Pouring concrete in winter has its own difficulties. The main problem is the normal hardening of the solution, the water in which can freeze and it will not gain technological strength. Even if this does not happen, the low drying rate of the composition will make the work unprofitable. Warming up the concrete with a PNSV wire will help resolve this issue.

In winter, this is the most convenient and cheapest way to achieve the desired hardness of the material. It is permitted by the standards of SP 70.13330.2012, and can be used when performing any construction work. After the concrete hardens, the wire remains inside the structure, so the use of cheap PNSV provides an additional economic effect.

Warming up concrete in winter with a cable makes it possible to solve two main problems. At temperatures below zero, the water in the solution turns into ice crystals, and as a result, the cement hydration reaction not only slows down, it stops completely. It is known that when water freezes, it expands, destroying the bonds formed in the solution, so after increasing the temperature it will no longer gain the required strength.

The solution hardens at optimal speed and maintains characteristics at a temperature of about 20°C. As temperatures drop, especially below freezing, these processes slow down, even though hydration produces additional heat. In order to meet technical conditions, in winter it is impossible to do without heating the concrete with a PNSV wire or another cable intended for this purpose in situations where:

• sufficient thermal insulation of the monolith and formwork is not provided;
• the monolith is too massive, which makes it difficult to heat it evenly;
• low ambient temperature at which water in solution freezes.

Wire characteristics

The cable for heating concrete PNSV consists of a steel core with a cross-section from 0.6 to 4 mm², and a diameter from 1.2 mm to 3 mm. Some types are galvanized to reduce the impact of aggressive components in mortars. Additionally, it is covered with heat-resistant polyvinyl chloride (PVC) or polyester insulation; it is not afraid of kinks, abrasion, aggressive environments, is durable and has high resistivity.
The PNSV cable has the following technical characteristics:

• The resistivity is 0.15 Ohm/m;
• Stable operation in the temperature range from -60°C to +50°C;
• Up to 60 m of wire is consumed per 1 cubic meter of concrete;
• Can be used at temperatures down to -25°C;
• Installation at temperatures down to -15°C.

The cable is connected to the cold ends through an aluminum autorecloser wire. Power can be supplied via a three-phase 380 V network by connecting to a transformer. With proper calculation, the PNSV can also be connected to a 220-volt household network; the length should not be less than 120 m. An operating current of 14-16 A should flow through the system located in the concrete mass.

Heating technology and laying scheme

Before installing the concrete heating system in winter, formwork and reinforcement are installed. After this, the PNSV is laid out with an interval between the wires of 8 to 20 cm, depending on the outside temperature, wind and humidity. The wire is not stretched and is attached to the fittings with special clamps. Bends with a radius of less than 25 cm and overlaps of current-carrying conductors should not be allowed. The minimum distance between them should be 1.5 cm, this will help prevent a short circuit.

The most popular installation scheme for PNSV is a “snake”, reminiscent of a “warm floor” system. It provides heating of the maximum volume of concrete mass while saving heating cable. Before pouring the solution into the formwork, you must make sure that there is no ice in it, the temperature of the mixture is not lower than +5°C, and the installation of the connection diagram is carried out correctly, and the cold ends are brought out to a sufficient length.

The PNSV wire comes with instructions, which you need to read before heating the concrete. The connection is made through sections of busbars in two ways through a “triangle” or “star” circuit. In the first case, the system is divided into three parallel sections connected to the terminals of a three-phase step-down transformer. In the second, three identical wires are connected into one node, then three free contacts are similarly connected to the transformer. The power supply is installed no further than 25 m from the connection point, the heated area is surrounded by a fence.

The system is connected after the entire volume of mortar has been completely filled. The technology for heating concrete with a PNSV heating cable includes several stages:

1. Heating is carried out at a rate of no more than 10°C per hour, which ensures uniform heating of the entire volume.
2. Heating at a constant temperature continues until the concrete reaches half its technological strength. The temperature should not exceed 80°C, the optimum is 60°C.
3. Cooling of the concrete should occur at a rate of 5°C per hour, this will help avoid cracking of the mass and ensure its solidity.

If the technological requirements are met, the material will gain a grade of strength corresponding to its composition. At the end of the work, the PNSV remains in the thickness of the concrete and serves as an additional reinforcing element.

It should be noted that using a KDBS or VET cable is much easier, since they can be connected directly to a 220 V network through a panel or socket. They are divided into sections, which helps to avoid overload. But these cables are more expensive than PNSV, so they are less often used in the construction of large facilities.

Another popular technology is the use of formwork with heating elements and electrodes, when the reinforcement is inserted into the solution and connected to the network using a welding machine or another type of step-down transformer. This heating method does not require a special heating cable, but is more energy-consuming, since water in concrete acts as a conductor, and its resistance increases significantly during hardening.

Length calculation

To calculate the length of the PNSV wire for heating concrete, several main factors must be taken into account. The main criterion is the amount of heat supplied to the monolith for its normal hardening. It depends on the ambient temperature, humidity, the presence of thermal insulation, volume and shape of the structure.

Depending on the temperature, the cable laying pitch is determined with an average loop length of 28 to 36 m. At temperatures down to -5°C, the distance between the cores or pitch is 20 cm, with a decrease in temperature for every 5 degrees, it decreases by 4 cm, at - At 15°C it is 12 cm.

When calculating the length, it is important to know the power consumption of the PNSV heating wire. For the most popular diameter of 1.2 mm, it is equal to 0.15 Ohm/m; for wires with a large cross-section, the resistance below a diameter of 2 mm has a resistance of 0.044 Ohm/m, and 3 mm – 0.02 Ohm/m. The operating current in the core should be no more than 16 A, therefore the power consumption of one meter of PNSV with a diameter of 1.2 mm is equal to the square of the current and the resistivity and is 38.4 W. To calculate the total power, you need to multiply this figure by the length of the laid wire.

The voltage of the step-down transformer is calculated in a similar way. If 100 m of PNSV with a diameter of 1.2 mm are laid, then its total resistance will be 15 Ohms. Considering that the current is no more than 16 A, we find the operating voltage equal to the product of the current and the resistance; in this case it will be equal to 240 V.

The use of PNSV wire is one of the cheapest ways to heat concrete. But it is more suitable for use by professional builders, since its connection requires special knowledge and equipment. This cable can also be used at home if you correctly calculate the power consumption. The use of thermal insulation materials will help reduce costs when heating the solution; in this case, heating will occur faster, and the temperature decrease will occur more evenly, which will improve the quality of concrete.

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 for the development of project planning and 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

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 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.

public corporation

I APPROVED

General Director, Ph.D.

S. Yu. Jedlicka

ROUTING
FOR HEATING MONOLITHIC REINFORCED CONCRETE STRUCTURES
LIQUID FUEL HEAT GENERATORS

48-03 TK

Chief Engineer

A. B. Kolobov

Department head

B. I. Bychkovsky

The map contains organizational, technological and technical solutions for heating monolithic structures with liquid fuel heat generators, the use of which in the production of monolithic concrete and reinforced concrete work at subzero air temperatures should help speed up work, reduce labor costs and improve the quality of constructed structures in winter conditions.

The technological map shows the scope of application, organization and technology of work, requirements for quality and acceptance of work, calculation of labor costs, schedule of work, the need for material and technical resources, decisions on safety and labor protection and technical and economic indicators.

The initial data and design solutions for which the map was developed were taken taking into account the requirements of SNiP, as well as the conditions and features characteristic of construction in Moscow.

The technological map is intended for engineering and technical workers of construction and design organizations, as well as work producers, foremen and foremen involved in the production of monolithic concrete and reinforced concrete work at subzero air temperatures.

Employees of PKTIpromstroy OJSC participated in the adjustment of the technological map:

Savina O. A. - computer processing and graphics;

Chernykh V.V. - technological support;

Kholopov V.N. - checking the technological map;

Bychkovsky B.I. - technical management, proofreading and standard control;

Kolobov A.V. - general technical management of the development of technological maps;

Ph.D. Jedlicka S. Yu. - general management of the development of technological maps.

1 AREA OF USE

1.1 The essence of the use of liquid fuel heat generators is the use of thermal energy released by heat generators and directed to open or formwork surfaces of structures for their heat treatment during concreting in winter conditions.

1.2 The scope of application of heat generators includes:

Warming of frozen concrete and soil foundations, reinforcement, embedded metal parts and formwork, removal of snow and ice;

Intensification of concrete hardening of structures and structures erected in sliding or volumetric-adjustable formwork, floor slabs and coverings, vertical and inclined structures concreted in metal formwork;

Preliminary heating of the joint zone of prefabricated reinforced concrete structures and acceleration of hardening of concrete or mortar when sealing joints;

Acceleration of hardening of concrete or mortar during the enlarged assembly of large-sized reinforced concrete structures;

Creation of thermal protection of surfaces inaccessible for thermal insulation.

1.3 The technological map contains:

Instructions for preparing structures for concreting and requirements for the readiness of previous work and building structures;

Schemes for organizing the work area during work;

Methods and sequence of work, description of the process of installing heating devices;

Temperature conditions that provide the necessary strength gain;

Professional number and qualification composition of workers;

Labor cost calculation;

Work schedule.

1.4 The number and qualification composition of workers, work schedule, calculation of labor costs, as well as the need for the necessary resources are determined in relation to the heating of monolithic structures with a surface module MP from 10 to 14*, erected in large panel formwork, the section sizes of which are 3.0 × 6.0 m.

* The surface modulus of a concrete structure is determined by the ratio of the sum of the areas of the structure’s cooled surfaces to its volume and has the dimension “M-1”.

1.5 Calculation of heating of structures was carried out taking into account the following conditions:

Outside air temperature - 20 °C

Wind speed 5 m/s

Temperature of laid concrete 15 °C

Isothermal heating temperature 40 °C

Concrete heating rate 2.5 °C/hour

Warm-up time 10 hours

Strength of concrete by the time of cooling to 0 °C 70% R28

The formwork structure is a steel sheet 4 mm thick, insulated on the outside with 50 mm thick mineral wool slabs and covered with 3 mm thick plywood.

1.6 When linking this technological map to other structures that are covered by its scope of application, the calculation part is subject to clarification, as well as the calculation of labor costs, the work schedule and the need for material and technical resources, taking into account the heating conditions.

2 ORGANIZATION AND TECHNOLOGY OF WORK EXECUTION

2.1 Before starting work on heating monolithic structures with heat generators, the following preparatory operations are performed:

Perform thermotechnical calculations for heating walls and ceilings using liquid fuel heat generators;

Install formwork, reinforcing mesh and frames, having previously cleared them of debris, snow and ice;

Install thermal insulation 50 mm thick on the side surfaces of the walls;

Install heat generators in the work area and test their operation;

Fences are installed and alarms are installed according to the work area organization diagram shown in the figure;

Install a fire shield with carbon dioxide fire extinguishers, place safety and labor protection instructions in the work area;

Check temporary lighting of workplaces;

Provide the worker with the necessary tools and personal protective equipment;

They provide instructions.

1 - heat generator TA-16 on liquid fuel - 3 pcs.; 2 - inventory fence; 3 - fire shield; 4 - continuous tarpaulin covering over the entire area of ​​the opening

Figure 1 - Scheme of organizing the working area for heating walls and ceilings using liquid fuel heat generators.

2.2 In order to accelerate the strength gain of monolithic structures, the thermal energy of heat generators is used, the number of which for heating a particular room is determined by thermal engineering calculations. An example of thermal engineering calculations for heating walls and ceilings using liquid fuel heat generators is given below.

2.3 A schematic diagram of the installation of formwork in a room with a height of 2.7 m to be heated by heat generators is shown in the figure.

1 - metal structure of volumetric adjustable formwork; 2 - steel deck = 4 mm; 3 - polyethylene film; 4 - thermal insulation (mineral wool mats) - 50 mm thick; 5 - plywood 3 mm thick

Figure 2 - Schematic diagram of formwork installation

2.4 The formwork and reinforcement are heated by turning on heat generators. In this map, according to the calculation, three mobile heat generators “Thermobile” are used for heating concrete, the technical characteristics of which are given in the table.

A general view of the Thermobile heat generator is shown in the figure.

Table 1

Characteristics of Thermobile heat generators

Figure 3 - General view of the Thermobile heat generator

The specified heat generator allows you to automatically control the combustion process. In case of overheating, smoke or lack of fuel, the heat generator turns off automatically. The heat generator is equipped with a thermostat that automatically maintains the set temperature in the room. Kerosene or diesel fuel can be used as fuel without additional settings. The average operating time on one gas station is 8 - 10 hours.

2.5 The necessary initial data for heating calculations include:

Type of construction - wall 200 mm thick

ceiling thickness 140 mm

Type of formwork - large-panel

The formwork structure is metal on the inside, not insulated, on the outside it is insulated with mineral wool mats 50 mm thick with a protective cover made of plywood 3 mm thick. Heat transfer coefficient of formwork Cop= 3.2 W/m2 °C

The construction of hydro- and thermal insulation is polyethylene film, mineral wool mats 50 mm thick. Heat transfer coefficient KP= 3 W/m2 °C

Outside air temperature - minus 20 °C

Wind speed - 5 m/sec

Initial concrete temperature - tbn= 15 °C

Isothermal heating temperature - tiz= 40 °C

The heating rate of the concrete mixture is 2.5 °C/hour

Warm-up time - 10 hours

Strength of concrete by the time of cooling to 0 °C - 70% R28

First, we determine the heating mode of the structure until the concrete reaches 70% R28.

During the heating period from 15 °C to 40 °C at an average concrete temperature of 27.5 °C in 10 hours, concrete will gain 15% R28.

The cooling time from 40 °C isothermal holding to 0 °C is determined by the formula:

(1)

Where WITH- specific heat capacity of concrete, kJ/kg °C (0.84)

g- volumetric mass of concrete, kg/m3 (2400)

MP- surface module, m-1 (11)

3.6 - conversion factor to hours

TO- heat transfer coefficient, W/m2 °C (11)

tisotherm- isothermal holding temperature, °C

toctiv.- temperature to which concrete cools, °C

tb.cp.- average concrete cooling temperature, °C

tn.v.- outside air temperature, °C

hours.

Considering that during cooling the concrete will gain insignificant strength, we assume that by the end of isothermal heating the concrete should gain 70% R28.

Based on the strength gain curve of the graphs, we determine that at an isothermal heating temperature of 40 °C, the remaining 55% of the strength of the concrete will gain in 54 hours. Thus, we get a heating time of 10 hours, an isothermal heating time of 54 hours and a cooling time of 4.6 hours.

The power required to heat the concrete mixture from 15 °C to 40 °C is determined by the formula

(2)

Where WITH- specific heat capacity of the concrete mixture, kJ/kg °C

g- volumetric mass of concrete, kg/m3

V- volume of concrete, m3

tiz.- isothermal heating temperature, °C

tb.n.- initial concrete temperature, °C

t- warm-up time, hour

kW

The power required to compensate for heat loss through the formwork, thermal protection and through the opening covered with a tarpaulin is determined by the formula

Where TO 1,2,3 - heat transfer coefficient of enclosing structures, W/m2 °C

S- cooling area

a- coefficient taking into account wind speed

tiz.- isothermal heating temperature, °C (40 °C)

tn.- outside air temperature, °C (minus 20 °C)

tvn.- indoor air temperature, °C (50 °C)

The total power requirement is 27.9 kW + 15.3 kW = 43.2 kW.

To heat concrete, we use three Thermobile 16 A heat generators with a capacity of 15.5 thousand kcal each.

The total power of all heat generators is 15.5 × 3 × 1.16 = 53.94 kW, which satisfies the total power requirement.

Thermal power consumption for heating concrete before purchasing 70% R28 will be

W= (3 × 15.5 × 1.16) × 10 + (2 × 15.5 × 1.16) × 54 = 2481.2 kWh

The specific thermal power consumption for heating 1 m3 of concrete will be

2481.2: 10.6 = 234.1 kWh

Fuel consumption will be

T= 1.8 × 3 × 10 + 1.8 × 2 × 54 = 248.4 l or 24.8 l/m3

2.6 Preparation of the base and laying of the concrete mixture are carried out taking into account the following requirements:

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;

The concrete mixture is laid continuously, without transferring, using means that ensure minimal cooling of the mixture during its supply. The temperature of the concrete mixture placed in the formwork should not be lower than plus 15 °C.

2.8 In case of interruptions in concreting, the concrete surface is covered and insulated, and, if necessary, heated.

2.9 Heating of concrete begins after laying and compacting the concrete mixture during the construction of monolithic walls and ceilings and devices for overlapping waterproofing and thermal insulation. When the structure begins to be heated, the open opening is covered with a tarpaulin.

2.12 The heating temperature of the concrete mixture is regulated by a thermostat equipped in the heat generator.

2.13 During heating of concrete, it is necessary to monitor the operating status of heat generators. If a malfunction is detected, the malfunction must be repaired immediately.

2.14 The cooling rate of concrete in accordance with the temperature schedule is 8 °C/h. For a design with surface module MP= 10 - 14 cooling rate is allowed no more than 10 °C/h. The outside air temperature is measured twice a shift, and the measurement results are recorded in the work log.

1 - monolithic structure; 2 - insulation; 3 - pencil case made of thin-walled steel tube; 4 - industrial oil; 5 - temperature sensor

Figure 5 - Installation of a temperature sensor in a heated structure

2.15 The strength of concrete is checked according to the actual temperature conditions. Compliance with the temperature schedule given in paragraph 1 allows you to obtain the required strength. After stripping, the strength of concrete at a positive temperature is recommended to be determined using a hammer designed by the Mosstroy Research Institute, ultrasonic testing, or drilling and testing cores. The strength gain of concrete at different temperatures is determined by the graph presented in the figure.

a, c - for class B25 concrete based on Portland cement with an activity of 400 - 500;

b, d - for concrete class B25 on Portland slag cement with an activity of 300 - 400

Figure 6 - Strength gain curves for concrete at different temperatures

2.16 Below is an example of determining the strength of concrete.

Determine the strength of concrete at a temperature rise rate of 10 °C per hour, an isothermal heating temperature of 70 °C, its duration of 12 hours and cooling at a rate of 5 °C per hour to a final temperature of 6 °C. Initial concrete temperature tn.b.= 10 °C.

1. Determine the duration of the temperature rise and the average temperature rise:

Duration of temperature rise = 6 hours

at average temperature = 40 °C

On the abscissa axis we plot the heating duration (6 hours) of point “A” according to the figure and draw a perpendicular until it intersects with the strength curve at 40 °C (point “B”).

The strength value during the temperature rise is determined by the projection of point “B” onto the ordinate axis (point “B”) and is 15%.

Figure 7 - Example of determining the strength of concrete

To determine the increase in strength during isothermal heating for 12 hours at a temperature of 70 ° C, from point “L” on the strength curve at 70 ° C, we lower the perpendicular to the abscissa axis (point “M”). From point “M” we set aside 12 hours (point “H”). Restoring the perpendicular from point “H”, we obtain point “K” on the strength curve at 70 °C. Projecting point “K” onto the ordinate axis, we obtain point “Z”. The segment “VZ” shows the tensile strength for 12 hours at a temperature of 70 ° C and is 46% R28.

To determine the increase in strength during a cooling period of 13 hours at an average temperature of 38 °C, from point “Z” we draw a straight line until it intersects with the strength curve at 38 °C and we obtain point “G”. From point “G” we lower the perpendicular to the abscissa axis and get point “E”, from which we set aside 13 hours and get point “D”. From point “D” we restore the perpendicular until it intersects with the strength gain curve at a temperature of 38 °C (point “D”). Projecting point “G” onto the ordinate axis, we obtain point “I”. The segment “ZI” gives us the value of the increase in strength during cooling of 9% R28.

Over the entire heat treatment cycle of 31 hours (6 + 12 + 13), the concrete acquires a strength of 15 + 46 + 9 = 70% R28.

For each specific concrete composition, the construction laboratory must clarify the optimal curing regime using prototype cubes.

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

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

20 °C for monolithic structures with MP < 5;

30 °C for monolithic structures with MP ≥ 5.

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

2.19 Work on thermal insulation of the heated surface, placement of heat generators and heating of concrete is performed by a team of three people, the distribution of operations between which for heating the walls and ceilings is presented in the table.

table 2

Distribution of operations by performers

2.20 Operations for concreting, thermal insulation and heating of monolithic structures are carried out in the following sequence:

The engine operator installs heat generators, fills them with fuel, and starts the heat generators;

Concrete workers lay concrete mixtures and cover exposed concrete surfaces with waterproofing and thermal insulation.

Before starting the heat generators, the section opening must be covered with a tarpaulin. The heat generator is put into operation only after all safety and labor protection requirements have been met.

In order to save fuel during work, it is recommended:

When determining the means and duration of transportation of the concrete mixture, exclude the possibility of its cooling more than the value established by the technical calculation;

Use concrete of higher relative strength with shorter heating duration;

Use the maximum permissible temperature for heating concrete, reduce the duration of heating by taking into account the increase in strength during cooling;

Arrange thermal insulation of the surface of concrete and formwork exposed to cooling;

Observe the thermotechnical mode of heating parameters;

Use chemical additives to shorten the warm-up time.

3 REQUIREMENTS FOR QUALITY AND ACCEPTANCE OF WORK

3.1 Quality control of heating of monolithic structures at negative air temperatures using heat generators is carried out in accordance with the requirements of SNiP 3.01.01-85 * “Organization of construction production” and SNiP 3.03.01-87 “Load-bearing and enclosing structures”.

3.2 Production control of heating quality is carried out by foremen and foremen of construction organizations.

3.3 Production control includes incoming control of equipment, operating materials, concrete mix and structures prepared for concreting, operational control of individual production operations and acceptance control of the required quality of a monolithic structure as a result of heating concrete using a heat generator.

3.4 During the incoming inspection of equipment, operating materials, concrete mix and prepared base, their compliance with regulatory and design requirements, as well as the presence and content of passports, certificates, acts for hidden work and other accompanying documents are checked by external inspection. Based on the results of the incoming inspection, the “Logbook of incoming accounting and quality control of received parts, materials, structures and equipment” must be filled out.

3.5 During operational control, compliance with the composition of the preparatory operations, the technology for setting up heat generators, laying concrete in the formwork structure in accordance with the requirements of working drawings, norms, rules and standards, the heating process, and the temperature in accordance with the calculated data are checked. The results of operational control are recorded in the work log.

The main documents for operational control are the technological map and the regulatory documents specified in the map, a list of operations controlled by the work manufacturer (foreman), data on the composition, timing and methods of control, the required strength indicators of monolithic walls and ceilings as a result of heating.

3.6 During acceptance inspection, the strength and geometric parameters of the walls and ceilings are checked as a result of heating the concrete by heat generators.

3.7 Hidden work is subject to inspection with the drawing up of reports in the prescribed form. It is prohibited to carry out subsequent work in the absence of inspection reports for previous hidden work.

3.8 The results of operational and acceptance control are recorded in the work log. The main documents for operational and acceptance control are this flow chart, the regulatory documents specified in it, as well as lists of operations and processes controlled by the foreman or foreman, data on the composition, timing and methods of control set out in the table.

Table 3

Composition and content of production quality control

	Foreman or foreman
Operations subject to control	Operations during incoming inspection	Preparatory operations		Operations during concreting of structures			Operations during acceptance control
Composition of control	Checking the performance of heat generators	Installation of protective fencing and lighting at the work site	Cleaning the base of the formwork, reinforcement from snow and ice. Insulation of the structure	Laying concrete in the construction of monolithic walls and ceilings	Concrete temperature control	Concrete strength control	Compliance of finished monolithic walls and ceilings with project requirements
Control methods	Visual and instrumental inspection			Visual and instrument			Visual-instrumental
Control time	Before concreting begins		Before and after concreting	During the concreting, heating and curing process			After heating
Who is involved in control	Construction company mechanic	Master, foreman		Laboratory			Laboratory, technical supervision

3.9 The temperature of heated concrete is controlled using technical thermometers or remotely using a temperature sensor installed in the well. The number of temperature measurement points is set on average at the rate of at least one point per 10 m2 of concrete surface. The temperature of the concrete is measured during the heating process at least every two hours.

3.10 The rate of temperature rise during heat treatment and the rate of concrete cooling at the end of heat treatment of monolithic structures should not exceed 15 °C and 10 °C per hour, respectively.

3.11 The strength of a monolithic structure is controlled according to the actual temperature conditions. The strength of concrete at the end of heating and cooling, which should be 70% R28, is achieved subject to compliance with the parameters of the schedule given in paragraph.

The strength of concrete as a result of heating is determined using a hammer designed by the Mosstroy Research Institute, using an ultrasonic method, or by drilling cores and testing.

4 OCCUPATIONAL SAFETY, ENVIRONMENTAL AND FIRE SAFETY REQUIREMENTS

4.1 When concreting structures and operating heat generators, the rules for safe work must be observed in accordance with SNiP 12-03-2001.

4.2 Installation sites for heat generators must be provided with fire-fighting equipment and inventory. Persons engaged in construction and installation work must be trained in safe methods of conducting work and obtaining the appropriate certificates, as well as the ability to provide first aid in case of injury or burns.

4.3 The construction and installation organization must have an engineering and technical worker responsible for labor protection and fire safety, safe operation of equipment, a certified motor mechanic trained in accordance with GOST 12.0.004-90.

4.4 Fuel for refueling the heat generator must be stored in a separate room equipped with primary fire extinguishing equipment.

4.5 Refueling is carried out only with the engines turned off and always cooled down. Only persons responsible for the operation of heat generators (motor operators) perform refueling.

4.6 During the entire period of operation of heat generators, safety signs in accordance with GOST R 12.4.026-2001 must be installed on construction sites. Refueling sites at night should be illuminated only by electric lamps or floodlights installed no closer than 5 m from the refueling site.

4.7 Technical personnel who heat concrete must undergo training at the Training Center and have their knowledge tested by a safety qualification commission and receive the appropriate certificates.

4.8 The area where heating is carried out is fenced. Warning posters, safety and labor protection rules, and fire-fighting equipment are placed in a prominent place. At night, the fence of the zone is illuminated, for which red light bulbs with a voltage of no more than 42 V are installed on it. A temporary lighting project is developed by a specialized organization at the request of the contractor.

The concrete heating area must be constantly under the supervision of an on-duty mechanic.

Access of unauthorized persons to the work area;

Place flammable materials near heated structures.

4.10 When carrying out work on heating monolithic structures with liquid fuel heat generators, it is necessary to strictly follow the safety and labor protection requirements in accordance with:

Table 4

List of requirements for machines, mechanisms, tools, materials

	Name				Technical specifications
	Heat generator	"Thermobile" TA16			Power, kcal/hour 16000 Distributor - small state enterprise "ETEKA"
	Technical thermometers				Measurement limit 140 °C
	Inventory mesh fencing				h= 1.1 m
	Polyethylene film				Thickness, mm 0.1 Width, m 1.4
	Mineral wool mats
	Fire shield				With carbon dioxide fire extinguisher
	Spotlight				Power, W 1000
	Concrete mix	According to the project
	Signal lights				Voltage, V 42
	Set of safety and labor protection signs

6 TECHNICAL AND ECONOMIC INDICATORS

6.1 Technical and economic indicators are given for the structure to be concreted and for 1 m3 of concrete indicated in the calculation.

6.2 Labor costs for heating monolithic structures with heat generators are calculated according to the “Unified Standards and Prices for Construction, Installation and Repair Work”, introduced in 1987 and are presented in table.

The calculation of labor costs was compiled for heating monolithic structures of walls and ceilings erected in large-panel formwork. Walls 200 mm thick, 2.7 m high. Floors 140 mm thick with plan dimensions 3 × 6 m. Total volume of concrete 10.6 m3.

Table 5

Labor cost calculation

	Name of works		Scope of work	Standard time		Labor costs
				workers, person-hours		workers, person-hours	machinists, man-hours, (machine work, machine-hours)
Experienced data	Heat generator installation
Experienced data from TsNIIOMTP	Installation of mesh fencing, safety posters, warning lights
E4-1-54 No. 10 (will apply)	Covering the opening with a tarpaulin
	Pre-heating of reinforcement and formwork
E4-1-49V No. 1v	Concreting walls
E4-1-49B No. 10	Concreting the floor
	Hydro- and thermal insulation device
Tariff and qualification guide	Heating of concrete mixture (including isothermal heating)
	Removing thermal insulation
E4-1-54 No. 12 (will apply)	Removing the shelter tarpaulin from the opening
Experienced data	Dismantling of heat generators

6.3 The duration of work for heating structures with heat generators is determined by the work schedule according to Table 6 78.9

Fuel consumption:

Per 1 m3 of concrete

Warm-up duration

Warm-up speed

Duration of isothermal exposure

"Load-bearing and enclosing structures." Occupational safety in construction. Industry standard instructions on labor protection.

8 Guide to electrical heat treatment of concrete. Research Institute for Reinforced Concrete Construction of the USSR State Construction Committee. Moscow, Stroyizdat, 1974

9 Guidelines for the production of concrete work in winter conditions, regions of the Far East, Siberia and the Far North. TsNIIOMTP Gosstroy USSR, Moscow, Stroyizdat, 1982

ENTERED INTO EFFECT by Order of the General Plan Development Department No. 6 of 04/07/98

annotation

The technological map for electrode heating of monolithic concrete structures at subzero air temperatures was developed by OJSC PKTIpromstroy in accordance with the minutes of the seminar-meeting “Modern winter concreting technologies”, approved by the First Deputy Prime Minister of the Moscow Government V.I. Resin, and the technical specifications for the development of a set of technological maps for the production of monolithic concrete works at subzero air temperatures, issued by the Moscow General Plan Development Department.

The map contains organizational, technological and technical solutions for electrode heating of monolithic concrete structures, the use of which should help speed up work, reduce labor costs and improve the quality of erected structures in winter conditions.

The technological map shows the scope of application, organization and technology of work, requirements for quality and acceptance of work, calculation of labor costs, work schedule, need for material and technical resources, safety decisions and technical and economic indicators.

The initial data and design solutions for which the map was developed were taken taking into account the requirements of SNiP, as well as the conditions and features characteristic of construction in Moscow.

The technological map is intended for engineering and technical workers of construction and design organizations, as well as work producers, foremen and foremen involved in the production of concrete work.

The technological map was developed by:

Yu.A.Yarymov - Ch. project engineer, work manager, I.Yu. Tomova - responsible executor, A.D. Myagkov, Ph.D. - responsible executor from TsNIIOMTP, V.N. Kholopov, T.A. Grigorieva, L.V. Larionova, I.B. Orlovskaya, E.S. Nechaeva - executors.

V.V.Shakhparonov, Ph.D. - scientific and methodological guidance and editing,

S.Yu.Jedlichka, Ph.D. - general management of the development of a set of technological maps.

1 area of ​​use

1.1. The scope of application of electrode heating of monolithic structures in accordance with the “Guide to the Electrical Heat Treatment of Concrete” (NIIZhB, Stroyizdat, 1974) is monolithic concrete and lightly reinforced structures. The use of this method is most effective for foundations, columns, walls and partitions, flat floors, and concrete preparations for floors.

Depending on the adopted arrangement and connection of electrodes, electrode heating is divided into through, peripheral, and using reinforcement as electrodes.

1.2. The essence of electrode heating is that heat is released directly in the concrete when an electric current is passed through it.

1.3. The technological map contains:

Electrode heating circuits;

Instructions for preparing structures for concreting, heating and requirements for the readiness of previous work and building structures;

Scheme of organizing the work area during the work;

Methods and sequence of work, description of installation and connection of electrical equipment and heating of concrete;

Electrical heating parameters;

Professional and numerical qualification composition of workers;

Work schedule and labor cost calculation;

Instructions for quality control and acceptance of work;

Safety solutions;

The need for the necessary material and technical resources, electrical equipment and operating materials;

Technical and economic indicators.

1.4. The technological map considers electrode through heating of a monolithic foundation with a volume of 3.16 m, plan dimensions of 1800x1800 mm and a height of 1200 mm using metal formwork.

1.5. The heating calculation was made taking into account the outside air temperature of -20 °C, the use of hydro- and thermal insulation in the form of polyethylene film and mineral wool mats 50 mm thick, metal formwork insulated with mineral wool mats 50 mm thick and protected by plywood 3 mm thick, electrical resistivity of the concrete mixture at the beginning of warming up 9 Ohm+..*m and the strength of concrete by the time it cools to 0 °C is 50%.

________________

* Defect of the original. - Database manufacturer's note.

1.6. The number and qualification composition of workers, work schedule and calculation of labor costs, as well as the requirements for the necessary material and technical resources and technical and economic indicators were determined based on the calculation of the heating of six foundations located on one part of the working area.

1.7. Electrode heating of monolithic structures can be combined with other methods of intensifying concrete hardening, for example, preheating the concrete mixture, using various chemical additives.

The use of antifreeze additives containing urea is not allowed due to the decomposition of urea at temperatures above 40 °C. The use of potash as an anti-frost additive is not permitted due to the fact that heated concrete with this additive has a significant (more than 30%) lack of strength and is characterized by reduced frost resistance and water resistance.

1.8. Linking this technological map to other designs and conditions of work at subzero air temperatures requires changes to the work schedule, calculation of labor costs, the need for material and technical resources and electrical heating parameters.

In modern conditions, there are many technologies that make it possible to continue the construction process even in winter. If the temperature drops, it is necessary to maintain a certain level of heating of the concrete mixture. In this case, the construction of houses and various objects does not stop for a minute.

The main condition for carrying out such work is to maintain a technological minimum at which the solution will not freeze. Electric heating of concrete is a factor that ensures compliance with technological standards even in winter. This process is quite complicated. But nevertheless, it is actively used everywhere at various construction sites.

Electric heating

Electrical heating of concrete is a rather complex and expensive process. However, to prevent the influence of low temperatures on the hardening cement mixture, it needs to provide a number of conditions. In winter, cement hardens unevenly. To prevent such deviation from the norm, electric heating technology should be used. It promotes a constant process of hardening of the mixture over the entire area.

Concrete is able to harden evenly at a temperature that will be close to +20 ºС. Forced electric heating is becoming an effective tool in the preparation of mortars.

Most often, electric heating technology is used for such purposes. If simply insulating the object is not enough, this alternative can solve the problem of unevenly hardening concrete.

Construction companies can choose from several approaches. For example, electrical heating can be carried out using a conductor such as a PNSV cable, or using electrodes. Also, some companies resort to the principle of heating the formwork itself. Currently, an induction approach or infrared rays can also be used for similar purposes.

Regardless of which method the management chooses, the heated object must be insulated. Otherwise, it will be impossible to achieve uniform heating.

Warming up with electrodes

The most popular method of heating concrete is the use of electrodes. This method is relatively inexpensive, because there is no need to purchase expensive equipment and devices (for example, wire type PNSV 1.2; 2; 3, etc.). The technology for its implementation also does not present any great difficulties.

The fundamental principle of the presented technology is the physical properties and characteristics of electric current. As it passes through concrete, it releases some thermal energy.

When using this technology, you should not apply voltage to the electrode system above 127 V if there is a metal structure (frame) inside the product. The instructions for electrical heating of concrete in monolithic structures allow the use of a current of 220 V or 380 V. However, it is not recommended to use a higher voltage.

The heating process is carried out using alternating current. If direct current is involved in this process, it passes through the water in solution and forms electrolysis. This process of chemical decomposition of water will prevent it from performing the functions that the substance has during the hardening process.

Types of Electrolytes

Electrical heating of concrete in winter can be carried out using one of the main ones. They can be string, rod or made in the form of a plate.

Rod electrolytes are installed in concrete at a short distance from each other. To create the presented product, scientists use metal reinforcement. Its diameter can range from 8 to 12 mm. The rods are connected to different phases. The presented devices are especially indispensable in the presence of complex structures.

Electrolytes, which are in the form of plates, are characterized by a fairly simple connection diagram. Their devices must be located on opposite sides of the formwork. These plates are connected to different phases. The current passing between them will heat the concrete. The plates can be wide or narrow.

String electrodes are necessary in the manufacture of other elongated products. After installation, both ends of the material are connected to different phases. This is how heating occurs.

Heating with PNSV cable

Electrical heating of concrete using PNSV wire, which will be discussed a little further, is considered one of the most effective technologies. In this case, the heater is a wire, not a concrete mass.

When laying the presented wire in concrete, it is possible to evenly heat the concrete, ensuring its quality when drying. The advantage of such a system is the predictability of the operating period. For high-quality heating of concrete in conditions of decreasing temperature, it is very important that it rises smoothly and evenly over the entire area of ​​the cement mortar.

The abbreviation PNVS means that the conductor has a steel core, which is packaged in PVC insulation. The cross-section of the wire when carrying out the presented procedure is selected in a certain way (PNSV 1,2; 2; 3). This characteristic is taken into account when calculating the amount of wire per 1 cubic meter of cement mixture.

The technology for heating concrete with wire is relatively simple. Electrical communications are allowed along the reinforcement frame. The wire should be secured in accordance with the manufacturer's recommendations. In this case, when feeding the mixture into the trench, formwork or mixture, the conductor will not be damaged by the pouring and operation of the hardened substance.

The wire should not touch the ground when laid out. After pouring, it is completely immersed in the concrete environment. The length of the wire will be influenced by its thickness, sub-zero temperatures in this climate zone, and resistance. The supplied voltage will be 50 V.

Cable application method

Electrical heating of concrete using PNSV wire, the technological map of which involves placing the product in a container immediately before pouring, is considered a reliable system. The wire must have a certain length (depending on its operating conditions). Due to good heating, the heat is smoothly distributed throughout the entire thickness of the material. Thanks to this feature, it is possible to increase the temperature of the concrete mixture to 40 ºС, and sometimes higher.

The PNSV cable can be powered into a network whose electricity is supplied by either 80/86. They have several levels of reduced voltage. One substation of the presented type is capable of heating up to 30 m³ of material.

To increase the temperature of the solution, it is necessary to spend about 60 m of PNSV 1.2 wire per 1 m³. In this case, the ambient temperature can be down to -30 ºС. Heating methods can be combined. This depends on the massiveness of the structure, weather conditions, and specified strength indicators. Also an important factor for creating a combination of methods is the availability of resources at the construction site.

If concrete can gain the required strength, it can resist destruction due to low temperatures.

Other wired heating options

The technology for heating concrete with a PNSV cable is effective provided that all instructions and requirements of the manufacturer are followed. If the wire extends beyond the concrete, it is likely to overheat and fail. Also, the wire should not touch the formwork or the ground.

The length of the wire shown will depend on the conditions in which the wire is used. They require the operation of a transformer to operate. If, using PNSV wire, the use of such a system is not very convenient, there are other types of conductor products.

There are cables that do not require power supply to operate. This makes it possible to save a little money on servicing the presented system. Ordinary wire has a wide range of applications. However, the PNSV wire, which was discussed above, has wider capabilities and scope of application.

Scheme of using a heat gun

Heating concrete with wire is considered one of the newest and most effective technologies. However, just recently no one knew about it. Therefore, a rather expensive but simple method was used. A shelter was built above the surface of the cement. For this method, the concrete base had to have a small area.

Heat guns were brought into the constructed tent. They pumped up the required temperature. This method was not without certain disadvantages. It is considered one of the most labor-intensive. Workers need to erect a tent and then monitor the operation of the equipment.

If we compare heating of concrete with wire and the method of using thermal units, it becomes clear that the old approach will require more costs. Most often, certain equipment of autonomous type is purchased. They run on diesel fuel. If there is no access to a regular fixed network on the site, this option will be the most advantageous.

Thermomat

The heating wire or can serve as the basis for creating special thermomats. They are quite effective. The only condition is a flat surface of the concrete base. Some types of heaters presented can work as a winding on columns, elongated blocks, poles, etc.

When using matte technology, a plasticizer is added to the solution itself, which speeds up the drying process. At the same time, they can also prevent the formation of water crystallization.

When using the presented technologies, it should be remembered that there are special documents regulating the electrical heating of concrete in winter. SNiP draws the attention of construction organizations to the need to constantly monitor the temperature indicators of this substance.

The cement mixture should not overheat above +50 ºС. This is just as unacceptable for its production technology as severe frosts. In this case, the rate of cooling and heating should not be faster than 10 ºС per hour. To avoid mistakes, the calculation of electrical heating of concrete is carried out in accordance with current standards and sanitary requirements.

Infrared mats can replace cable counterparts. They can be used for wrapping figured columns and other elongated objects. This approach is characterized by low energy consumption. Concrete structures exposed to infrared rays begin to quickly lose moisture. To prevent this from happening, you need to cover the surfaces with regular plastic film.

Heated formwork

Electric heating of concrete in winter can be carried out immediately in the formwork. This is one of the new ways that is very effective. Heating elements are installed in the formwork panels. If one or more of them fails, the faulty equipment is dismantled. It is replaced with a new one.

Equipping the mold in which concrete hardens with infrared heaters has become one of the successful decisions made by managers of construction companies. This system is able to provide the required conditions to the concrete product located in the formwork, even at a temperature of -25 ºС.

In addition to high efficiency, the presented systems have a high efficiency rate. Very little time is spent preparing for heating. This is extremely important in severe frost conditions. The profitability of heating formwork is determined to be higher than that of conventional wired systems. They can be used repeatedly.

However, the cost of this type of electric heating is quite high. It is considered unprofitable if you need to heat a building of non-standard dimensions.

Principle of induction and infrared heating

In the above systems of thermomats and heated formwork, the principle of infrared heating can be used. To better understand the operating principle of these systems, it is necessary to delve into the question of what infrared waves are.

Electrical heating of concrete using the presented technology is based on the ability of sunlight to heat opaque, dark objects. After heating the surface of the substance, the heat is evenly distributed throughout its entire volume. If the concrete structure is wrapped in a transparent film in this case, when heated it will transmit rays into the concrete. In this case, heat will be retained inside the material.

The advantage of infrared systems is that there are no requirements for the use of transformers. Experts say that the disadvantage is the inability of the presented heating to evenly distribute heat throughout the entire structure. Therefore, it is used only for relatively thin products.

The induction approach in modern construction is used quite rarely. It is more suitable for structures such as purlins and beams. This is influenced by the complexity of the presented equipment.

The principle of induction heating is based on the fact that a wire is wound around a steel rod. It has a layer of insulation. When an electric current is connected, the system produces an inductive disturbance. This is how the concrete mixture is heated.

Having considered the electrical heating of concrete, as well as its basic methods and technologies, we can conclude that it is advisable to use one or another method in production conditions. Depending on the type of manufactured structures and production conditions, technologists choose the appropriate option. A meticulous approach to the technology of hardening the concrete mixture allows us to produce high-quality products, screeds, foundations, etc. Every builder should know the rules for working with cement in winter.