Technological map for heating concrete with electrodes. Technological map Technological map for electrode heating of monolithic concrete structures

TYPICAL TECHNOLOGICAL CARD (TTK)

ELECTRODE HEATING OF STRUCTURES MADE OF MONOLITIC CONCRETE AND REINFORCED CONCRETE

1 AREA OF USE

1.1. A standard technological map (hereinafter referred to as TTK) was developed for winter concreting using the method of electric heating with string electrodes when installing monolithic reinforced concrete structures in the construction of a residential building. The essence of electrode heating is that heat is released directly into the concrete when an electric current is passed through it. The use of this method is most effective for foundations, columns, walls and partitions, flat floors, as well as concrete preparations for floors.

1.2. The standard technological map is intended for use in the development of Work Production Projects (WPP), Construction Organization Projects (COP), other organizational and technological documentation, as well as for the purpose of familiarizing workers and engineers with the rules for the production of concrete work in winter on a construction site .

1.3. The purpose of creating the presented TTK is to provide a recommended flow chart for concrete work in winter.

1.4. When linking the Standard Flow Chart to a specific facility and construction conditions, production schemes and volumes of work, technological parameters are specified, changes are required to the work schedule, calculation of labor costs, and the need for material and technical resources.

1.5. Standard technological maps are developed according to drawings of standard designs of buildings, structures, certain types of work on construction processes, parts of buildings and structures, regulate technological support means and rules for performing technological processes during the production of work.

1.6. The regulatory framework for the development of technological maps is: SNiP, SN, SP, GESN-2001, ENiR, production standards for material consumption, local progressive standards and prices, labor cost standards, material and technical resource consumption standards.

1.7. Working technological maps are developed on the basis of technical specifications according to the drawings of the Detailed Design for a specific structure, structure, are reviewed and approved as part of the PPR by the Chief Engineer of the General Contracting Construction and Installation Organization, in agreement with the Customer’s organization, the Customer’s Technical Supervision and the organizations that will be in charge of the operation of this building.

1.8. The use of TTK helps to improve the organization of production, increase labor productivity and its scientific organization, reduce costs, improve quality and reduce the duration of construction, safe performance of work, organize rhythmic work, rational use of labor resources and machines, as well as reducing the time required to develop PPR and unify technological solutions.

1.9. The work performed sequentially during electrode heating of concrete and reinforced concrete structures in winter includes:

Determination of the cooling surface module;

Installation of string electrodes;

Electrical heating of the structure.

1.10. When electrically heating concrete and reinforced concrete structures using the electrode method, the main material used is string electrodes manufactured on the construction site from reinforcing steel of periodic profile A-III, with a diameter of 8-12 mm, a length of 2.5-3.5 m and rod electrodes made of reinforcing steel of periodic profile of grade A-III, with a diameter of 6-10 mm and a length of up to 1.0 m.

1.11. The work is carried out in winter and is carried out in three shifts. The working hours during a shift are:

Where 0.828 is the coefficient of TP utilization by time during the shift (time associated with preparing the TP for work and conducting ETO - 15 minute breaks associated with the organization and technology of the production process).

1.12. Work should be performed in accordance with the requirements of the following regulatory documents:

SNiP 12-01-2004. Organization of construction;

SNiP 12-03-2001. Occupational safety in construction. Part 1. General requirements;

SNiP 12-04-2002. Occupational safety in construction. Part 2. Construction production;

SNiP 3.03.01-87. Load-bearing and enclosing structures;

GOST 7473-94. Concrete mixtures. Technical conditions.

2. TECHNOLOGY AND ORGANIZATION OF WORK

2.1. In accordance with SNiP 12-01-2004 “Construction Organization”, before the start of work on the site, the Subcontractor must, according to the act, accept from the General Contractor the prepared construction site, including the finished reinforcement frame of the structure being constructed.

2.2. Before starting work on electrode heating of the concrete mixture, the following preparatory measures must be completed:

A person responsible for the quality and safety of work has been appointed;

Team members were instructed on safety precautions;

A thermal engineering calculation of the electrode heating of the structure was carried out;

The work area has been fenced with warning signs;

The routes for personnel movement along the electrical heating area are indicated on the diagram;

Floodlights were installed, a fire shield with a fire control unit was installed;

The necessary electrical equipment has been installed and connected;

The necessary installation equipment, equipment, tools and a household trailer for workers' rest were delivered to the work area.

2.3. Installation and operation of electrical equipment is carried out in accordance with the following instructions:

The transformer substation is installed near the work area, connected to the power supply network and tested at idle;

Inventory sections of busbars were manufactured (see Fig. 1) and installed near heated structures;

The busbars are interconnected by cable and connected to the transformer substation;

All contact connections are cleaned and checked for tightness;

The contact surfaces of switches, main and group distribution boards are ground;

The tips of the connected wires are cleaned of oxides, damaged insulation is restored;

The arrows of electrical measuring instruments on the panels are set to zero.

Fig.1. Busbar section

1 - connector; 2 - wooden stand; 3 - bolts; 4 - conductors (strip 3x40 mm)

2.4. In order to accelerate the strength gain of monolithic structures, the thermal energy released directly in the concrete during electrode heating is used. The number of electrodes required to warm up a particular structure is determined by thermal engineering calculations. To do this, it is necessary to determine the cooling surface module of a given design (see Table 1).
Cooling surface modules

Table 1


Name	Surface sketch	Magnitude
Cube		- cube side
Parallelepiped		- parallelepiped sides
Cylinder		- diameter
Pipe		- diameter
Wall, slab		- thickness

Specific consumption of electrodes per 1 mheated concrete in kg

table 2


Name of electrodes	designs
	4	8	12	15
Strings	4	8	12	16
Rod	4	10	14	18

2.5. Before laying the concrete mixture, the formwork and reinforcement are installed in working position. Immediately before concreting, the formwork must be cleared of debris, snow and ice, and the surfaces of the formwork must be coated with lubricant. Preparation of bases, products and laying of concrete mixture is carried out taking into account the following general requirements:

Use a plastic concrete mixture with a mobility of up to 14 cm along a standard cone;

Lay concrete mixture with a temperature of at least +5 °C in a structure with a cooling surface module of 14, as well as in cases where the placement and installation of electrodes has already been carried out;

When the cooling surface module is more than 14 and in cases where the installation and assembly of electrodes must be carried out after laying the concrete mixture, its temperature must not be lower than +19 ° C;

The concrete mixture is laid continuously, without transferring, using means that ensure minimal cooling of the mixture during its supply;

At air temperatures below minus 10 °C, reinforcement with a diameter of more than 25 mm, as well as reinforcement of rolled products and large metal embedded parts if they have ice on them, are pre-heated with warm air to a positive temperature. Removing ice using steam or hot water is not allowed;

Start electrical heating at a temperature of the concrete mixture not lower than +3 °C;

In places where the heated concrete comes into contact with frozen masonry or frozen concrete, place additional electrodes to provide enhanced heating of the area adjacent to the cold surface;

When interrupting electrical heating work, cover the joints of heated surfaces with heat-insulating materials.
2.6. Immediately after laying the concrete mixture into the formwork, the exposed surfaces of the concrete are covered with waterproofing (polyethylene film) and thermal insulation (mineral wool mats 50 mm thick). In addition, all fittings outlets and protruding embedded parts must be additionally insulated.

2.7. For electrical heating of a small volume of side surfaces of massive structures (peripheral heating) and intersections of prefabricated reinforced concrete structures, rod electrodes, which are manufactured on the construction site from reinforcing steel of periodic profile of grade A-III, with a diameter of 6-10 mm and a length of up to 1.0 m.

Rod electrodes are driven into the concrete mixture through layers of hydro- and thermal insulation or holes drilled into the formwork of structures at a distance of , depending on the applied voltage and power.

Fig.2. Installation of rod electrodes

2.8. The specific resistance of concrete during the hardening process increases sharply, which leads to a significant decrease in the flowing current, power and, consequently, to a decrease in the heating temperature, i.e. to extend the curing time of concrete. In order to reduce these periods, various concrete hardening accelerators are used. To maintain the current value during electrical heating of concrete and maintain its constant temperature, it is necessary to regulate the voltage. Regulation is carried out in two to four steps ranging from 50 to 106 V. The ideal mode is smooth voltage regulation.

It is especially important to regulate the tension when heating reinforced concrete. Steel reinforcement distorts the current path between the electrodes, because The resistance of the reinforcement is significantly less than the resistance of concrete. Under these conditions, overheating of concrete is possible, which is especially harmful for openwork structures.

The location of the electrodes in the concrete should provide heating conditions, namely:

The temperature difference in the electrode zones should not exceed +1 °C per 1 cm of zone radius;

Heating of the structure must be uniform;

At a given voltage, the power distributed in the concrete must correspond to the power required to implement a given heating mode. To do this, it is necessary to observe the following minimum distances between the electrodes and the fittings: 5 cm - with a voltage at the beginning of warming up of 51 V, 7 cm - 65 V, 10 cm - 87 V, 15 cm - 106 V;

If it is impossible to maintain the specified minimum distances, arrange local insulation of the electrodes.

2.9. Group placement of electrodes eliminates the risk of local overheating and helps equalize the temperature of the concrete. At a voltage of 51 and 65 V, at least 2 electrodes are installed in a group, at a voltage of 87 and 106 V - at least 3, at a voltage of 220 V - at least 5 electrodes in a group.

Fig.3. Installation of group electrodes

When heating reinforced concrete structures with dense reinforcement, allowing the placement of the required number of group electrodes, single electrodes with a diameter of 6 mm should be used, with a distance between them no more than:

20-30 cm at a voltage of 50-65 V;

30-42 cm at a voltage of 87-106 V.

A voltage of 220 V for electrical heating can be used in the group method only for unreinforced structures, and special attention must be paid to compliance with safety regulations. When electrically heating using a voltage of 220 V, temperature control is carried out by turning on and off part of the electrodes or periodically turning off the entire section.

The distance between the electrodes is taken depending on the outside temperature and the accepted voltage according to Table 3.
Table 3


Outside air temperature, °C	Supply voltage, V	Distance between electrodes, cm	Specific power, kW/m
-5	55	20	2,5
	65	30
	75	50
-10	55	10	3,0
	65	25
	75	40
	85	50
	65	15	3,5
	75	30
	85	45
	95	55
-20	75	20	4,5
	85	30
	95	40

2.10. For electrical heating of massive slabs with single reinforcement, lightly reinforced walls, columns, beams, string electrodes, manufactured on the construction site from reinforcing steel of periodic profile of grade A-III, with a diameter of 8-12 mm, a length of 2.5-3.5 m.
When using string electrodes, special attention should be paid to the correctness and reliability of their installation. If during concreting the electrode comes into contact with the reinforcement, the structure cannot be heated, because It is impossible to correct the position of the string electrode after concreting.

When heating columns with symmetrical single reinforcement, one electrode (string) up to 3.5 m long is installed in the center parallel to the structure. The end of the electrode is released for connection to the electrical circuit. The second electrode is the reinforcement itself. If the distance from the electrode to the reinforcement is more than 200 mm, then a second or several such electrodes are installed.

Fig.4. Installation of string electrodes

Fig.5. Diagrams of a concreting section using electrical heating

1 - heated design; 2 - fence; 3 - warning notice; 4 - box with sand; 5 - fire shield; 6 - distribution board; 7 - signal light; 8 - soffits; 9 - cable type KRT or insulated wire type PRG-500; 10 - PZS-35 type spotlight; 11 - path of maintenance personnel along the electric heating area, which is energized

2.11. Before applying voltage to the electrodes, check the correctness of their installation and connection, the quality of the contacts, the location of temperature wells or installed temperature sensors, the correct installation of insulation and supply cables.

Voltage is supplied to the electrodes in accordance with the electrical parameters specified in Table 3. Voltage supply is allowed after concrete has been placed in the structure, the necessary thermal insulation has been laid and people have left the fence.

Immediately after applying voltage, the electrician on duty re-checks all contacts and eliminates the cause of the short circuit, if it occurs. During heating of concrete, it is necessary to monitor the condition of contacts, cables and electrodes. If a malfunction is detected, you must immediately turn off the voltage and eliminate the malfunction.

2.12. The rate of concrete heating is controlled by increasing or decreasing the voltage on the low side of the transformer. When the outside air temperature changes during the warming up process above or below the calculated value, the voltage on the low side of the transformer is reduced or increased accordingly. Warming up is carried out at a reduced voltage of 55-95 V. The rate of temperature rise during heat treatment of concrete should not be higher than 6 °C per hour.

The cooling rate of concrete at the end of heat treatment for structures with surface modulus =5-10 and >10 is no more than 5 °C and 10 °C per hour, respectively. The outside air temperature is measured once or twice a day, and the measurement results are recorded in a log. At least twice a shift, and in the first three hours from the start of concrete heating, the current and voltage in the supply circuit are measured every hour. Visually check that there is no sparking at the electrical connections.

The strength of concrete is usually checked by the actual temperature conditions. After stripping, the strength of concrete at a positive temperature is recommended to be determined by drilling and testing cores.

2.13. Thermal insulation and formwork can be removed no earlier than the moment when the temperature of the concrete in the outer layers of the structure reaches plus 5 °C and no later than the layers have cooled to 0 °C. Freezing of formwork, hydro- and thermal insulation to concrete is not allowed.

To prevent the appearance of cracks in structures, the temperature difference between the exposed concrete surface and the outside air should not exceed:

20 °C for monolithic structures with surface modulus up to 5;

30 °C for monolithic structures with a surface modulus of 5 and higher.

If it is impossible to comply with the specified conditions, the concrete surface after stripping is covered with tarpaulin, roofing felt, boards, etc.

TYPICAL TECHNOLOGICAL CARD (TTK)

ELECTRIC HEATING OF CONCRETE

1 AREA OF USE

A typical flow chart has been developed for electrical heating of concrete.

1. Electric heating is used when concreting structures at outdoor temperatures below -5 ° C, as well as at positive (“plus”) outdoor temperatures, when there is a need to sharply speed up the process of concreting a building or structure. As a rule, the goal of electrical heating is to obtain 50% of the brand strength of concrete at the end of electrical heating.

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, 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 prestressed reinforcement - 80% of the design strength; for structures subject to alternating freezing and thawing or located in the zone of seasonal thawing of permafrost, - 70% of the design strength; for structures loaded with design load - 100% of design strength.

The duration of concrete curing and its final properties largely depend on the temperature conditions in which the concrete is exposed. As the temperature rises, the activity of the 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 (Fig. 1).

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, special methods of preparing, feeding, laying and curing concrete are used.

When preparing a concrete mixture in winter conditions, its temperature is increased to 35... 40 ° C by heating the aggregates and water. The fillers are heated to 60°C with steam registers, in rotating drums, in installations with flue gases blown through the filler layer, and with hot water. Water is heated in boilers or hot water boilers to 90 °C. 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 soils. 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 at temperatures below -10°C are heated to a positive temperature.

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 that involves the use of the initial heat content added to the concrete mixture during its preparation or before laying it in the structure, and the heat release of cement that accompanies the hardening of concrete - the so-called “thermos” method; methods based on artificial heating of concrete placed in a structure - electric heating, contact, induction and infrared heating, convective heating; methods that use the effect of lowering the eutectic point of water in concrete using special antifreeze 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.

2. The choice of method for electrical heating of concrete depends on the nature and massiveness of the structures, determined by the surface modulus MP, equal to the ratio of the cooled surface of the structure in microns to its volume in m, as well as on the timing of the work, the type of cement and insulation materials. For electrical heating of monolithic structures with a surface modulus above 6, it is advisable to use the electrode heating method.

3. In order to save energy, electrical heating should be carried out as quickly as possible at the maximum permissible temperature for a given structure and the concrete should be kept under current only until it reaches 50% of its design strength.

4. With the electrode method of electric heating, the heated concrete is included in the electrical circuit as resistance, using electrodes made of reinforcing or grade steel, placed inside the concrete or located on its surface. Since direct current causes electrolysis of water, only alternating current is applicable for electrode heating.

5. For the electrode heating method, a nominal voltage (49-121 V) is used, which ensures more accurate compliance with the specified concrete curing regime.

Special transformers are used as a source of electricity.

The use of increased voltage (up to 220 V) is allowed when heating unreinforced concrete and in exceptional cases when heating lightly reinforced structures containing no more than 50 kg. reinforcement per 1 m of concrete.

When performing construction work in winter conditions, it is necessary to use artificial heating of concrete. Electrical energy is widely used for these purposes. Electrical heat treatment of concrete turns out to be in some cases more profitable than other heating methods (steam, hot air, etc.).

Electrical heat treatment of concrete is based on the conversion of electrical energy into thermal energy directly inside the concrete by passing alternating electric current through it using electrodes (electrode heating) or in various types of heating devices.

The most effective and economical method of electrical heat treatment is electrode heating. The use of direct current is not allowed in this case, since it causes electrolysis of water and other components contained in concrete.

During electrode heating, concrete is connected to an alternating current circuit using steel electrodes. One of the main initial parameters when calculating the electrode heating of concrete is its electrical resistivity.

The electrical resistivity of concrete is determined mainly by the amount of water, the concentration of electrolytes in it and temperature. During the first 2-5 hours of concrete heating, its initial electrical resistivity decreases to a minimum value, and then increases.

The value of the initial electrical resistivity of concrete ranges from 400 to 2500 Ohm-cm (minimum - from 200 to 1800 Ohm-cm). When calculating the electrode heating of concrete, the calculated resistivity is taken as the initial parameter

Maintaining the temperature of concrete in accordance with a given mode of electrical heat treatment can be carried out in the following ways:

changing the voltage supplied to the electrodes or electric heating devices;

disconnecting the electrodes or electric heaters from the network at the end of the temperature rise;

periodic switching on or off of voltage on electrodes or electric heaters.

The listed methods of maintaining a given mode can be carried out both automatically and manually.

Special power transformers are used to electrically heat concrete. Depending on the required power, both three-phase and single-phase transformers can be used.

The three-phase transformer TMT-50 with a power of 50 kVA has two secondary windings with different numbers of turns. When connecting these windings in a star or triangle, you can respectively obtain voltages of 50.5 or 87.5 V and 64.5 or 106.6 V.

A three-phase transformer of the TMOA-50 type with an aluminum winding with a power of 50 kVA is widely used. Unlike the TMT-50 transformer, voltage regulation in it is carried out by changing not only the connection diagram of the secondary winding, but also the transformation ratio. In this case, the secondary voltage can vary from 49 to 127 V.

In addition to the transformer, the mobile installation for heating concrete contains a distribution board with switching, protective and measuring equipment. The electrical circuit diagram of such an installation is shown in Fig. 2. The distribution board is designed to connect several outgoing lines to spotlights - devices used to connect electrodes.

Very often, installations for electrical heating of concrete are equipped with single-phase transformers TB-20 with a power of 20 kVA. It has a primary winding designed to be connected to a network with a voltage of 380 or 220 V, and two secondary windings, connecting which in series or parallel, you can get 102 and 51 V.

Welding transformers can also be used to heat concrete. It must be taken into account that welding transformers are designed for intermittent short-term operation. Therefore, in a long-term mode of concrete heating, the load on welding transformers should not exceed 60-70% of the nominal one.

6. When the surface modulus of structures is within 6-15, electrical heating should be carried out in a three-stage mode

1) warming up;

2) isothermal heating;

3) cooling;

In this case, the specified strength of concrete will be ensured by the end of the cooling stage. In this case, the temperature should be raised as quickly as possible, and isothermal heating should be carried out at the maximum permissible temperature for a given design.

7. The rise in the temperature of concrete structures with a surface modulus of less than and large extent should not exceed 5 °C per hour, and with a modulus of more than 5 - no more than 8 °C per hour. For structures of short length (6-8 m) and heavily reinforced, as well as for welded reinforced concrete, the rate of temperature rise can be increased to 15 °C per hour.

To avoid an unacceptably sharp rise in concrete temperature at the beginning of heating and to reduce the peak power during heating, a voltage of 50-60 V is initially used, increasing it as the concrete hardens.

8. The duration of isothermal heating is set by the construction laboratory and depends on the outside air temperatures in Table 1.

8. The cooling rate of concrete at the end of isothermal heating should not exceed 3° per hour for structures with a module up to 3-6 °C; per hour - with a module from 3 to 8; 8° per hour - with a modulus of more than 8.

The intensity of concrete cooling is regulated by changing the voltage, current or periodically turning it on.

...

Concrete is a very popular building material today, for the production of which components such as cement, water, aggregate and water are used. But it’s one thing when you pour concrete in the summer, because the warm season has a beneficial effect on the process of gaining strength. What happens in winter? In severe frosts, the development of strength characteristics stops, and this is extremely undesirable. In this case, it is necessary to apply a number of measures that will allow the concrete to warm up. To do this, you need to know all the features of the technological map of concrete for the winter period and the current methods of heating.

Technological map and methods of heating concrete

Warm up with a welding machine

This heating method involves the use of the following materials:

pieces of reinforcement;
incandescent lamps and a thermometer for measuring temperature.

The process of installing pieces of reinforcement is carried out parallel to the circuit, with adjacent and straight wires, between which the pouring lamp is mounted. It is thanks to it that it will be possible to measure voltage.

To measure temperatures, you should use a thermometer. This process takes a long time, approximately 2 months. At the same time, during the entire heating process it is necessary to protect the structure from the influence of cold and water. It is advisable to use heating with a welding machine when there is a small volume of concrete and excellent weather conditions.

Infrared method

The meaning of this method is that equipment is being installed that operates in the infrared range. As a result, it is possible to convert radiation into heat. It is thermal energy that is introduced into the material.

Infrared heating of concrete mixture represents electromagnetic vibrations, the wave propagation speed of which will be 2.98 * 108 m/s and the wavelength 0.76-1,000 microns. Very often, tubes made of quartz and metal are used as a generator.

The main feature of the presented technology is the ability to supply energy from conventional alternating current. When infrared heating of concrete, the power parameter may change. It depends on the required heating temperature.

Thanks to the rays, energy can penetrate into deeper layers. To achieve the required efficiency, the heating process must be carried out smoothly and gradually. It is forbidden to work here at high power levels, otherwise the top layer will have a high temperature, which will ultimately lead to a loss of strength. It is necessary to use this method in cases where it is necessary to heat up thin layers of the structure, as well as prepare a solution to speed up the adhesion time.

What are the pros and cons of a house made of aerated concrete?

Induction method

To implement this method, it is necessary to use alternating current energy, which will be converted into heat in formwork or reinforcement made of steel.

The converted thermal energy will then be distributed to the material. It is advisable to use the induction heating method when heating reinforced concrete frame structures. These can be crossbars, beams, columns.

If you use induction heating of concrete on the outer surfaces of the formwork, then it is necessary to install successive turns that are isolated from the inductors by wire, and the number and pitch are determined by calculation. Taking into account the results obtained, it is possible to produce templates with grooves.

When the inductor has been installed, it is possible to heat the reinforcement frame or joint. This is done in order to remove ice before concreting occurs. Now the open surfaces of the formwork and structure can be covered with thermal insulation material. Only after the wells have been constructed can the actual work begin.

When the mixture reaches the required temperature, the heating procedure is stopped. Make sure that the experimental indicators differ from the calculated ones by at least 5 degrees. The cooling rate can maintain its limits of 5-15 C/h.

Application of transformers

To increase the temperature in concrete, you can use such an inexpensive and simple method as the PNSV heating wire.

The design of this cable includes two elements:

round single-wire conductor made of steel;
insulation, for which you can use PVC plastic or polyethylene.

If you need to heat a mixture of 40-80 m3, then it will be enough to install just one transformer substation. This method is used when the air temperature outside has reached -30 degrees. It is advisable to use transformers for heating monolithic structures. For 1 m of weight, a 60 m wire will be enough.

Which manufacturers of autoclaved aerated concrete exist are indicated in this

This manipulation is performed according to the following instructions:

A heating wire is laid inside the concrete. It is connected to the station or transformer terminals.
With the help of an electric current, the array begins to gain temperature, as a result of which it manages to harden.
Since the material has excellent thermal energy conductivity properties, heat begins to move at high speed throughout the entire mass.

Table 1 – Characteristics of PNSV brand wires

1	AC voltage, V	380
2	Length of cable section for voltage 220 V:
	– PNSV1.0 mm, m	80
	– PNSV1.2 mm, m	110
	– PNSV1.4 mm, m	140
3	Cable heat dissipation power:
	– for reinforced installations, W/l.m.	30-35
	– for non-reinforced installations, W/l.m.	35-40
4	Recommended supply voltage, V	55-100
5	Average core resistance value:
	– PNSV1.2 mm, Ohm/m	0,15
	– PNSV1.4 mm, Ohm/m	0,10
6	Method parameters:
	– Specific power, kW/m3	1,5-2,5
	– Wire consumption, lm/m3	50-60
	– Cycle of thermos aging of structures, days	2-3

The heating wire, which is laid inside the concrete, should heat the structure up to 80 degrees. Electrical heating occurs using transformer substations KPT TO-80. This installation is characterized by the presence of several low voltage stages. Thanks to this, it becomes possible to adjust the power of the heating cables, and also adjust it according to the changed air temperature.

Using the cable

Using this heating option does not require large amounts of electricity or additional equipment.

The whole process proceeds according to the following scheme:

The cable is being installed on the concrete base before the mortar is poured.
Secure everything using fasteners.
Be careful during cable installation and operation to ensure that its surface does not become damaged.
Connect the cable to the low-voltage electrical cabinet.

Antifreeze additives

With the addition of antifreeze additives, concrete is able to withstand the most aggressive precipitation. The components included in such a mixture can be very different, but the main role is assigned to antifreeze. This is a liquid that prevents water from freezing.

If it is necessary to cock reinforced concrete structures, the mixture must contain sodium nitrite and sodium format. The main feature of antifreeze mixtures remains the preservation of anti-corrosion and physico-chemical properties at low temperatures.

When constructing ready-mixed concrete or producing curbs, it is necessary to use a mixture that contains calcium chloride. This component allows you to achieve fast hardening speed and resistance to low temperatures.

The ideal antifreeze additive remains a chemical such as potash. It dissolves very quickly in water, and there is no corrosion. If you use potash when heating concrete in winter, you will be able to save on building materials.

If you use antifreeze additives, it is very important to adhere to all safety standards. For example, you should not use concrete with such components when the structure is under tension and monolithic chimneys are being erected.

SNiP

All installation and construction activities must be carried out in accordance with established standards. The concreting process in winter is no exception. Warming up of a concrete structure at low air temperatures occurs in accordance with the following documents:

SNiP 3.03.01-87 – Load-bearing and enclosing structures
SNiP 3.06.04-91 – Bridges and pipes

The video shows concrete heating in winter, technological map:

Despite the fact that the presented documentation only indirectly touches on the topic related to heating concrete, it contains certain sections in which there is a technology for pouring concrete mortar in the frosty season.

Timing

When calculating the heating of concrete, it is necessary to take into account factors such as the type of structure, the total heating area, the volume of concrete and electrical power.

During heating work with concrete, it is worth developing a technological map. It will include all the values of laboratory observations, as well as the heating time and hardening time of the material.

Calculation of concrete heating begins with the selection of a scheme. For example, the four-stage method is most often chosen. The first stage involves curing the material. After this, the temperature indicators are increased to a specific value, heating and cooling are carried out; the duration of holding before the start of the event is approximately 1-3 hours at a low temperature. After this, you can proceed to the calculation of heating, which is directly dependent on the speed and final temperature.

Throughout the entire process, it is worth monitoring the temperature, noting all results when it rises after 30-60 minutes, and when cooling, monitoring is carried out once per shift. If the mode is violated, it is necessary to maintain all parameters by turning off the current and increasing the voltage. In this case, the actual indicators and those obtained during the calculation may not coincide. After this, a graph of the dependence of time on strength is constructed, where the required value of time and heating temperature is indicated, and then the required value of strength is found.

The process of heating concrete is a very important event, without which the concrete structure will simply cease to gain strength in cold weather, resulting in a decrease in grade and further destruction. It is not difficult to carry out all these activities; you just need to determine which of the presented ones suits you best.

Warming up concrete is a mandatory procedure in low temperature conditions. It is necessary to ensure optimal conditions under which concrete can harden normally. Otherwise, the structure of the material is disrupted and it begins to lose its properties. It is dangerous to allow the mixture to freeze during the setting period.

Why do you need to warm up?

Warming up the concrete in winter is necessary so that the existing water in the solution does not turn into ice crystals. Otherwise, the pressure inside the pores of the cement will increase, which will lead to the destruction of the material that has already hardened. It will no longer meet the high strength requirements.

The need to heat the material is also due to other reasons related to ongoing processes in the solution:

when freezing, water increases in volume by 10-15%, which leads to the destruction of the edges of the pores, and the material becomes loose;
icing of reinforcement caused by exposure to low temperatures disrupts the metal-cement bond, which worsens the technical characteristics of the structure.

To prevent the solution from freezing, it is necessary to create a temperature at which the concrete will naturally harden. An increased temperature of the material during heating is also undesirable, since it leads to accelerated interaction between concrete and water, and more specifically to its evaporation.

Ways to warm up in winter

You can avoid freezing of the solution in the cold season using special equipment. All possible methods of heating the material are established in SNiP 3.03.01-87 (Load-bearing and enclosing structures, section 7.57) and SNiP 3.06.04-91 (Bridges and pipes, section 6.37). The main methods include: heating in the formwork, thermos, the use of electrodes, heating wires, infrared heaters, etc. Each method is unique and requires the use of different equipment.

Heating concrete with electrodes is the most common method. Electric current conductors are installed in different places of the poured mass. Current passing through an electrical circuit generates heat. This is how concrete is electrically heated.

There are several options for connecting electrodes to the concrete mixture. In each case, the connection diagram used is individual. When choosing it, it is taken into account that electrolysis in water and concrete solution is caused by direct current, and in the process of electrical heating it is recommended to use three-phase alternating current.

Important! When reinforcing concrete with metal or iron rods, using a network voltage of more than 127V is prohibited. The exception is certain areas for which projects have been specially developed.

Heating of concrete can be done using different types of electrodes:

strings - used for pouring of a large length (columns or piles);
rod - used for joints of structures of complex configurations;
strip - used to heat concrete from different sides of the structure;
plate - electrodes attached to the back side of the formwork are connected to different phases, due to this an electric field is formed.

Use of wire

To minimize time, a special wire is used for heating the concrete - PNSV. It is a steel core insulated in polyethylene or PVC.

When choosing this method, you cannot do without a transformer for heating the concrete. The essence of the method is that the equipment heats the wires, and the heat from them is transferred to the concrete composition. Due to the high thermal conductivity of the material, energy is quickly distributed throughout the array. One station can heat up to 80 m³ of concrete mixture. This method is used to heat monolithic structures in 30-degree frosts.

The main advantage of using wire for heating is the ability to adjust the temperature depending on weather conditions. The cable is capable of raising temperatures up to 80 ºС. A transformer for heating concrete must have several low voltage stages. This will allow you to regulate the power of the heating wires and adjust its value in accordance with changes in air temperature.

The need to use a transformer to heat the concrete significantly increases the cost of construction. TMO and TMTO equipment for heating concrete is expensive (90-120 thousand rubles), rent is 10-15% of the cost. There is no point in purchasing it for a one-time fill.

To warm up concrete in winter, you will need a technological map. It is developed by a power engineer for each individual project, although there are also standard samples of this document.

Based on the technological map, the number of transformer stations is calculated, their favorable location is determined, as well as the order of placement of the cable for heating the concrete. On average, processing 1 m³ of solution requires up to 60 meters of cable. To carry out a uniform load across the phases, it is necessary to test the wire.

Instructions for heating with heating wire

For effective heating, the heating wire must have a cross-section of at least 1.2 mm, and the operating current must be at least 12 A.

Electrical heating of concrete is carried out as follows:

the cable for heating the concrete is placed inside the structure in such a way that the conductors do not touch each other and do not extend beyond the edges of the concrete;
soldering cold ends to the heating wire and bringing them outside the heating zone;
checking the assembled electrical circuit with a megohmmeter;
supplying voltage to the assembled system and heating the structure.

This is a passive method, focused not on the transfer of thermal energy, but on its conservation. Its essence comes down to insulating a concrete structure from the outside using heat-insulating materials.

From an economic point of view, this method is the most profitable, since cheap sawdust can be used as thermal insulation materials. But insulating the structure is not always enough to create natural conditions for the mixture to harden. Additional use of other methods will be required.

Warming up with IR emitters

Infrared heating devices have low power consumption. They are directed to the heated area, and in the concrete structure the infrared rays are converted into heat.

The main advantage of the method is the ability to heat individual sections of the structure. However, with a thick concrete layer, heating is uneven, which can lead to a decrease in the strength of the structure.

IR emitters have found application in processing joints or creating thin-walled elements.

The method is based on the phenomenon of electromagnetic induction. The energy of the electromagnetic field is converted into thermal energy, which is transferred to the heated surface. This process takes place in steel formwork or on reinforcement.

Induction heating is only possible for closed loop designs. The reinforcement coefficient with iron or steel elements must be at least 0.5. To create an indicator, wrap the entire structure with insulated wire. An electric current passed through it creates an electromagnetic field that heats up all metal elements. From them heat is transferred to concrete.

The essence of the method comes down to passing steam through pipes pre-installed into the structure or between the walls of the formwork. If the temperature of concrete in a steam-saturated state during heating exceeds 70 ºС, then the material will gain the same strength in a few days as it did in 10-12 days.

Steam must be released 30 minutes before pouring the concrete mixture to warm up the structure.
This method is highly effective, but requires significant costs to implement.

How much does it cost to heat concrete?

The source of cost estimates is the technological map. To calculate how much electric heating costs, you need to know the following parameters: volume of concrete, material consumption and process duration.

The most economical methods are heating the mixture using the “thermos” method or using IR emitters using a small amount of electricity. As for the efficiency, these methods are lower than when heating with heating wires, electrodes or steam.

TYPICAL TECHNOLOGICAL CARD (TTK)

ELECTRODE HEATING OF STRUCTURES MADE OF MONOLITIC CONCRETE AND REINFORCED CONCRETE

1 AREA OF USE

1.3. The purpose of creating the presented TTK is to provide a recommended flow chart for concrete work in winter.

1.8. The use of TTK helps to improve the organization of production, increase labor productivity and its scientific organization, reduce costs, improve quality and reduce the duration of construction, safe performance of work, organize rhythmic work, rational use of labor resources and machines, as well as reduce the time required for the development of project planning and the unification of technological solutions.

1.9. The work performed sequentially during electrode heating of concrete and reinforced concrete structures in winter includes:

Determination of the cooling surface module;

Installation of string electrodes;

Electrical heating of the structure.

1.10. When electrically heating concrete and reinforced concrete structures using the electrode method, the main material used is string electrodes manufactured on the construction site from reinforcing steel of periodic profile A-III, with a diameter of 8-12 mm, a length of 2.5-3.5 m and rod electrodes made of reinforcing steel of periodic profile of grade A-III, with a diameter of 6-10 mm and a length of up to 1.0 m.

1.11. The work is carried out in winter and is carried out in three shifts. The working hours during a shift are:

where 0.828 is the coefficient of TP utilization by time during the shift (time associated with preparing the TP for work and conducting ETO - 15 minute breaks associated with the organization and technology of the production process).

1.12. Work should be performed in accordance with the requirements of the following regulatory documents:

SNiP 12-01-2004. Organization of construction;

SNiP 12-03-2001. Occupational safety in construction. Part 1. General requirements;

SNiP 12-04-2002. Occupational safety in construction. Part 2. Construction production;

SNiP 3.03.01-87. Load-bearing and enclosing structures;

GOST 7473-94. Concrete mixtures. Technical conditions.

2. TECHNOLOGY AND ORGANIZATION OF WORK