Pouring concrete at sub-zero temperatures without heating. Technological features of pouring concrete at sub-zero temperatures. The problem of organizing brickwork in cold weather

In most of the territories of our country, cold or cool temperatures persist for more than half of the year. If we take into account that during concrete work, “winter” begins with a temperature drop to +5 o C, then the “window” for concrete work 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 frame is not connected to the concrete stone. The strength of such a base after thawing and final hardening will be several times lower.

From all these processes it follows that the less water in a non-cohesive 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 the maturation of some compositions by 20%, for others, all 100% is required.

For reinforced concrete with non-stressed 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. Optimum temperature the aging 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 of the 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 just 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 entered into warm room for him to accept room temperature but 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. Because using everything 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 inserted inside. concrete mortar. 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. Inside they put a stove, a heat gun, etc., with their help they maintain a 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 chemicals. Some of them accelerate curing by initial stage 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 temperature 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, it is problematic to concrete slab foundations in winter: 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).

Another 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.

Please note that the maximum temperature of the 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? Depends on its composition, but for private traders it’s real, albeit with at great expense, 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.

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 the weather conditions during the process, the quality and brand of the solution, 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 the installation 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 unnecessary internal stresses foundation structures.

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 prevail 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 of the region.
• In a number of areas, due to the poor development of road networks, delivery large volumes construction 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 prices for the necessary materials are reduced during this period. 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 solution, 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 consumption 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 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 the most solid basis for the construction of walls and in the shortest possible time.

• In the case when the mortar 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, as gaps may 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 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 high voltage currents (up to 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 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 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 under optimal conditions, one can speak about the readiness of the foundation no earlier than after 4 weeks - the foundation 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° FROM 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.

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

Type of antifreeze additive	average temperature during ripening	Week 1	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 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° FROM 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 until ripe with a film. 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 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:

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. Therefore, it is necessary to plan construction works to move during the season and to erecting 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?

Although modern building technology, in principle, allow pouring the foundation at temperatures far enough 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 on pouring and care of concrete, in relation to the real temperature regime.

Video: the complexity of the foundation pouring processes in winter

The foundation is the fundamental structure, the quality of which determines the geometric, technical and performance characteristics erected structure. Due to the specifics of the curing process by pouring concrete and reinforced concrete foundations it is undesirable to engage in winter in order to avoid their deformation and premature destruction. Minus thermometer readings significantly limit construction in our latitudes. However, if necessary, pouring concrete at low temperatures can still be successfully carried out if the right way and with precision observed technology.

Features of winter "national" filling

The vagaries of nature often make adjustments to development plans in the domestic territory. Either heavy rain interferes with digging a ditch, then a heavy wind interrupts, or it hampers the onset of the summer season.

The first frosts generally radically change the course of work, especially if it was planned to pour a concrete monolithic base.

The concrete foundation structure is obtained as a result of hardening of the mixture poured into the formwork. It consists of three almost equal components: aggregate and cement with water. Each of them makes a significant contribution to the formation of a durable reinforced concrete structure.

In terms of volume and mass, aggregates prevail in the body of the created artificial stone: sand, gravel, gruss, crushed stone, broken brick etc. According to the functional criteria, the leader is the binder - cement, the share of which in the composition is less than the share of the filler by 4-7 times. However, it is he who binds the bulk components together, but acts only in tandem with water. In fact, water is just as important a component of the concrete mix as cement powder.

Water in the concrete mixture envelops the fine particles of cement, involving it in the hydration process, followed by the crystallization stage. The concrete mass does not harden, as they say. It hardens by the gradual loss of water molecules from the periphery to the center. True, in the "transition" of the concrete mass into fake diamond not only the components of the solution are involved.

The environment has a significant influence on the correct course of processes:

• At average daily temperatures from +15 to +25ºС, the hardening of the concrete mass and curing takes place at a normal pace. In the specified mode, concrete turns into stone after 28 days specified in the regulations.
• With average daily thermometer readings of +5ºС, hardening slows down. The concrete will reach the required strength in about 56 days, if no noticeable temperature fluctuations are foreseen.
• Upon reaching 0ºС, the hardening process stops.
• At negative temperatures, the mixture poured into the formwork freezes. If the monolith has already managed to gain critical strength, then after thawing in the spring it will concrete again enter the hardening phase and continue it until a full set of strength.

Critical strength is closely related to the brand of cement. The higher it is, the less days the concrete mixture is needed before it is set.

In case of insufficient strength development before freezing, the quality of the concrete monolith will be very doubtful. Freezing in concrete mass the water will crystallize and increase in volume.

As a result, internal pressure will arise, destroying the bonds inside the concrete body. The porosity will increase, due to which the monolith will pass more moisture into itself and resist frost more weakly. As a result, operational periods will be reduced or you will have to do work from scratch again.

Minus temperature and foundation

It is pointless to argue with weather phenomena, you need to competently adapt to them. Therefore, the idea arose of developing methods for constructing reinforced concrete foundations in our difficult climatic conditions, possible for implementation in the cold period.

Note that their use will increase the construction budget, therefore in most situations it is recommended to resort to more rational options foundation devices. For example, use the bored method or carry out factory production.

At the disposal of those who are not satisfied alternative ways, there are several proven successful practices. Their purpose is to bring concrete to a state of critical strength before freezing.

According to the type of impact, they can be conditionally divided into three groups:

• Security external care behind the concrete mass poured into the formwork up to the stage of gaining critical strength.
• Increasing the temperature inside the concrete mass until the moment of sufficient curing. It is carried out by means of electrical heating.
• The introduction of modifiers into the concrete solution, lowering the freezing point of water or activating processes.

The choice of the method of winter concreting is influenced by an impressive number of factors, such as the power sources available on the site, the forecast of weather forecasters for the hardening period, the ability to bring a heated solution. Based on local specifics, the best option is selected. The most economical of the listed positions is considered the third, i.e. pouring concrete at sub-zero temperatures without heating, which predetermines the introduction of modifiers into the composition.

How to pour a concrete foundation in winter

To know which method is better to use for curing concrete to critical strength indicators, you need to know their characteristic features, familiarize yourself with the minuses and pluses.

Note that a number of methods are used in combination with any analogue, most often with preliminary mechanical or electric heating concrete mix components.

External conditions "for ripening"

Favorable external conditions for hardening are created outside the object. They consist in maintaining the temperature of the environment surrounding the concrete at a standard level.

Concrete poured "in minus" is cared for in the following ways:

• Thermos method. The most common and not too expensive option, which consists in protecting the future foundation from external influences and heat loss. The formwork is extremely quickly filled with a concrete mixture heated above standard indicators, quickly covered with vapor barrier and thermal insulation materials. The insulation prevents the concrete mass from cooling down. In addition, during the hardening process, concrete itself releases about 80 kcal of thermal energy.
• Keeping the flooded object in greenhouses - artificial shelters that protect from the external environment and allow carrying out measures for additional heating of the air. Tubular frames are erected around the formwork, covered with a tarpaulin or sheathed with plywood. If braziers or heat guns for the supply of heated air, then the method goes into the next category.
• Air heating. It involves the construction around the object of a closed space. At a minimum, the formwork is covered with curtains made of tarpaulin or similar material. It is desirable that the curtains be thermally insulated to increase the effect and reduce costs. In the case of curtains, steam or air flow from the heat gun is supplied to the gap between them and the formwork.

It is impossible not to notice that the implementation of these methods will increase the construction budget. The most rational "thermos" is to force you to buy covering material. The construction of a greenhouse is even more expensive, and if you also rent a heating system for it, then you should think about the cost figure. Their use is advisable if there is no alternative type and it is necessary to fill monolithic slab frozen and spring defrosting.

It should be remembered that repeated defrosting is destructive for concrete, therefore, external heating must be brought to the required hardening parameter.

Methods for heating concrete mass

The second group of methods is used mainly in industrial construction, because. needs a source of energy, accurate calculations and participation professional electrician. True, craftsmen, in search of an answer to the question of whether it is possible to pour ordinary concrete into formwork at sub-zero temperatures, found a very ingenious way out with the supply of energy welding machine. But even this requires at least initial skills and knowledge in difficult building disciplines.

In the technical documentation, the methods of electric heating of concrete are divided into:

• Through. According to what concrete warms up electric currents, which supply electrodes laid inside the formwork, which can be rod or string. Concrete in this case plays the role of resistance. The distance between the electrodes and the applied load must be accurately calculated, and the feasibility of their use has been unequivocally proven.
• Peripheral. The principle is to heat the surface zones of the future foundation. Thermal energy is supplied by heating devices through tape electrodes attached to the formwork. It can be strip or sheet steel. Heat is distributed inside the array due to the thermal conductivity of the mixture. Effectively, the concrete thickness warms up to a depth of 20 cm. Further, it is less, but at the same time, stresses are formed that significantly improve the strength criteria.

The methods of through and peripheral electrical heating are used in non-reinforced and lightly reinforced structures, because fittings affect the heating effect. With a dense installation of reinforcing bars, the currents will be closed to the electrodes, and the generated field will be uneven.

The electrodes at the end of the warm-up remain forever in the structure. In the list of peripheral methods, the most famous is the use of heating formwork and infrared mats laid on top of the base being constructed.

Most in a rational way heating of concrete is recognized as curing with an electric cable. The heating wire can be laid in structures of any complexity and volume, regardless of the frequency of reinforcement.

The disadvantage of heating technologies is the ability to dry out concrete, therefore, calculations and regular monitoring of the temperature state of the structure are required for carrying out.

Introduction of additives into concrete solution

The introduction of additives is the easiest and cheap way concreting at sub-zero temperatures. According to it, concrete pouring in winter can be carried out without the use of heating. However, the method may well complement heat treatment indoor or outdoor type. Even when used in conjunction with heating the hardening foundation with steam, air, electricity, there is a reduction in costs.

Ideally, the enrichment of the solution with additives is best combined with the construction of the simplest "thermos" with a thickening of the heat-insulating shell in areas with a thinner thickness, at corners and other protruding parts.

Additives used in "winter" concrete solutions are divided into two classes:

• Substances and chemical compounds that lower the freezing point of a liquid in solution. Provide normal hardening at sub-zero temperatures. These include potash, calcium chloride, sodium chloride, sodium nitrite, combinations thereof, and the like. The type of additive is determined based on the requirements for the temperature of the solution hardening.
• Substances and chemical compounds that accelerate the hardening process. These include potash, modifiers with a base of a mixture of calcium chloride with urea or calcium nitrite-nitrate, with sodium chloride, one calcium nitrite-nitrate, etc.

Chemical compounds are introduced in a volume of 2 to 10% by weight of the cement powder. The amount of additives is selected based on the expected hardening temperature of the artificial stone.

In principle, the use of antifreeze additives allows concreting even at -25ºС. But such experiments are not recommended for builders of private sector facilities. In fact, they run late autumn with single first frosts or in early spring, if concrete stone must be cured by a certain date, and there are no alternatives.

Common antifreeze additives for pouring concrete:

• Potash or otherwise potassium carbonate (K 2 CO 3). The most popular and easy-to-use modifier of "winter" concrete. Its use is a priority due to the absence of reinforcement corrosion. For potash, the appearance of salt stains on the surface of concrete is not typical. It is potash that guarantees the hardening of concrete at thermometer readings down to -25°C. The disadvantage of its introduction is to accelerate the setting rate, which is why it will be necessary to cope with pouring the mixture in a maximum of 50 minutes. In order to maintain plasticity for the convenience of pouring into the solution with potash, soap naphth or sulphite-alcohol bard is added in a volume of 3% by weight of the cement powder.
• Sodium nitrite, otherwise a salt of nitrous acid (NaNO 2). Provides concrete with a stable set of strength at temperatures up to -18.5 ° C. The compound has anti-corrosion properties, increases the intensity of hardening. The downside is the appearance of efflorescence on the surface of the concrete structure.
• Calcium chloride (CaCl 2), which allows concreting at temperatures down to -20 ° C and accelerates the setting of concrete. If it is necessary to introduce a substance into concrete in an amount of more than 3%, it is necessary to increase the grade of cement powder. The disadvantage of the application is the appearance of efflorescence on the surface of the concrete structure.

Preparation of mixtures with antifreeze additives is carried out special order. First, the aggregate is mixed with the main part of the water. Then, after light mixing, cement and water with chemical compounds diluted in it are added. The mixing time is increased by 1.5 times compared to the standard period.

Potash in the amount of 3-4% by weight of the dry composition is added to concrete mortars, if the ratio of binder to aggregate is 1:3, nitrite nitrate in the amount of 5-10%. Both antifreeze agents are not recommended for use in pouring structures operated in a watered or very humid environment, because. they contribute to the formation of alkalis in concrete.

In pouring critical structures, it is better to use cold concrete, prepared mechanically in the factory. Their proportions are calculated with accuracy based on the specific temperature and humidity of the air during the pouring period.

Prepare cold mixes hot water, the proportion of additives is introduced in strict accordance with weather conditions and the type of structure being built.

Methods for pouring concrete in winter:

Winter concreting with a greenhouse device:

Antifreeze agent for winter concreting:

Before pouring solutions with antifreeze additives, it is not necessary to warm up the bottom of the pit or trench dug under the foundation. Before pouring heated compositions, warming up the bottom is mandatory in order to avoid unevenness that may result from ice melting in the ground. Filling should be carried out in one day, ideally in one step.

If interruptions cannot be avoided, the intervals between grouting should be kept to a minimum. If the technological subtleties are observed, the concrete monolith will gain the necessary margin of safety, will be mothballed for the winter and continue to harden with the advent of warm weather. In the spring, it will be possible to start building walls on a ready-made reliable foundation.

Industrial construction does not stop during the cold season, but hardening of concrete at low temperatures is problematic. Crystallization of water leads to a decrease in strength, which negatively affects the quality of structures. The use of antifreeze additives in concrete, a wide choice of which the industry offers today, will help to solve the problem.

The use of antifreeze additives allows pouring mortars at temperatures up to -50ºС. They are liquid or powder formulations that are added to the mixture. It is important to know that concrete additives added during frost provide only 30% hardening. The final hardening occurs after the defrosting of the monolith.

There are several types of antifreeze additives that differ in the principle of action:

• Compositions that lower the freezing point of the liquid, as a result, the process of cement hydration continues, setting proceeds according to the standard mechanism;
• Additives based on sulfates accelerate the compaction of concrete, while generating excess heat, which increases the rate of hydration of the cement;
• Complex additives increase the solubility and activity of cement laitance, while the compounds that appear as a result of reaction with water reduce its crystallization temperature.

Varieties

There are various antifreeze additives, each of which has its own mechanism of action. A popular additive is sodium carbonate, otherwise known as potash. This is a powdery crystalline composition that accelerates the hardening of the concrete mixture. The use of this type of additive reduces the technical characteristics of the material, including strength. To reduce this effect, sodium tetraborate is added to potash.

Attention! Potash belongs to dangerous substances, therefore, when working with it, safety requirements are observed.

Sodium tetraborate is a complex substance consisting of sodium and calcium salts, with the addition of ammonium. It is an additional antifreeze additive used with sodium carbonate. Without it, the structure can lose up to 30% of its strength after thawing and complete curing.

Sodium nitrite is an effective antifreeze additive that reduces the crystallization temperature of water, accelerates the hardening of the composition, and has an anticorrosive effect. Its use is dangerous, since sodium nitrite powder is a flammable, explosive, toxic substance. It is used in frosts down to -25ºС. It cannot be miscible with lignosulfonic acids, as it releases toxic gas when interacting with them. Calcium nitrite-nitrate has similar properties.

Sodium formate is an antifreeze additive for concrete that reduces the crystallization temperature of water and accelerates the hydration of cement. It is added in a proportion of not more than 6% of the total mass of concrete. To improve plasticization, naphthalene lignosulfonate is added to the additive.

Urea - PMD, prolongs the liquid phase of water, practically does not affect the setting speed.

Calcium and sodium chlorides, ammonia water reduce the freezing point, but have an increased corrosive effect. They strongly influence metal elements therefore not recommended for use in reinforced concrete products.

How to use

The choice of additive and the method of its application depends on the conditions and material where it will be introduced. Any concrete additives used at sub-zero temperatures are introduced into the solution with water, according to the manufacturer's instructions. After thorough mixing, it is recommended to wait a while for this component to diffuse in the composition.

According to SP 70.13330.2012, in order for the composition to achieve the desired strength, it is important that before the temperature of the composition reaches the mark for which the additive is designed, it gains no more than 20% of the planned strength.

Consumption of antifreeze additive per 1 cubic meter material depends on the average daily ambient temperature. Up to -5 degrees, it is recommended to add up to 2% of the additive by weight of the solution, up to -10 degrees, this figure increases to 3%, up to -15 degrees, no more than 4%. At severe frosts calculation is made individually for each type. The rate of hardening of the solution is reduced, and the maximum strength is achieved after the end of frost.

When adding plasticizers and PMD, it is necessary to observe certain rules work. The recommended range of the poured solution is from +15 to +25ºС. To dissolve additives, a certain amount of water is required, which must be heated, this ensures complete dissolution of substances. Sand and gravel used in the solution are also heated immediately before adding. Cement must not be heated, as it will lose its astringent properties. The poured solution must be covered, this is especially true during snowfall.

Advantages and disadvantages of additives

The main advantage of concrete with antifreeze additives is the ability to perform work all year round. Picked up in correct proportion, they improve the adhesion of the components, improving the quality of the solution. They also have other advantages:

• increasing the service life due to the compaction of concrete;
• increase the plasticity of mixtures, making it easier to work with them;
• the frost resistance of ready-made concrete increases, which is important for elements of load-bearing structures;
• the use of PMD is the cheapest way to fill at low temperatures;
• the use of additives reduces shrinkage during solidification, while maintaining the integrity of the structure;
• antifreeze additives fill the pores of concrete, thereby significantly increasing its water resistance;
• some compositions significantly increase the corrosion resistance of the monolith, extending the service life of structures, buildings and structures at times.

The use of antifreeze additives has its drawbacks. If used incorrectly, the strength characteristics of concrete are reduced, therefore, when working, it is necessary to strictly adhere to the instructions. Some additives are flammable and toxic, which must be taken into account when working with them. Even with additives, the hardening rate will be relatively slow in frost. To achieve the desired strength when laying in winter, more cement is required, which increases the cost of construction.

Do-it-yourself anti-frost additives

In private construction, if it was not possible to carry out all the work in the warm season, it is necessary to continue in the winter. Therefore, it is allowed to prepare anti-frost additives for concrete with your own hands at home. To do this, you only need table salt or sodium chloride. Such an additive reduces the freezing point of water and reduces the time of critical solidification of the mixture.

To do antifreeze additive into concrete with your own hands, salt is dissolved in water and added to the solution. The concentration up to -5 degrees is 2% by weight of the solution, up to -15 degrees, mass fraction chloride reaches 4%.

The disadvantage of this additive is its corrosive activity towards metal, so it is not suitable for reinforced concrete structures. It should be borne in mind that the rate of solidification of the solution at negative temperatures will be, on average, 3 times lower than under normal conditions.

As early as the beginning of the last century, work with concrete mixtures were 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 chips, 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 the most optimal for obtaining high-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? Of course. 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. 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 into summer period, it should be noted the fact of a decrease in the strength of the product at +30 degrees. A practical way out is to moisten the concrete surface with water. In summer, due to evaporating moisture, concrete is made more liquid. And of course, it should be reported at what temperature to pour concrete in winter - it is recommended to perform all work up to -15C.

Reacting to the effects of temperature, concrete sets more evenly in wet and cool weather in summer. And if the work is carried out during the rainy period, then the resistance of the material to moisture is increased with special cement. To prevent the solution from being washed away, the site is covered with polyethylene. However, outdoor concrete work is not recommended during heavy rains. 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.