down, and if negative values it stops, which is due to the crystallization of water, which is part of the cement slurry. This leads to the destruction of the concrete structure, which becomes unusable. Despite this, most craftsmen with experience in the field monolithic construction, are faced with the need to continue the cycle of concrete work in the winter, and therefore, they face the question: “How to extend the liquid phase of concrete, and, consequently, its life. To solve this problem, experts suggest using antifreeze additives in concrete, the technical characteristics and main varieties of which will be discussed in this article.

Antifreeze additives in concrete: main varieties

Antifreeze additives in concrete are Chemical substance in the form of a dry mixture or solution, which, through involvement in the process of concrete crystallization maximum number water, accelerate the process of hydration of the concrete mixture, contributing to the hardening of concrete in conditions of negative temperatures. However, the main purpose of the antifreeze additive is to maintain liquid state concrete solution and the subsequent acceleration of its hydration, which significantly slows down at low temperatures.

Important! When using antifreeze additives in concrete, it is important to remember that the strength of concrete with antifreeze additives under conditions of negative temperatures does not exceed 30% of the maximum possible design strength, the remaining 70% of strength is gained by concrete during thawing. In this regard, structures, the concreting of which took place in the winter period, should not be subjected to high loads.

In accordance with the chemical basis, the following types of antifreeze additives in concrete are distinguished:

• Antifreeze;
• sulfates;
• Antifreeze additives-accelerators.

Let us consider in more detail the characteristics of each presented variety.

• Antifreeze is an antifreeze additive in concrete, which helps to reduce the crystallization temperature of the liquid that is part of the solution, and also increases or slightly reduces the setting rate of the solution. However, it does not have any effect on the rate of formation of structures.
• Sulfate based concrete additives are another popular antifreeze component that provides top speed formation of a dense solution. A characteristic feature of antifreeze additives based on sulfates is the active release of heat, which begins after they are added to the solution and is accompanied by the interaction of the concrete solution with hydration products. Due to the fact that sulfate-based additives are characterized by strong binding to sparingly soluble compounds, they cannot be used to lower the freezing point of the working mixture.
• At the heart of the action antifreeze additives-accelerators lies in the increase in the degree of solubility of the silicate components of cement, which, reacting with the products of its hydration, form double and basic salts, which reduce the freezing point of the liquid component of the concrete mortar.

Important! Modern complex antifreeze additives for concrete not only regulate the kinetics of its strength gain, but also correct its rheological properties. By lowering the crystallization temperature of the liquid component of the solution, they reduce the time of its initial setting, influencing the hardening of the cement stone and increasing its grade strength.

There are several types of accelerator additives, each of which has a certain set of chemical and operational properties. Let's consider them in more detail.

Potash or calcium carbonate, which is a crystalline substance, is a strong antifreeze component, significantly accelerating the process of setting and subsequent hardening of concrete. Like any antifreeze additive, calcium carbonate reduces the strength of the concrete structure, and in order to minimize this negative impact on the building, experts recommend combining potash with sodium tetraborate or sulfide-yeast mash, the concentration of which should not exceed 30%. Due to the fact that calcium carbonate is a potential dangerous substance, during its operation it is necessary to observe certain safety measures;

Sodium tetraborate, also called brown or sulfate-yeast mash, is a mixture of sodium, calcium, ammonium or lignosulfonic acid salts. Experts recommend adding this substance as an admixture when using calcium carbonate, which helps prevent the loss of strength characteristics of concrete structures after they are thawed. Otherwise, one can observe not only the appearance of cracks in the structures, but also a decrease in their water resistance and frost resistance. Thus, the use of potash as an antifreeze additive without the addition of sodium tetraborate will reduce the strength characteristics of the structure by 20-30%;

sodium nitrite- crystalline powder used as an antifreeze additive to concrete mortar. Given that sodium nitrite is a flammable poisonous substance, during its operation it is important to observe the maximum permissible concentration of the substance, which is determined empirically and usually does not go beyond 0.1 - 0.42 l / kg of cement mortar, provided that ambient temperature will be from 0 to -25 degrees. At the enterprise, in the process of working with sodium nitrite, the maximum permissible concentration of the substance in the workplace should not exceed 0.005 mg/l. In accordance with the requirements of the Research Institute of Concrete and Reinforced Concrete, the container that was used for the transportation, storage and manufacture of sodium nitrite must be marked "POISON". The joint use of sodium nitrite and lignosulfonic acids is prohibited, since their interaction is accompanied by the formation of toxic gases;

sodium formate- white crystalline powder, which also acts as an antifreeze accelerator. In most cases, it is used in conjunction with naphthalene lignosulfonate to improve water-reducing and plasticizing characteristics. Sodium formate is an antifreeze additive in concrete, the consumption of which does not exceed 2-6% of the total mass of cement.

Important! In addition to the above substances, sodium formate on alcohol, calcium chloride, ammonia water and urea can be used as antifreeze additives under conditions of negative temperatures.

Advantages and disadvantages of antifreeze additives in concrete mortar

Advantages of antifreeze additives in concrete

• Using antifreeze additives in concrete, you will be able to carry out concrete work at the construction site even in winter;
• Due to the fact that antifreeze additives increase the degree of adhesion of the mortar components, they significantly increase the strength of the monolith;
• Due to the high strength of products made with the use of antifreeze additives in concrete, they can be used for industrial purposes;
• They have a positive effect on the durability of the mixture, extending the life of the building;
• Increases plasticizing and stabilizing characteristics cement mixture- the use of concrete with increased plasticity makes it possible to manufacture structures that will not crack after the working composition has solidified;
• Increases the frost resistance of the concrete mix. This indicator is especially important for concrete intended for the construction of critical structures, for example, bridge supports. In most cases, it is directly dependent on the density of concrete. More dense grades of concrete are characterized by large quantity possible freeze and thaw cycles;
• Unlike alternative methods increasing the frost resistance of concrete, the use of antifreeze additives is characterized by a relatively low cost;
• Using antifreeze additives, you will significantly reduce the risk of concrete shrinkage. monolithic construction;
• Improving the moisture resistance of concrete structures by filling the pores with plasticizers that prevent the penetration of water;
• The acceleration of the process of hardening of the concrete mortar is the main point due to which the mortar can "not be afraid" of the cold;
• By opting for an antifreeze additive in concrete, you will reliably protect the reinforcement used from corrosion processes that occur due to water that is part of the concrete solution.

Disadvantages of antifreeze additives in concrete

• The desire to increase the reliability of the strength characteristics of concrete, it is necessary to increase the consumption of cement;
• The individual components that make up the additives are poisonous;
• In some cases, the declared power of concrete is reduced;
• In the case of using antifreeze additives in concrete, the speed of the set of strength characteristics of the concrete structure is reduced.

Experts advise introducing an antifreeze additive into the concrete solution along with water. It is important to note that it is desirable to do this with the last third of the liquid. It is not recommended to add additives to the dry mix. After adding an antifreeze additive to the solution, wait a certain period of time during which a uniform distribution of the components will occur.

When carrying out installation activities in conditions of negative temperatures, follow the instructions below:

• If you are working in snowy conditions, take care to organize appropriate shelters;
• The temperature of the solution that came out of the mixer should not go beyond the recommended range from +15 to +25 degrees;
• To prepare the working mixture, experts recommend using heated water;
• As for the heating of aggregates, it is recommended to do it before direct use.

Important! Specialists in the construction industry recommend paying attention to SNIP 3.03.01, according to which, in order to achieve the necessary strength characteristics of the concrete solution, it is necessary to comply with the requirements for the care of concrete in the winter. In the process of performing these measures, by the time the temperature for which the additive dosage was calculated was reached, it is not recommended to achieve a structural strength exceeding 20% ​​of the declared design strength.

Dosage and consumption of antifreeze additive in concrete

The dosage of the antifreeze additive in concrete, the consumption of which is an extremely variable parameter, is selected taking into account each specific situation through tests in production and laboratory conditions.

The consumption of antifreeze additive depends on the following factors:

• Ambient temperature under which installation activities will be carried out;
• Declared grade strength of the cement used;
• Chemical-mineralogical and material composition of the cement used in the process of work, as well as its estimated rate of curing;
• The temperature of the solution, which it will reach at the outlet of the mixer;
• Maintenance conditions for concrete structures.

Important! In the case of long-term use or storage of a solution in which additives were added, it is necessary to check its homogenization by periodically stirring. Calculation required amount antifreeze additive is produced with an error of 2%.

Do-it-yourself antifreeze additive in concrete

If the warm days have already passed, but you are suddenly faced with the need to pour a monolithic structure, you can not do without the use of an antifreeze additive in concrete. The most preferable option, in this case, would be to purchase an antifreeze additive in a specialized store, which is explained by their relative cheapness, low consumption and the ability to significantly improve the properties of the concrete solution, subject to minimal negative consequences. If the proposed scope of work is small, and you plan to carry out installation activities at a temperature of at least -10 degrees, this option is the most optimal.

However, if you do not have the opportunity to purchase a ready-made antifreeze additive in concrete, you can easily make it yourself, since the only material that you need in the process of work is chlorides (salts). Chloride salts reduce the freezing point of the solution, reduce the time of its initial setting and reduce the consumption of cement. However, experts are sure that a self-made chloride-based antifreeze additive can only be used for non-reinforced structures, which is due to corrosion processes developing under the action of chlorides.

Benefits of an antifreeze additive based on chlorides

• Low cost;
• No effect on the rate of hardening of concrete, due to which, the preparation of the solution can be carried out in advance;
• No effect on the structure of the cement mortar;
• Increased particle mobility, thanks to which you can give the cement mortar the desired shape.

Disadvantages of antifreeze additive based on chlorides

• A high level of corrosive activity, as a result of which, an antifreeze additive based on chlorides cannot be used for the manufacture of structures in the structure of which metal and reinforcement are present. The latter will oxidize under the influence of chlorides and exfoliate from the concrete structure, violating its integrity.

How does ambient temperature affect chloride consumption?

• Calculation of the proportion of chlorides in ready solution produced according to the following scheme:
• If installation activities are carried out at an average daily temperature of at least -5 degrees, the optimal proportion of chlorides in the finished solution should not exceed 2%;
• If work is carried out at lower temperatures (-6 to -15 degrees), the optimal proportion of chlorides should be 4% of the total mass of the solution.

Important! In this case, the scheme for gaining the expected strength of the structure during drying under conditions of negative temperatures will look like this:

For the first option, where the salt concentration is 2%:

• 30% after a week;
• 80% after a month;
• The construction will reach 100% strength only after 3 months.

For the second option (salt concentration is 4%), these figures will be 15%, 35%, 50%, respectively.

Important! Despite the fact that salt is an independent antifreeze additive, experts recommend using it in conjunction with calcium chloride, the mass fraction of which, when used at temperatures up to - 5 degrees, is 0.5% by weight of the solution, and 2% - if used at temperatures from -6 to -15 degrees.

Precautions when working with antifreeze additives

• In the process of working with antifreeze additives, it is necessary to use protective gloves;
• In case of contact with exposed skin, wash with soap and water. Avoid contact of the antifreeze additive with the eyes, if this cannot be avoided, rinse the eyes with plenty of water and consult a doctor immediately.
• Disposal of the additive is carried out in accordance with local regulations, which is explained by the presence of harmful components in the composition of antifreeze additives. As a result, it is forbidden to pour the mixture into the soil, waterways or sewers.

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 at an 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, it is necessary to create 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. Composite-balanced 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 temperature outside air, a certain concentration of the solution of ammonia mixing water is assigned (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 stains 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 in the process of curing concrete can migrate and accumulate in certain areas of 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 salt crystallization 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 fittings and process 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.

The minimum temperature for pouring concrete is approximately 5°C. That is, this indicator is considered critical - up to this figure, any work with concrete can be carried out as usual. However, it must be remembered that the optimum temperature for pouring concrete is 17-25 ° C, since such conditions allow obtaining the maximum strength of the solution. If the temperature approaches 5°C, the grout will likely be less solid and less durable and will not reach the brand specifications. Even when carrying out work, it should be remembered that the hardening time can increase significantly.

However, sometimes it becomes necessary to carry out construction work in winter at low temperatures. There may be several reasons:

• limited construction time in summer due to unfavorable climatic conditions;
• the need to pour concrete on loose soils;
• seasonal decline in prices for materials, including cement;
• reduction in the cost of work by specialized firms due to low demand for their services.

There are two main technologies with which you can carry out concreting in the cold season.

The first is that concrete is poured at low temperatures on heated fittings, which can be ordinary electrodes. A standard 12-volt cable is connected to the armature, through which current flows. This is quite enough to metal rods heated up and heated the solution, preventing it from freezing. The disadvantages of this method are obvious - in addition to the use of special materials, you will have to spend a hefty amount on electricity.

In addition, pouring concrete at low temperatures can be carried out using warm formwork. The essence of this technology is to insulate the solution with special heat-insulating materials, such as foam or mineral wool. But there are also disadvantages here - it is necessary to cover concrete from all sides, which leads to high consumption. auxiliary materials and increase the time of work.

Pouring concrete at low temperatures: general recommendations

Thus, we can conclude that pouring concrete at negative temperatures should be carried out only if it is not possible to postpone this stage of construction work until more favorable conditions occur.