GOSSTROY USSR
CENTRAL RESEARCH FACILITY
AND DESIGN AND EXPERIMENTAL INSTITUTE
ORGANIZATION, MECHANIZATION AND TECHNICAL ASSISTANCE FOR CONSTRUCTION
(TsNIIOMTP)
ROUTING
FOR ELECTRIC HEATING
HEATING WIRES
MONOLITHIC CONCRETE STRUCTURES
MOSCOW - 1985
Recommended for publication by the decision of the section “Technology of construction production” of the Scientific and Technical Council of the Central Scientific Research Institute of Transport and Equipment of the State Construction Committee of the USSR Technological map for electric heating of monolithic concrete structures with heating wires. M., 1985. (Gosstroy of the USSR. Central scientific research and design and experimental institute of organization, mechanization and technical assistance to construction. TsNIIOMTP). Technological solutions for electrical heating with heating wires of monolithic concrete and reinforced concrete structures and their parts erected in winter conditions are presented. Recommendations are given for the selection of the main technological parameters for electric heating of concrete at subzero outside temperatures, as well as layout diagrams for wire electric heaters in monolithic structures. The technological map was prepared by employees of the concrete works department of the TsNIIOMTP Gosstroy of the USSR (N.S. Musatova, Ph.D. A.D. Myagkov, Ph.D. V.V. Shishkin) and department No. 7 of the Implementation Bureau of the TsNIIOMTP (B Y. Gubman, B.A. Lomtev, G.S. The card is intended for construction and design organizations.
1 . APPLICATION AREA
1.1. The technological map has been developed for electrical heating with heating wires of various unified monolithic reinforced concrete structures erected in winter conditions. 1.2. Examples are given of electrical heating of foundations, grillages, retaining walls and other monolithic structures using heating wires. 1.3. The essence of the method is to transfer the heat generated by the wires into the concrete by contact. Wires with a metal current-carrying insulated conductor, connected to the electrical network, work as resistance heaters. Heating wires can be laid directly into an array of a monolithic structure or used in inventory flexible flat electric heating devices (GED) for external electrical heating of concrete (Fig. 1). 1.4. The work covered by the map includes: preparing the work area and structure for concreting and electrical heating of concrete; laying the heating wire into the structure; concreting the structure; electrical heat treatment of concrete; concrete quality control.Rice. 1 . Heating flat element (HEP)
2. ORGANIZATION AND TECHNOLOGY OF THE CONSTRUCTION PROCESS
2.1. Before concreting the structure, the following preparatory work is carried out: formwork, reinforcing mesh and frames are installed; in this case, the soil foundation under the structure must be heated and protected from freezing (the use of inventory formwork of various designs and types is allowed; when used in winter conditions, it is insulated with mineral wool mats, polystyrene foam, polyurethane foam, etc., and the heat transfer coefficient of the insulation should be no more than 2 W/m 2 × °C); on a flat area no more than 25 m from the monolithic structure being erected, a transformer substation of the KTP-63-OB type is installed; Soffits are installed at a distance of up to 1.5 m from the structure - inventory sections of three-phase busbars (Fig. 2);
Rice. 2. Inventory section of busbar trunking (outermost section):
1 - connector; 2 - wooden stand; 3 - bolts; 4 - conductors (lane 3 ´ 40 mm)
Install fencing of the work area and provide alarm and lighting; Wooden flooring covered with rubber mats is installed near the transformer substation and distribution cabinets, a fire shield with carbon dioxide fire extinguishers is installed, and safety signs are hung in the work area; connect the transformer substation to the supply network and test it at idle, and also check the operation of temporary lighting and automatic temperature control systems; provide the working unit with the necessary tools, personal protective equipment, and provide instructions; clean the formwork and reinforcement of the structure being built from debris, snow and ice. 2.2. After completing the preparatory work, concreting begins with electrothermal treatment of concrete. Work is performed in a certain sequence. Before concreting, heating wires are placed in the structure: in reinforced concrete structures, the wire is wound onto reinforcement frames and meshes, in concrete structures - onto templates laid during concreting, and the length of the wire heaters, depending on the operating voltage, is taken according to the nomogram (Fig. 3).
Rice. 3. Nomogram for determining the length of wire heaters
The heating wire is wound into the structure without strong tension (with a force of up to 30 - 50 N). In corners with cutting edges, additional insulation made of roofing felt or bituminized paper is installed under the wire. The wires are fastened to the fittings with binding wire, and in order to avoid burning of the insulation, a short circuit to ground in densely reinforced structures and the burning of the ends of the heating wire from the concrete outwards, conclusions are arranged from the mounting wire with a cross-section of 2.5 - 4 mm (Fig. 4). The terminals are located on one side of the structure, and the connection points are carefully insulated. The formwork is installed partially uninstalled in order to be able to lay the heating wires into the structure. Heating wires are connected to inventory sections of busbars connected by cable to the transformer substation. After this, they begin to concrete the structure, observing measures to prevent damage to the insulation and breaks in the heating wires, in particular, sharp blows and rapid lowering of the working part of the vibrator into the formwork are not allowed, as well as the use of bayonets and other equipment with cutting edges and etc. The horizontal surfaces of the finished product are covered with waterproofing materials (film, bituminized paper, etc.), and if there is a large area of open surfaces, flexible flat electric heaters (FELs) and insulation are also laid. To insulate heated concrete, it is recommended to use inventory flexible thermal insulation coatings (TIGP), which are a moisture-proof cover made of rubberized fabric, inside of which an insulating canvas-stitched glass material of the CPS brand is enclosed.
Rice. 4 . Concrete heating wire leads:
1 - heating wires; 2 - installation wires; 3 - concrete
To regulate the concrete heating temperature, an external temperature sensor of the automation system is installed in a special well and voltage is supplied to the wire electric heaters. The duration of heating is determined depending on the temperature and the required final strength of concrete according to the graphs in Fig. 5.
Rice. 5 . Strengthening curves for concrete at different temperatures:
a, c - for concrete M200 on Portland cement with an activity of 400 - 500;
b, d - for M200 concrete based on Portland slag cement with an activity of 300 - 400
2.3. Work on laying the heating wire in the structure and electrical heating of monolithic concrete is performed by a team of four people: electrician of the 5th category - 1, electrician of the 3rd category - 1, concrete worker of the 3rd category - 1, reinforcement worker of the 3rd category - 1. 2.4. When laying concrete mixture in horizontal layers in massive structures and reinforced concrete structures of significant height (walls, columns, etc.), separate wire heaters should be placed in the area of these layers. After covering the next layer with concrete mixture, the heaters placed in it are connected to the electrical network (the thickness of the laid layer should not exceed 50 cm). 2.5. The calculation of labor costs was compiled for electrical heating with heating wires of a structure with a module Mp = 10 m -1 with an area of 70 m 2 . Structure thickness 200 mm; wire spacing 100 mm; double-sided heating (wires and gas transmission); linear load 25 W/m. The duration of heat treatment at a maximum isothermal holding temperature of 60 - 70 °C is taken from the condition that the concrete reaches 50% of its design strength by the end of heating. When changing the massiveness of the structure (module) and the installation pitch of wire electric heaters, correction factors should be used that increase or decrease labor costs and the cost of the structure.
Calculation of labor costs for electrical heating of structures with an area of 70 m 2 with heating wires using a module Mp = 10 m -1
|
Rationale |
Name of works |
Scope of work |
Standard time per unit of measurement, |
Labor costs for the entire volume of work, |
Prices per unit of measurement, rub.-kop. |
Cost of labor costs for the entire scope of work, rubles-kopecks. |
Team composition and mechanisms used |
EniR, 1979, § 23-2-28, tab. 2, paragraph 1, 2 | Installation of a transformer substation using a truck crane in the concreting area | Electricians 5 grades - 1, | 3 size - 1 | Truck crane AK-7.5-1 | EniR, 1979, § 1-4 | Carrying and replacing inventory sections of three-phase busbars with a section weight of 10 kg | Concrete worker 3 grades. - 1 | ENiR, 1979, § 23-7-26, paragraph 3c | Installation of bolted safety mesh fencing using a separate frame over 2 m2 | Concrete worker 3 grades. - 1 Electrician 3rd grade. - 1 | ENiR, 1979, § 23-2-18, paragraph 1a | Attaching safety posters | Electrician 3 grade - 1 | ENiR, 1979, § 23-4-6, paragraph 2a, note. 3 | Winding onto a reinforcing frame of a heating wire with a cross-section of up to 4 mm 2 - with fastening at individual points | Concrete worker 3 grades. - 1 fitter 3 grade. - 1 Electrician 3rd grade. - 1 | ENiR, 1980, § 4-1-38, paragraph 1 | Installation of flexible flat elements (FLE) and thermal insulation coatings for heating exposed concrete surfaces | Concrete worker 3 grades. - 1 fitter 3 grade. - 1 Electricians: 5 grades. - 1 3 sizes - 1 | EniR, 1979, § 23-7-34, paragraph B | Connecting a transformer substation and busbar sections to the network using cables with a cross-section of up to 16 mm 2 |
100 ends |
Electrician 5 grade - 1 | EniR, 1979, § 23-4-15, paragraph 4 | Checking the insulation condition of cables and wires with a megger before and after installation | Electricians: 5 grades - 1 3 sizes - 1 | EniR, 1979, § 23-7-34, tab. 1, item a | Connecting heating wires to the terminals of busbar sections |
100 ends |
Electrician 3 grade - 1 | Tariff 3 times | Electrician duty during electrical processing of concrete | Electrician 3 grade - 1 | Total: | The same, per 1 m 3 of concrete |
Correction factors for monolithic structures of various masses
Correction factors for different pitches of wire electric heaters
2.6. Quality control Before concreting the structure, it is necessary to check the presence of insulating materials, wire heaters and GEP in the volume provided for in the technological map. It is necessary to check the functionality and absence of mechanical damage to the insulation of wires, electric power transmission, switching network, transformers and other electrical equipment and automatic temperature control systems; availability of current clamps, voltmeter, dielectric mats, gloves, etc. Before laying the concrete mixture, the quality of clearing snow and ice from the base, formwork and reinforcement must be checked. After concreting, it is necessary to check the reliability of covering the horizontal surfaces of the structure with waterproofing material and the thickness of the insulation. At least twice a shift, it is necessary to measure the temperature of the concrete mixture in the bodies of dump trucks and in bunkers at a depth of 5 - 10 cm, and after laying each layer in the structure - at a depth of 5 cm. The temperature of heated concrete should be monitored with mercury thermometers. The number of temperature measurement points is set at the rate of at least one point per 3 m 3 of concrete. The temperature of the concrete during the heating process is measured every hour. At least twice per shift, and in the first three hours of warm-up - three times, the current and voltage in the supply circuit should be measured. The absence of sparking at electrical connections is checked by visual inspection. The strength of concrete can be controlled based on the actual temperature conditions of the least heated areas. After stripping, the strength of heated concrete at a positive temperature is determined (using a NIImosstroy hammer, a Kashkarov hammer, an ultrasonic method, or by drilling cores and testing). General requirements for concrete quality control must comply with SNiP Sh-15-76. 2.7. Safety precautions When operating the HEP (heating element), heating wires and power supply electrical equipment, in addition to the general rules for safe work in accordance with SNiP Sh-4-80 “Safety in Construction”, you should be guided by the “Rules for Technical Operation and Safety of Electrical Installations of Industrial Enterprises”. Electrical safety at the construction site, work sites and workplaces must be ensured in accordance with the requirements of GOST 12.1.013-78. Persons engaged in construction and installation work must be trained in safe methods of carrying out work, as well as be able to provide first aid in case of electrical injury. A construction and installation organization should have an engineer and technical worker responsible for the safe operation of the organization’s electrical equipment, who has a safety qualification group of at least IV. Responsibility for the safe performance of specific construction and installation works using electrical installations rests with the engineering and technical workers supervising the execution of these works. When installing electrical networks on a construction site, it is necessary to provide for the possibility of disconnecting all electrical installations within individual objects and work areas. Work related to connecting (disconnecting) wires must be performed by electrical engineering specialists with the appropriate safety qualification group. During the entire period of operation of electrical installations, safety signs in accordance with GOST 12.4.026-76 must be installed on construction sites. Technical personnel conducting electrical heating of concrete must undergo training and knowledge testing by a qualification commission on safety precautions and receive the appropriate certificates. Electricians on duty must have qualifications of at least Group III. Workers engaged in electrical heating of concrete are provided with rubber boots or dielectric galoshes, and electricians are also provided with rubber gloves. Connecting the heating wires and measuring the temperature with technical thermometers is carried out with the voltage turned off. The area where electrical heating of concrete is carried out must be fenced; Warning posters, safety regulations, and fire-fighting equipment should be placed in a visible place; at night, the area should be well lit, for which red lights are installed on the fence, which automatically light up when voltage is applied to the heating line. Walking by people and placing foreign objects on the surface of energized heating elements is prohibited. Access of unauthorized persons to the heating zone is prohibited. All metal non-current-carrying parts of electrical equipment and fittings should be reliably grounded by connecting the neutral wire (core) of the power cable to them. When using a protective ground loop, before turning on the voltage, you must check the loop resistance, which should be no more than 4 ohms. Near transformers, switches and distribution boards, floorings covered with rubber mats are installed. Checking the insulation resistance of wires using a megger is carried out by personnel whose safety qualification group is not lower than III. The ends of wires that may be live must be insulated or shielded. The area where concrete is electrically heated must be constantly supervised by an electrician on duty. IT IS PROHIBITED: to move the GEP by dragging it behind the cable outlets; lay the GEP on an unprepared surface that has pins or cutting edges, which can damage the integrity of the dielectric insulation of wire heaters; lay GEP with an overlap on one another, as well as on surfaces that have depressions or holes that disrupt heat transfer and cause local overheating; connect electric power transmission and heating wires to a network with a voltage exceeding the operating voltage for specific objects; connect heating wires exposed to the air to the electrical network that are not partially or completely concreted into the structure or not buried in the ground; connect electrical power supply and heating wires with mechanical damage to the insulation, as well as unreliably made switching connections; connect heaters to a network with a voltage above 220 V. It is allowed to measure the temperature manually with thermometers and concrete monolithic structures, including layer-by-layer laying of concrete mixture, with the electric power supply and heating wires not disconnected from a network with a voltage of no more than 60 V, subject to the following requirements: in there are no energized heating wires or outlets in the operating area of the deep vibrator; the fittings are grounded; personnel qualification group not lower than II; personnel perform work in rubber dielectric shoes and gloves; work is carried out under the supervision of an electrician.3. TECHNICAL AND ECONOMIC INDICATORS (per 1 m 3 of concrete)
|
Name |
For double-sided heating of monolithic structures with heating wires, thickness, mm |
||||
| Labor costs, person-hours | |||||
| Salary, rub.-kop. | |||||
| Cost of machine time, machine-hour | |||||
| Output per worker per shift, m 3 concrete | |||||
4 . MATERIAL AND TECHNICAL RESOURCES
Need for machines, equipment, tools and supplies
|
Name |
Brand (GOST, TU) |
Quantity |
Technical specifications |
|
| Complete transformer substation for heating concrete | KTP-63-05 | Power 63 kW; maximum current on the LV side - 520 A | ||
| Automatic temperature control unit | ART-2 | Regulation range - from 20 to 100 °C | ||
| Air heater | VPT-400 | |||
| Heating flat elements | GEP | Specific power up to 600 W/m; heating temperature 70 ° C | ||
| Flexible thermal insulation coatings | TIGP | Thickness 30 mm; reduced mass 3 kg/m2 | ||
| Clamp meter | Ts-91 | |||
| Dielectric | ||||
| rug | ||||
| galoshes | ||||
| gloves | ||||
| Heating wire | POSHV, TU 16-505.524-73 | Broadcasting wires of the brands PPZh, PVZh, PRSP, etc. can be used. | Inventory sections of three-phase busbars | Section length 1.5 m; weight 10 kg | Cable |
KRPT 3 ´ 10 mm 2, GOST 13497-68 |
Inventory mesh fencing | Height 1.5 m |
| Fire shield | With carbon dioxide fire extinguishers | |||
| Signal lights (red) | For voltage 36 V | |||
| Spotlight | Power 1 kW | |||
| Heat-shrinkable polyethylene tubing or insulating tape | ||||
| Technical mercury thermometers | Temperature measurement limit 40 - 100 °C |
Grillage electrical heating diagram. Fragment of the plan
Sheet 1
1 - inventory three-phase section of busbars; 2 - dielectric mat; 3 - transformer substation KTP-63-06; 4 - block attachment ART-2; 5 - inventory fence; 6 - red signal lights; 7 - spotlight; 8 - grillages
Grillage electrical heating circuit
Sheet 2
1 - thermal insulating flexible coating (TIGP); 2 - heating flat elements (HEP); 3 - wooden insulated shield; 4 - metal void former; 5 - heating wires; 6 - temperature sensor
KnotIcm . Sheet 3Grillage electrical heating circuit
Sheet 3
1 - hairpin; 2 - wooden insulated shield; 3 - inventory connector; 4 - heat-resistant installation wires; 5 - protective frame; 6 - tubular electric heaters; heating elements; 7 - asbestos cord; 8 - clamps
Sheet 4
1 - inventory three-phase section of busbars; 2 - spotlight; 3 - block attachment ART-2; 4 - transformer substation KTP-63-06; 5 - dielectric mat; 6 - inventory fence; 7 - red signal light
Section A - A see Sheet 5Scheme of electrical heating of floor slabs
Sheet 5
1 - heating flat elements (HEP); 2 - thermal insulating flexible coating (TIGP); 5 - temperature sensor; 4 - block - ART-2 attachment; 5 - wooden portable shields; 6 - transformer substation NTL-63-06; 7 - heating wires; 8 - insulated formwork; 9 - concrete slab
Sheet 6
1 - transformer substation KTP-63-06; 2 - block - ART-2 attachment; 3 - inventory fence; 4 - spotlights; 5 - red signal light; 6 - dielectric mat; 7 - inventory three-phase section of busbars
Section A - A see Sheet 7Retaining wall electrical heating circuit
Sheet 7
1 - heating flat elements (GEL); 2 - heating wires; 3 - temperature sensor; 4 - thermal insulating flexible coating (TIGP)
Sheet 8
1 - transformer substation KTP-63-06; 2 - block - ART-2 attachment; 3 - dielectric mat; 4 - sliding formwork
Section A - A see sheet 9.KnotIsee sheet 10Electrical heating circuit for a hyperbolic cooling tower
Sheet 9
1 - block - prefix ART-2; 2 - transformer substation KTP-63-05; 3 - spotlight; 4 - sliding formwork; 5 - thermal insulating flexible coating (TIGP)
Electrical heating circuit for a hyperbolic cooling tower
Sheet 10
1 - main branch; 2 - main cable; 3 - heating wire
| 1 area of use. 1 2. Organization and technology of the construction process. 2 3. Technical and economic indicators. 10 4. Material and technical resources.. 11 5. Schemes for electrical heating of concrete during the construction of certain types of concrete structures |
Warming up concrete is a mandatory procedure in low temperature conditions. It is necessary to ensure optimal conditions under which concrete can harden normally. Otherwise, the structure of the material is disrupted and it begins to lose its properties. It is dangerous to allow the mixture to freeze during the setting period.
Why do you need to warm up?
Warming up the concrete in winter is necessary so that the existing water in the solution does not turn into ice crystals. Otherwise, the pressure inside the pores of the cement will increase, which will lead to the destruction of the material that has already hardened. It will no longer meet the high strength requirements.
The need to heat the material is also due to other reasons related to ongoing processes in the solution:
- when freezing, water increases in volume by 10-15%, which leads to the destruction of the edges of the pores, and the material becomes loose;
- icing of reinforcement caused by exposure to low temperatures disrupts the metal-cement bond, which worsens the technical characteristics of the structure.
To prevent the solution from freezing, it is necessary to create a temperature at which the concrete will naturally harden. An increased temperature of the material during heating is also undesirable, since it leads to accelerated interaction between concrete and water, and more specifically to its evaporation.
Ways to warm up in winter
You can avoid freezing of the solution in the cold season using special equipment. All possible methods of heating the material are established in SNiP 3.03.01-87 (Load-bearing and enclosing structures, section 7.57) and SNiP 3.06.04-91 (Bridges and pipes, section 6.37). The main methods include: heating in the formwork, thermos, the use of electrodes, heating wires, infrared heaters, etc. Each method is unique and requires the use of different equipment.
Heating concrete with electrodes is the most common method. Electric current conductors are installed in different places of the poured mass. Current passing through an electrical circuit generates heat. This is how concrete is electrically heated.
There are several options for connecting electrodes to the concrete mixture. In each case, the connection diagram used is individual. When choosing it, it is taken into account that electrolysis in water and concrete solution is caused by direct current, and in the process of electrical heating it is recommended to use three-phase alternating current.
Important! When reinforcing concrete with metal or iron rods, using a network voltage of more than 127V is prohibited. The exception is certain areas for which projects have been specially developed.
Heating of concrete can be done using different types of electrodes:
- strings - used for pouring of a large length (columns or piles);
- rod - used for joints of structures of complex configurations;
- strip - used to heat concrete from different sides of the structure;
- plate - electrodes attached to the back side of the formwork are connected to different phases, due to this an electric field is formed.
Use of wire
To minimize time, a special wire is used for heating the concrete - PNSV. It is a steel core insulated in polyethylene or PVC.
When choosing this method, you cannot do without a transformer for heating the concrete. The essence of the method is that the equipment heats the wires, and the heat from them is transferred to the concrete composition. Due to the high thermal conductivity of the material, energy is quickly distributed throughout the array. One station can heat up to 80 m³ of concrete mixture. This method is used to heat monolithic structures in 30-degree frosts.
The main advantage of using wire for heating is the ability to adjust the temperature depending on weather conditions. The cable is capable of raising temperatures up to 80 ºС. A transformer for heating concrete must have several low voltage stages. This will allow you to regulate the power of the heating wires and adjust its value in accordance with changes in air temperature.
The need to use a transformer to heat the concrete significantly increases the cost of construction. TMO and TMTO equipment for heating concrete is expensive (90-120 thousand rubles), rent is 10-15% of the cost. There is no point in purchasing it for a one-time fill.
To warm up concrete in winter, you will need a technological map. It is developed by a power engineer for each individual project, although there are also standard samples of this document.
Based on the technological map, the number of transformer stations is calculated, their favorable location is determined, as well as the order of placement of the cable for heating the concrete. On average, processing 1 m³ of solution requires up to 60 meters of cable. To carry out a uniform load across the phases, it is necessary to test the wire.
Instructions for heating with heating wire
For effective heating, the heating wire must have a cross-section of at least 1.2 mm, and the operating current must be at least 12 A.
Electrical heating of concrete is carried out as follows:
- the cable for heating the concrete is placed inside the structure in such a way that the conductors do not touch each other and do not extend beyond the edges of the concrete;
- soldering cold ends to the heating wire and bringing them outside the heating zone;
- checking the assembled electrical circuit with a megohmmeter;
- supplying voltage to the assembled system and heating the structure.
This is a passive method, focused not on the transfer of thermal energy, but on its conservation. Its essence comes down to insulating a concrete structure from the outside using heat-insulating materials.
From an economic point of view, this method is the most profitable, since cheap sawdust can be used as thermal insulation materials. But insulating the structure is not always enough to create natural conditions for the mixture to harden. Additional use of other methods will be required.
Warming up with IR emitters
Infrared heating devices have low power consumption. They are directed to the heated area, and in the concrete structure the infrared rays are converted into heat.
The main advantage of the method is the ability to heat individual sections of the structure. However, with a thick concrete layer, heating is uneven, which can lead to a decrease in the strength of the structure.
IR emitters have found application in processing joints or creating thin-walled elements.
The method is based on the phenomenon of electromagnetic induction. The energy of the electromagnetic field is converted into thermal energy, which is transferred to the heated surface. This process takes place in steel formwork or on reinforcement. 
Induction heating is only possible for closed loop structures. The reinforcement coefficient with iron or steel elements must be at least 0.5. To create an indicator, wrap the entire structure with insulated wire. An electric current passed through it creates an electromagnetic field that heats up all metal elements. From them heat is transferred to concrete.
The essence of the method comes down to passing steam through pipes pre-installed into the structure or between the walls of the formwork. If the temperature of concrete in a steam-saturated state during heating exceeds 70 ºС, then the material will gain the same strength in a few days as it did in 10-12 days.
Steam must be released 30 minutes before pouring the concrete mixture to warm up the structure.
This method is highly effective, but requires significant costs to implement.
How much does it cost to heat concrete?
The source of cost estimates is the technological map. To calculate how much electric heating costs, you need to know the following parameters: volume of concrete, material consumption and process duration.
The most economical methods are heating the mixture using the “thermos” method or using IR emitters using a small amount of electricity. As for the efficiency, these methods are lower than when heating with heating wires, electrodes or steam.
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 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.
Concrete is a very popular building material today, for the production of which components such as cement, water, aggregate and water are used. But it’s one thing when you pour concrete in the summer, because the warm season has a beneficial effect on the process of gaining strength. What happens in winter? In severe frosts, the development of strength characteristics stops, and this is extremely undesirable. In this case, it is necessary to apply a number of measures that will allow the concrete to warm up. To do this, you need to know all the features of the technological map of concrete for the winter period and the current methods of heating.
Technological map and methods of heating concrete
Warm up with a welding machine
This heating method involves the use of the following materials:
- pieces of reinforcement;
- incandescent lamps and a thermometer for measuring temperature.
The process of installing pieces of reinforcement is carried out parallel to the circuit, with adjacent and straight wires, between which the pouring lamp is mounted. It is thanks to it that it will be possible to measure voltage.
To measure temperatures, you should use a thermometer. This process takes a long time, approximately 2 months. At the same time, during the entire heating process it is necessary to protect the structure from the influence of cold and water. It is advisable to use heating with a welding machine when there is a small volume of concrete and excellent weather conditions.
Infrared method
The meaning of this method is that equipment is being installed that operates in the infrared range. As a result, it is possible to convert radiation into heat. It is thermal energy that is introduced into the material.
Infrared heating of concrete mixture represents electromagnetic vibrations, the wave propagation speed of which will be 2.98 * 108 m/s and the wavelength 0.76-1,000 microns. Very often, tubes made of quartz and metal are used as a generator.
The main feature of the presented technology is the ability to supply energy from conventional alternating current. When infrared heating of concrete, the power parameter may change. It depends on the required heating temperature.
Thanks to the rays, energy can penetrate into deeper layers. To achieve the required efficiency, the heating process must be carried out smoothly and gradually. It is forbidden to work here at high power levels, otherwise the top layer will have a high temperature, which will ultimately lead to a loss of strength. It is necessary to use this method in cases where it is necessary to heat up thin layers of the structure, as well as prepare a solution to speed up the adhesion time.
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Induction method
To implement this method, it is necessary to use alternating current energy, which will be converted into heat in formwork or reinforcement made of steel.
The converted thermal energy will then be distributed to the material. It is advisable to use the induction heating method when heating reinforced concrete frame structures. These can be crossbars, beams, columns.
If you use induction heating of concrete on the outer surfaces of the formwork, then it is necessary to install successive turns that are isolated from the inductors by wire, and the number and pitch are determined by calculation. Taking into account the results obtained, it is possible to produce templates with grooves.
When the inductor has been installed, it is possible to heat the reinforcement frame or joint. This is done in order to remove ice before concreting occurs. Now the open surfaces of the formwork and structure can be covered with thermal insulation material. Only after the wells have been constructed can the actual work begin.
When the mixture reaches the required temperature, the heating procedure is stopped. Make sure that the experimental indicators differ from the calculated ones by at least 5 degrees. The cooling rate can maintain its limits of 5-15 C/h.
Application of transformers
To increase the temperature in concrete, you can use such an inexpensive and simple method as the PNSV heating wire.
The design of this cable includes two elements:
- round single-wire conductor made of steel;
- insulation, for which you can use PVC plastic or polyethylene.
If you need to heat a mixture of 40-80 m3, then it will be enough to install just one transformer substation. This method is used when the air temperature outside has reached -30 degrees. It is advisable to use transformers for heating monolithic structures. For 1 m of weight, a 60 m wire will be enough.
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This manipulation is performed according to the following instructions:
- A heating wire is laid inside the concrete. It is connected to the station or transformer terminals.
- With the help of an electric current, the array begins to gain temperature, as a result of which it manages to harden.
- Since the material has excellent thermal energy conductivity properties, heat begins to move at high speed throughout the entire mass.
Table 1 – Characteristics of PNSV brand wires
| 1 | AC voltage, V | 380 |
| 2 | Length of cable section for voltage 220 V: | |
| – PNSV1.0 mm, m | 80 | |
| – PNSV1.2 mm, m | 110 | |
| – PNSV1.4 mm, m | 140 | |
| 3 | Cable heat dissipation power: | |
| – for reinforced installations, W/l.m. | 30-35 | |
| – for unreinforced installations, W/l.m. | 35-40 | |
| 4 | Recommended supply voltage, V | 55-100 |
| 5 | Average core resistance value: | |
| – PNSV1.2 mm, Ohm/m | 0,15 | |
| – PNSV1.4 mm, Ohm/m | 0,10 | |
| 6 | Method parameters: | |
| – Specific power, kW/m3 | 1,5-2,5 | |
| – Wire consumption, lm/m3 | 50-60 | |
| – Cycle of thermos aging of structures, days | 2-3 |
The heating wire, which is laid inside the concrete, should heat the structure up to 80 degrees. Electrical heating occurs using transformer substations KPT TO-80. This installation is characterized by the presence of several low voltage stages. Thanks to this, it becomes possible to adjust the power of the heating cables, and also adjust it according to the changed air temperature.
Using the cable
Using this heating option does not require large amounts of electricity or additional equipment.
The whole process proceeds according to the following scheme:
- The cable is being installed on the concrete base before the mortar is poured.
- Secure everything using fasteners.
- Be careful during cable installation and operation to ensure that its surface does not become damaged.
- Connect the cable to the low-voltage electrical cabinet.
Antifreeze additives
With the addition of antifreeze additives, concrete is able to withstand the most aggressive precipitation. The components included in such a mixture can be very different, but the main role is assigned to antifreeze. This is a liquid that prevents water from freezing.
If it is necessary to cock reinforced concrete structures, the mixture must contain sodium nitrite and sodium format. The main feature of antifreeze mixtures remains the preservation of anti-corrosion and physico-chemical properties at low temperatures. 
When constructing ready-mixed concrete or producing curbs, it is necessary to use a mixture that contains calcium chloride. This component allows you to achieve fast hardening speed and resistance to low temperatures.
The ideal antifreeze additive remains a chemical such as potash. It dissolves very quickly in water, and there is no corrosion. If you use potash when heating concrete in winter, you will be able to save on building materials.
If you use antifreeze additives, it is very important to adhere to all safety standards. For example, you should not use concrete with such components when the structure is under tension and monolithic chimneys are being erected.
SNiP
All installation and construction activities must be carried out in accordance with established standards. The concreting process in winter is no exception. Warming up of a concrete structure at low air temperatures occurs in accordance with the following documents:
- SNiP 3.03.01-87 – Load-bearing and enclosing structures
- SNiP 3.06.04-91 – Bridges and pipes
The video shows concrete heating in winter, technological map:
Despite the fact that the presented documentation only indirectly touches on the topic related to heating concrete, it contains certain sections in which there is a technology for pouring concrete mortar in the frosty season.
Timing
When calculating the heating of concrete, it is necessary to take into account factors such as the type of structure, the total heating area, the volume of concrete and electrical power.
During heating work with concrete, it is worth developing a technological map. It will include all the values of laboratory observations, as well as the heating time and hardening time of the material. 
Calculation of concrete heating begins with the selection of a scheme. For example, the four-stage method is most often chosen. The first stage involves curing the material. After this, the temperature indicators are increased to a specific value, heating and cooling are carried out; the duration of holding before the start of the event is approximately 1-3 hours at a low temperature. After this, you can proceed to the calculation of heating, which is directly dependent on the speed and final temperature.
Throughout the entire process, it is worth monitoring the temperature, noting all results when it rises after 30-60 minutes, and when cooling, monitoring is carried out once per shift. If the mode is violated, it is necessary to maintain all parameters by turning off the current and increasing the voltage. In this case, the actual indicators and those obtained during the calculation may not coincide. After this, a graph of the dependence of time on strength is constructed, where the required value of time and heating temperature is indicated, and then the required value of strength is found.
The process of heating concrete is a very important event, without which the concrete structure will simply cease to gain strength in cold weather, resulting in a decrease in grade and further destruction. It is not difficult to carry out all these activities; you just need to determine which of the presented ones suits you best.
Using a technological map for heating concrete in winter, you can combine ensuring efficiency with compliance with safety standards. This document contains information about heating concrete structures and technological solutions that will help speed up work and reduce labor costs without compromising the quality of structures erected in winter.
Application area
The technological map is relevant when it is necessary to warm up lightly reinforced monolithic concrete structures. The described methods are the most effective for the following parts of the structure:
There are several types of heating. The most commonly used are:
- peripheral;
- through;
- reinforcement
All methods differ only in the elements used as electrodes, and their principle is the same - when electricity is passed through, heat is released, which heats the concrete from the inside.
The technological map for electrical heating of concrete contains the necessary diagrams, as well as description of all elementary operations:
- recruitment of workers with the required qualifications;
- calculation of labor costs;
- drawing up a work schedule;
- calculation of material costs for machinery and equipment;
- preparation for concreting and heating;
- organization of work area;
- installation of electrical equipment and its connection.
It also provides safety regulations and tips for saving energy.
Work organization
Electrical heating of concrete using PNSV wire according to the technological map begins with preparation. First, the complex transformer substation is installed on a flat surface and tested at idle by connecting the device to the power supply. Then sections of busbars are prepared and mounted near structures that require heating. The installed sections are then connected with suitable cables and connected to the substation circuit.
If necessary, remove ice, debris or snow from the work site.
The concrete mixture is placed in formwork, open surfaces are insulated with polyethylene film and mineral wool mats. Electrodes are driven into the points indicated in the diagram - steel rods with a diameter of 6 millimeters and a length of 1 meter - while the visible ends should be longer than 10 and shorter than 20 centimeters, the distance between them depends on the air temperature and the selected voltage. All this is regulated by the tables given in the technological map. The electrodes are connected and connected to the busbars.
Check before turning on electricity a few important points:
- compliance of the actual installation of electrodes with the diagram;
- correct connection of electrodes and their connection;
- presence of temperature sensors;
- quality of contacts;
- compliance with the rules for laying insulation.
If everything is in order, then current is supplied to the converter. If a short circuit occurs, the electrician on duty diagnoses and corrects the cause of the fault. In any case, the specialist is obliged to check the condition of the contacts again - this is a safety standard.
The temperature sensor readings are first checked once an hour; normally, the measurement results change by 6 degrees each time. When the isothermal phase ends and the concrete begins to heat up, this is done half as often. At each stage, it is necessary to check not only the instrument readings, but also the condition of the taps and connections.
If it is necessary to adjust the warm-up speed, then the voltage of the low side of the electrical transformer is changed. The same applies to situations when the outside air temperature becomes different from the calculated one, which is checked twice a day by recording the thermometer readings in a log. With the same frequency, the characteristics of the electric current are measured - strength and voltage - and the connections are inspected to prevent sparking.
Thermal insulation, like formwork, is removed only after the top layers have cooled to 5 degrees, but before the temperature drops to zero degrees, otherwise they may freeze to the concrete, which is unacceptable. To avoid cracks, monitor the difference in temperature between the surface and the air, which should not exceed 20-30 degrees. If it is impossible to achieve such conditions, the concrete is protected with roofing felt or tarpaulin. The cooling rate should be in the range of five to ten degrees per hour.
The result is greatly influenced by following a few simple rules. When laying the base, workers must not allow the concrete to freeze due to contact with the base or deform it without acquiring the required strength. You cannot remove ice from a structure that has already been insulated with hot water or steam. The concrete mixture is poured evenly, while the mass should cool slowly and not reach a temperature below five degrees.
This technique is presented as a demonstration of an approximate sequence of actions and features of electric heating; it is not a manual. To warm up the concrete, you need to download the technological map and follow it.
Energy Saving
For effective energy saving, several conditions must be met. It is important not to allow the concrete mixture to cool at the transportation or placement stage by more than the value established by the technological calculation. Portland cement (especially fast-hardening cement) will contribute to savings. This mixture has high relative strength, which means it takes less time to warm up. A chemical additive can be included in another type of mass, which will reduce the duration of heat treatment by increasing the electrical conductivity or strength of concrete.
The structure should be heated to the maximum permissible temperature, because strength increases mainly during the cooling stage. Poor-quality thermal insulation or its getting wet, cables of inappropriate density or broken contacts - all this leads to wasted energy.
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