Filling the solution at sub-zero temperatures. Pouring concrete at sub-zero temperatures - features and technology. Concreting using the mortar's own heat

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 a common place is determined, the internal boundaries of the trench are marked, 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. 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, 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 moving heavy special equipment 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 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 without 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, 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 winter period carried out only when absolutely necessary.

Below is a table that clearly shows how the ambient temperature affects the ripening time and the full set of the required strength of 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 and safe terms for stripping the poured concrete structure.

- 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 will only reduce the negative impact of 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 is a big 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 winter filling concrete foundation it is quite possible to solve the use of special salt additives in the prepared solution. Judging by the numerous advertisements, it is worth adding this component when mixing, and then everything will go on as usual. This should be dealt with in more detail.

Video: one of the options for antifreeze additives in concrete

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

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

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

As stated above, optimum temperature for chemical reactions 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:

Firstly, the expansion of frozen water not bound by cement causes internal pressure on the porous structure of 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 begins, it rains or relative humidity air jumped above the 60% mark.
The use of such additives does not exempt from necessary activities on the device of the "greenhouse" after pouring, and maintaining the desired temperature with the help of heat guns. If this is not possible, then the foundation etc it is necessary to cover with heaters (for example, with a layer of dry sawdust, grass or turf, about 300 mm thick, and then cover with a film until ripe. in the spring, at the end of stable cold weather.

Care behind the cast foundation

It would be a serious mistake to 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. Thus, it is necessary to plan the construction work in such a way that during the season we can move on to the construction of walls on created basis.
Immediately after the concrete structure has set the required 100% strength, it is recommended to carry out and without delay. More about these technological processes described in the relevant articles of our portal.

What can be the general conclusion?

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

Video: the complexity of the foundation pouring processes in winter

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 the maturation of some compositions 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 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 electric heating is also used with the help of various kinds of 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 with inside to the formwork, and after its removal, 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. The 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, you need a large number of lamps.

Additives and Additives

Another way to pour concrete at low temperatures is to use chemical substances. 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 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).

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

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

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

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

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

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

Pouring concrete at low temperatures: general recommendations

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

Pouring concrete is an integral part of most construction work. As is known, ideal period for repair and construction work - this is a warm season and pouring concrete is no exception to this rule. However, what should those who decide to continue construction despite the frosty weather do?

Pouring concrete at sub-zero temperatures in most cases is contraindicated by SNiP standards, since extreme cold does not allow the compositions that contain liquid to fully polymerize. But in some cases, the circumstances are such that the concrete must be poured, no matter what. And the question is quite natural, is it possible to pour concrete at sub-zero temperatures?

For areas where there is no warm season or situations in which winter construction is vital, there are several technological solutions that can be used to form concrete structures without compromising quality. In this article, you will be given detailed instructions how to pour concrete at sub-zero temperatures.

Pouring concrete

At sub-zero temperatures, the liquid in cement mortar freezes and the ice crystals break crystal lattice concrete, thereby weakening the strength future design. After thawing, the bond structure is not restored, and the structure loses a significant part of the strength potential, up to complete destruction.

The direct impact of sub-zero temperatures on a concrete structure is permissible only after it acquires the required amount of strength, frost resistance, etc. In construction terminology, this value is called the critical strength threshold - after overcoming it, frost does not negative impact on the concrete structure.

The threshold of critical strength is determined depending on the brand of concrete solution. The standard cement mix of the M400 brand must gain at least 30% of its maximum strength, and the lower the concrete grade, the higher the percentage of gained should be.

Before pouring concrete in sub-zero temperatures, it is necessary to create conditions that will prevent it from freezing. There are several technologies that help create or maintain the temperature necessary to overcome the critical strength threshold.

Among them are the following:

Mixture heating. Before pouring concrete at sub-zero temperatures, the solution is heated to a certain state, which gives it the opportunity to overcome the critical strength limit as it cools;
Internal heating. Maintain positive temperature in reinforced reinforced concrete structures it is possible through internal electric heating, which is carried out at the expense of electricity and metal components;
External heating. It is possible to create the temperature necessary to achieve critical strength using external steam or;
thermal insulation. Pouring concrete at minus 5 degrees can be carried out using high-quality insulated formwork, which does not release heat from the inside and allows cold to penetrate inside;
The use of specialized additives that prevent water from freezing.

Each of these technologies has its own characteristics, which must be taken into account, giving them their preference. Consider the above options in more detail.

Mixture heating

The concrete solution is kneaded on heated water, the temperature of which can be from 60 to 90 degrees. This technology can only be used if the ambient temperature does not fall below -15 degrees. This circumstance should be taken into account if you are wondering, “At what sub-zero temperature can concrete be poured?”

Specialty Blends

AT cement mixture various antifreezes are added to prevent the formation of ice crystals. Most often, calcium chloride or sodium chloride (table salt) is used for this purpose, which can be no more than 2% of the total mass of the components. If the temperature does not fall below 15 degrees below zero, then modifying additives will save the structure from freezing.

Note!
Despite the fact that the price of modifying components is quite affordable, it is best to purchase factory mixtures that guarantee technical qualities product.
A homemade recipe mixed with your own hands can be a waste of resources.

thermal insulation

If you want to pour the foundation at sub-zero temperatures, then you can do without heating or third-party additives. It is enough to qualitatively insulate the formwork with sheet or loose insulation.

During the crystallization of the concrete solution, heat is released, which is quite enough to overcome the critical strength threshold if the formwork is assembled according to the thermos principle. However this technology undesirable to use when too low temperatures.

Note!
Heat-insulating formwork can be used in conjunction with the above technologies - in this way, you will increase their effectiveness.

Internal and external heating

Internal heating is carried out by connecting to the reinforcing frame alternating current from a special transformer - the metal is heated and heats the cement mixture. External heating consists of a specialized mobile formwork, which contains heating elements that are powered by steam or electricity.

The disadvantages of this technology are the high cost of construction - the enormous consumption of resources for heating affects. Also, this technology cannot be used independently, since without the presence necessary knowledge and equipment, it is impossible to achieve the desired effect.

Among the advantages, it should be noted that you may not think about the question, “Up to minus how much concrete can be poured?” - concrete heating allows pouring at temperatures up to -25 degrees.

Outcome

The correct approach to the question of how to pour concrete in sub-zero temperatures makes it possible to obtain a high-quality result even in the most adverse conditions. In the future, eliminate small errors and establish additional elements concrete cutting will help you diamond circles and diamond drilling of holes in concrete. More detailed information on this topic you can get by watching the video in this article.

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

Features of concrete hardening

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

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

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

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

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

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

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

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

Stage of production and transportation in winter

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

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

Note! High temperature reduces the mobility of concrete.

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

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

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

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

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

Operation at sub-zero temperatures

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

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

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

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

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

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

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

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

Antifreeze additives

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

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

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

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

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