When carrying out construction, there is often a need to concrete foundations, reinforcement or other areas in the winter season. In this case, it is necessary to prevent the water contained in the concrete from freezing. If this happens, the ice crystals will significantly reduce performance characteristics material and its strength.

Basic Rules

In order for winter concreting to be successful and the quality of the concrete not to deteriorate, it is necessary to adhere to several basic rules for carrying out the process in the cold season:

1. First of all, you should use special antifreeze additives that will prevent freezing and increase its strength.
2. In the absence of additives, the concrete mixture should be diluted only with heated water, and the prescribed methods should be used to ensure high quality designs.
3. Machines that will transport concrete in the cold season must have insulation.
4. Before starting work, the concrete base must be thoroughly cleaned of dust and dirt and heated.
5. Snow and ice should be removed from reinforcement and formwork that will be used during the concreting process. If the reinforcement has a diameter of more than 25 mm or is made of a rolled profile, at an air temperature below -10 degrees it is heated until it reaches a positive temperature. The same operation must be carried out with large metal embedded parts.
6. Concreting work must be carried out at an accelerated pace, continuously, to prevent cooling of the concrete layer placed first.
7. After pouring concrete, its entire surface must be insulated with wooden panels or mats.

Compliance with these simple conditions will allow you to obtain high-quality concreting that maintains strength and reliability.

Methods for curing concrete mortar

Modern construction uses several methods of keeping concrete mortar at sub-zero temperatures, which should be considered quite effective and cost-effective.

Winter concreting methods can be divided into 3 groups:

• Thermos method, based on the conservation of heat introduced into the concrete solution during its manufacture or before pouring into the structure;
• electric heating carried out by contact, induction or infrared heaters after laying the solution;
• the use of special chemical antifreeze agents, with the help of which the effect of lowering the eutectic point of water present in the mixture is achieved.

These methods, when concreting in winter, can be used separately or combined if necessary. The choice of the method used when carrying out construction work is influenced by such factors as the massiveness and type of structure, the composition and required strength of concrete, natural conditions at a certain time of the year, the equipment of the construction site with one or another type of power equipment and some others.

For example, the thermos method is recommended for use when working with highly exothermic Portland quick-hardening cements. They have the greatest heat release, ensuring the high heat content of the created structure. At the same time, curing the concrete solution based on the method can be done in combination - a “thermos with additives”, where it occurs due to chemical accelerators, or using the “hot thermos” method, where serious electrical power is required to heat the concrete to high positive temperatures.

Unlike the thermos method, artificial heating of the concrete solution involves not only increasing the temperature of the laid material to the maximum permissible, but also maintaining it for the time necessary for the concrete to gain a given strength. Typically, the artificial heating method is used when working with structures that have a high level of massiveness, where the specified strength cannot be achieved only by using the thermos method.

Anti-frost chemicals are added to concrete solutions in amounts from 3 to 16%, depending on the desired result and the weight of the mixture, and ensure stable hardening of the material at negative temperatures. As a rule, the choice of type of additives depends on the type of structure, the amount of reinforcement used, the presence of stray currents and aggressive media, as well as the temperature at which the process occurs.

Today, the following agents are used as antifreeze additives:

• sodium nitrite;
• calcium chloride combined with sodium nitrite;
• calcium chloride combined with sodium chloride;
• calcium nitrate-nitrite in combination with urea;
• calcium nitrate in combination with urea;
• calcium nitrite-nitrate in combination with calcium chloride;
• nitrate-nitrite-calcium chloride in combination with urea;
• potash.

Besides, in modern construction in the cold season, the antifreeze additive sodium formate is often used, but its use is limited in prestressed structures with steel reinforcement intended for use in gas or water environments with air humidity above 60%. It should be noted that the use of this additive is prohibited when constructing structures with reactive silica or used in industrial plants that consume direct electric current.

It should be added that all chemical additives are strictly prohibited to be used during concreting reinforced concrete structures electrified railways and industrial enterprises where stray electric current occurs.

Warm-up methods

All of the above methods have been successfully applied on large and well-equipped construction sites. Some of them require the organization of quite expensive additional equipment or equipment.

In small conditions construction work for foundation concreting country house, greenhouse or paving, not all of the proposed methods look appropriate. In this case, winter concreting may be accompanied by such actions as the construction of a temporary shelter at the work site, where the required area will be heated with a heat gun, or the use of PVC film and other warming materials.

Covering concrete mixture recommended in cold weather at temperatures from -3 to +3 degrees. PVC film and other insulation materials allow you to accumulate heat inside concrete structure, which leads to faster solidification and hardening of the solution.

If the air temperature reaches -5 to -15 degrees, experts recommend using electric or gas heat guns. They are arranged as follows:

• on wooden frame the PVC film layer is strengthened, creating a reinforcement in the form of a tent;
• Heat guns are installed in the tent.

The higher the temperature in the tent, the faster the concrete mixture will set, and, accordingly, the shorter the warming up time will be.

As a rule, heating for 1-3 days is sufficient for concrete to acquire primary strength, allowing further work to be carried out.

Guidelines

So, you need to carry out concrete laying work on your summer cottage. What algorithm of actions should be chosen to ensure concreting winter conditions was it successful?

First of all, you should purchase concrete. In addition, it is allowed self-production concrete mixture. To prepare M200 grade material you will need:

• 3 parts of M500 cement (it is forbidden to use wet or hard cement);
• 5 parts of sand (the use of both quarry and washed sand is allowed; the use of sand with clay or other additives is strictly prohibited);
• 7 parts crushed stone (it is recommended to use washed crushed gravel with fractions from 5 to 20 mm; the use of crushed limestone, as well as pebbles and unwashed crushed stone is prohibited);
• water (should make up about 25% of the total mixture).

To use concrete in winter, chemical anti-frost elements and plasticizers can be added to it.

If average daily temperature during work is no more than -5 degrees, the following actions must be taken:

1. Carefully check all the materials used to prepare the concrete mixture - crushed stone, sand and water - for the absence of snow and ice and be sure to warm them up.
2. Build a frame from lumber and cover it with insulating material, creating a tent.
3. Check the tent for any gaps through which cold air can enter.
4. If the tent meets all the necessary requirements, you can connect a heat gun or heat generator.
5. should be carried out until it acquires a light white color. When touched, the mixture should be warm, which indicates the presence of a reaction to set and gain strength. If the concrete turns dark gray, this indicates that it has frozen and lost its properties. Such a solution must be crushed and concreting work must be done again.

What to do if the re-concreting process is not possible? In this case, the structure should be carefully covered with PVC film. This will keep the top layer of concrete intact during frosts and thaws. Perhaps in the spring the concrete will be able to continue the hydration process. Of course, its strength will become as low as possible, but doing this is better than simply leaving the structure in the rain and snow.

Excerpts from SNiP related to concrete work in winter: transportation, laying concrete mix, how to pour concrete in winter at subzero temperatures.

SNiP. PRODUCTION OF CONCRETE WORK AT NEGATIVE AIR TEMPERATURES

2.53. These rules are followed during the period of concrete work when the expected average daily outside air temperature is below 5 °C and the minimum daily temperature is below 0 °C.

2.54. The preparation of the concrete mixture should be carried out in heated concrete mixing plants, using heated water, thawed or heated aggregates, ensuring the production of a concrete mixture with a temperature not lower than that required by calculation. It is allowed to use unheated dry aggregates that do not contain ice on the grains and frozen lumps. In this case, the duration of mixing the concrete mixture should be increased by at least 25% compared to summer conditions.

2.55. Methods and means of transportation must ensure the prevention of a decrease in the temperature of the concrete mixture below that required by calculation.

2.56. The condition of the base on which the concrete mixture is laid, as well as the temperature of the base and the method of laying must exclude the possibility of the mixture freezing in the area of ​​contact with the base. When curing concrete in a structure using the thermos method, when preheating the concrete mixture, as well as when using concrete with antifreeze additives, it is allowed to lay the mixture on an unheated, non-heaving base or old concrete, if, according to calculations, freezing will not occur in the contact zone during the estimated period of curing the concrete.

At air temperatures below minus 10 °C, concreting of densely reinforced structures with reinforcement with a diameter greater than 24 mm, reinforcement made of rigid rolled sections or with large metal embedded parts should be carried out with preliminary heating of the metal to a positive temperature or local vibration of the mixture in the reinforcement and formwork areas, with the exception of cases of laying preheated concrete mixtures (at a mixture temperature above 45 ° C). The duration of vibration of the concrete mixture should be increased by at least 25% compared to summer conditions.

2.57. When concreting elements of frame and frame structures in structures with rigid coupling of nodes (supports), the need to create gaps in the spans depending on the heat treatment temperature, taking into account the resulting temperature stresses, should be agreed upon with the design organization. Unformulated surfaces of structures should be covered with steam and thermal insulation materials immediately upon completion of concreting.

Reinforcement outlets of concrete structures must be covered or insulated to a height (length) of at least 0.5 m.

2.58. Before laying the concrete (mortar) mixture The surfaces of the joint cavities of precast reinforced concrete elements must be cleared of snow and ice.

2.59. Concreting of structures on permafrost soils should be carried out in accordance with SNiP II-18-76.

Acceleration of concrete hardening when concreting monolithic bored piles and embedding bored piles should be achieved by introducing complex antifreeze additives into the concrete mixture that do not reduce the freezing strength of concrete with permafrost soil.

2.60. The choice of concrete curing method for winter concreting of monolithic structures should be made in accordance with the recommended Appendix 9.

2.61. Concrete strength control should be carried out, as a rule, by testing samples made at the place where the concrete mixture is laid. Samples stored in the cold must be kept for 2-4 hours at a temperature of 15-20 °C before testing.

It is allowed to control the strength by the temperature of the concrete during its curing.

2.62. The requirements for work at subzero air temperatures are set out in the table. 6

6. Requirements for the production of concrete work at subzero temperatures.

Parameter	Parameter value	Control (method, volume, type of registration)
Pour concrete at sub-zero temperatures.
1. Strength of concrete of monolithic and prefabricated monolithic structures at the moment of freezing:		Measuring according to GOST 18105-86, work log
for concrete without antifreeze additives:
structures operating inside buildings, foundations for equipment not subject to dynamic influences, underground structures	Not less than 5 MPa
structures exposed atmospheric influences during operation, for class:	Not less, % of design strength:
B7.5-B10	50
B12.5-B25	40
B30 and above	30
structures subject to alternating freezing and thawing in a water-saturated state at the end of curing or located in the seasonal thawing zone of permafrost soils, subject to the introduction of air-entraining or gas-forming surfactants into the concrete	70
in prestressed structures	80
for concrete with antifreeze additives	By the time the concrete has cooled to the temperature for which the amount of additives is designed, at least 20% of the design strength
2. Loading structures design load allowed after concrete reaches strength	At least 100% design	-
3. Temperature of water and concrete mixture at the outlet of the mixer, prepared:		Measuring, 2 times per shift, work log
on Portland cement, slag Portland cement, pozzolanic Portland cement of grades below M600	Water no more than 70 °C, mixtures no more than 35 °C
on quick-hardening Portland cement and Portland cement grade M600 and higher	Water no more than 60°C, mixture no more than 30°C
on aluminous Portland cement	Water no more than 40 C, mixtures no more than 25 ° C
Temperature of the concrete mixture placed in the formwork at the beginning of curing or heat treatment:		Measuring, in places determined by the PPR, work log
with the thermos method	Set by calculation, but not lower than 5°C
with antifreeze additives	Not less than 5 C above the freezing point of the mixing solution
during heat treatment	Not lower than 0 °C
5. Temperature during curing and heat treatment for concrete at:	Determined by calculation, but not higher, °C:	During heat treatment - every 2 hours during the period of temperature rise or on the first day. In the next three days and without heat treatment - at least 2 times per shift. The rest of the holding period - once a day
Portland cement	80
slag Portland cement	90
6. Rate of temperature rise during heat treatment of concrete:		Measuring, every 2 hours, work log
for structures with surface modulus:	No more than, °C/h:
up to 4	5
from 5 to 10	10
St. 10	15
for joints	20
7. Concrete cooling rate at the end of heat treatment for structures with surface modulus:		Measuring, work log
up to 4	Determined by calculation
from 5 to 10	No more than 5°C/h
St. 10	No more than 10°C/h
8. The temperature difference between the outer layers of concrete and air during stripping with a reinforcement coefficient of up to 1%, up to 3% and more than 3% should be, respectively, for structures with a surface modulus:		Same
from 2 to 5	No more than 20, 30, 40 °C
St. 5	No more than 30, 40, 50 °C

At concreting And pouring concrete in construction winter conditions are considered under which the average daily outside air temperature drops to +5°C, and during the day the temperature drops below 0°C. They are determined not by the calendar, but by the temperature of the phase transition to the solid state of water, as one of the strategically important building materials. IN northern regions In Russia, such a season can last for most of the year. It is obvious that at this time the costs of capital construction are increasing, but freezing it in the literal and figurative sense, even for a shorter period, will lead to immeasurably large and unjustified losses.

A classic building concrete mixture consists of thoroughly mixed components:

• Binder - cement of the required grade
• Water
• Coarse aggregate - crushed stone of the required fraction
• Fine aggregate - construction sand of appropriate quality
• Various additives necessary to use the concrete mixture and achieve the proper properties of the concrete

Setting of the concrete mixture occurs due to the hydration of particles of the binder - in our case, aluminosilicate Portland cement. For thermodynamic reasons, the rate of any chemical reaction, including hydration, decreases approximately by half when the temperature drops by 10 o C.

At temperatures below 0 o C, chemically unbound water turns into ice and increases in volume by approximately 9%. As a result, in the thickness concrete tensions arise that destroy its structure. The frozen concrete mixture has some strength, but only due to the adhesion of ice crystals. When thawing, the process of cement hydration resumes, but due to structural disturbances, the concrete cannot gain its design strength, i.e. its strength characteristics will be significantly lower than those of concrete that has not been frozen. Experiments have established that the process of concrete strength gain is significantly influenced by hardening conditions. Namely, if the concrete manages to gain 30-50% of its design strength before freezing, depending on its brand, excess water is squeezed out of its thickness, and further exposure to low temperatures no longer affects its physical and mechanical characteristics. However, further ripening will occur several times slower than under normal conditions. At the same time, it must be remembered that critical load-bearing structures (beams, lintels, crossbars, ceilings, etc.) can be loaded only after reaching 70% strength. If the reinforcement of the monolith was prestressed in at least one direction, then 100% of the design strength will be required.

How can you achieve full quality? monolithic concrete at laying concrete mixture in winter conditions ? The answer is obvious - ensuring such thermodynamic conditions under which the water involved in the chemical process will be in the liquid phase. Fundamentally, this can be achieved in two ways - either by increasing the temperature of the reaction zone, or by decreasing the crystallization temperature of water. Let's consider ways to achieve both effects in conjunction with the components of the concrete mixture, and in the same order in which they are listed above.

1. The standard setting time for classic Portland cement under normal conditions is 28 days. Along with it, there are highly active fast-hardening cements that can ensure complete maturation of concrete within 2-3 days or even faster. If the monolith is massive enough, then its freezing will not take place during this time due to the high heat capacity of water and the exothermic nature of the hydration reaction. For example, this type of cement is used in dry mixes such as “Cast concrete grade 300”. After just 4 hours, structures made from it (slabs, screeds, steps, etc.) can be walked on. Disadvantages: high cost and lack of time for delivery and laying of ready-mixed concrete. As a result, these concretes have not found large-scale use.
2. As you know, water at sea level boils at +100 o C. It would seem that at a temperature of +99 o C, concrete would harden almost instantly. However, as experience shows, the rate of its hardening drops sharply after +50 o C, although the process continues. This temperature is considered technologically optimal. If it is possible to somehow provide exactly this in the thickness of classical concrete, then in most cases the formwork can be removed within 1-2 days. When mixing a ready-mixed concrete mixture, manufacturers use water heated up to +50 o C. Water is needed not only for the chemical reaction, but also for the workability of the mixture. At subzero temperatures, ice crystals form precisely from excess water. To reduce its content, vacuum suction is used using rigid shields or flexible mats. Something similar happens naturally due to capillary forces when laying a layer of masonry mortar on a porous brick. That is why building codes and regulations allow concrete pouring and concreting in winter . Such a cement-sand mortar gains its final strength after thawing. Frail reinforced concrete suffers the most from freezing. Steel reinforcing rods are excellent “cold bridges” and intensively remove heat from the thickness of concrete. The water around them freezes, and the ice, expanding, pushes away the plastic concrete mixture. New water flows from it into the gaps formed between the crystals, which in turn also freezes and the process is repeated until all the water freezes, mainly around the rods. It is clear that when it thaws, reinforced concrete will lose the properties of a composite material.
3. To heat crushed stone to +60 o C, ready-mixed concrete producers use special registers through which heated water or even steam is passed.
4. The same goes for sand. Heating cement is prohibited to avoid “cooking”.
5. To increase ductility and, as a result, workability concrete in winter, plasticizers are added to the concrete mixture, both mineral (for example, lime) and organic (various polymer gels, dispersions, etc.). It is possible to use special additives, for example, to reduce pore formation in the thickness of concrete. This has a positive effect on the water and frost resistance of concrete stone. There are reinforcing and structuring additives, for example fibers - polymer, metal or mineral, which increase the strength characteristics of concrete stone. In the issue under consideration, the most interesting are antifreeze additives, or, as they are also called, additives. In conditions where heating is impossible, and there is enough time, to preserve the structure of concrete, you can reduce the freezing point of water by adding electrolytic reagents. The most common in construction are potash, calcium chloride, sodium salts - sulfate, nitrate and nitrite, chloride, etc. However, it must be taken into account that with increasing temperature and thawing of water in the environment, these salts, due to osmotic processes, will diffuse to the surface of the concrete and form so-called efflorescence. In addition, the rate of concrete ripening will drop to critical levels due to the low temperature of the liquid phase (down to -20 o C) and the increase in the ionic strength of the saline solution. Electrolytic additives are prohibited in concrete with stressed or thermally strengthened reinforcement (due to electrochemical corrosion), as well as in structures located in places where stray currents occur (electrified objects - railways, etc., due to increased conductivity).

If at negative temperatures during concrete works do not preheat the components for winter concreting, then to achieve a given temperature, the concrete mixture can be prepared in forced-action concrete mixers with steam heating, while sacrificing some time that could have been spent on delivery and placement. It must be remembered that at a temperature of +40 o C, hydration occurs at least four times faster than under normal conditions. That's why in winter conditions All working with concrete mixture should be done as quickly as possible. It is optimal to produce heated concrete mixture directly on site. She's the best would be better suited For laying concrete in winter using the “thermos” method, in which the formwork and concrete surface are passively insulated. Often, 2% of the already familiar calcium chloride is added to the concrete mixture, which accelerates the initial setting, while simultaneously lowering the crystallization temperature of water to -3 o C. There are other additives that accelerate setting of concrete in winter. The main thing is that it does not take place entirely during the preparation or transportation of the concrete mixture due to an overdose of additives.

According to building codes Maximum temperature concrete mixture should not exceed +70°C for quick-hardening cement, +80°C for Portland cement and +90°C for slag Portland cement and pozzolanic Portland cement.

Warming up, heating and heating concrete during winter concreting

To maintain the required temperature of the concrete mixture in artificial conditions, the most widespread is the forced supply of heat to the concrete structure. Distinguish heating, heating and heating of hardening concrete.

• Warming up concrete in winter carried out by introducing heating elements into the thickness of concrete. These can be tubes with a coolant circulating in them (water, steam or air), but the most widespread are insulated electric heating wires of the PNSV type. They are wound in groups on volumetric frame reinforced concrete structure even before laying the concrete mixture, and upon its completion, the groups are connected to a source of alternating or direct current safe voltage (transformer). The winding pitch is determined by the cross-section of the wire and must be such that the ohmic resistance of the wire provides the necessary heat generation. When connecting, you must ensure that the ends of the wires coming out of the formwork are short, otherwise they will burn out in the air without heat removal.
• For heating concrete during winter concreting Hot houses are used as heating structures. Essentially, these are greenhouses made of film or woven materials, built around a structure, inside which there is a functioning heat gun or fan. For electric wave heating of concrete thickness, electrodes (plates, rods, strips and strings - depending on the design) are used. As a result of connecting opposite electrodes to different phases of alternating current, an electromagnetic field is formed in the concrete mixture, under the influence of which the mass is heated to the required temperature and its heat is maintained for the required time. The plates are hung on the inside of the side formwork, reinforcement rods with a diameter of 6-12 mm are placed in the thickness of the concrete with a calculated pitch. Strip electrodes can be placed on one side of the structure or on both. String electrodes are most effectively used when winter concreting columns
• For heating At the ends and bottom of the monolith, thermoactive formwork is sometimes used, consisting of steel panels (or multilayer panels) with heating elements and thermal insulation mounted on them. When directly heating the concrete surface, infrared generators are used - metal tubular or carborundum rods. Thermal energy from the surface, due to thermal conductivity, spreads throughout the entire volume of the hardening monolith. Sometimes infrared heating is carried out through the formwork; for this purpose it is coated with black matte varnish. Along with radiant energy, electromagnetic (induction) energy has found wide application for these purposes. Induction heating is carried out using successive turns of an insulated wire (inductor), which is laid out along the surface to be heated. The number of turns and heating intensity are pre-calculated in laboratory conditions for a given specific case and carefully adjusted throughout the entire process. The efficiency of induction heating of reinforced concrete is increased by a closed steel frame.

Blowing heated steam or air over a formwork monolith is effective only for thin-walled structures and has not been widely used.

With any method of heating and/or (heating, heating), winter concreting is carried out as follows:

• Snow and ice are removed from the formwork surfaces
• For the same purpose, the reinforcement frame is heated
• equipment corresponding to the chosen method is installed
• concrete mixture is laid and compacted
• surfaces of the structure that come into contact with air must be insulated

Then comes the stage of constructing wells to measure the temperature, and only after that the heating itself begins, which stops as soon as design temperature will be achieved. For the first eight hours, you need to monitor the temperature of the laid concrete every two hours, and then at least once per shift (with recording in a log).

After isometric heating is completed, under no circumstances should the structure be abruptly cooled; this may result in serious damage to the monolith. Rapid cooling causes enormous stress in the concrete and leads to cracking. The heating temperature can exceed the calculated one by only 5°C. The cooling rate of concrete after the end of heating should not exceed 15°C/hour; for reinforced concrete monoliths it is 2-3°C/hour.

Dismantling of the formwork (stripping) is carried out only after the concrete reaches the required strength. It varies from 40% to 70% and even 100% depending on the grade of concrete and the purpose of the structure.

In any case, you need to remember that only compliance with technological requirements can guarantee the proper quality of a monolithic structure.

If it is necessary to carry out concreting in winter conditions, then the main problem is low temperatures, due to which building materials freeze. According to SNiP 3.03.1, winter concreting conditions are temperatures below 5 degrees Celsius.

Features of work in winter

All technologies used for concreting at low temperatures are designed to prevent this freezing. We can point out 2 main features that make the process of laying concrete at low temperatures quite difficult.

This:

• Freezing of water in concrete pores. Frozen water expands, which increases internal pressure. This makes the concrete less strong. In addition to all this, ice films can form around the aggregates, which in turn leads to a disruption of the connection between the components of the mixture.
• Cement hydration slows down at low temperatures, which means that the time it takes for concrete to gain hardness increases greatly.

Important!
Concrete gains approximately 70% of its design strength in a week at temperatures environment at 20 degrees.
In winter conditions, this period can be 3-4 weeks.

Freezing water

It is necessary to dwell in more detail on such an important factor as water freezing. The period when the water froze is of great importance for the strength of the entire structure. There is a direct relationship: the earlier the concrete was frozen, the more fragile the concrete will be.

The period when the concrete mixture sets is the most critical and decisive. The technology of concreting in winter conditions states that if the concrete mixture freezes immediately after laying in the formwork, then its further strength will depend only on the strength of the frost.

As the temperature rises, the hydration process will certainly continue. But the strength of such a structure will be significantly inferior to a similar structure whose mixture was not frozen during installation.

If concrete has managed to gain a certain strength before freezing, then it can easily withstand further freezing without structural changes or internal defects. It is also necessary to try to avoid so-called cold seams. To achieve this, concrete must be laid continuously.

Strength value

When working in low temperature conditions, it is important to remember the critical strength of concrete. This value is equal to 50% of the declared brand strength. It is important to remember this indicator, because with modern winter concreting, the mixture is protected from freezing until it reaches this same value of 50%.

If we are talking about an object of special importance, then protection from freezing is carried out until the mixture reaches the 70% mark.

Winter concreting methods

At the moment, there are 3 main methods of laying concrete in conditions low temperatures. The use of anti-frost additives. This is the cheapest and most technologically sound method for protecting the mixture from frost. All supplements of this kind are divided into 3 main groups, depending on their mode of action.

The peculiarities of concreting in winter conditions are such that it is often impossible to get by only with antifreeze additives. It is necessary to take a number of measures that will enhance the effect of the chemicals used and speed up the hardening time.

Such additional measures are:

• Preliminary cleaning of formwork and reinforcement from snow and ice. Iron fittings must be heated to positive temperatures.
• All work must be carried out at the fastest possible pace.
• Direct transportation of the mixture must be carried out in a machine equipped with a double bottom, into which exhaust gases must flow for heating.
• During unloading, it is necessary to protect construction site from gusts of wind, and the unloading means themselves should be as insulated as possible.
• After installation is completed, it is necessary to cover the mixture with mats to retain heat for as long as possible.
• Ideally, all components of the mixture should be preheated.

Important!
When preheating components, it is necessary to use a special loading order into the mixer to avoid “steeping the mixture.”
At low temperatures, water is first poured into the mixer, then coarse aggregate is supplied, the drum is turned several times, and only then sand and cement are poured.
These instructions must be strictly followed.

Thermos method

This method involves placing the mixture, which has a positive temperature, in insulated formwork. There is also a similar “hot thermos” method, in which the mixture is preheated for a short period of time to 60-80 degrees.

Then it is compacted in this heated state. Additional heating is recommended. The mixture is heated most often using electrodes.

Heating and heating concrete using electricity and infrared radiation

It is used when the “thermos method” is insufficient. Its essence is to warm up the concrete and maintain heat until it reaches the required strength margin, such that it may then require cutting the reinforced concrete with diamond wheels.

Most often, the solution is heated using electric current. The concrete becomes part of the electrical circuit and provides resistance. As a result, it heats up and the goal is achieved.

Conclusion

Don’t be afraid of working with concrete even in subzero temperatures. After all, if all the rules are followed, it will be possible to maintain the strength characteristics of materials for high level, and the video in this article will help you understand many of the nuances

The concept of “winter conditions” in the technology of monolithic concrete and reinforced concrete is somewhat different from the generally accepted one - calendar. Winter conditions begin when the average daily outside air temperature drops to +5°C, and during the day there is a drop in temperature below 0°C.

At subzero temperatures, water that has not reacted with cement turns into ice and does not enter into a chemical combination with cement. As a result, the hydration reaction stops and, therefore, the concrete does not harden. At the same time, significant internal pressure forces develop in the concrete caused by an increase (by about 9%) in the volume of water as it turns into ice. When concrete freezes early, its fragile structure cannot withstand these forces and is damaged. During subsequent thawing, frozen water again turns into liquid and the process of cement hydration resumes, but the destroyed structural bonds in concrete are not completely restored.

Freezing of freshly laid concrete is also accompanied by the formation of ice films around the reinforcement and aggregate grains, which, due to the influx of water from less cooled areas of the concrete, increase in volume and squeeze the cement paste away from the reinforcement and aggregate.

All these processes significantly reduce the strength of concrete and its adhesion to reinforcement, and also reduce its density, resistance and durability.

If concrete acquires a certain initial strength before freezing, then all the processes mentioned above do not have an adverse effect on it. The minimum strength at which freezing is not dangerous for concrete is called critical.

The value of the standardized critical strength depends on the class of concrete, the type and operating conditions of the structure and is: for concrete and reinforced concrete structures with non-prestressing reinforcement - 50% of the design strength for B7.5...B10, 40% for B12.5...B25 and 30% for B 30 and above, for structures with prestressing reinforcement - 80% of the design strength, for structures subject to alternate freezing and thawing or located in the seasonal thawing zone of permafrost soils - 70% of the design strength, for structures loaded with the design load - 100% design strength.

The duration of concrete hardening and its final properties largely depend on temperature conditions, in which concrete is kept. As the temperature rises, the activity of water contained in the concrete mixture increases, the process of its interaction with the minerals of the cement clinker accelerates, and the processes of formation of the coagulation and crystalline structure of concrete intensify. When the temperature decreases, on the contrary, all these processes are inhibited and the hardening of concrete slows down.

Therefore, when concreting in winter conditions, it is necessary to create and maintain such temperature and humidity conditions under which the concrete hardens until it acquires either critical or specified strength in the shortest possible time with the least labor costs. For this purpose they use special methods preparing, feeding, placing and curing concrete.

When preparing a concrete mixture in winter conditions, its temperature is increased to 35...40C by heating the aggregates and water. Fillers are heated to 60C by steam registers, in rotating drums, in installations with flue gases blown through a layer of filler, hot water. Water is heated in boilers or hot water boilers up to 90C. Heating cement is prohibited.

When preparing a heated concrete mixture, a different procedure is used for loading the components into the concrete mixer. In summer conditions, all dry components are loaded simultaneously into the mixer drum, pre-filled with water. In winter, in order to avoid “brewing” of cement, water is first poured into the mixer drum and coarse aggregate is loaded, and then, after several revolutions of the drum, sand and cement are added. The total duration of mixing in winter conditions is increased by 1.2...1.5 times. The concrete mixture is transported in closed, insulated and heated containers (tubs, car bodies) before starting work. Cars have a double bottom, into the cavity of which exhaust gases from the engine enter, which prevents heat loss. The concrete mixture should be transported from the place of preparation to the place of placement as quickly as possible and without overload. Loading and unloading areas must be protected from the wind, and the means of supplying the concrete mixture to the structure (trunks, vibrating trunks, etc.) must be insulated.

The condition of the base on which the concrete mixture is laid, as well as the laying method, must exclude the possibility of freezing at the junction with the base and deformation of the base when laying concrete on heaving pounds. To do this, the base is heated to positive temperatures and protected from freezing until the newly laid concrete acquires the required strength.

Before concreting, formwork and reinforcement are cleared of snow and ice, reinforcement with a diameter of more than 25 mm, as well as reinforcement made of rigid rolled profiles and large metal embedded parts are heated to a positive temperature at temperatures below - 10 ° C.

Concreting should be carried out continuously and at a high rate, and the previously laid layer of concrete should be covered before its temperature drops below the specified level.

The construction industry has an extensive arsenal of effective and economical methods for curing concrete in winter conditions, which ensure high quality structures. These methods can be divided into three groups: a method involving the use of the initial heat content introduced into the concrete mixture during its preparation or before laying it in a structure, and the heat release of cement accompanying the hardening of concrete - the so-called “thermos” method; methods based on artificial heating of concrete , laid in the structure - electric heating, contact, induction and infrared heating, convective heating, methods using the effect of lowering the eutectic point of water in concrete using special anti-freeze chemical additives.

These methods can be combined. The choice of one method or another depends on the type and massiveness of the structure, the type, composition and required strength of concrete, meteorological conditions of the work, energy equipment of the construction site, etc.

Thermos method

The technological essence of the “thermos” method is that the concrete mixture, which has a positive temperature (usually within 15...30°C), is placed in insulated formwork. As a result, the concrete of the structure gains a given strength due to the initial heat content and exothermic heat release of the cement during cooling to 0°C.

During the hardening process of concrete, exothermic heat is released, which quantitatively depends on the type of cement used and the curing temperature.

High-quality and fast-hardening Portland cements have the greatest exothermic heat release. The exotherm of concrete provides a significant contribution to the heat content of the structure maintained by the “thermos” method.

Concreting using the “Thermos with accelerator additives” method

Some chemical substances(calcium chloride CaCl, potassium carbonate - potash K2CO3, sodium nitrate NaNO3, etc.), introduced into concrete in small quantities (up to 2% by weight of cement), have the following effect on the hardening process: these additives accelerate the hardening process in the initial period of curing concrete. Thus, concrete with the addition of 2% calcium chloride by weight of cement already on the third day reaches a strength 1.6 times greater than concrete of the same composition, but without the additive. The introduction of accelerator additives, which are also anti-freeze additives, into concrete in the specified quantities lowers the freezing temperature to -3°C, thereby increasing the cooling time of concrete, which also helps concrete acquire greater strength.

Concrete with accelerator additives is prepared using heated aggregates and hot water. In this case, the temperature of the concrete mixture at the outlet of the mixer fluctuates between 25...35°C, decreasing to 20°C by the time of laying. Such concretes are used at outdoor temperatures of -15... -20°C. They are placed in insulated formwork and covered with a layer of thermal insulation. Hardening of concrete occurs as a result of thermos curing in combination with the positive effects of chemical additives. This method is simple and quite economical; it allows the use of the “thermos” method for structures with MP

Concreting "Hot thermos"

It consists of short-term heating of the concrete mixture to a temperature of 60... 80°C, compacting it while hot and holding it in a thermos or with additional heating.

Under construction site conditions, the concrete mixture is heated, as a rule, by electric current. To do this, a portion of the concrete mixture is included in an alternating current electrical circuit using electrodes as a resistance.

Thus, both the power released and the amount of heat released over a period of time depend on the voltage supplied to the electrodes (direct proportionality) and the ohmic resistance of the heated concrete mixture (inverse proportionality).

In turn, the ohmic resistance is a function of the geometric parameters of the flat electrodes, the distance between the electrodes and the specific ohmic resistance of the concrete mixture.

Electro-razofev of the concrete mixture is carried out at a voltage of 380 and less often 220 V. To organize the electro-razofev at the construction site, a post with a transformer (voltage on the low side is 380 or 220 V), a control panel and a switchboard is equipped.

Electric heating of the concrete mixture is carried out mainly in buckets or in the bodies of dump trucks.

In the first case, the prepared mixture (at a concrete plant), having a temperature of 5...15°C, is delivered by dump trucks to the construction site, unloaded into electric buckets, heated to 70...80°C and placed in the structure. Most often, ordinary tubs (shoes) with three electrodes made of steel 5 mm thick are used, to which the wires (or cable cores) of the power supply network are connected using cable connectors. To ensure uniform distribution of the concrete mixture between the electrodes when loading the bucket and better unloading of the heated mixture into the structure, a vibrator is installed on the body of the bucket.

In the second case, the mixture prepared at the concrete plant is delivered to the construction site in the back of a dump truck. The dump truck enters the heating station and stops under the frame with electrodes. With the vibrator running, the electrodes are lowered into the concrete mixture and voltage is applied. Heating is carried out for 10... 15 minutes until the temperature of the mixture is 60°C for quick-hardening Portland cements, 70°C for Portland cements, 80°C for slag Portland cements.

To heat the mixture to this high temperatures Large electrical powers are required in a short period of time. Thus, to heat 1 m of mixture to 60°C in 15 minutes, 240 kW is required, and in 10 minutes - 360 kW of installed power.

Artificial heating and heating of concrete

The essence of the method of artificial heating and heating is to increase the temperature of the laid concrete to the maximum permissible and maintain it during the time during which the concrete gains critical or specified strength.

Artificial heating and heating of concrete is used when concreting structures with MP > 10, as well as more massive ones, if in the latter it is impossible to obtain the specified strength in a timely manner when cured only by the thermos method.

The physical essence of electric heating(electrode heating) is identical to the method of electrical heating of a concrete mixture discussed above, i.e., the heat released in the laid concrete when an electric current is passed through it is used.

The generated heat is spent on heating the concrete and formwork to a given temperature and compensating for heat loss to the environment that occurs during the curing process. The temperature of concrete during electrical heating is determined by the amount of electrical power built into the concrete, which should be assigned depending on the selected heat treatment mode and the amount of heat loss that occurs during electrical heating in the cold.

For summing up electrical energy Various electrodes are used for concrete: plate, strip, rod and string.

The following basic requirements are imposed on the designs of electrodes and their placement schemes: the power released in concrete during electrical heating must correspond to the power required by thermal calculation, the electric and, therefore, temperature fields should be as uniform as possible, the electrodes should be placed, if possible, outside the heated structure to ensure minimal metal consumption, the installation of the electrodes and the connection of wires to them must be done before laying the concrete mixture (when using external electrodes).

Plate electrodes satisfy the stated requirements to the greatest extent.

Plate electrodes belong to the category of surface electrodes and are plates made of roofing iron or steel, sewn onto the internal surface of the formwork adjacent to the concrete and connected to opposite phases of the power supply network. As a result of current exchange between opposing electrodes, the entire volume of the structure is heated. Using plastic electrodes, lightly reinforced structures are heated correct form small sizes(columns, beams, walls, etc.).

Strip electrodes are made from steel strips 20...50 mm wide and, like plate electrodes, are sewn onto the inner surface of the formwork.

Current exchange depends on the connection scheme of the strip electrodes to the phases of the supply network. When opposite electrodes are connected to opposite phases of the power supply network, current exchange occurs between the opposite faces of the structure and the entire mass of concrete is involved in heat generation. When adjacent electrodes are connected to opposite phases, current exchange occurs between them. In this case, 90% of all supplied energy is dissipated in peripheral layers with a thickness equal to half the distance between the electrodes. As a result, the peripheral layers are heated due to Joule heat. The central layers (the so-called “core” of concrete) harden due to the initial heat content, exothermic cement and partly due to the influx of heat from the heated peripheral layers. The first scheme is used for heating lightly reinforced structures with a thickness of no more than 50 cm. Peripheral electric heating is used for structures of any massiveness.

Strip electrodes are installed on one side of the structure. In this case, adjacent electrodes are connected to opposite phases of the supply network. As a result, peripheral electrical heating is realized.

One-sided placement of strip electrodes is used for electrical heating of slabs, walls, floors and other structures no more than 20 cm thick.

For complex configurations of concreted structures, rod electrodes are used - reinforcing bars with a diameter of 6...12 mm, installed in the concrete body.

It is most advisable to use rod electrodes in the form of flat electrode groups. In this case, a more uniform temperature field in the concrete is ensured.

When electrically heating concrete elements of small cross-section and considerable length (for example, concrete joints up to 3...4 cm wide), single rod electrodes are used.

When concreting horizontally located concrete or reinforced concrete structures with a large protective layer, floating electrodes are used - reinforcing bars 6 ... 12 mm embedded in the surface.

String electrodes are used to heat structures whose length is many times greater than their size. cross section(columns, beams, purlins, etc.). String electrodes are installed in the center of the structure and connected to one phase, and metal formwork (or wood with roofing steel deck sheathing) to the other. In some cases, working fittings can be used as another electrode.

The amount of energy released in concrete per unit time, and therefore temperature regime electrical heating depends on the type and size of the electrodes, the layout of their placement in the structure, the distances between them and the connection diagram to the power supply network. In this case, a parameter that allows arbitrary variation is most often the supplied voltage. The released electrical power, depending on the parameters listed above, is calculated using the formulas.

Current is supplied to the electrodes from the power source through transformers and distribution devices.

As main and switching wires, insulated wires with a copper or aluminum core are used, the cross-section of which is selected based on the condition of passing the calculated current through them.

Before turning on the voltage, check the correct installation of the electrodes, the quality of the contacts on the electrodes and the absence of short circuits to the fittings.

Electrical heating is carried out at low voltages within 50... 127 V. Average specific consumption electricity is 60... 80 kW/h per 1 m3 of reinforced concrete.

Contact (conductive) heating. This method uses the heat generated in a conductor when an electric current passes through it. This heat is then transferred by contact to the surfaces of the structure. Heat transfer in the concrete structure itself occurs through thermal conductivity. For contact heating of concrete, thermoactive (heating) formworks and thermoactive flexible coatings (TAGF) are mainly used.

The heating formwork has a deck made of metal sheet or waterproof plywood, on the back of which there are electrical heating elements. In modern formworks, heating wires and cables, mesh heaters, carbon tape heaters, conductive coatings, etc. are used as heaters. The most effective is the use of cables that consist of constantan wire with a diameter of 0.7 ... 0.8 mm, placed in heat-resistant insulation . The insulation surface is protected from mechanical damage by a metal protective stocking. To ensure uniform heat flow, the cable is placed at a distance of 10... 15 cm branch from branch.

Mesh heaters (a strip of metal mesh) are insulated from the deck with an asbestos sheet, and on the back side of the formwork panel - also with an asbestos sheet and covered with thermal insulation. To create an electrical circuit, individual strips of the mesh heater are connected to each other by distribution bars.

Carbon tape heaters are glued with special adhesives to the shield deck. To ensure strong contact with the commutating wires, the ends of the tapes are copper-plated.

Any warehouse with a deck made of steel or plywood can be converted into heating formwork. Depending on the specific conditions (heating rate, ambient temperature, thermal protection power of the rear part of the formwork), the required power density can vary from 0.5 to 2 kV A/m2. Heating formwork is used in the construction of thin-walled and medium-mass structures, as well as when embedding units of prefabricated reinforced concrete elements.

Thermoactive coating (TRAP) is a lightweight, flexible device with carbon tape heaters or heating wires that provide heating up to 50°C. The basis of the coating is fiberglass, to which the heaters are attached. For thermal insulation, staple fiberglass is used with shielding with a layer of foil. Rubberized fabric is used as waterproofing.

The flexible coating can be manufactured in various sizes. To fasten individual coverings to each other, holes are provided for passing through tape or clips. The coating can be placed on vertical, horizontal and inclined surfaces of structures. After finishing work with the coating in one place, it is removed, cleaned and rolled up for ease of transportation. It is most effective to use TRAP when constructing floor slabs and coverings, making preparations for floors, etc. TRAP is manufactured with a specific electrical power of 0.25... 1 kV-A/m2.

Infrared heating uses the ability of infrared rays to be absorbed by the body and transformed into thermal energy, which increases the heat content of this body.

They generate infrared radiation by heating solids. In industry, infrared rays with a wavelength of 0.76... 6 microns are used for these purposes, while maximum flow waves of this spectrum are possessed by bodies with a temperature radiating surface 300...2200°С.

Heat from the source of infrared rays to the heated body is transferred instantly, without the participation of any heat carrier. Absorbed by irradiated surfaces, infrared rays are converted into thermal energy. From the surface layers heated in this way, the body warms up due to its own thermal conductivity.

For concrete work, tubular metal and quartz emitters are used as infrared radiation generators. To create a directed radiant flux, the emitters are enclosed in flat or parabolic reflectors (usually made of aluminum).

Infrared heating is used for the following technological processes: heating of reinforcement, frozen bases and concrete surfaces, thermal protection of laid concrete, acceleration of concrete hardening when constructing interfloor ceilings, erection of walls and other elements in wooden, metal or structural formwork, high-rise structures in sliding formwork (elevators, silos, etc.).

Electricity for infrared installations usually comes from a transformer substation, from which a low-voltage cable feeder is laid to the work site, powering the distribution cabinet. From the latter, electricity is supplied via cable lines to separate infrared installations. Concrete is treated with infrared rays if available automatic devices, providing specified temperature and time parameters by periodically turning on and off infrared installations.

Induction heating of concrete uses the heat generated in reinforcement or steel formwork located in the electromagnetic field of an inductor coil through which an alternating electric current flows. To do this, an insulated inductor wire is laid in successive turns along the outer surface of the formwork. An alternating electric current passing through an inductor creates an alternating electromagnetic field. Electromagnetic induction causes eddy currents in the metal (reinforcement, steel formwork) located in this field, as a result of which the reinforcement (steel formwork) heats up and the concrete heats up from it (conductively).