If you need to pour a foundation or carry out other similar works at negative temperatures, then you can’t do without heating procedures. Moreover, they must be carried out in accordance with building regulations. How concrete is heated in winter time according to SNIP No. 3_03_01-87, you will find out now.

Why do you need to heat concrete?

As already noted, concrete is poured not only in summer, but also in winter. The difference is that in winter period cement composition heating is required, the price of which can be quite high.

This process is necessary for the following reasons:

• at negative temperatures;
• the structure of the material is destroyed, due to which deformed areas are formed on it, and it ultimately becomes less durable.

Advice! Cutting reinforced concrete with diamond wheels will help you remove protruding irregularities. In this case, it is necessary to use protective equipment in the form of a respirator and special glasses. As for small depressions, cleaning them will require diamond drilling holes in concrete and subsequent filling of the recesses with cement mortar.

These processes can be avoided, but this will require equipment for heating the concrete in winter. You can do without it only if, before the appearance low temperatures the composition managed to gain a certain strength. For convenience, the data is included in the table:

Types of concrete heating

SNiP number 3_03_01-87 establishes which methods of heating concrete in winter should be used for certain structures.

These methods include:

• thermos;
• preheating the composition;
• heating in the formwork;
• induction method;
• electrode heating;

• use of heating wires;
• thermos with antifreeze components;
• infrared heating.

We will look at the most common of them.

Heating concrete with heating wire

To minimize the heating time of concrete in winter, a special heating wire is used - PNSV.

His components are:

1. steel core consisting of one wire;
2. insulating layer made of polyethylene or PVC.

This heating method is based on the use transformer substations, which heat up the wires greatly. From them, heat is transferred to the concrete composition. It should be noted that this method is very convenient, since it allows you to adjust the heating level depending on weather conditions.

To install such a system you will need routing warming up concrete in winter. It is usually compiled by an energy specialist who is an employee construction organization. There are also standard samples of such a document.

This map determines the number and location of heating stations, as well as the order of placement and the number of heating wires. As calculations for heating concrete in winter show, heating 1m³ of solution requires an average of 50-60 meters of cable.

Implemented this technology in the following way:

1. the heating wire is placed inside the structure being built - this is done so that the conductors are placed evenly, do not touch the formwork, do not extend beyond the edges of the concrete and do not come into contact with each other;

In the photo - laying the wire

1. The cold ends are soldered to the heating wire - after that they are taken out of the heating zone;

Advice! To maintain a thermal field in the soldering area, wrap this area with foil.

1. wire terminals are connected to transformer equipment in accordance with the instructions contained in the technological maps:
2. collected electrical circuit checked with a megohmmeter;
3. voltage is supplied to the created system and the heating process begins, for correct implementation which will be required temperature graph heating of concrete in winter, contained in the technological map.

Thermos method

As the name implies, this method is not intended to transfer, but to retain heat. It consists of protecting concrete with the help of thermal insulation materials placed outside it. Thanks to them, the mixture used loses heat more slowly and gains strength faster ().

The advantage of the method under consideration is its affordable cost, because even ordinary sawdust can be used as insulation. However, it should be noted that passive heat conservation alone may not be enough. In this case, in addition to it, it will be necessary to use additional methods of heating the concrete in winter.

Infrared heating of concrete structures

This method is based on the use of infrared heaters. They are installed in such a way that the radiation emanating from them is directed towards the open concrete surface or on the formwork. The energy they transmit causes heating cement mortar and its accelerated hardening.

Advice! Do not use this method to warm up a structure that has a large volume. Infrared rays will not be able to heat it evenly, which will lead to a decrease in the strength of the material. Therefore, for massive products it is better to use other types of concrete heating in winter.

Induction heating

This method uses the phenomenon of electromagnetic induction to generate heat. With its help, the energy of the electromagnetic field is modified and becomes thermal radiation, which is transferred to the material being processed. This transformation occurs in steel formwork or on reinforcement.

The instructions for implementing this method state that it can only be used in those structures that have a closed loop. In addition, they must have dense reinforcement with a reinforcement coefficient of over 0.5. Another necessary condition- Availability metal formwork or the ability to wrap a cable around the structure to create an inductor.

Conclusion

When carrying out reinforced concrete work in frosty weather, it is necessary to use heating. Without it, the resulting structure will be less strong and durable ().

The most common heating methods include the use of heating wires, infrared emitters, the use of electromagnetic induction, as well as thermal insulation. The video in this article will tell you more about how concrete is heated in winter.

Excerpts from SNiP related to concrete work in winter: transportation, laying concrete mixture, how to pour concrete in winter at sub-zero temperatures.

SNiP. PRODUCTION OF CONCRETE WORK AT NEGATIVE AIR TEMPERATURES

2.53. These rules are followed during the period of concrete work when the expected average daily outside air temperature is below 5 °C and the minimum daily temperature is below 0 °C.

2.54. The preparation of the concrete mixture should be carried out in heated concrete mixing plants, using heated water, thawed or heated aggregates, ensuring the production of a concrete mixture with a temperature not lower than that required by calculation. It is allowed to use unheated dry aggregates that do not contain ice on the grains and frozen lumps. In this case, the duration of mixing the concrete mixture should be increased by at least 25% compared to summer conditions.

2.55. Methods and means of transportation must ensure the prevention of a decrease in the temperature of the concrete mixture below that required by calculation.

2.56. The condition of the base on which the concrete mixture is laid, as well as the temperature of the base and the method of laying must exclude the possibility of the mixture freezing in the area of ​​contact with the base. When curing concrete in a structure using a thermos method, when preheating the concrete mixture, as well as when using concrete with antifreeze additives It is allowed to lay the mixture on an unheated, non-heaving base or old concrete, if, according to calculations, freezing does not occur in the contact zone during the estimated period of curing the concrete.

At air temperatures below minus 10 °C, concreting of densely reinforced structures with reinforcement with a diameter greater than 24 mm, reinforcement made of rigid rolled sections or with large metal embedded parts should be carried out with preliminary heating of the metal to a positive temperature or local vibration of the mixture in the reinforcement and formwork areas, with the exception of cases of laying preheated concrete mixtures (at a mixture temperature above 45 ° C). The duration of vibration of the concrete mixture should be increased by at least 25% compared to summer conditions.

2.57. When concreting elements of frame and frame structures in structures with rigid coupling of nodes (supports), the need to create gaps in the spans depending on the heat treatment temperature, taking into account the resulting temperature stresses, should be agreed upon with the design organization. Unformulated surfaces of structures should be covered with steam and thermal insulation materials immediately upon completion of concreting.

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

2.58. Before laying the concrete (mortar) mixture The surfaces of the joint cavities of precast reinforced concrete elements must be cleared of snow and ice.

2.59. Concreting of structures on permafrost soils should be carried out in accordance with SNiP II-18-76.

Acceleration of concrete hardening when concreting monolithic bored piles and embedding bored piles should be achieved by introducing complex antifreeze additives into the concrete mixture that do not reduce the freezing strength of concrete with permafrost soil.

2.60. Choosing a concrete curing method for winter concreting monolithic structures should be carried out in accordance with recommended Appendix 9.

2.61. Concrete strength control should be carried out, as a rule, by testing samples made at the place where the concrete mixture is laid. Samples stored in the cold must be kept for 2-4 hours at a temperature of 15-20 °C before testing.

It is allowed to control the strength by the temperature of the concrete during its curing.

2.62. The requirements for work at subzero air temperatures are set out in the table. 6

25. Technology for the production of concrete work in winter

A feature and requirement for winter concreting is the creation of such a mode of laying and hardening of concrete in which by the time of freezing it acquires the necessary strength, called critical. The limits of such strength are indicated in SNiP.

Methods for laying concrete in winter determined by the methods used to maintain it. In practice, both unheated curing methods (thermos method) and methods of artificial heating or heating of structures are used (electrical heat treatment of concrete, the use of heating formwork and coatings, heating with steam, hot air or in greenhouses).

1. General methods for accelerating strength gain include: use of high activity cements; minimum W/C value; high frequency starting materials; long duration of mixing the mixture; thorough compaction of the concrete mixture.

2. Application of antifreeze additives (sodium chloride in combination with calcium chloride, sodium nitrate, potash, etc.), providing hardening at low temperatures. This allows you to transport the mixture in an uninsulated container and lay it out in the cold. The mixture with antifreeze additives is placed in structures and compacted in compliance with general rules laying concrete.

3. Heating of materials at the site of concrete preparation (the “thermos” method): heating of raw materials with steam (in stacks in a warehouse, in intermediate bins, in supply bins); insulated formwork (40 mm thick boards and 1...2 layers of roofing felt, double hollow formwork with a layer of sawdust, etc.); electrical heating of the concrete mixture before placement in special buckets.

4. Heating of concrete at the site of laying into blocks: electric heating (surface and deep electrodes, in thermoactive formwork, electric heating devices). Electrode heating of concrete is provided through electrodes located inside or on the surface of the concrete. Adjacent or opposite electrodes are connected to wires different phases, as a result of which between the electrodes in concrete there is electric field, warming it up. The current in reinforced structures is passed at a voltage of 50-120 V, and in non-reinforced ones - 127-380 V. When the current passes, the concrete heats up for 1.5-2 days. acquires formwork strength; heating in greenhouses and tents (air is heated inside the tent) is an effective and progressive method winter concreting; heating with warm air from air heaters; steam heating with special formwork.

26. Defects in concrete masonry and ways to eliminate them. Caring for laid concrete mixture

The reasons for the appearance of defects in the laying of concrete mixture: non-compliance of the concrete mixture with the requirements of GOST or the conditions of the laying block (dimensions, reinforcement); violation of concrete laying technology.

Laying defects: sinkholes, concrete delamination, sagging, surface wear, hairline cracks. Sinks are voids in a block that are not filled with concrete or filled with lean concrete (gravel without cement mortar). The reasons for their appearance are the arrival at the laying site of concrete containing gravel of unacceptable size in terms of the size of the block and the density of its reinforcement; due to leakage of cement mortar through cracks in the formwork and at the joints of the formwork; due to poor sealing. Most often they appear in difficult-to-work parts of blocks. External sinks are revealed when stripping the formwork, but inside the block they cannot be detected.

To eliminate internal cavities, cementation is used by injecting cement mortar with mortar pumps through holes made in concrete. External sinks are ripped out, the thin porous concrete is removed to healthy concrete and sealed with concrete containing fine gravel.

The reasons for concrete delamination are excessively prolonged vibration during compaction, dropping it into a block with high altitude. The delamination defect cannot be eliminated. Concrete laid with such a defect must be removed and replaced.

Sludges of cement laitance and a spongy concrete surface appear at the junction between the concrete surface and the formwork as a result of leakage of cement laitance during compaction of the overlying layers of concrete and pinching of air bubbles. They are eliminated when preparing the surface of a building block for concreting the adjacent block.

Hairline cracks in concrete appear as a result of its shrinkage and indicate an irrational composition of the concrete mixture (in particular, excess cement), oversized building blocks and high temperature stresses or poor care(quick drying out). This defect cannot be eliminated.

Elimination of removable defects consists of cutting out low-quality concrete, cleaning the cut-out area from dirt, dust to healthy concrete and preparing the surface in the same way as in a construction joint. Concrete newly laid in a defective area must be maintained in accordance with the previously stated rules until it reaches the required strength.

Maintenance of laid concrete is to protect it from mechanical damage, premature loads, in maintaining it in a wet state, in removing excess heat from large blocks, maintaining positive temperatures in winter, and preventing premature removal of formwork. Without care or poor care of hardening concrete, a sharp decrease in its strength is observed. Freshly laid concrete should be protected from walking and driving over it for 10...12 hours until initial strength is achieved, as well as from shock during the operation of construction machines.

In the first days after installation, it should be in a warm and humid environment. Best Temperature hardening 15...20°C. Therefore, during the concrete maintenance stage, it is watered and covered from the sun with straw mats, matting, and tarpaulin.

Moisten the concrete from hoses with a diffuse stream in the form of rain. This operation begins immediately after it has been established that cement particles will not be washed out of the set concrete when exposed to water.

Concrete is watered at air temperatures above 5°C, starting it under normal conditions after 10...12 hours, and in hot dry weather 2...4 hours after laying and continuing for 3...14 days with an interval of 3 to 8 hours. Water consumption for irrigation is at least 6 l/m2.

While the concrete is in the formwork, it is wetted. After stripping, wet and protect the stripped surface. At temperatures below 5°C, watering is stopped and the concrete is covered with matting or tarpaulin.

Caring for concrete is greatly simplified by covering it with moisture-proof films, painting in 1...2 layers with one of the following materials: bitumen or tar emulsions, petroleum bitumen solutions, ethinol varnish, synthetic rubber latex, etc. Film-forming materials are applied to the dried surface of the laid concrete. Material consumption from 300 to 700 g/m2. After the layer has dried, the concrete surface is covered with a layer of sand 3...4 cm thick for 20...25 days.

Coating with film-forming materials is permissible only in structural joints and on the uppermost open part of the concrete structure. Painting is not allowed in construction joints.

Today, thanks to the fact that modern technologies allow us to continue construction works even in winter, finished objects can be delivered exactly on time and with a guarantee of quality. Even in winter, concrete structures continue to be built and work does not stop due to weather conditions.

When concreting any structures in the winter season at sub-zero elevation, special requirements must be observed temperature conditions for hardening concrete. The main condition for high-quality hardening of the concrete mixture is not to allow the temperature to drop below the technically determined level.

In order to ensure the strength of concrete during its hardening, it is necessary to observe and withstand temperature regime.

Why warm up concrete in winter?

To understand where and why cracks appear in concrete, you need to know the principle of pouring it and the rules for its hardening. When mixing concrete mortar manually, water is added to the dry mixture. It is the excess water at sub-zero temperatures that freezes in the solution, which causes the formation of large ice crystals, and also creates strong pressure in the pores cement mixture, all this leads to the destruction of the uncured concrete solution and a strong decrease in its strength after hardening. The most critical is freezing during setting.

The main condition that must be met when curing and hardening concrete is the correct temperature regime. If all requirements are met, the strength of concrete will be maximum. When the temperature decreases, water interacts with cement more slowly, and when the temperature rises, it accelerates. Therefore, when concreting large monolithic structures in winter, it is necessary to observe the correct temperature and humidity conditions, which will allow the concrete to gain maximum strength in a minimum period of time.

Concreting method in winter

There are several methods for pouring concrete. Its types depend on weather conditions, as well as on the type of structure that is being built. Among the most common:

1. Thermos can also be added with anti-frost components.
2. Heating formwork.
3. Heating using electrodes.
4. IR or induction heating.
5. Warming up with wires.

To have a clearer idea of ​​heating the concrete mixture, we will consider the most relevant methods separately.

Electric heating of concrete in winter

The most common method that saves heat artificial method, is heating the solution using electrodes. The method is based on transmission electric current through the concrete solution, due to which heat is released. To supply current to the concrete mixture, it is just right to use Various types electrodes that have individual scheme connections. Because of D.C. provokes electrolysis of water in solution; during the warm-up period, single-phase and three-phase alternating current can be used.

Types of electrodes used for heating:

1. Rod electrode. It is made from reinforcement and placed in concrete mortar with calculated step. The edge must be located 3 centimeters from the formwork. With the help of such electrodes you can heat up the most complex structure.
2. Plate electrode. Such plates are attached to the inside of the formwork and by connecting electrodes opposite to each other, an electric field is created, under the influence of which the concrete mixture will be heated to desired temperature and hold for the required time.
3. String electrode. This type is usually used when heating concrete columns.
4. Strip electrode. Such strips can be attached to the required sides of the structure.

The next fairly common heating method is a heating wire. This technology is currently most used by large construction companies, both domestic and foreign. Note that quite a few objects in Moscow were heated during construction using this particular method.

This method consists of attaching a heating wire of the required length to the reinforcement frame before laying the mass in the formwork. This method involves the use of PNSV wire, its rod is galvanized steel, the diameter of which is 1.2 mm. The heat generated from such a wire, when electricity passes through it, is distributed evenly throughout the concrete mixture, and allows it to be heated to 40 degrees. The wires are powered by electricity using special substations that have several stages of reduced voltage. One such substation is capable of heating up to 3 cubic meters of concrete. To warm up 1 cubic meter concrete, about 60 meters of wire are required. This method allows you to heat concrete structures of any complexity at temperatures down to -30 degrees.

Large today construction companies Several types of heating are used simultaneously. The need for such a combination depends on many factors, the main ones being:

• size of the construction project;
• required concrete strength;
• weather;
• availability of energy resources at construction sites.

The method of heating the formwork involves its design with heating elements that are initially placed in it. This method is similar to the plate heating method, only the heating does not come from inside formwork, but from its interior or exterior.

This method is not used very often in winter due to its complexity. When pouring the foundation, the formwork cannot come into contact with the entire concrete structure, so only part of the concrete mass is heated.

The induction method is used extremely rarely. It is usually used in beams, purlins, and crossbars. The principle of this method is that it is wrapped around metal reinforcement. insulated wire, which creates induction and heats the metal rod itself.

Electric heating concrete structure used in winter due to the fact that IR rays are able to heat the entire surface of an opaque object and distribute heat over the entire area. When choosing this method, you should take into account that the structure needs to be enveloped plastic film so that the rays pass through it and the heat does not escape too quickly. The advantage of this method is that it does not require special substations, but the disadvantage is uneven heating of the concrete structure. This method is most suitable for heating a thin structure.

When starting the construction of a particular facility, make sure that all the rules, recommendations are followed and all the nuances are taken into account, otherwise you can get not only a poor-quality result, but also do it in a year major renovation the entire concrete structure.

Concrete will only be strong if it is laid the right technique and stood up to standards.

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Basics of winter concreting

Concrete works when the average daily outside air temperature is below 5°C and the minimum daily temperature is below 0°C, perform according special rules, installed for work in winter conditions(SNiP III-15-76).

In winter conditions, the main task is to prevent premature freezing of the laid concrete. It is necessary that the concrete maintains a positive temperature (above O0) during placement and maintenance until its strength reaches a certain value, called the “critical” strength

For structures that are immediately subjected to alternating freezing and thawing immediately after curing, the critical strength of concrete, regardless of its class, must be at least 70%. and in prestressed structures - at least 80% of the design strength.

For structures that are immediately exposed to design water pressure upon completion of curing (reservoirs, retaining walls), as well as structures that are subject to special requirements in terms of frost resistance and water resistance, the critical strength must be at least 100% of the design strength.

For massive structures special purpose(dams, supports, bridges, etc.) the conditions and periods of permissible freezing of concrete are established in the project. The requirements listed above are due to the fact that concrete at negative temperatures (below 0°C) does not harden, since the water in it turns into ice and the physical and chemical processes of interaction between cement and mixing water practically stop. However, when the frozen concrete thaws, the hardening processes resume, and if freezing occurs no earlier than it reaches critical strength, then the concrete will subsequently acquire the specified (design) strength. If the concrete is allowed to freeze earlier, a partially irreversible loss of strength will occur (mainly due to a breakdown in adhesion between the coarse aggregate and the cement mortar).

The loss of strength will be greater, the younger the concrete was at the time of freezing (for example, Portland cement concrete, which reaches strength on the 28th day and is frozen a day after laying, irreversibly loses up to half of its strength). Concrete frozen when it reaches the above critical strength values ​​must be maintained after thawing in conditions that ensure that it obtains the design strength until the structure is loaded with the design load.

By the time the load-bearing formwork of concrete and reinforced concrete structures it is required that the concrete strength is 50...100% of the design. Such structures, after stripping, can in many cases be exposed to low temperatures without harm, but in each specific case It is still necessary to compare the stripping and critical strength. In cases where, due to the conditions of multiple turnover of the formwork, the latter (for example, side panels foundation formwork, pillars, walls, etc.) are removed before the concrete reaches critical strength, the stripped surfaces should be temporarily covered.

The same has to be done in cases where the temperature difference between the concrete surface and the outside air exceeds the following values: 20С - for structures with a surface module from 2 to 5 and; Ш°С - for structures with a surface module 5 and above. Otherwise, during rapid cooling, temperature cracks will form on the surface of the beyun.

Stripping of structures is carried out at a positive concrete temperature; Under no circumstances should the formwork be allowed to freeze to the concrete.

For hardening in winter conditions of concrete prepared in ordinary water(without introducing into it chemical additives of salts that lower the freezing point of the resulting saline solution), it is necessary, first of all, that the mixture be placed in the formwork warm and all its components have a positive temperature. For example, it is impossible to place concrete mixture prepared on frozen sand and crushed stone into formwork. When such a mixture is heated after laying, the moisture contained in the frozen state in sand and crushed stone will thaw and occupy a smaller volume (it is known that water increases when freezing and, conversely, ice decreases in volume by about 10% when thawing." 13 the result of this is loose , porous, and therefore low-strength concrete.

Therefore, in winter, the concrete mixture is prepared using heated water; fillers (sand, crushed stone) are also heated or thawed to a positive temperature. An exception may be made for dry crushed stone or gravel that does not contain ice on the grains and frozen lumps (humidity not higher than 1...1.5%). Such filler can be loaded into the mixer unheated, provided that upon exiting the mixer the concrete mixture will have a given positive temperature. The cement is not heated, since when mixed with water and aggregates it quickly takes on a positive temperature.

Transportation and placement of the concrete mixture is carried out quickly so that its temperature in the formwork is positive.