PUBLIC CORPORATION

DESIGN AND TECHNOLOGY
INDUSTRIAL CONSTRUCTION INSTITUTE

OJSC PKTIpromstroy

ROUTING
FOR ELECTRODE HEATING
STRUCTURES FROM SOLID CONCRETE

Put into effect by the Order of the General Plan Development Department
No. 6 dated 04/07/98

Moscow - 1997

ANNOTATION

The technological map for electrode heating of monolithic concrete structures at negative air temperatures was developed by OJSC PKTIpromstroy in accordance with the protocol of the seminar-meeting " Modern technologies winter concreting”, approved by the First Deputy Prime Minister of the Government of Moscow V.I. Resin, and the terms of reference for the development of a set of technological maps for the production of monolithic concrete work at negative air temperatures, issued by the General Plan Development Department of Moscow. 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 structures being built in winter conditions. The technological map shows the scope, organization and technology of work, the requirement for quality and acceptance of work, labor cost calculation, work schedule, the need for material and technical resources, safety solutions and technical and economic indicators. Initial data and Constructive decisions, for which the map was developed, were adopted taking into account the requirements of SNiP, as well as the conditions and features typical for construction in Moscow. The technological map is intended for engineering and technical workers of construction and design organizations, as well as foremen, foremen and foremen associated with 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. Nechaev - performers. V.V. Shakhparonov, Ph.D. - scientific and methodological guidance and editing, S.Yu. Edlichka, Ph.D. - general management of the development of a set of technological maps.

1 AREA OF USE

1.1. Scope of electrode heating monolithic structures in accordance with the "Guidelines for the electrical heat treatment of concrete" (NIIZhB, Stroyizdat, 1974) are monolithic concrete and low-reinforced structures. The application of this method is most effective for foundations, columns, walls and partitions, flat ceilings, concrete preparations for floors. Depending on the adopted arrangement and connection of electrodes, electrode heating is divided into through, peripheral and using fittings as electrodes. 1.2. The essence of electrode heating is that heat is released directly in concrete when an electric current is passed through it. 1.3. The technological map contains: - schemes of electrode heating; - instructions for preparing structures for concreting, heating and requirements for the readiness of previous work and building structures; - scheme of organization of the working area for the duration of the work; - methods and sequence of work, a description of the installation and connection of electrical equipment and the implementation of concrete heating; - electrical parameters of heating; - professional and numerical-qualification structure of workers; - work schedule and labor cost calculation; - instruction on quality control and acceptance of works; - safety solutions; - the need for the necessary material and technical resources, electrical equipment and operating materials; - recommendations for energy saving; - technical and economic indicators. 1.4. The technological map considers electrode through heating of a monolithic foundation with a volume of 3.16 m 3 with plan dimensions of 1800 ´ 1800 mm and a height of 1200 mm using metal formwork. 1.5. The calculation of warm-up was made taking into account the outdoor temperature of -20 °C, the use of hydro- and thermal insulation in the form of a 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 concrete mix at the beginning of heating 9 Ohm × m and concrete strength by the time of cooling to 0 ° C - 50% R 28. 1.6. The number and qualification composition of workers, the work schedule and the calculation of labor costs, as well as the need for the necessary material and technical resources and technical and economic indicators are determined based on the calculation of the heating of six foundations located on one work area. 1.7. Electrode heating of monolithic structures can be combined with other methods of concrete hardening intensification, for example, pre-heating of 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 antifreeze additive is not allowed due to the fact that heated concrete with this additive has a significant (more than 30%) lack of strength, is characterized by reduced frost resistance and water resistance. 1.8. Linking this flow chart to other designs and work conditions at negative air temperatures requires changes to the work schedule, labor cost calculation, the need for material and technical resources and electrical heating parameters.

2. ORGANIZATION AND TECHNOLOGY OF WORK PERFORMANCE.

2.1. Prior to the start of work on the electrode heating of the concrete mixture, the following preparatory operations are performed: - on a flat area near the grip, a complete transformer substation KTP TO-80/86 is installed; - connect the KTP TO-80/86 to the mains supply and test it at idle; - make inventory sections of busbars (Fig. 1); - install sections of busbars near heated structures (Fig. 2); - carry out safety measures; - connect the busbars to each other with a cable of the brand KRPT 3 ´ 25; with a cable of the KRPT 3 ´ 50 brand, they are connected to the complete substation KTP TO-80/86 or other transformers used for these purposes; - clean from debris, snow, ice and set the formwork and reinforcement into working position. 2.2. Immediately after the concrete mixture is placed in the formwork, the open surfaces of the concrete are covered with waterproofing (polyethylene film) and thermal insulation (mineral wool mats 50 mm thick). 2.3. Through the layers of hydro- and thermal insulation, electrodes are driven into the concrete mixture according to the scheme (Fig. 3). 2.4. Steel rods with a diameter of 6 mm and a length of 1000 mm were taken as electrodes. 2.5. The electrodes are installed in such a way that their ends protrude from the concrete by 10 - 20 cm. The distance between the electrodes is taken depending on the outside temperature and the received voltage (table 1). 2.6. The electrodes are switched between each other and connected to the busbar sections (Fig. 3). 2.7. Connect the busbars to the mains supply (Fig. 4). 2.8. Before applying voltage to the electrodes, check the correctness of their installation and connection, the quality of the contacts, the location of the temperature wells or installed temperature sensors, and the correct laying of the insulation. 2.9. Apply voltage to the electrodes in accordance with the electrical parameters (table 1). 2.10. Immediately after applying voltage, the electrician on duty re-checks all contacts, eliminates the cause of the short circuit, if any. 2.11. If it is necessary to turn off the rod electrode, a new one is installed next to it and connected.

Electrical parameters of electrode heating

Table 1

Outside air temperature, °С	Supply voltage, V	Distance between electrodes, cm	Specific power, kW / m 3

2.12. Every two hours during isothermal heating measure the temperature of the concrete. Special wells are arranged for temperature measurements (Fig. 5, 6). 2.13. Warming up of the concrete mixture is carried out in accordance with the schedule below at a temperature rise rate of -6 °C/hour. During heating, the temperature of the concrete is controlled at least after 1 hour.

2.14. During the period of temperature rise, at the stage of isothermal heating, and also after each voltage switch, it is necessary to monitor the readings of measuring instruments, the state of contacts and taps. 2.15. The concrete heating rate is controlled by increasing or decreasing the voltage on the low side of the transformer. 2.16. When the outside air temperature changes during the warm-up process above or below the calculated value, the voltage on the low side of the transformer is respectively reduced or increased. 2.17. Heating is carried out at a reduced voltage of 55 - 95 V. 2.18. The set of strength of concrete at different temperatures of its aging is determined by the graph (Fig. 7). An example of determining the strength according to the schedule is shown in fig. 8.2.19. The rate of cooling of concrete at the end of heat treatment for structures with surface modulus Mn = 5 - 10 and Mn > 10 is not more than 5 °С and 10 °С per hour, respectively. The outside air temperature is measured once or twice a day, the measurement results are recorded in the journal. 2.20. At least twice a shift, and in the first three hours from the beginning of concrete heating every hour, measure the current and voltage in the supply circuit. Visually check the absence of sparks at the electrical connections. 2.21. The strength of concrete is usually tested according to the actual temperature regime. After stripping, the strength of positive-temperature concrete is recommended to be determined using a hammer designed by NIIMosstroy, using an ultrasonic method, or by drilling and testing cores. 2.22. 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 cool down to 0. It is not allowed to freeze the hydro- and thermal insulation formwork to concrete. 2.23. To prevent the appearance of cracks in structures, the temperature difference between the open surface of concrete and the outside air should not exceed: a) 20 ° C for monolithic structures with Mn< 5; б) 30 °С для монолитных конструкций с Мп >5. If it is impossible to comply with the specified conditions, the concrete surface after stripping is covered with tarpaulin, roofing felt, shields, etc. 2.24. The preparation of the bases and the laying of the concrete mix into the structure at negative air temperatures is carried out taking into account the following requirements: the condition of the bases on which the concrete mix is ​​laid, as well as the method of laying, should exclude the possibility of deformation of the base and freezing of concrete in contact with the base until it acquires the required strength; remove ice from the reinforcement formwork with steam or hot water not allowed. At air temperatures below -10 ° C, reinforcement with a diameter of more than 25 mm, as well as reinforcement of rolled profiles and large metal embedded parts, should be heated to a positive temperature. All protruding embedded parts and outlets must be insulated; the laying of the concrete mixture is carried out continuously, without transshipments, by means that ensure the minimum cooling of the mixture during its supply; the temperature of the concrete mixture laid in the formwork must not be lower than +5 °C. 2.25. Electrode heating of foundation concrete is performed by a team of 3 people (Table 2).

Distribution of operations by performers

table 2

2.26. Heating of monolithic foundations is carried out in the following sequence: the concrete worker prepares electrodes of the required length and in the required quantity from steel with a diameter of 6 mm; electrician V p. cuts the ends of the cable cores, connects it to the transformer substation KTP TO-80/86; electrician III p. arranges inventory sections of busbars along the grip, connects them to each other; electrician V p. connects the busbar sections to the transformer substation, makes grounding and tests the idling operation. After laying the concrete mixture into the formwork, the concrete worker covers the upper surfaces of the structure with hydro and thermal insulation; electricians V and III p. the electrodes are placed in the structure according to the selected scheme, the electrodes are switched between each other and connected to the sections of the busbar. Apply voltage to the electrodes. Recommendations for energy saving. In order to save energy during electrode heating of monolithic structures, it is recommended: - when determining the means and duration of transportation of the concrete mixture, it is not possible to cool it more than it is established by the technological calculation, violation of uniformity and decrease in the specified mobility at the place of laying; - use concrete mixtures of higher relative strength with a short warm-up time (Portland cement, quick-hardening Portland cement); - use chemical additives to reduce the duration of heat treatment, improve the electrical conductivity of concrete mixtures and obtain increased strength acquired by concrete immediately after heating; - apply the maximum allowable temperature for the heat treatment of concrete, taking into account the increase in the strength of concrete during cooling; - monitor the quality and density of connections of contacts; - prevent the heat-insulating layers from getting wet; - reliably insulate the surface of concrete and formwork subjected to cooling; - observe the mode of electrical processing.

3. REQUIREMENTS FOR QUALITY AND ACCEPTANCE OF WORKS

3.1. The quality control of the electrode heating of a monolithic structure at negative air temperatures is carried out in accordance with the requirements of SNiP 3.01.01-85 * "Organization construction industry”, SNiP III-4-80 * “Safety in construction” and SNiP 3.03.01-87 “Bearing and enclosing structures”. 3.2. Production quality control of electrode heating is carried out by foremen and foremen, with the participation of specialists from the energy services of construction organizations. 3.3. Production control includes input control of electrical equipment, operating materials and concrete mixture, operational control of individual production operations and acceptance control of the required quality of a monolithic structure. 3.4. During the input control of electrical equipment, operating materials and concrete mix, they are checked by external inspection for their compliance with regulatory and design requirements, as well as the presence and content of passports, certificates and other accompanying documents. During operational control, they check compliance with the composition of the preparatory operations, the technology for setting up electric heating equipment and devices, laying concrete into the formwork of the concreted structure in accordance with the requirements of SNiP, the process of electrode heating, temperature, current strength and voltage in accordance with the calculated data. During acceptance control, the quality of a monolithic structure is checked as a result of electrode heating: The results of operational control are recorded in the work log. The main documents for operational control is this routing and indicated in the map regulations, lists of operations controlled by the manufacturer of works (foreman), data on the composition, timing and methods of control, the required strength indicators of the foundation as a result of heating (Table 3). 3.5. The temperature control of the heated concrete should be carried out with technical thermometers or remotely using temperature sensors installed in the well. The number of temperature measurement points is set on average at the rate of at least one point for every 3 m 3 of concrete, 6 m of structure length, 50 m 2 of floor area, 40 m 2 of floor preparation area, etc. The temperature of the concrete is checked at least every 2 hours. At least twice a shift, and in the first three hours from the beginning of concrete heating every hour, measure the current and voltage in the supply circuit. There must be no sparking at the junctions of the wires. 3.6. The rate of temperature rise during the heat treatment of concrete is not higher than 6 °C/h; - concrete cooling rate at the end of heat treatment for structures with modulus 5 - 10 - 5 °С/h over 10 - 10 °С/h 3.7. The control of the strength of concrete is carried out by the temperature of the concrete during curing. The strength of heated concrete, which has a positive temperature, is determined using a hammer from NIIMosstroy, using an ultrasonic method, or by drilling cores and testing.

COMPOSITION AND CONTENT OF INDUSTRIAL QUALITY CONTROL

Table 3

Who controls	Foreman or master
Operations subject to control	Operations during input control	Preparatory operations		Operations for the device of the foundation and heating of concrete				Operations during acceptance control
Composition of control	checking the insulation of wires and the operability of switching equipment, transformers and other electrical equipment used in the work	installation of protective fencing and light signaling at the work site	cleaning the formwork base, reinforcement from snow, ice. Installation of rod electrodes. Construction insulation	laying concrete in a monolithic foundation structure	control of the magnitude of the current and voltage of the supply circuit	concrete temperature control	concrete strength control	compliance of the finished monolithic foundation with the requirements of the project
Control methods	visual-instrumental check			visual and instrument				visual-instrumental
Control time	before concreting		before and after concreting	in the process of electric heating of concrete				after electrical heating
Who is in control	power engineer construction organization	master, foreman		electricians and laboratory				laboratory, technical supervision

4. CALCULATION OF LABOR COSTS

Calculation of labor costs is made for electrode heating of six foundations with a total concrete volume of 19 m 3 .

Table 4

Rationale	Name of works		Scope of work	Norm of time, man-hour	Labor costs man-hour	The composition of the link
ENiR 1987 § E23-6-2 p. 35	Installation of a transformer substation in the heating zone					Electricians V p. – 1 person III p. - l pers.
ENiR 1987 § E1-19 p. 2 "a"	Carrying and installation in place of the inventory sections of the busbar with a mass of sections of 10 kg
E22-1-40 p. 1 "a"	Preparation of electrodes	10 cuts				Concrete worker III p. - 1 person
Experimental data of TsNIIOMTP	Installing a safety fence					Concrete worker III p. - 1 person electrician III p. - 1 person
E4-1-50 p. 2	Installing the main line and connecting electrodes to it, connecting a transformer substation, laying electrodes in the body of concrete. Removing the supply wires of the line after warming up	1 m 3 heated concrete				Electrician V p. - 1 person III p. - 1 person
ENiR 1987 § E23-4-14 tab. 3 p. 2	Checking the condition of the cable with a megger					Electrician V p. - 1 person
Tariff and qualification guide	Electric heating of the concrete mix					Electrician III p. - 1 person
ENiR 1987 E4-1-54; item 10	Hydro and thermal insulation device					Concrete worker III p. - 1 person
ENiR 1987 E4-1-54 p. 12	Removal of hydro and thermal insulation					Concrete worker III p. - 1 person
E22-1-40 p. 1 "a"	Shearing electrodes	10 cuts				Concrete worker III p. - 1 person
ENiR 1987 § E23-6-16 p. 3 K = 0.3	Detaching busbar sections	100 ends				Electrician III p. - 1 person

5. WORK SCHEDULE

6. REQUIREMENTS FOR MATERIAL AND TECHNICAL RESOURCES

Table 5

	Name	Brand (GOST, TU)			Technical specifications
	Complete transformer substation for concrete heating	KTP TO-80/86			Power - 80 kW Max. current 490 A Voltage 55, 65, 75, 85, 95 V
	Clamp meters
	Inventory sections of busbars				Section length - 1.5 m, weight 10 kg
	Cable	KRPT - 3 ´ 25 + 1 ´ 16			GOST 13497-68
		KRPT - 3 ´ 50
		KRPT 3 ´ 25
		KRPT - 3 ´ 16
		APR - 4 mm 2
	Reinforcing steel - electrodes	GOST 5781-82			Æ 6 mm
	Inventory mesh fencing				h = 1.5 m
	Insulating tape
	Polyethylene film Тс 0.1 ´ 1400	GOST 10354-82			thickness d = 0.1 mm width B = 1.4 m
	Dielectric	TU 38-106359-79
	gloves
	galoshes
	mat
	fire shield				With carbon dioxide fire extinguishers
	spotlight				Power - 1000 W
	Mineral wool	GOST 9573-82 Grade - 50

7. SAFETY SOLUTIONS

7.1 When operating rod electrodes made of reinforcing steel and power supply electrical equipment, in addition to the general requirements of the rules safe production works in accordance with SNiP III-4-80 * "Safety in construction" should be guided by the "Rules technical operation and safety of electrical installations of industrial enterprises. 7.2 Electrical safety on construction site, work sites and workplaces must be provided in accordance with the requirements of GOST 12.1.013-78 “Construction. Electrical safety. General requirements ". Persons employed in construction and installation works must be trained in safe ways of conducting work, as well as be able to provide first aid in case of electrical injury. 7.3 The construction and installation organization must have an engineering and technical worker responsible for the safe operation of the organization's electrical facilities, having a safety qualification group of at least IV. 7.4 When installing electrical networks, it is necessary to provide for the possibility of shutting down all electrical installations within individual sections and objects of work. 7.5 Work related to the connection (disconnection) of wires must be carried out by electrical engineering specialists with the appropriate safety qualification group. 7.6 During the entire period of operation of electrical installations, safety signs must be installed at construction sites in accordance with GOST 12.4.026.76. Duty electricians must have a qualification of at least group III. 7.8 Workers employed in the heating of concrete are provided with rubber boots or dielectric galoshes, and electricians, in addition, with rubber gloves. Connection of heating wires, temperature measurements with technical thermometers are carried out with the voltage turned off. 7.9 The area where the concrete is heated must be fenced off. Warning posters, safety regulations, fire fighting equipment are placed in a conspicuous place; at night, the zone fence must be illuminated, for which red lamps are installed on it, which automatically light up when voltage is applied to the heating line. 7.10 All metal current-carrying parts of electrical equipment and fittings should be reliably grounded by connecting the neutral wire of the supply cable to them. When using a protective ground loop, before turning on the voltage, it is necessary to check the resistance of the loop, which should be no more than 4 ohms. Floors covered with rubber mats are installed near transformers, circuit breakers and switchboards. 7.11 Checking the insulation resistance of wires using a megohmmeter is carried out by personnel whose safety qualification group is not lower than III. The ends of wires that may be energized must be insulated or shielded. The concrete heating area must be constantly supervised by an electrician on duty. 7.12 It is forbidden: to connect wires with mechanical damage to the insulation, as well as unreliably made switching connections; carry out work on warming up in wet weather, during a thaw, without fencing the heating zone; work in case of a detected fault in the wiring; lay wires directly on the ground; place flammable materials near installations for concrete heating, access of unauthorized persons to the heating zone.

8. TECHNICAL AND ECONOMIC INDICATORS

Rice. 1. Inventory section of busbar trunking (end section):

1 - connector; 2 - wooden stand; 3 - bolts; 4 - conductors (strip 3 ´ 40 mm)

Rice. 2. Scheme of the organization of the working area

1 - complete transformer substation KTP TO-80/86; 2 - spotlight; 3 - busbar sections; 4 - cable KRPT 3 ´ 2.5; 5 - cable KRPT 3 ´ 50; 6 - dielectric mat; 7 - inventory fence; 8 - red signal lamp

Rice. 3. Scheme of connecting electrodes to busbars

Rice. 4. Scheme of connecting busbars to the mains

Rice. 5. Installation of a temperature sensor in a heated structure

1 - monolithic structure; 2 - insulation;

3 - a case made of a thin-walled steel tube;

4 - industrial oil; 5 - temperature sensor

Note: 1. During heating and isothermal heating, the temperature of concrete is measured in wells No. 1 and 2, during cooling, in wells No. 1, 2, 3. 2. Electrodes are conventionally not shown.

Rice. 6. Scheme of arrangement of temperature wells

Rice. 7. Curves of curing concrete at different temperatures of its aging:

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

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

Example: Determine the strength of concrete in a structure with Mn = 4 on Portland cement grade 400 at a temperature rise rate of 10 ° C per hour, an isothermal heating temperature of 70 ° C, its duration is 12 hours and cooling at a rate of 5 ° C per hour to a final temperature of 8 ° FROM. Solution: 1. Determine the value of the relative strength for the period of temperature rise, the duration of the temperature rise at an average temperature To do this, from point "A" (see graph) we draw a perpendicular to the intersection with the strength curve at 40 ° C (point "B"). The value of strength during the rise in temperature is determined by the projection of point "B" on the ordinate axis (point "C") and is 15%. We determine the increase in relative strength during isothermal heating for 12 hours as a projection of the section (points "L" and "K") of the strength curve at 70 ° C (segment "B3"), which corresponds to 46% R 28. We determine the increase in the strength of concrete for 12 hours of cooling according to the strength curve at 38 ° C as the projection of the "ZHG" section on the y-axis. The segment "ZI" corresponds to 9% R 28. For the entire heat treatment cycle, concrete acquires a strength of 15 + 46 + 9 = 70% R 28. For each specific composition of concrete, the construction laboratory should clarify the optimal curing mode on prototype cubes.

Rice. 8. An example of determining the strength of concrete according to the schedule

LITERATURE

1. SNiP 3.01.01-85* "Organization of construction production". 2. SNiP 3.03.01-87 "Bearing and enclosing structures". 3. SNiP III-4-80* "Safety in construction". 4. Handbook on electric heating of concrete of monolithic structures (to SNiP III -15-76) NIIZhB Gosstroy USSR, Moscow, Stroyizdat, 1985. 5. Guidelines for electrical heat treatment of concrete. NIIZhB Gosstroy USSR, Moscow, Stroyizdat, 1974 6. Guidelines for the production of concrete work in winter conditions, areas of the Far East, Siberia and the Far North. TsNIIOMTP Gosstroya USSR, Moscow, Stroyizdat, 1982. 7. Temporary instructions for induction heating of reinforced concrete structures (VSN-22-68). Technical department of Glavmosstroy, Moscow, 1969

Concrete is very popular today. construction material, for the manufacture 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 favorably affects the process of curing. What happens in winter? In severe frosts, the set of strength characteristics stops, and this is highly 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 concrete flow chart 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 fittings is carried out in parallel with the circuit, with adjacent and straight wires, between which a pouring lamp is mounted. It is thanks to her that it will be possible to make voltage measurements.

Use a thermometer to measure temperatures. In terms of time, this process takes a long time, about 2 months. At the same time, for 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 with a small volume of concrete and excellent weather conditions.

infrared method

The meaning of this method is that equipment is being installed, the operation of which is performed 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 the concrete mixture is an electromagnetic oscillation, in which the wave propagation speed will be 2.98 * 108 m / s and a wavelength of 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 possibility of power supply from conventional alternating current. With 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 high temperature, which will eventually 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 coupling time.

What are the pros and cons of a house made of aerated concrete, indicated in this

Induction method

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

After the converted thermal energy will be distributed to the material. It is advisable to use the induction heating method when heating reinforced concrete frame structures. It 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, which are isolated from inductors and 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 reinforcing cage or joint. This is done in order to remove frost before concreting takes place. Now the exposed surfaces of the formwork and structure can be covered with heat-insulating material. Only after the arrangement of the wells can you start direct work.

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 keep 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:

• single-wire conductor of round shape, made of steel;
• insulation, for which PVC plastic or polyethylene can be used.

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

Which manufacturers of autoclaved aerated concrete exist are indicated in this

Such a manipulation is performed according to the following instructions:

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

Table 1 - Characteristics of wires of the PNSV brand

1	AC voltage, V	380
2	Cable section length for voltage 220 V:
	– PNSV1.0 mm, m	80
	– PNSV1.2 mm, m	110
	– PNSV1.4 mm, m	140
3	Specific heat dissipation power of the cable:
	- for reinforced installations, W/r.m.	30-35
	– for non-reinforced installations, W/r.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 thermal curing 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 with the help of transformer substations KPT TO-80. Such an installation is characterized by the presence of several stages of low voltage. Thanks to this, it becomes possible to adjust the power of the heating cables, as well as adjust it according to the changed air temperature.

Cable use

The use of this heating option does not require large expenditures of electricity and additional equipment.

The whole process proceeds as follows:

1. The cable is being installed on a concrete base before pouring the mortar.
2. Fix everything using fasteners.
3. Be careful during cable installation and operation to avoid damage to its surface.
4. 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 atmospheric precipitation. The components included in such a mixture can be very different, but the role of the main one is assigned to antifreeze. It is a liquid that does not allow water to freeze.

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

When erecting ready-mixed concrete, the production of curbs, it is necessary to use a mixture that contains calcium chloride. This component allows you to achieve a fast hardening speed, resistance to low temperature conditions.

Potash remains the ideal antifreeze additive. 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, monolithic chimneys are being erected.

SNiP

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

• SNiP 3.03.01-87 - Bearing and enclosing structures
• SNiP 3.06.04-91 - Bridges and pipes

On the video - warming up concrete in winter, technological map:

Although the documentation provided only indirectly touches on the topic of concrete heating, it contains certain sections in which there is pouring technology concrete mortar during the frosty season.

Timing

When calculating the warming up of concrete, factors such as the type of structure, the total heating area, the volume of concrete and the electrical power must be taken into account.

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

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

Throughout the process, it is worth monitoring the temperature, noting all the results with an increase in 30-60 minutes, and when cooling down, control is carried out 1 time 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 that, a graph of the dependence of time on strength is built, where the required value of time and temperature of heating 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 during frost, as a result of which this will lead to a decrease in the grade and further destruction. It is not difficult to carry out all these activities, it is enough just to determine which of the presented ones suits you best.

The requirements of SNiP 3-03-01-87 establish standards for warming up concrete in winter, which is carried out on the condition that the daily minimum air temperature is less than 0 ° C. Technological heating of concrete in winter is necessary to prevent freezing of the liquid concrete solution and to prevent the appearance of ice in the structure and around the reinforcing bars.

Water in the solution, as an element of the hydration reaction, in the solid state is not able to activate and begin to accelerate the hardening of concrete. Rather, on the contrary, ice begins to destroy the material, as it increases the internal pressure in the structure. As the temperature rises, the hydration process continues, but the quality of the concrete element and its durability are lost. Therefore, concrete heating methods have been developed, the basics of which are described below. All methods of warming up concrete in winter are constantly and actively exploited, but which of them will be most effective for a particular construction site needs to be clarified on site.

This concrete heating technology is based on the action of directed infrared radiation. That is, the heated material is processed exactly in the place to which the rays are directed. The equipment is installed in the place where the heating will be carried out, while the formwork does not interfere. It is possible to heat the concrete surface itself, and the radiation power is regulated by changing the distance between the infrared installation and the heated object. In practice, infrared heating of concrete is used on small objects.

Infrared concrete heating is a highly efficient technology, the equipment is easy to use, energy costs are low. Also of the advantages should be noted the mobility of the equipment.

Disadvantages - the high cost of equipment, as well as the fact that it is impossible to heat concrete in winter with one installation if the object is large or voluminous. That is, multiple installations may be required. Also, during the operation of radiating equipment in the autumn period, moisture evaporates too quickly, which negatively affects the quality and reliability of the object. This phenomenon can be combated, which causes additional financial and time costs. The most affordable and economical option is plastic film.

PNSV wire in construction

Technological heating of concrete with the PNSV wire is simple. Before pouring the solution into the formwork or form, the PNSV heating cable is laid there according to a pre-calculated scheme. A voltage is supplied to the circuit from a step-down transformer, as a result of which the concrete mixture is evenly and constantly heated.

Such a scheme for heating concrete has its advantages: it is not too high a power consumption and a low cost of the method - the costs are only for the PNSV wire and the transformer. For example, a connection scheme with a 80 kW transformer can heat up an area up to 90 m 3.

The disadvantage is a long and labor-intensive preparation for heating the surface: it is necessary to properly lay (at the desired depth) and connect the cable (an example is shown in the diagram).

Heating with electrodes

What does heating concrete with electrodes mean? The PNSV wire is replaced by wire or reinforcing electrodes Ø 8-12 mm. Such heating of concrete in winter with electrodes is suitable only for pouring vertical or voluminous objects, since the electrodes for heating concrete are stuck vertically into the solution, and they, like the circuit from the PNSV wire, are energized from a step-down transformer. The distance between the electrodes is 0.6-1 m.

Advantages: ease of installation. Disadvantages: high power consumption and high cost of the circuit, since all electrodes remain in the structure.

Heating formwork (thermos)

The method of heating formwork is the heating of concrete with special heating elements. Calculations for such heating show that the amount of heat in the solution should not be less than the amount of heat loss during the cooling of the structure for the entire time required to obtain the final hardness of concrete.

The heating element is an electric film. The advantages of this method are the possibility of heating several areas or one large surface at the same time, low energy consumption and mobility. The disadvantage of heating formwork is the high cost of construction.

Induction heating

Such electrical heating of concrete in winter is based on the operation of a simple induction coil. The method of induction for heating is used in closed circuit designs, where the length of the object is greater than the size of its section. Induction heating should be carried out with the connection of a step-down transformer for 12-36 V.

The coils of the inductor are laid out in advance according to the template, then the cable is laid in the grooves made in the solution, and the concrete mixture is poured. After connecting the device, the temperature of the concrete must be controlled, and when the maximum value is reached, the inductor turns off. If this is not enough, then the further method of electrical heating is the thermos method. You can also switch the inductor to pulse mode.

The advantages of this method are: uniform heating of the entire structure, savings on fittings and electrodes, low energy consumption (electricity consumption per 1 m³ - up to 150 kW / h).

Disadvantages: small heating area with one device. With an increase in the size of the inductor, the consumption of electricity increases.

Heating with thermomats

The way to heat concrete with thermoelectromats is good because the device itself works autonomously, and its operation does not need to be controlled. Thermomats consume very little electricity - less than with the method of heating with a wire or an inductor, and the result is better, since with uniform heating of the solution there are no local overheating zones, the formation of which can lead to the appearance of microcracks in the structure.

The advantages of heating a concrete solution with thermoelectromats are the ease of use of the devices, and the easily connected thermomat is a reusable equipment that can last up to 12 months with active constant work. The next advantage is the high quality of the results due to great depth warming up: in one work shift, concrete reaches 70-80% of its standard branded strength.

The disadvantage is that the thermomat is expensive, as a result of which a lot of fake low-quality equipment is thrown into the market.

thermal tent

This method has been known for a long time, as it is the very first of all existing methods of heating concrete in winter. It consists in the fact that a frame made of any material, for example, wooden bars or metal pipes, is arranged over a concrete structure, and this frame is covered with a tarpaulin or other rolled material. The frame can be made by one worker.

Any heating device, for example, a gas gun, is installed inside the resulting tent. It can also be an electric or diesel gun, and even a primitive fire, which will heat the volume of the constructed tent.

The advantages of this method are obvious - low cost, efficiency, minimal energy consumption. Of the shortcomings - only one: in this way you can warm up a small amount of concrete.

Calculation of concrete heating

To calculate the length of the PNSV wire for one section, as well as the required number of such sections for a particular concrete structure, the technical characteristics of the wire itself and the operating voltage of the step-down transformer are taken into account. For example, if the voltage on the transformer is 220V, the length of one section of the PNSV wire with a cross section of 1.2 mm will be 110 meters. With a decrease in voltage, a proportional reduction in the length of the cable segment in the section occurs.

If we take the average wire consumption of 50-60 m / m³ for one heating section, then the radiated heat can warm up the concrete mass up to 80 ° C.

To begin the calculation of the empirical dependence of the average value of the concrete temperature during cooling on the surface area, it is necessary to take into account the following factors and calculations:

1. The average annual weather forecast for the winter period in the region for several years. The predicted value of the average air temperature for the current winter period is also taken into account.
2. The module of the working heated surface is calculated, and, based on these calculations, the corresponding thermos exposure of the solution is determined.
3. According to the established formula, the average temperature of the structure during its cooling is calculated.
4. Information is required on the temperature of the delivered ready mix concrete and its exothermic characteristics. This information can be obtained from the manufacturer.
5. According to the established formulas, heat losses are determined during the transportation of the mixture and its unloading.
6. It is also necessary to determine the temperature of the solution from the beginning of its laying, taking into account the heat transfer to warm up the formwork and reinforcement.
7. Relying on regulatory requirements strength of concrete, calculate the cooling time of the solution.

This method of calculation works when predicting the setting time of concrete, taking into account heat losses during pouring the mixture, and heat radiation from the working surface, but such calculations are approximate.

Heating of concrete with electrodes technology updated: August 31, 2017 by: Artyom

With the onset of cold weather, many construction sites either close or move on to work that can be performed in a given period of time without disturbing the technical process. However, installation using cement-based slurries is sometimes very difficult to postpone without stopping the entire production and cannot be carried out at sub-zero temperatures. Therefore, a special technology for heating concrete was developed, which allows to cope with the task in any frost.

Kinds

To begin with, it must be said that today there are many different methods for maintaining the temperature in the solution. All of them have their own specific characteristics and corresponding cost. However, professional masters recommend paying attention to the four most popular of them ().

Base

First of all, it should be noted that, to begin with, a technological map is created for heating concrete with wires or other selected means, which fully describes all the cycles of the process and the temperature in them.

• The fact is that all such an operation is carried out only in order to accelerate the solidification of the mixture and save it from the appearance of air bubbles, which are caused by the freezing of water.
• Given all this, it is necessary not only to heat the composition, but also to prevent the temperature from becoming too high. Therefore, when using active funds, you need to acquire special regulators and controllers.

Thermos

It is believed that this technological heating of concrete is the simplest and does not require large financial costs.

However, it is not always suitable for severe frosts and does not allow for constant monitoring.

• It is based on the fact that first waterproofing with a reflective surface inside is laid in the formwork.. The same material is also prepared in advance for covering the structure.
• After that, the solution is heated to a temperature of 75 degrees and, having added antifreeze additives to it, it is poured into a mold.

• At the next stage, the installation instructions require closing the surface with maximum tightness, which will create the effect of a thermos.

Warm formwork

This method is based on the fact that when creating a mold for pouring, special panels are used that have the ability to raise and maintain the temperature.

• It should be noted that a technological map is not required for such heating of concrete. It is very conditional and is suitable only for products with small dimensions.
• Special attention deserves the fact that there are special panels for such use, which are reusable and have a certain shape.

Advice! This method is very well suited for the manufacture of flights of stairs, since some companies create special panels of such geometric proportions as marches. They are easy to use and quite practical.

Cable heating

It is worth saying that the price of this method is quite high, but it is he who is the most effective and reliable.

Thanks to him, all structures were erected in modern Moscow, not paying attention to the season and the cold.

• This method requires a pre-prepared project, which must indicate the brands of cables and control devices used.
• The essence of such heating is that the heating elements are placed in the formwork in a certain way, using coils or a spiral. After that, they are connected to the control equipment.

• It should be noted that do-it-yourself this way better not to reproduce. It requires a certain heating with a specific rate of temperature rise and the same cooling. At the same time, it is important to constantly monitor that the process proceeds evenly and in the same parameters.
• It is important to remember that after solidification, the cables will remain inside the structure and become a kind of reinforcement.

Advice! It is better not to use a similar method when creating reinforced products or to wind a heating element directly on its structure, since iron has greater degree expansion when heated, and sinks or cracks may appear.

Electrode method

The principle of operation of this method is based on the use of electric current, which will be directed from one electrode to another.

In this case, it is not necessary to use diamond drilling of holes in concrete or other fixing principles, since the contacts are fixed on special racks or directly on the formwork.

• It should be noted that this technique is also very effective and does not require large financial costs. However, in order to create the necessary magnetic field that would heat the solution, it is necessary to accurately place all the electrodes in the correct position and at a certain distance from each other.
• It is worth noting that some types of such contacts involve subsequent removal from the structure, although they mostly remain inside the product, which should be taken into account if cutting reinforced concrete with diamond wheels is subsequently planned.

Advice! This technique uses open currents that can affect various devices and even simple wires inside the building. Therefore, it is very important to comply with all safety requirements and strictly follow the instructions in the manual.

• When using a cable for heating, professional craftsmen try to lay it immediately from the reel in turns to avoid kinks or breaks.
• When warm formwork is used, it is recommended to wrap it in a heat-resistant film in order to extend the life of this structure.
• The thermos method is best combined with other heating systems in order to achieve maximum effect even in the most severe frosts.
• Quite often, large voltage drops appear on the construction site. Therefore, experts advise using a voltage regulator to protect the system and be able to make adjustments.