Pouring concrete at sub-zero temperatures without heating. At what temperature can concrete be poured - construction in winter and summer. Concentration of ammonia mixing water, %

In winter, when the temperature outside drops below zero, the construction of reinforced concrete, monolithic and concrete structures becomes more difficult. The liquid, which is part of the cement mortar, begins to crystallize, as a result, the process of concrete hardening almost completely stops.

As soon as the warm season sets in, the hydration of the cement resumes, and the mortar that has frozen during the construction phases has a high thermal conductivity and low strength. This is due to the fact that the frozen water expands, making the concrete loose. In order to eliminate such consequences, it is customary to use antifreeze additives. They resist liquid crystallization sub-zero temperature.

Additives in concrete

Having visited hardware store, you can find concrete additives in a large assortment. For example, sodium nitrite is in the form of a liquid or white crystals. Carbamide is known by its second name - urea - and is produced in the form of colorless crystals. You can find compounds of calcium nitrate and urea. The kind of white powder is sodium formate.

The dark brown liquid is a compound that suggests the presence of:

• acetylacetone;
• nitrochloractinide;
• acetoacetic ether.

It is noteworthy that this composition is used at temperatures up to -25 ° C. Asol-K is made from special modifiers and potash solution. It is able to withstand temperatures down to -10 °C, while when the temperature changes above 0 °C, concrete hardening is accelerated. High-quality antifreeze for mixtures is Hydrozim, which provides hydration at temperatures below -15 ° C. From NH3 gas and ordinary water, a composition is made that can withstand frosts down to -30 ° C. Hydroconcrete has plasticizing properties, which can be used at temperatures not lower than -15 °C.

Additives for cement slurries

Antifreeze additive in the solution allows you to use the latter in the construction monolithic structures, buildings and manufacturing reinforced concrete elements. Today, such additives are presented in a wide range, but the most popular are electrolyte compositions and their compounds. They provide antifreeze effect and accelerate the hardening of the solution. Among the most common are:

• HK, HH1.
• NNHC.

sodium formate

Formate is an antifreeze additive that has the appearance of a crystallized white powder that dissolves well in water. An additive is made from sodium salts of formic acid. The main actions are aimed at reducing the temperature of the water in the solution and prolonging the hydration of the concrete.

The additive does not have plasticizing properties and is capable of creating tension in the structure due to salt crystallization. In order to avoid this, you should additionally use the C-3 plasticizer, which is available in liquid and powder form.

Consumption

The consumption of an antifreeze additive must be known in order to purchase the right amount of the product. In the manufacture of a solution with an antifreeze additive FN + S-3, warm water, into which sodium formate and a plasticizer should be poured. The amount of additive is determined depending on weather conditions. If the temperature varies from 0 to -5 ° C, then the consumption will be 3% of the total mass of cement.

When the temperature changes in the range from 0 to -10 ° C, an additive in the amount of 4% by weight of cement should be used. The amount of additive increases to 6% if the ambient temperature drops to -15 °C. The volume of C-3 plasticizer always remains at the level of 0.8-1% by weight of cement, which does not depend on air temperature. For an antifreeze substance, the consumption will be different. This parameter will be influenced by the properties of the additive and the air temperature.

Application of additives

Antifreeze additives must be added to the solution along with the liquid. It is desirable to do this with the last third of the water. Additives should not be added to the dry mix. As soon as the additive is added, you should wait a certain time, which is necessary for the distribution of the components. Antifreeze additives are used at sub-zero temperatures according to certain rules.

If you have to work in snowy conditions, then it is important to take care of shelter. When the solution leaves the mixer, it is important to keep its temperature in the range from +15 to +25 °C. The working mixture should be prepared using heated water. Aggregates are heated before direct use.

Application restrictions

Antifreeze additives of some types can be used subject to restrictions. They apply to prestressed structures and products that will be subjected to dynamic loads. Use when sealing joints reinforced concrete structures solutions of chloride salts are not allowed. This applies to products that have embedded parts and fittings. The rule is relevant if chemical protection has not been carried out.

Mixtures of antifreeze additives should be used after preliminary testing of samples for corrosive attack. The described means can cause efflorescence on the concrete surface, as well as reduce the strength characteristics of structures. Concrete with additives must have a positive temperature by the beginning of curing.

This parameter should be higher than the outside air temperature by 5 °C or more with a plus sign. For example, if the air temperature is -13 °C, then the concrete temperature should be +18 °C. Unburied surfaces of structures should be thermally insulated to prevent freezing of water from these areas.

The use of antifreeze additives requires compliance with the outside temperature limits. If chloride salts (sodium nitrate) are used, the temperature should be -15°C or higher. As for potash, this limit will be -25 ° C. The technology of using the described additives has one important virtues, which consists in the minimum material and physical costs for implementation. The disadvantage of technology is a long period acquisition of critical strength by the structure. If the rules for using additives are violated, some problems may arise, such as efflorescence on the surface and corrosion of the reinforcement.

When an antifreeze additive is used in concrete, complex methods of maintaining structures are often used. Thermos technology is used to reduce the hardening time. To do this, the mixture is prepared on the heated components of the liquid with the thermal insulation of concrete.

Why You Should Use a Chloride-Based Supplement and When You Should Avoid It

Antifreeze additives made on the basis of chlorides have many advantages, among them should be highlighted:

• low cost;
• no effect on the structure of the solution;
• no effect on the rate of solidification, so that the solution can be prepared in advance;
• an increase in the mobility of the particles, due to which it is possible to give the solution the desired shape.

The above additive has a minus, which is expressed in high level corrosive activity. As a result, such a tool can be used only for those structures that do not have reinforcement and metal in the structure. The latter begins to oxidize, exfoliate from the structure, thereby violating the integrity.

Precautions for use

Wear protective gloves when handling additives. If the product got on open areas skin, they should be washed with soap and water. The additive should not get into the eyes, but if this cannot be avoided, then they should be rinsed with plenty of water and consult a doctor. Additives must be disposed of in accordance with local regulations, as these formulations contain harmful components. This indicates that the mixture must not be poured onto the soil, discharged into sewers or waterways.

Conclusion

Using antifreeze additives, it is possible to increase the structural strength by almost 2 times. These ingredients allow you to save on cement, because it can be of a lower class, which means it will cost less. Structures can be made of less impressive thickness, while strength and other properties will not be lost.

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 season, 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 it is also well compacted. Very often in winter, both sand and gravel are in a state “stuck” by frost. There is a good chance that as the temperature rises, the pad may lose the required density, even with the highest quality compaction.

• 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. On the 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 unnecessary internal stresses foundation structures.

Nevertheless, as already mentioned, there are situations when, for one reason or another, the construction of a foundation in 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, delivery large volumes building materials or the movement of heavy special equipment is possible only on frozen ground.
• Sometimes to winter construction are resorted to in order to save money, since during this period 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 very often added in cases where it is necessary to fill the foundation with negative temperatures air.

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 maximum 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 engaged in the manufacture, delivery and unloading required solution into the prepared formwork.

• It is not recommended to pour concrete in layers, because between them, due to low temperatures, gaps may form, 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 winter period carried out only when absolutely necessary.

Below is a table that clearly shows how temperature affects environment for the time of maturation 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.

hardening time of concrete, days	-3°C	0°C	+5° С	+10° С	+20° С	+30° С
1	3%	5%	9%	12%	23%	35%
2	6%	12%	19%	25%	40%	55%٭
3	8%	18%	27%	37%	50%٭	65%
5	12%	28%	38%	50%٭	65%	80%٭٭
7	15%	35%	48%٭	58%	75%٭٭	90%
14	20%	50%٭	62%	72%٭٭	90%	100%
28	25%	65%	77%٭٭	85%	100%	-

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). In inner space install reinforcing structures that 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 everything financial risks and justification for such haste.

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 pouring a concrete foundation in winter can be completely solved by using 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 mentioned above, the optimum temperature for the 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, and developed special additives. 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.

For example, the table below shows the approximate maturation time of concrete in conditions of negative temperatures with the use of antifreeze additives:

Type of antifreeze additive	average temperature during ripening	1 Week	2 weeks	4 weeks	3 months
sodium nitrite	-5°C	30%	50%	70%	90%
	-10°C	20%	35%	55%	70%
	-15°C	10%	25%	35%	50%
Potash	-5°C	50%	65%	85%	100%
	-10°C	30%	50%	70%	90%
	-15°C	25%	40%	60%	80%
	-20°C	23%	35%	55%	70%
	-25°C	20%	30%	50%	60%

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 plot to “get involved” with winter styling foundation (if this is not caused by some special circumstances), spend significant material resources, physical efforts 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 at all exempt from the necessary measures for the construction of a "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 assume that even in ideal conditions you can only fill the formwork with concrete, and then maturation. 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 solidifying foundation as much as possible from temperature extremes, including daily ones.
• Do not allow the poured solution to dry out, the rapid evaporation of unbound water - this will most likely end in cracking the surface.
• 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 the brand strength, that is, when optimal timing 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:

Actions taken to ensure normal maturation of concrete	less - 3 ° С	from - 3° to + 5° С	from + 5° to + 10° С	from + 10° to + 15° С	from + 15° to + 25° С	over + 25° С
Covering with a film, constant moistening of the poured surface and formwork, covering concrete with moisture-saving material	No	No	No	No	in severe windy weather	always
Covering with a film, moistening the concrete surface	No	No	Yes	Yes	Yes	not enough
Covering with film and thermal insulation material	No	Yes	No	No	No	No
Covering with a film and thermal insulation material, creating a greenhouse with maintaining a temperature of +10 ° C for at least 3 days	Yes	No	No	No	No	No
Permanent maintenance of a thin layer of water on the concrete surface	No	No	Yes	Yes	Yes	Yes

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 details about these technological processes are 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 would still be to plan such work 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

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 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 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 in the 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 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.

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 outdoor concrete work 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.

In most of the territories of our country, cold or cool temperatures persist for more than half of the year. Considering that at concrete works ah "winter" begins with a temperature drop to +5 o C, then the "window" for work with concrete is very small. However, it can be extended, and significantly, through the use of various means. These are the so-called technologies of winter pouring of concrete.

During the normal course of the concrete curing process, moisture serves as a "gluing" element for cement particles. Upon her transition to solid state all processes are stopped.

But this is not the only problem. It is known that when freezing, the volume of water increases by about 9%. As a result, inside the mass of concrete is formed high blood pressure. If the grains of cement up to this point have not yet gained a certain level of strength, they are destroyed under pressure. After freezing, they will no longer fully acquire their properties and the concrete will not be strong enough.

There is another unfavorable moment in the winter pouring of reinforced foundations. Steel is an excellent conductor of heat, and it facilitates the removal of heat from the thickness of the concrete. Possessing good heat-conducting properties, the rods quickly cool down. Around them, the water freezes first. Ice pushes concrete particles aside, and in their place comes water that has not yet frozen from the still warm layers. It also freezes, pushing the concrete further back. As a result, the array is no longer a monolith: the framework is not connected to concrete stone. The strength of such a base after thawing and final hardening will be several times lower.

Of all these processes it follows that than less water in an incoherent state will be at the time of freezing, the less will be the loss of strength. Through various experiments and calculations, the boundary strength values ​​were determined at which concrete can be frozen. They are called the point of critical strength. Depending on the class of concrete and the purpose of the building, the type of use of the structure, it is required to wait for some compositions to mature by 20%, for others, all 100% is required.

For reinforced concrete with non-tensile reinforcement (the type used in private housing construction), it is 50%, for foundations that will be subjected to alternate defrosting / freezing (baths and country houses without heating) - 70%. After reaching this point, the foundation can be frozen. After thawing, all processes in it will resume. The loss of strength in this case is no more than 6%.

Concreting methods in winter conditions

The speed of the hardening process depends on the temperature of the solution. With its increase, the activity of water increases significantly, the rate of curing increases. Therefore, when carrying out concrete work in winter or at temperatures below +5 o C, it is important to create and maintain the required level of heating. The optimum aging temperature of the solution is from +20 o C to +30 o C. There are several ways to do this:

• make the solution warm;
• insulate the formwork;
• use additives and additives that accelerate hardening and/or lower the freezing point of water;
• preheat the already poured concrete mass.

All of these methods work well. They are used singly or in combination.

First of all, it is necessary to choose the right cement for winter concreting foundation. It is known that during the hardening of concrete, reactions occur in which heat is released. For winter, it's a great feature. At the same time, fast-hardening Portland cements and compositions of high grades emit a greater amount of heat. Therefore, for kneading at low or sub-zero temperatures, it makes sense to buy them.

Only this will allow you to fill the foundation with a strip or slab foundation at positive temperatures during the day, and slight frosts at night. But at the same time, it will be necessary to make the batch warm (read below), and after pouring the foundation, the formwork will need to be thermally insulated: covered with mats, straw, etc. If you have already purchased a heat insulator, you can use it, you only need to monitor its condition, cover it with a film or other moisture-proof materials.

Temperature increase during kneading

During the winter pouring of the foundation, the temperature of the solution is adjusted to 35-40 o C. To do this, heat the water and backfill. In no case should the cement be heated: it will “brew” and become practically useless.

It is good if it is possible to use an electrically heated concrete mixer: it is connected to the network and the drum is heated. In another case, it is advisable to preheat it by scrolling through the heated water.

When kneading, water is heated to 90 o C. Crushed stone and sand must be heated to 60 o C. This is done by blowing hot air, heating in special ovens. Furnaces are for a private builder from the realm of fantasy, but you can arrange hot air blowing. For example, from a stove or a fire, stretch several duct pipes into a pile of rubble or sand.

Once again, we pay attention: do not heat the cement. It can be brought into a warm room so that it takes room temperature, but it cannot be heated.

During the winter mixing of the solution, the order of laying the components changes: water is poured, crushed stone and sand are poured into it. After several revolutions cement is added.

In addition, it is necessary to increase the kneading time. It should be longer by 20-50%: due to better mixing, reactions are activated and the temperature rises during hardening.

Warming and heating solution

To extend the cooling time of concrete, it is required to retain heat to the maximum. Therefore, using all possible means and available materials, carry out the insulation of the walls of the formwork. You can use tarpaulins, mats, some old warm clothes, fill the gap between the walls of the formwork and the soil, straw. Anything, as long as the heat doesn't leak into the air.

One of the tasks is to keep the heat of the solution

During construction in industrial scale electrical heating is also used different kind electrodes. They can be located on the surface, fixed on the formwork or introduced into the concrete solution. The method is effective, but rarely implemented in private construction. This pleasure is very expensive: the consumption of electricity for heating a cubic meter of concrete is 60-80 kW / h. At the same time, it is necessary to strictly control the temperature: measure it every two hours (or more often) and turn it off when it reaches +30 o C. Then turn it on again after a while. Control must be around the clock.

When pouring the foundation with your own hands in winter, it is realistic to use only heating cables. They are attached from the inside to the formwork, and after it is removed, they are dismantled. There is a second option - to "drown" the wire in concrete. Both methods work well, but only if the walls are insulated from the cold.

There are also on sale special heating mats for heating concrete. They are laid out on the surface, included in the network. Its cost is 2.5 thousand rubles / m2.

To maintain the temperature, there are greenhouses above the object. These are structures very much reminiscent of greenhouses. And their task is similar: to keep warm. A frame is erected, it is covered with a film or other similar materials. Putting a stove inside heat gun etc., with their help maintain positive temperature. But at the same time, it is also necessary not to forget about moistening so that the moisture from the solution does not evaporate.

Another method of heating concrete is using infrared emitters. This method is good because, under the influence of waves, the solution itself is heated directly. The emitters are covered with aluminum casings, creating a directed flow. However, for effective heating, a large number of lamps will be needed.

Additives and Additives

Another way to pour concrete at low temperatures is to use chemical substances. Some of them accelerate the hardening at the initial stage of the process. The mass fraction of all additives is not more than 2% by weight of cement. Large quantities can adversely affect the quality of concrete, so stick to the recipes.

One of the methods of winter concreting is the addition of special antifreeze additives to the batch.

The most common additive that increases the “frost resistance” of concrete and accelerates its hardening is calcium chloride. Potash and sodium nitrate are also used. If you add them during normal mixing, the freezing point will drop to -3 o C.

One "BUT". Chlorides cannot be used for reinforced concrete - they provoke the rapid destruction of steel. So the most common concrete hardening accelerator - calcium chloride - is not suitable for pouring the foundation.

Pouring concrete at sub-zero temperatures is possible if the solution is heated with the same additives. In this case, you can work at -15 o C. But for the normal quality of the foundation, you will need to insulate the fill and follow simple but mandatory rules.

Rules for winter pouring concrete

The solution is poured into the prepared formwork. Preparation consists in removing ice and snow, heating the reinforcement and the bottom of the foundation. This is the most difficult stage. Scraping off ice is half the trouble, but warming up the reinforcement and the entire perimeter of the foundation is a problem. The temperature should not be high, but it is necessary to achieve its positive values.

As an option, you can consider the device of portable braziers, which are lowered into the pit and kindled there. It is possible to use heat guns powered by gas cylinders. The use of other means is difficult due to their high cost.

For this reason, concrete in winter slab foundations problematic: such areas cannot be heated. For this type of bases, "winter" will be limited to light frosts at night and positive daytime temperatures. Filling can be started after the reinforcement and the bottom have a positive temperature.

The strip foundation can also be poured during frosts: it is realistic to heat such a base and reinforcement in a limited volume. Not easy, but possible.

Everything can be done step by step. Break the whole tape into small areas, start heating at the same time or with a certain time interval on several of them (two or three, depending on the time required for kneading and heating the pit). Start pouring one area by moving the braziers further. While the first heated area is being poured, the next one will reach the required temperature. The flooded area is immediately covered with heat-insulating materials and proceed to the next, and moving along the entire perimeter.

It is imperative to knock down the ice and heat the reinforcement - this is the only way the foundation will be strong

The mechanism is clear. So it is possible to pour the foundation with concrete at -15 o C (but with appropriate additives, "hot" mixing and measures to preserve heat).

One more important condition- work must be carried out continuously. In winter, it is impossible to fill the foundation in parts. This is 100% correct. The gap between fillings should be such that a film does not have time to form on the surface of the previous part, and even more so that moisture does not freeze. Work must be carried out continuously until the end of the pour. The flooded parts must immediately be covered with heat-insulating mats. As you can see, several people are needed for this work. One cannot cope with all tasks.

note that Maximum temperature solution should be 35-40 o C. Its excess leads to a slowdown in the curing process. The situation will, of course, be better than when freezing, but not by much.

Results

Pouring the foundation in winter is not an easy task, but it is possible even with your own hands. Need helpers and careful preparation, but you can make a normal base at sub-zero temperatures. At what temperature can concrete be poured? It depends on its composition, but for private traders it is realistic, albeit at high cost, to achieve normal quality at temperatures not lower than -10-5 o C. Filling at positive temperatures during the day and frost at night will turn out to be less expensive.

Let's try to answer this question, taking into account the characteristics of concrete and its behavior at sub-zero temperatures.

To begin with, water in concrete is needed not only for its fluidity, it also plays a very important role. important role during the hardening process.

At sub-zero ambient temperatures, water begins to turn into ice, as a result of which it not only stops the hardening process, but also contributes to the destruction of the bonds that formed before it freezes.

You should be aware that concrete can freeze at a temperature of about -4 0 C. At temperatures below +5 0 C, the process of its hardening slows down several times, and strength gain is delayed until it becomes warmer. So, the longer the stagnation during the hardening process, the less strength it will gain in the end.

This means only one thing - at sub-zero temperatures, we need to create conditions under which concrete will not only not freeze, but will continue to gain its strength.

Let's try to figure out how to do it.

Technology of pouring concrete at sub-zero temperatures

There are several technologies for pouring concrete when the ambient temperature is below zero:

• By adding antifreeze additives to concrete
• With the help of electrical heating of concrete from the inside
• With the help of insulation of the pouring object

The use of additives in pouring concrete

The use of antifreeze additives in prepared concrete is very common. This is due, first of all, to low costs and their availability. And if you use purchased concrete, then all the necessary antifreeze additives will already have to be in it, in accordance with the values ​​​​of sub-zero temperatures.

These additives contain various chemical impurities, like salts of monocarboxylic acids, sodium nitrite and formate, etc., which significantly accelerate the hardening process, increase the final strength of concrete, and most importantly, prevent the water contained in concrete from freezing.

The main disadvantages of using various kinds of additives in concrete is that they do their job well only at low sub-zero temperatures - about -5 0 C.

In severe frosts, some of the additives will work, but all the same, hardening will slow down significantly and eventually there will be a shortage of concrete strength up to 30%. Also, some additives are aggressive to metal reinforcement, and act destructively on it.

The technology of heating concrete during its pouring

Heating of concrete with electricity is carried out in two ways:

• Passing current through concrete using electrodes
• By passing current through a pre-laid wire inside the concrete

If someone is interested in the technology of electric heating of concrete in winter, I described it in detail in one of my articles. Therefore, I will not focus on this technology much attention here.

The first method is much cheaper, since fittings can act as electrodes.

If you follow the rules, then additional special equipment is needed for these purposes, but some craftsmen, having inserted reinforcing electrodes into concrete, connect a welding machine to them, thereby passing current through the concrete and heating it.

It is worth noting that reinforced concrete they are heated with voltage up to 127 V, and concrete without metal reinforcement can be heated with voltage up to 380 V.

The second method involves additional costs for special heating cable, which, as a rule, must be laid along the reinforcing cage and connected to it with current through special substations.

This method is much more expensive, but at the same time - much more efficient than heating concrete with electrodes.

The main disadvantage of using this method is significant financial costs, and at the same time, when heating concrete, it is necessary to constantly monitor its temperature. High temperatures will cause the concrete to dry out and can cause significant damage to the future structure.

In severe frosts additional insulation formwork or concrete.

Insulation of concrete during curing

In addition to creating artificial heating of concrete, at a slight sub-zero temperature, standard methods of insulation can be applied.

As a rule, the formwork itself is insulated, if any, as well as all visible parts of the concrete. Warming is carried out various materials, from conventional sawdust to the use of Styrofoam.

One of the most popular methods of insulation is the creation of a so-called tent around the concreting object. Additionally, heating guns can be installed inside such a tent.

The disadvantage of this method is that the concrete must be supplied already heated, and conventional insulation, without additional heating, will be ineffective in severe frosts.

So we examined the main methods of pouring concrete at sub-zero temperatures, to all this I would like to add that often for this purpose they use not one specific method, but a set of measures is planned in advance, ranging from the addition of antifreeze additives to heating with subsequent insulation .

But despite this, the process of pouring concrete in sub-zero temperatures is not cheap, despite the fact that manufacturers make huge "winter discounts" on building materials.

In addition to this, it is very difficult to maintain a constant comfortable temperature with the help of artificial heating, and what is not unimportant - the temperature must be evenly distributed throughout the entire volume of the fill.

Attention! Pouring concrete at sub-zero temperatures is only possible if you have no other choice, because minimum temperature environment in the process of hardening, without additional interventions, in spring should be +5 0 С, and in autumn - +10 0 С.

For autumn, the requirements are stricter, because the main hardening period is 28 days, and during this time the autumn air temperature can drop significantly.