Snip concrete work in winter. Electrical heating of concrete in winter: methods, technologies, equipment. Acid and alkali resistant concretes

Concrete heating methods in winter period at sub-zero temperatures today are numerous. They differ in observance of specific rules and requirements in the application of technologies. The choice depends on local conditions, air temperature during the period of the year when the work is carried out.

Whichever method is chosen, when heating concrete in winter, the conditions of the process should be thoroughly observed, combining a set of measures used in the construction of monolithic and any other type of structures.

The main requirement for winter work for concreting - the implementation of the process at a given pace and in strict sequence. Thanks to the unmistakable actions in compliance with technological regulations achieve guaranteed quality of structures and foundations, poured at sub-zero temperatures. Conditions for professional concrete work are regulated by:

norms and rules of SNiP 3.03.01-87;
SNiP 3.06.04-91;
several other documents on the basis of which the building standards for cold climate areas.

It is forbidden to warm up concrete in winter time with deviations from the construction project.

The main methods of heating concrete

There are several methods for warming up concrete in winter. It should be understood that when applying technologies, the price is not always the leading parameter. Often, with a slight increase in costs, results are obtained many times more technologically advanced and stronger than analogues.

Thermos method

One of the oldest and inexpensive ways concreting in the cold - the thermos method. It is based on the effect of hydration. It is based on the fact that the exothermic heat released during the curing of concrete is added to the heat introduced into the mixture during the manufacture of concrete at the factory.

Concrete brought from the factory is delivered to the object with the highest possible temperature.
In this case, the solution should be quickly placed in the formwork prepared in advance and covered with thermal insulation.
During the hydration process, 1 kg of the mixture releases approximately 80 kilocalories of heat, which contributes to the production of concrete products with critical strength acquired by the time of freezing.

Method based on complex antifreeze additives

When choosing antifreeze additives it is necessary to strictly observe the technology and adhere to the following requirements:

the thermal resistance of the formwork must be higher than the calculated value (only in this case is the concrete able to reach the critical strength mark);
thin structural elements, protrusions and other parts that cool down / harden faster than the base must be heated additionally (this is how uniform concrete hardening is achieved);
the surface of the structure that is not protected by formwork to prevent moisture loss or, conversely, to prevent waterlogging due to excessive snow ingress after hardening, must be covered with waterproofing (use polyethylene or other dense materials);
with a clear threat of a temperature drop below the calculated value (follow the forecasts for the area), the structure must either be insulated or heated.

Electrical heating of concrete

Most economical way heat treatment of concrete - electrical heating, namely, electrode heating of concrete. The electric current passes through the conductor, which is concrete, and heats up the entire volume of the solution from the inside. The method has proven itself well in reinforced and low-reinforced blocks, foundation grillages.

Important: the use of electrodes for structures with large quantity reinforcement is highly undesirable.

Peripheral heating is performed using tape electrodes made of wide strips of roofing steel, fixed on the formwork. Smooth steel fittings with a thickness of 5 mm or more are used as rod electrodes.

The electrodes are connected by taps (branches). The connection of the tap to the electrode is by twisting, using loops, rings or clamps. To connect, you must use a step-down transformer or welding machine. After the concrete has hardened, the electrodes remain inside, the contacts that look outwards are cut off.

An alternative to the electrode method of heating are innovative thermoelectromats "FlexiHIT". They reduce energy costs by 4.4 times.

When using a thermomat, infrared rays evenly heat the structure. Branded concrete gains strength in 11 hours, which it would acquire in 28 days under natural conditions.
With their help, they get rid of unnecessary structures. Important characteristic thermomat - laying speed. Equipping foundations and grillages with thermomats for heating bored concrete piles, the hydration rate increases.
The master will need only half an hour to install thermomats, and when connecting the electrodes, it takes at least half a day to assemble the circuit and connect it to a voltage source.

Heating concrete in the formwork

The method of heating formwork involves the transfer of heat from it to the outer layers of the concrete structure. From there, the heating goes in the thickness of the concrete due to thermal conductivity. An alternative to heating formwork is the installation of the same FlexiHIT thermomats with similar benefits.

Both methods are used for thin-walled and medium-sized concrete walls with and without reinforcement.
The heat from the formwork or IR heating with a thermomat compensates heat loss wall layers of concrete in large monolithic blocks of large mass and volume. It is based on the principle of "adjustable thermos" for the foundation.
However, if heating wires and carbon-graphite tapes insulated with fiberglass 10 cm in size are used in the form of a heating formwork for concrete, then the use of a thermomat consists in a snug fit of the product to the surface of the grillage.

In both cases, to maintain the isothermal process, it is necessary to avoid the appearance of air gaps, and, if possible, insulate the structure. Installation of equipment for heating takes place with outer side formwork.

Application for heating a heating wire, 2-segment or one-piece thermomat

At the core traditional way- heat release from the conductor located in the structure. Heating is by conductive heat.

The newest method used for the manufacture of columns in winter is based on the use of one-piece thermomats or 2-segment infrared heaters for heating concrete columns. The devices are equipped with a built-in thermostat in each segment of the heating device.

One-piece thermomat is used if the size of the column is known in advance. In the production of ceilings and beams, thermoelectromats are placed in the lower part of the concreted product.

Air heating method

The method of air heating of concrete refers to the convective type and consists in uniform heating of the structure from the supply from the outside. warm air. Use a flexible hose or a rubberized sleeve for this. The air is produced by a heat generator powered by an alternating voltage mains or running on diesel fuel.

Air heating is used to pour concrete formwork into closed space with air circulation, reinforced by a fan for uniform heating of concrete. For air heating, it is recommended to use insulated tarpaulin airtight materials to create a greenhouse over a concrete structure.

Supervision of concrete work in winter

According to the norms of SNiP 152-01-2003, the quality of concrete products is confirmed after carrying out control measures. Used control:

input (the mixture is checked for the presence of all components);
operational control (performed during the execution of laying and other works);
acceptance control (checking the quality of the structure as a whole).

Thus, the correctness of the principle of concreting the foundation and erection is checked. monolithic structures in winter time.

There are many ways of concreting in winter. They are widely used in cold climate areas. Modern methods using infrared heating are more effective and safe, which is why they are increasingly chosen by qualified craftsmen.

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Construction is a year-round process, and, in order to avoid major losses, should not depend on weather conditions. The main criterion for high-quality concreting in winter is the warming up of the concrete.

According to SNiP, it is regulated technological heating concrete, if the minimum daily air temperature drops below 0°C. Its purpose is to prevent the freezing of raw concrete mix, which entails the formation of ice films in the thickness of the material and around the reinforcement.

Water is directly involved in the process of preparing concrete, but, turning into ice, it ceases to be part of the chemical hydration, preventing the mixture from hardening. In addition, as the ice expands, it creates internal pressure and breaks bonds in freshly poured concrete. After the liquid thaws, the hydration process can resume, but some compounds are lost forever, which leads to a decrease in the quality of the material and the durability of the structure.

Concrete heating methods

The choice of heating method depends not only on the type of structure and weather conditions, but also on the economic feasibility and urgent framework for the completion of concreting. There are such types of heating:

preliminary;
thermos;
electrode;
heating formwork;
infrared;
heating loops;
induction.

preheating

It involves heating the concrete mixture to a temperature of approximately 50 ° C using electric current with a voltage supply of 220-380 V, for 5-10 minutes. After the hot concrete is poured, it cools down according to the thermos method.

For preheating, the site requires electrical power more than 1000 kW for 3-5 cubic meters of concrete mix.

Keeping the concrete mix by the thermos method

The most economical and simple of all, this method has become widespread in construction. The mixture, at a temperature of 25-45°C, is delivered to the site and placed in the formwork. If it is heated to a higher temperature, then during transportation there is a risk of its solidification.

Immediately after pouring, the structure is covered on all sides with heat-insulating material. As a result, the concrete hardens due to insulation from cold air, the heat of the mixture itself, and also as a result of the exothermic reaction of the cement.

The amount of heat that concrete receives from these sources can be calculated, and in accordance with the value, select the desired layer of insulation. It should be enough to withstand concrete at positive temperatures until it hardens and the formwork is dismantled, regardless of external temperature conditions.

However, not all designs can be warmed by the thermos method. The most suitable are those in which the cooling area is relatively small. That is, if the mixture is prepared from medium activity Portland cements, thermos aging is suitable if the surface modulus is not higher than 8.

In winter, it is recommended to use fast-hardening highly active cements, as well as to introduce into them special additives- chemical hardening accelerators. The use of additives containing urea is not allowed, since at temperatures above 40 ° C it decomposes and the concrete strength is reduced by up to 30%, which is expressed in low frost resistance and water permeability. Such measures make it possible to use the thermos method on surfaces with a modulus of 10 to 15.

In accordance with the thermotechnical calculation, which is carried out when designing a thermos cover, the amount of heat in the concrete mixture should not be lower than the amount of heat loss during cooling for the entire period required for the hardening of concrete.

As a heater, boards and plywood with a layer of foam, sawdust, cardboard, mineral wool etc. Particular care should be taken to insulate structures with level differences, corners and thin elements. Formwork and thermal protection are removed when the outer layer of concrete reaches 0°C.

Electrode heating method

A method for accelerating the curing of concrete by passing an electric current into it. It is widely used in the construction of monolithic structures made of concrete and reinforced concrete in the winter, as well as in the production of modular elements. Among the advantages are the reliability and simplicity of the method, the rapid heating of the mixture. The disadvantages include the need for a high power source on the site: from 1000 kW per 5 m³ of concrete and a constant increase in the heating temperature as the material hardens.

Electrode winter warming up concrete is peripheral, through and using fittings as transmitting electrodes. It is most often used when working with weakly reinforced structures: foundations, walls, partitions, columns, ceilings. It can often be combined with concrete preheating and thermos method using chemical hardeners.

Entering the concrete for a certain period of time, the current heats it evenly over the entire plane, regardless of the thickness of the segment. This is especially important when working with lightweight concrete difficult to heat up. The effect of the current on the hardening of the mass is due to an increase in the temperature inside the material and the electrolysis of water, and resistivity concrete changes at different stages of its formation.

Concrete is heated by electrodes using at least two metal pins. Connected to anti-phase wires, they transfer current between them. In this case, the specified voltage is very important: it can be increased (220-380 V) or reduced (60-128 V). Electric heating over 127 V is used only for non-reinforced structures and with strict observance of safety regulations. AT reinforced concrete in case of high voltage supply, local overheating may occur, causing evaporation of moisture and short circuits.

After pouring, metal rods are stuck into walls or columns, to which low voltage is supplied from the transformer. The electrodes are metal bars or strings, whose length is determined by the place of use. Their diameter is from 6 to 10 mm. Depending on the weather, the step between the electrodes can be from 0.6m to 1m.

If the transformer is three-phase, one electrode will be enough for one column. Quick installation and effective heating on the one hand, on the other hand, turns into the high cost of disposable katane electrodes and energy costs.

Heating formwork method

Direct contact of electrodes with concrete is useful for warming up vertical structures, while the method of heating formwork is more suitable for jellied ones, but the essence of the procedure does not change from this.

The principle of electrode heating of a monolithic structure is the flow of heat from the surface of the formwork into the concrete due to its thermal conductivity. Heating elements, carbon fiber, mica-plastic and mesh heaters are used as heat transmitters.

To create a uniform temperature contour, all exposed surfaces and ends should be insulated. It is preferable to pour the concrete mixture into preheated formwork: this reduces the heating time of concrete and reinforcement, and prevents form deformation.

Before starting the laying of the mixture, the formwork should be turned off. The mode of supply of electricity to all panels must be the same, and this is set manually. The temperature of preheated concrete should not exceed 60°C, as moisture may begin to evaporate, which will increase the viscosity of the mass.

The mixture is laid in layers and immediately covered with heat-insulating materials. Before turning on the electrodes, the concrete is kept for some time to evenly distribute the temperature. Then, carefully, one at a time, shields are connected.

To achieve 80% strength, the total heating time of concrete at a temperature of 80°C is 13-15 hours. In order to save (almost one and a half times), the temperature can be lowered to 60 ° C, but the solidification time will be 20-23 hours.

Concrete heating scheme:

The control panel is installed and connected, connecting cables are unwound.
Plug-in connectors are connected along the entire perimeter of the formwork and to temperature sensors.
Signal lights are connected to the control panel. After the switch is turned on, the voltage will be applied to both the power and signal circuits, which control the presence of voltage in the phases. The network current is monitored by a voltmeter on dashboard remote control.
Installation starts. Using switches, the sensors in the formwork panels are connected to the electronic temperature controller.
If one of the shields overheats, the power supply stops, as evidenced by the signal of the corresponding lamp.
When the warm-up is over, the unit will automatically turn off.

infrared heating

This method uses the principle of peripheral use of thermal energy obtained from infrared emitter. They can be both metal (heating elements) and carborundum emitters. Infrared transmitters combined with reflectors and other devices constitute an infrared installation.

The optimal distance from the emitter to the heated surface is 1.2 m. For better heat absorption, the formwork can be coated with black matte paint. To avoid evaporation of moisture from the surface, the structure is covered with plastic wrap, roofing felt or glassine.

The process of heating concrete with infrared rays is divided into three stages: exposure of the mixture and its heating, active heating, cooling.

The approximate consumption of electricity for heating 1 m³ is 120-200 kW / h.

Infrared heat is directed to the outer areas of the heated structure and contributes to the following processes:

warming up frostbitten soil and layers of concrete, mortgages, fittings, cleaning them from ice and snow;
acceleration of the hardening process of floors, monolithic structures, inclined and vertical structures;
pre-heating of the zones of docking of frozen and fresh mixtures;
heating hard-to-reach places for warming.

Using heating loops

The method with heating wires consists in laying out on a reinforcement frame in the formwork right amount heating wires (PNSV). Their number is calculated depending on the heat transfer and the fill area.

Then spread on top concrete mass, and when current is passed through the wires, it, due to its thermal conductivity, warms up to 40-50 ° C. As heating loops, wires for concrete PNSV with PVC insulation and galvanized steel core with a diameter of 1.2 mm are used. You can also use PTPZh in polyethylene insulation with two 1.2 mm wires.

Electricity is supplied through step-down transformers such as KTP-63/OB or KTP-80/86, where you can adjust the heating power depending on changes outside temperature. At a time, one substation is enough to heat up to 30 cubic meters of concrete at air temperatures down to -30°C.

Heating 1 m³ requires an average of 60 m of heating wire.

Induction heating

This method of heating concrete in winter is based on the use of a magnetic component in an alternating electromagnetic field, where an electric current is generated as a result of induction. With this heating, the energy magnetic field, directed to the metal, is converted into heat, from where it is transferred to concrete. The intensity of heating depends on the magnetic and electrical properties heat source (metal) and magnetic field strength.

The induction method is applied to structures with closed loop, where its length is greater than the size of the section, to reinforced concrete with dense reinforcement or structures with metal formwork. In accordance with safety regulations, heating is carried out at a reduced voltage of 36-12 V.

Before pouring the mixture, a template is laid out along the contour of the structure, where the coils of the inductor will be placed. Next, an insulated wire is laid in the grooves, where concrete is then poured. As with any heating method, first it is kept for 2-3 hours at a minimum temperature of about 7 ° C, for this the inductor is activated for 5-10 minutes every hour. The temperature of the concrete begins to rise at a rate of 5-15 ° C and upon reaching the limit mark, the inductor can be turned off, then further heating is carried out by the thermos method or switches to a pulsed mode, periodically maintaining the desired level of heat.

The advantages of this method include uniform heating along the entire length and cross section of the structure, the possibility of heating the reinforcement and saving on electrodes.

Approximate energy consumption per 1 m³ is about 120-150 kWh.

Calculation of concrete heating

As for determining the length of the wire per section and the number of such sections in the structure, it depends on the characteristics of the wire and the voltage of the transformer.

For example, when a current of 220V is applied, the length of the PNSV section of 1.2 mm is 110 m. If the voltage decreases, the length of the wire in the segment is proportionally reduced.

The heat received from the heating section with an average wire consumption of 50-60 m/m³ is capable of heating the poured concrete up to 80°C.

To obtain the average temperature of concrete during cooling, an empirical relationship is used. An approximate calculation of cooling is determined as follows:

Based on the meteorological weather forecast for the entire winter period in the required area, the expected average outdoor temperature is set.
The surface modulus is determined, according to which a suitable thermos holding is calculated.
Using a formula, calculate average temperature concrete for the entire cooling time.
The cement supplier receives data on ready mix at what temperature it will be delivered and what exothermic characteristics it has.
The formulas are used to calculate heat losses during delivery and unloading.
The initial temperature of the concrete is determined from the time of laying, taking into account the transfer of its heat to the heating of the reinforcement and formwork.
Based on the strength requirements, the duration of cooling of the concrete mixture is determined.

This method of calculation is used to predict the age of concrete, take into account heat loss during pouring, and heat radiation from the surface, but it should be remembered that the data are approximate.

The most common method of heating concrete during pouring in winter is electrical heating, which is used in cases where conventional insulation of the object is not enough. It is about him that we will talk today.

There are several ways to warm up concrete in winter:

1. Heating of concrete with electrodes.
2. Electric heating of concrete with wire PNSV
3. Electrical heating of the formwork
4. Heating by induction method
5. Infrared

It should be noted that regardless of the method, electric heating of concrete must be accompanied by its insulation or at least the creation of a thermos around the object. Otherwise, uniform heating may not work, and this will not have a very good effect on its final strength.

Warming up concrete with electrodes - connection diagram

Concrete heating with electrodes is the most common method of electric heating in winter. This is due, first of all, to simplicity and cheapness, because, in some cases, there is no need to spend money on heating wires, expensive transformers, etc.

The principle of operation of this method of electrical heating is based on physical properties an electric current that, when passing through a material, releases a certain amount of heat.

In this case, the conductive material is the concrete itself, in other words, when the current passes through the water-containing concrete, it heats it up at the same time.

Attention! If the concrete structure contains a reinforcing cage, it is not recommended to apply a voltage of more than 127 V to the electrodes. In the absence of metal frame, both 220 V and 380 V can be used. Higher voltage is not recommended.

There are several types of electrodes for heating concrete in winter:

Rod electrodes. To create them, metal fittings d 8 - 12 mm are used. Such rods are inserted into concrete at a short distance and connected to different phases, as in the diagram. In cases complex structures, such electrodes for heating concrete will be indispensable. Fiberglass reinforcement is not suitable for such purposes, because it is a dielectric.

Plate electrodes. Sometimes they are called plate electrodes. The connection scheme for such heating is very simple - the plates are located on both opposite inner sides of the formwork and are connected to different phases, and the passing current will heat the concrete. Instead of wide plates, narrow strips are sometimes used, the principle of operation of these strips is the same.

String electrodes. Used when pouring columns, beams, pillars and similar structures. The principle of operation is still the same, the strings are connected to different phases, thereby heating the concrete in the winter.

Heating of concrete with electrodes must be carried out only alternating current, as D.C., passing through the water, contributes to its electrolysis. In other words, water will chemically decompose without fulfilling its main function in the hardening process.

Electric heating of concrete with PNSV wire: technology and scheme

If heating concrete with electrodes is one of the cheapest options for electric heating in winter, then, in turn, heating with a PNSV wire is one of the most effective.

This is due to the fact that not the concrete itself is used as a heater, but the PNSV heating wire, which releases heat when current passes through it. Using such a wire, it is much easier to achieve a smooth increase in the temperature of the concrete, and in general such a wire will lead predictably, which will facilitate the necessary gradual increase in temperature in winter.

It is worth mentioning the PNSV wire itself (P - wire, H - heating, C - steel core, B - PVC insulation). There are various sections 1.2, 2, 3. Depending on the section used, its quantity is selected per 1 meter of cubic concrete mix.

The technology of electric heating of concrete with the PNSV wire, as well as the connection diagram, is very simple. The wire without tension is passed along the reinforcing cage, and it is attached to it. It is necessary to fix it so that when concrete is fed into the trench or formwork, it is not damaged.

When electrically heating concrete with the PNSV wire in winter, it is laid so that it does not touch the ground, the formwork, and also does not go beyond the concrete itself. The length of wire used depends entirely on its thickness, resistance, expected sub-zero temperature, and the applied voltage, with the help of a special transformer, is usually about 50 V.

There are also cables that do not require the use of a transformer. Using them will save you some money. It is very convenient to use, but still, a conventional PNSV wire has more wide opportunities for application.

Electrical heating of formwork in winter

This method of electrical heating involves the manufacture of formwork with pre-laid heating elements in it, which, when heated, will give off as necessary for concrete warmly. It resembles the heating of concrete with plate electrodes, only the heating is not carried out on inside formwork, and inside it, or outside.

Electrical heating of the formwork in winter is not often used, given the complexity of the structure, especially since when pouring the foundation, for example, the formwork does not come into contact with the entire concrete structure. Thus, only part of the concrete will heat up.

Induction and infrared methods of concrete heating

The induction method of heating concrete is used extremely rarely, and even then, mainly in beams, crossbars, girders, due to the complexity of its device.

It is based on the fact that an insulated wire wrapped around a steel bar of an armature will create induction and heat the armature itself.

Electrical heating of concrete in winter infrared rays is based on the ability of such rays to heat the surface of opaque objects, followed by heat transfer throughout the volume. When using this method, it is necessary to provide for wrapping the concrete structure with a transparent film, which will pass the rays through itself, preventing the heat from leaving so quickly.

The advantage of this method is that it is not necessary to use special transformers. The disadvantage is that infrared radiation is not capable of uniform heating of large structures. This method is only suitable for thin structures.

Do not forget that regardless of the method of electric heating of concrete in winter, it is necessary to constantly monitor its temperature, because too high (more than 50 0 С) is just as dangerous for it as too low. The rate of heating of concrete, as well as the rate of cooling, should not exceed 10 0 C per hour.

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

SNiP. PRODUCTION OF CONCRETE WORKS AT NEGATIVE AIR TEMPERATURES

2.53. These rules shall be complied with during the period of concrete work, with the expected average daily outdoor temperature below 5 °C and the minimum daily temperature 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 the calculation. It is allowed to use unheated dry aggregates that do not contain frost on grains and frozen clods. At the same time, 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 that the temperature of the concrete mixture does not drop below the 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 laying method, must exclude the possibility of freezing of the mixture in the zone of contact with the base. When curing concrete in a structure by 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-porous base or old concrete, if, according to the calculation, in the contact zone during the estimated period of curing the concrete does not freeze.

At air temperatures below minus 10 °C, concreting of densely reinforced structures with reinforcement with a diameter of more than 24 mm, reinforcement from rigid rolled profiles 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 vibrating 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 for gaps in spans, depending on the heat treatment temperature, taking into account the resulting thermal stresses, should be agreed with the design organization. Unformed surfaces of structures should be covered with steam and thermal insulation materials immediately after concreting.

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

2.58. Before laying concrete (mortar) mix surfaces of joint cavities of prefabricated reinforced concrete elements must be free 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 of laying the concrete mixture. Samples stored in frost should 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. Requirements for the performance of work at negative air temperatures are set in the table. 6

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

Parameter	Parameter value	Control (method, scope, type of registration)
Pouring concrete at low temperatures.
1. The strength of concrete of monolithic and precast-monolithic structures by the time of freezing:		Measuring according to GOST 18105-86, journal of works
for concrete without antifreeze additives:
structures operated inside buildings, foundations for equipment that are not subject to dynamic effects, underground structures	Not less than 5 MPa
structures exposed to atmospheric influences during operation, for the class:	Not less than, % of design strength:
B7.5-B10	50
B12.5-B25	40
B30 and above	30
structures subjected to alternating freezing and thawing in a water-saturated state at the end of curing or located in the zone of seasonal thawing of permafrost soils, provided that air-entraining or gas-forming surfactants are introduced into the concrete	70
in prestressed structures	80
for concrete with antifreeze additives	By the time the concrete is cooled to the temperature for which the amount of additives is calculated, at least 20% of the design strength
2. Loading of structures with the design load is allowed after the concrete reaches strength	At least 100% of design	-
3. The 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 grades below M600	Water not more than 70 °С, mixtures not more than 35 °С
on fast-hardening Portland cement and Portland cement grade M600 and higher	Water not more than 60°С, mixtures not more than 30 °С
on aluminous Portland cement	Water not more than 40 C, mixtures not more than 25 °C
The temperature of the concrete mixture laid 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°С
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, °С:	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 exposure time - once a day
Portland cement	80
slag Portland cement	90
6. The rate of temperature rise during the heat treatment of concrete:		Measuring, every 2 hours, work log
for structures with surface modulus:	No more, °С/h:
up to 4	5
5 to 10	10
St. ten	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
5 to 10	Not more than 5°С/h
St. ten	Not more than 10°С/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
2 to 5	Not more than 20, 30, 40 °С
St. 5	Not more than 30, 40, 50 °С

Construction in modern conditions does not stop even in the cold season: in winter, this process becomes more complicated due to weather conditions and begins to require the use of certain technologies. For example, for a high-quality setting of concrete, it must be warmed up, but how can this be done in winter?

There are many methods for warming up concrete in winter. These are quite complicated and expensive methods, however, if you ignore them, the concrete will not gain strength and will not meet the design requirements. To warm up concrete, PNSV wires are most often used. To start the process, you need a transformer or welding machine. The second option is weaker and will not give a quick and high-quality effect, like the first.

Thermomats for heating concrete

The thermomat for heating concrete is not some new invention: it has been actively used for more than ten years at all construction sites in the country. The method is especially popular in northern regions, where the need to warm up the structure is more acute. The method has proven itself well, but has been improved over the years of existence.

Thermoelectric mats are devices that can work autonomously. The warm-up time is set automatically, and a person does not need to monitor the equipment on and off. The devices consume much less electricity than when the structure is heated with wires. The method allows to warm up the material qualitatively. Heating occurs evenly, there is no local overheating: this means that the concrete will harden without microcracks and will have high strength.

The advantages of this method:

Easy to use;
No equipment required complex care;
It is not required to control the heating temperature, the control is carried out automatically;
High quality heating
In 12 hours, the mixture reaches 70% brand strength.

Disadvantages:

Thermomats are expensive, and not every developer can purchase them;
Most of the goods on the market are fake, which are not suitable for heating concrete, as they consist of a Korean heating film designed for use as a warm floor. The power of such devices is too small to warm up the concrete mixture.

It is quite possible to distinguish a fake: you need to pay attention to how the film is applied. In devices for underfloor heating, it is applied in stripes, in devices for heating concrete, a film layer is applied evenly.

Warming up concrete in winter with the PNSV wire

This is a fairly simple way to warm up. It is used in 70% of cases, as it is very affordable. In order to make it possible, it is necessary to take care of the installation of the wires in advance, so the PNSV wire is laid first, and then the concrete mixture is poured. The cable is heated by a transformer, which creates a low voltage.

Advantages:

Low cost of the procedure. The transformer consumes significantly less energy than other equipment, so it is very relevant if the budget is limited. Buying it is also optional: it is quite possible to rent the necessary equipment for a while.
A step-down transformer 80 kW is suitable for warming up the concrete mix. With the help of such equipment, 90 m 3 of concrete warms up without problems.
Wire laying is possible in any weather.

The method is not without drawbacks:

It is necessary to take care of the warm-up procedure in advance, lay the wire, lay the heating loops (the wire is laid using a special technology: it is not enough just to concrete it, it is necessary that the structure covers all the concrete, for which it is laid with loops, which are fixed in a special way, similar to laying underfloor heating) .
The method requires physical effort from workers.

Warming up concrete in winter with electrodes

It is not necessary to use the PNSV wire for heating: fittings tied with 8-10 mm wire rod are suitable for this purpose. This method is not suitable if you need to fill slab foundation or concrete slab. It is usually used when pouring columns, diaphragms, walls: this method of heating is quite convenient and does not require extra costs.

It also needs a transformer to work. Metal rods are connected to it, which are connected to a concrete structure. The step-down transformer will supply a reduced voltage, which will heat up the metal parts of the structure.

Temperature environment is an important factor to consider when determining the spacing between electrodes. The standard interval is 0.6-1 meter. Warming up of concrete is carried out due to the moisture contained in its mass. The transformer supplies three phases to the structure. The areas between the installed electrodes are heated. If it is necessary to warm up the column, then it will be enough to install one electrode, since the heating of concrete in winter will occur due to the contact of the structure with the transformer phase and the ground.

The advantages of this method:

Fast, uncomplicated heating installation;
Inexpensive materials used for mounting.

The disadvantages include the following:

High energy consumption of the electrodes. One electrode requires approximately 45-50 amps
A step-down transformer with a power of 80 kW cannot be connected to a large number electrodes. Its power may not be enough. To solve the problem, it is recommended to use several transformers.
Reinforcement and wire cannot be pulled out of the structure after warming up, it will remain there forever.

Formwork for heating concrete

For this method, formwork is used, into the panels of which a heating element. The convenience of the design lies in the fact that, if necessary, you can easily replace it defective elements. If the house is monolithic, then with the help of such formwork you can warm it up completely. If you warm up the floors in stages, then the formwork can be rearranged, going to desired area work. You can use this method even at an ambient temperature of -25 degrees.

The advantages of this technique:

High performance with relatively low energy consumption;
Requires a little time for preparation, installation;
Can be used in very coldy;
Can be used multiple times.

Disadvantages:

High price.
It is inconvenient if the structure is non-standard.

Induction heating of concrete in winter

This method of heating is used quite rarely and is less than ten percent. The heating of the material is carried out due to magnetic induction, which is converted into heat. This process is possible through the use of turns insulated wire and built into the design metal parts.

The main difficulty of the process is that it is necessary to accurately calculate the turns of the wire, taking into account the amount of metal in the structure. Often it is almost impossible to do this, which is why the method of magnetic induction is unpopular.

Infrared concrete heating

Guided infrared installations can greatly facilitate the heating of concrete in winter. The unit does not need to be mounted anywhere: heating can take place directly through the formwork of the structure. Infrared installation allows you to qualitatively warm up the exposed surfaces of concrete. It is suitable for working with any design, regardless of its shape. Adjusting the heat is quite simple: it is carried out by moving the heating element away or approaching the structure.

Advantages:

The method efficiently consumes electricity and warms up the concrete efficiently.

Disadvantages:

High price equipment. If the volume of production is large, then a lot of infrared installations are required, which is unprofitable for the developer.
The method etches moisture from concrete, which can weaken its strength. To avoid this problem, it is recommended to cover the structure with a film.

Teplyak for warming up concrete

It's pretty old way warming up: a frame is built over the concrete structure, covered with a tarpaulin. Placed inside the ball thermal plant.

Advantages of the method:

Warming up is relatively fast;
Low energy consumption, gas or other fuel can be used.

Disadvantages:

Labor-intensive method, especially on large areas.

Most often, a step-down transformer is used on construction sites. This is the most affordable and effective way to quickly warm up concrete in winter at an affordable price.

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