Concreting at low temperatures. Pouring concrete at sub-zero temperatures - possible options for concreting. How cement mortar behaves in frost

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

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

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

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

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

• In addition, there is no complete certainty that during the general thawing of the soil in the spring and its possible movements, the entire structure of the foundation under construction will not “lead”, and this can lead to the formation 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.

• Fixed formwork made of extruded polystyrene foam can also be used, which, in addition to its direct function, also performs the role of a heater.

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

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

• This may be due to the nature of the soil. If sandy loose soils 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 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 winter construction is resorted to in order to save money, since prices for necessary materials. This will be beneficial if the work is carried out independently.
• Often there is an opportunity to save money by reducing the cost of services by construction companies, due to a sharp decrease in demand for their activities in the cold season.

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

Pouring the foundation with concrete

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

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

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

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

Below is a table that clearly shows how temperature affects environment at the time of maturation and 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-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 of the children's designer "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 when sub-zero temperatures, from above it is necessarily closed with a dense polyethylene film. To do this, erect a temporary structure made of wood or reinforcing bars over it, which is also tightened with polyethylene. In the resulting closed room, outwardly resembling a greenhouse, the temperature necessary for high-quality solidification is maintained with the help of heat guns. It is clear that this will require very significant material costs.

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

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

Are antifreeze supplements a "panacea"?

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

Video: one of the options for antifreeze additives in concrete

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

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

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

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

The danger of negative temperatures lies in several reasons:

• First, the expansion of frozen, uncemented water causes internal pressure on the porous structure of the concrete, which leads to its destruction. This becomes especially noticeable after the temperature rises to positive levels - such concrete is neither strong nor goes nothing compared to "normal".
• Secondly, the complex chemical process of cement hydration itself requires a certain temperature regime. Already at temperatures below +5° 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.

The table below, for example, shows the approximate maturation of concrete in conditions of negative temperatures using 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 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 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 the branded strength, that is, when optimal timing for formwork removal (as indicated in the first table).

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

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

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

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

A few more important notes:

• Even if shallow strip foundation poured in ideal conditions for this, you should not leave it “naked” and unloaded for the winter. This will inevitably lead to the onset of surface erosion processes, and the structure will lose its strength, begin to crack and crumble. 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 technologies, in principle, allow pouring the foundation at temperatures that are quite far from the optimal 15-25 degrees, the best option nevertheless, there will be planning of such works in the most favorable conditions. So the foundation will turn out to be guaranteed reliable and of high quality. If, nevertheless, circumstances force a “zero cycle” to be carried out in other conditions, all technological recommendations for pouring and caring for concrete should be strictly followed, in relation to the actual temperature regime.

Video: the complexity of the foundation pouring processes in winter

Construction in modern world you can start not only in spring, summer and early autumn, but now late autumn and even in winter.

And this became possible with the invention of antifreeze additives. We will talk about this interesting additive later.

Application conditions

Such additives make it possible to use low-quality instead of expensive high-quality ones, and this makes it possible to reduce the cost of concrete products. Due to this, it is also possible to reduce the thickness of the product, for example, floor slabs, moreover, without compromising quality.

About the use of antifreeze additives

How to use

How to cook containing a special additive?

An antifreeze additive is added to concrete along with other ingredients (sand, water, plasticizer). Better supplement add along with water, because the additive can replace water. Everything must be mixed well with a concrete mixer for an hour, until a homogeneous mass.

The additive is introduced into the mixture according to the instructions on the package. This amount is approximately 0.2% of the amount of cement at an air temperature not exceeding or decreasing the value indicated on the package.

In the event of a decrease in temperature, the dosage of the additive should be increased at the rate of 0.05% per degree. On the packaging, the dosage is indicated in grams.

In the event that the dosage of the additive is indicated in milliliters, then in order to determine the amount of the substance, its mass is divided by 1.45. The result obtained will be the volume in milliliters. Then we measure right amount and add to the mixture.

Antifreeze additives are used only at sub-zero ambient temperatures. They cannot be used under other conditions! After pouring concrete, structures made of this material are moistened for about two days.

Construction objects

The use of antifreeze additives makes it possible to build at any time of the year and under any weather conditions. Due to this, the terms of construction of facilities are reduced, the cost is also reduced due to the rational operation of construction equipment and labor.

Antifreeze additives are used:

• in the manufacture of heavy and light iron concrete structures,
• in the construction of monolithic reinforced concrete structures,
• it is added to
• when laying pavement and production of road elements,
• build bridges, oil and gas platforms, various buildings, dams, dams and much more.

Antifreeze additives are applicable at air temperatures from -15 to approximately -25 ° C, depending on the substance used.

Antifreeze additives are delivered to stores and retail chains in containers with a volume of 1 m3 or 5 liters each.

If you are thinking of building in the cold season, we recommend that you use antifreeze additives and be sure to carefully read the instructions on the packaging of additives.

We hope that this article was useful to you. We wish you good luck in your endeavors!

General provisions. The concept of "winter conditions" in production concrete work different from the calendar. "Winter conditions" for a specific construction site begin when the average daily outdoor temperature drops to + 5 ° C, and during the day it falls below zero.

At temperatures below 0°C, hydration processes in concrete stop, i.e. interaction of cement minerals with water. The hardening of concrete is suspended, as the concrete freezes, turning into a monolith, the strength of which is determined by the forces of freezing. Internal stresses appear in concrete, caused by an increase in the volume of free water by about 9% during freezing. These stresses break the fragile adhesive bonds between the individual components of concrete, reducing its strength. Free water, freezing on the surface of aggregate grains in the form thin film, prevents the adhesion of the cement paste to the aggregate. It also worsens the strength properties of concrete.

After thawing of concrete, hardening at a positive temperature resumes, but the strength is lower than the design one, i.e. the one that would be achieved by hardening in normal conditions. Other properties of concrete also decrease: density, durability, adhesion to reinforcement, etc. The properties of concrete deteriorate the more significantly, the earlier it freezes after laying. If concrete gains a certain strength by the time of freezing, then the negative effect of freezing on its properties is small: after thawing, the strength of concrete can reach the design value. In this case, the adhesive bond between the cement paste and the aggregate is much greater than the internal stresses. Therefore, the probability of deformations in the contact zone is less.

The minimum strength of concrete at the time of its freezing, sufficient to achieve its design strength after thawing, is called critical. This strength for concrete in structures with non-stressed reinforcement should be at least 30 ... 50% of the design, depending on the class of concrete, and not less than 50 kg / cm2. In prestressed structures, it should be at least 70% of the design value. If the structures are supposed to be loaded in the winter period, then by the time of freezing, the concrete strength in them should reach 100% of the design value.

To receive in winter conditions For concrete of design quality, it is necessary to provide for it a temperature and humidity regime in which the physical and chemical processes of hardening are not disturbed and do not slow down. The duration of maintaining such a regime should ensure the achievement of critical or design strength.

The task of "winter" concreting: to obtain concrete of a given strength. For this, general activities and various technologies for ensuring the normal mode of concrete hardening.

General events:

a) Work is carried out on a heated concrete mix. This mixture at the time of laying in the structure must have a positive temperature, the opposite of the ambient air temperature. This is achieved by heating water, crushed stone and sand (steam) during the preparation of the concrete mixture at the plant.

b) To prevent cooling on the way, the body of the dump truck is closed from above with shields, and from below it is heated by exhaust gases from the car engine through the arranged double bottom of the body.

c) Buckets and bunkers are covered with insulated wooden lids, and sheathed on the outside. At severe frosts they are periodically heated with steam. Concrete pumps are installed in heated rooms. Before starting work, it is pumped through the concrete pipeline hot water. Pipe links of the main concrete pipeline at a temperature below minus 10 ° C are enclosed in thermal insulation together with a heating rough pipeline.

d) Before laying the concrete mixture, the formwork and reinforcement are cleaned of debris, snow, ice. To do this, if necessary, hot air blowing from heaters or steam is used, as well as hot steam washing followed by hot air blowing.

e) In frosts below minus 15°C, reinforcement from rods with a diameter of more than 25 mm and rolled profiles is heated to plus 5°C to ensure good adhesion of concrete to reinforcement. For the same purpose, protruding beyond the insulated formwork metal elements after heating, they are insulated at a length of at least 1.5 m from the block.

f) The quality of concrete is strongly influenced by the condition of the base on which it is laid. It is important to exclude early freezing of concrete at the junction with the base and subsequent deformation of the heaving soils of the base.

Before concreting the foundations, heaving soils are heated by steam, fire or electricity. Non-heaving soils do not warm up. The temperature of the mixture to be laid must be at least 10°C higher than the temperature of the substrate. It is not allowed to lay the concrete mixture on frozen ground (“frozen” base).

If it is necessary to lay the concrete mix on the previously laid and frozen concrete, it is warmed up to a depth of at least 400 mm and protected from freezing until fresh concrete acquires critical strength.

g) When concreting, to reduce heat losses, the concrete mixture is laid in small sections along the length and width, so that the previously laid layers overlap with new ones faster, and the concrete temperature does not have time to fall below the calculated one.

h) Concreting is carried out around the clock without interruptions, since the preparation of frozen working joints is very laborious and the required quality is not always ensured.

Technologies that ensure the normal mode of concrete hardening:

1. Application of chemical additives.

Chemical additives lower the freezing point of the liquid part of the concrete mixture, which ensures concrete hardening at temperatures below 0°C, which increases the curing time.

This method is relatively inexpensive (additional cost compared to normal conditions(price increase) about 16%) and is widely used in construction. As additives are used: sodium chloride, calcium chloride, potassium carbonate (potash), sodium nitrite, etc.

Additives are introduced into the concrete mixture during its preparation. Depending on their number, a given effect is obtained:

At 1-2% by weight of cement - acceleration of concrete hardening;
- at 3–5% by weight of cement – ​​lowering the freezing point by 5–10°С;
- at 10–15% of the weight of cement - complete elimination of freezing " cold concrete”, but at the same time, curing continues for 40–90 days.

2. Concrete heating.

a) Thermos method. The heat generated during chemical reactions concrete hardening. For this, the structure is additionally insulated.

The method is effective for massive structures of a simple shape, especially for buried structures and structures on the ground and in the ground (foundations, basement walls, foundations for equipment, floors on the ground, etc.). To enhance the effect in the preparation of the mixture, cements with increased heat release are used.

b) Steam heating. A "shirt" of roofing material, wooden or steel panels is arranged around the concreted structure, under which steam is supplied (Fig. 4.52). The “jacket” provides the necessary heating of the structure and humidity (does not dry out the concrete).

Low pressure steam 0.5–0.7 atm is used. with a temperature of 80–90°С. Approximate mode of steam heating: rate of temperature rise (gradient) no more than 5–10 deg/h; isothermal heating at a temperature of 80°C for concretes on ordinary Portland cement and 95°C on slag Portland cement and pozzolanic cement. The cooling rate (gradient) of concrete should be 10 deg/h. Steam heating of concrete can be carried out until it gains design strength, which is especially important for our eastern and northern regions, where the "winter period" is
8... 10 months.

The method is used to warm up various concrete structures, but only where there is steam in the required amount.

in) Electrical heating. Internal - with the help of electrodes. Heat is released when passing electric current through wet concrete. Electrodes can be embedded in freshly laid concrete or heating wires are embedded in the structure prior to concreting. The number of electrodes, heating wires in each case is determined by calculation.

The advantage of the method is simplicity. Disadvantages - the complexity of control (round-the-clock surveillance) and high cost.

External - heat is released by "heating" formwork or heating flexible electrical cords.

3. Concreting in greenhouses. Above the concreted structure or part of it is arranged light frame fencing made of tarpaulin, film, etc. (tent) and warm air is supplied under it or heaters are placed inside the tent. Under the tent (temperature plus 5–10 °C), concreting is carried out under normal conditions.

Depending on the task, the greenhouse can “work” for 3–16 days, until concrete reaches 50% of the design (design) strength, or all of the design 28 days.

4. Concrete heating infrared rays(penetrating heating).

The peculiarity of the method is that the transfer of heat to concrete (warming up) occurs throughout the entire thickness of the structure simultaneously and with the same intensity (Fig. 4.53).

For heating monolithic concrete, heating elements of the NVSZh type (heat-resistant air drying heater) or NVS (air drying heater) are used. The power of these heaters per 1 m of length ranges from 0.6 to 1.2 kW, the temperature of the radiating surfaces is from 300 to 600°C. Heating elements operate at a voltage of 127, 220 and 380 V.

Carborundum emitters have a power of up to 10 kW / h, and their working temperature reaches 1300–1500 °C.

The optimal distance between the infrared unit and the heated surface is 1–1.2 m.

It is possible to heat with infrared emitters both open surfaces of concrete and through the formwork. For better absorption of infrared radiation, the surface of the formwork is covered with black matte varnish. The temperature on the concrete surface should not exceed 80-90°C. To exclude intensive evaporation of moisture from concrete, open surfaces are covered with plastic wrap, glassine or roofing material.

Infrared installations are placed at such a distance from each other to warm up all sections of the concrete surface. The heating of concrete by infrared rays is conventionally divided into three periods: holding of concrete and its heating; isothermal heating; cooling down.

The method is used for heat treatment of concrete in thin-walled structures with a large surface modulus (for example, walls concreted in sliding formwork, slabs, beams). This method is also used to thaw frozen concrete in working joints, when laying concrete in grooves, as well as to thaw reinforcement, embedded parts and the “active” surface of the formwork-cladding before placing concrete into it.

Source: Technology of building processes. Snarsky V.I.

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, increased pressure is formed inside the mass of concrete. 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. 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 foundation. It is known that during the hardening of concrete, reactions occur in which heat is released. For winter, it's a great feature. Wherein large quantity heat is emitted by fast-hardening Portland cements and compositions of high grades. 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. Therefore, using all possible means and available materials, carry out the insulation of the walls of the formwork. You can use tarpaulins, mats, some old warm clothes, fill the gap between the walls of the formwork and the soil, straw. Anything, as long as the heat doesn't leak into the air.

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

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

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

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

To maintain the temperature, there are greenhouses above the object. These are structures very much reminiscent of greenhouses. And their task is similar: to keep warm. 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 concrete heating 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- 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.

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 will help to solve the problem, wide selection offered by the industry today.

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 is classified as a hazardous substance, 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 have a strong effect on metal elements, so they are 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.

The consumption of antifreeze additive per 1 cubic meter of 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%. In severe frosts, the 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 make a do-it-yourself antifreeze additive in concrete, 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, the mass fraction of 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.