Can concrete be poured in winter? Concreting in winter: methods, features, necessary measures.

If necessary, winter concreting main problem are low temperatures environment that lead to freezing building materials. Accordingly, the technology of concreting in winter conditions is aimed at preventing the freezing of water and other materials.

Requirements for winter concreting are determined by SNiP 3.03.01, according to which temperatures below 5°C are considered winter conditions.

Features of winter concreting

There are two important reasons that complicate the process of laying concrete in winter.

• At low temperatures the process of cement hydration slows down, which is the reason for the increase in the time for concrete to harden.

At an ambient temperature of 20 0 C, concrete gains about 70% of its design strength within a week. When the temperature drops to 5 0 C, it will take 3-4 times more time to gain such a level of strength.

• Another undesirable process is the development of internal pressure forces that arise due to the expansion of frozen water. This phenomenon leads to softening of concrete. In addition, ice films form around the aggregates from frozen water, breaking the bond between the components of the mixture.

When water freezes, significant pressure develops in the pores of the hardening mixture, which leads to the destruction of the structure of weak concrete and a decrease in its strength characteristics.

The decrease in strength is the more significant, the earlier the age of the concrete, the water froze. The most dangerous is the setting period concrete mix. If the mixture freezes immediately after laying it in the formwork, then its strength at low temperatures will be due only to freezing forces. With an increase in temperature, the cement hydration process will resume, but the strength of such concrete will be significantly inferior to the same characteristic of the material that has not been frozen.

Only concrete that has already gained a certain strength value can withstand freezing without structural damage. It is important to follow the rule of continuous concrete placement to avoid cold joints.

AT modern construction in world practice, the most common method of winter concreting, when the concrete mixture is protected from freezing during its setting and gaining a certain strength value, which is called critical.

Under the critical value of the strength of concrete, the strength is taken, which is equal to 50% of the brand. In critical structures, concrete is protected from freezing until 70% of the design strength is reached.

In modern construction, several methods of concreting are used in winter period:

• the use of antifreeze additives;
• covering the concrete mixture with PVC film and other heaters;
• electric and infrared heating of concrete.

Regardless of what you are building, the question is, ? We know how to choose a brand depending on the type of object, the load and the nature of the soil.

The basic law of concrete strength, described, allows you to correctly plan construction work.

The most popular, concrete mixes and components.

The use of antifreeze additives

Technologically, the most convenient and cost-effective method of winter concreting is the use of antifreeze additives. This unheated method is much cheaper than concreting with preliminary fencing and insulation of the structure, heating with electricity and infrared rays.

Antifreeze modifiers can be used alone or in combination with various methods heating.

All existing "winter" additives in concrete can be divided into three main groups.

• The first group includes additives that either slightly accelerate or slightly slow down the processes of setting and hardening of the mixture. Representatives of this class are strong and weak electrolytes, non-electrolytes and compounds of organic origin - urea and polyhydric alcohols.
• Modifiers based on calcium chloride belong to the second group. These substances have the ability to greatly accelerate the setting and hardening processes and have significant antifreeze properties.
• The third group includes substances that have weak antifreeze properties, but are strong setting and hardening accelerators with strong heat release immediately after pouring. The scope of these additives is small, but they are of interest from a scientific point of view. Such additives include trivalent sulfates based on aluminum and iron.

Measures that increase the effectiveness of the use of antifreeze additives

Antifreeze additives perform important role- activate the processes of hardening of the mixture and reduce the freezing point of the liquid phase. But to obtain an effective result, along with the use of modifiers, it is necessary to perform a number of related activities.

• The preheating of its components contributes to the creation of internal heat in the concrete mix.
• After the laying is completed, the concrete surface must be insulated with mats, which will retain the heat generated as a result of the exothermic reaction of cement and water and maintain conditions suitable for hardening.
• In winter, it is most effective to use Portland cements and high-quality fast-hardening cements.

• In the manufacture of a concrete mix from heated components, a different order of loading of all elements is used than in traditional summer conditions, when all dry components are simultaneously loaded into a mixer drum filled with water. In winter, in order to avoid cement brewing, water is first poured into the drum, then coarse aggregate is poured in, and then the drum is rotated several turns and sand and cement are poured.

The duration of mixing of the components in winter time should be increased by about one and a half times.

• Transportation of the mixture should be carried out in an insulated car with a double bottom, where exhaust gases enter. Places for loading and unloading the concrete mixture must be isolated from the effects of wind, and the means of supplying the mixture must be carefully insulated.
• The formwork and reinforcement must be cleared of snow and ice, the reinforcement must be warmed up to a positive temperature.
• A prerequisite for winter concreting is the rapid pace of its implementation.

Thermos method

Technologically, the "thermos" method is carried out by laying a mixture of positive temperature in an insulated formwork. Concrete gains strength due to the initial heat content and exothermic release during the cement hydration reaction.

Maximum heat dissipation is provided by Portland cements and high-quality cements. The "thermos" method in combination with antifreeze additives is especially effective.

Concreting by the "hot thermos" method consists in short-term heating of the mixture to 60-80 0 С, compacting it in a hot state and keeping it in a "thermos" or using additional heating.

In the conditions of the construction site, the concrete mixture is heated using electrodes. The mixture acts as resistance in the alternating current circuit. Electrical heating is carried out in the bodies of dump trucks or tubs.

Methods of artificial heating and heating of concrete

The essence of this method is to create and further maintain the temperature of the mixture at the maximum allowable value until the concrete gains the required strength. This method is used in cases where the "thermos" method is not enough.

There are several options to achieve the desired result:

• The physical meaning of electrode heating is similar to the above described method of electrode heating of the mixture. In this case, heat is used, which is released by the mixture when an electric current is passed through it. Several types of electrodes are used to supply electric current to concrete: plate, string, strip, rod. The most effective are plate electrodes made of roofing steel. The plates are sewn onto the formwork surface, which is in direct contact with concrete, and connected to opposite phases of the network. Current exchange occurs between the opposite electrodes, as a result of which the entire concrete structure is heated.
• The essence of contact or conductive heating is to use the heat generated in the conductor during the passage of an electric current through it. By contact method, heat is transferred to all surfaces of the concrete element. From the surfaces, heat is distributed throughout the structure.

For contact heating of concrete, thermoset flexible coatings or thermoset formworks are used.

• The method of infrared heating is based on the ability of infrared rays, when absorbed by the body, to transform into thermal energy. Heat from the radiator to the heated body is carried out instantly without the use of a heat carrier. Quartz and tubular metal emitters are used as infrared wave generators. Infrared heating is used to warm up fittings that are frozen concrete surfaces, thermal protection of the laid concrete mix.
• At induction heating heat is used, which is released in the steel formwork or reinforcing parts and products located in the electromagnetic field of the coil-inductor. This method is used to warm up previously made concrete structures at any ambient temperature and in any formwork.

GD Star Rating
a WordPress rating system

In winter conditions (the average daily temperature of the outside air is below +5 ° C), free water freezes, which stops the process of cement hydration, its increase in volume (up to 9%) destroys the concrete structure. This leads to the fact that after thawing, the concrete can no longer gain design strength.

It has been established that if concrete gains 30 ... 50% of the design strength before freezing, then further exposure to low temperatures does not affect its physical and mechanical characteristics. This value of strength is called critical. Depending on the brand of concrete, it is equal to: 50% M - for M200, 40% M - for M300 and 30% M - for M400 and above.

The winter methods of concreting, which ensure the achievement of critical strength by concrete, include: heating the concrete during its preparation; keeping concrete in insulated formwork (thermos method); the introduction of chemical additives into concrete that reduce the freezing point; thermal effect of heating formwork on freshly laid concrete; electrode heating; exposure to infrared heat sources, etc. Select technological methods depending on economic efficiency, concreting conditions, type of structures and features of the concretes used, availability of cheap heat sources.

During the preparation of concrete mixtures at the plants, the heating of the components and mixing water is organized, the preparation process itself is carried out in a warmed room, which ensures the output of the concrete mixture of a given temperature. To heat sand and gravel, special registers are used through which water or steam heated to 90 ° C is passed. Mixing water is heated to a temperature of 40 ... 80 ° C (depending on the type of cement) mainly by steam in water heaters.

The concrete mixture is transported in winter in insulated concrete trucks, special containers, dump trucks with body heating by exhaust gases. The body is covered with a tarpaulin or insulated shields, tubs and bunkers - with wooden insulated covers.

Winter concreting with unheated curing of concrete includes the “thermos” method, which is based on laying a concrete mixture heated to a temperature of 20 ... 80 ° C into insulated formwork. Exposed concrete surfaces protect against cooling. The amount of heat introduced into the concrete mixture and released during the exothermic reaction of the cement is quite sufficient for the concrete to achieve critical strength.

Transportation to the place of concreting of the heated concrete mixture is accompanied by significant heat losses, an increase in the rigidity of the mixture and a decrease in its workability. In order to eliminate these shortcomings, it is more expedient to heat concrete directly at the place of work. For this, special electrodes are used, which are immersed in the concrete mixture located in the back of a dump truck or in a bunker. Leading up to them electricity 380 V, the mixture is heated for 5 ... 10 minutes to a temperature of 75 ... 90 ° C.

In practice, the method of electrical heat treatment of concrete is widely used. It is based on the conversion of electrical energy into thermal energy directly inside the concrete or in various kinds electrical heating devices. The following methods have been mastered in construction: electrode heating (electric heating itself); heating in an electromagnetic field (induction); heating with various electric heating devices.

The electrode method of heating is divided into through and peripheral. With through heating, rod electrodes with a diameter of up to 6 mm are used, placing them over the entire cross section, with peripheral heating - floating frame and plate, sewn-in plate and string electrodes. In each specific case, the layout of the electrodes and the voltage on them are calculated. When heating concrete, they strictly monitor the rate of rise in its temperature (8 ... 15 ° C / h) and the time of isothermal heating.

For contact electrical heating are used different kind heating formwork, which is divided into hard (wooden, metal) and soft (from tarpaulin or asbestos fabric, rubber, plastic, etc.). Thermoactive formwork is installed with separate panels or enlarged panels. The heat sources in the shields are rod, tubular-rod and angle-rod electric heaters, strip electrodes, electrodes made of wire or foil, pressed into an electrically conductive composition.

To heat concrete with steam, a so-called “steam jacket” is created around the concreted structure, which provides the required temperature and humidity conditions for concrete hardening. Heating temperature 70...95° С.

Induction heating of concrete occurs due to the release of heat during the passage of eddy currents in the metal formwork and the structure, which are in the electromagnetic field of the inductor (multi-turn coil), through which alternating current industrial frequency with a voltage of 36 ... 120 V. Heat from reinforcement and metal formwork is transferred to concrete and heats it up. Induction heating is mainly used for heat treatment of concrete of small-section structures: columns, beams, joints, structures erected in sliding, climbing and horizontally moving formwork.

Heating elements with a power of 0.6 ... 1.2 kW, ceramic rod radiators with a diameter of 6 ... 50 mm with a power of 1 ... 10 kW, quartz tubular radiators and other means serve as sources of heating with infrared rays. Infrared emitters complete with reflectors are used for heating thin-walled capacitive structures, concrete preparation, monolithic joints and knots, etc. During heating, the temperature on the concrete surface should not exceed 80 ... 90 ° C.

The use of chemical additives in concrete reduces the freezing point of water and thus ensures concrete hardening at low temperatures. Potash (P), sodium nitrite (NN), calcium nitrate (NK), calcium nitrate compound with urea (NKM), calcium nitrite-nitrate (NNK), calcium chloride (CH) with sodium chloride (CN) are used as antifreeze additives. , calcium chloride (CC) with sodium nitrite (NN), etc. The choice of antifreeze additives and their optimal amount depend on the type of structure to be concreted, its degree, the presence of aggressive agents and stray currents, and the ambient temperature.

The concept of "winter conditions" in the technology of monolithic concrete and reinforced concrete is somewhat different from the generally accepted calendar one. Winter conditions begin when the average daily outdoor temperature drops to +5°C, and during the day there is a drop in temperature below 0°C.

At negative temperatures, water that has not reacted with cement turns into ice and does not enter into a chemical combination with cement. As a result, the hydration reaction stops and, consequently, the concrete does not harden. At the same time, significant internal pressure forces develop in concrete, caused by an increase (by about 9%) in the volume of water when it passes into ice. With early freezing of concrete, its fragile structure cannot withstand these forces and is broken. Upon subsequent thawing, the frozen water again turns into a liquid and the cement hydration process resumes, however, the destroyed structural bonds in concrete are not completely restored.

Freezing of freshly laid concrete is also accompanied by the formation of ice films around the reinforcement and filler grains, which, due to the influx of water from less cooled concrete zones, increase in volume and squeeze the cement paste from the reinforcement and filler.

All these processes significantly reduce the strength of concrete and its adhesion to reinforcement, and also reduce its density, durability and durability.

If concrete acquires a certain initial strength before freezing, then all the processes mentioned above do not adversely affect it. The minimum strength at which freezing is not dangerous for concrete is called critical.

The value of the normalized critical strength depends on the class of concrete, type and operating conditions of the structure and is: for concrete and reinforced concrete structures with non-stressed reinforcement - 50% of the design strength for B7.5 ... B10, 40% for B12.5 ... B25 and 30% for B 30 and above, for structures with prestressed reinforcement - 80% of the design strength, for structures , subjected to alternate freezing and thawing or located in the zone of seasonal thawing of permafrost soils - 70% of the design strength, for structures loaded with the design load - 100% of the design strength.

The duration of hardening of concrete and its final properties largely depend on temperature conditions in which concrete is kept. As the temperature rises, the activity of water contained in the concrete mixture increases, the process of its interaction with cement clinker minerals accelerates, and the processes of formation of the coagulation and crystalline structure of concrete intensify. When the temperature drops, on the contrary, all these processes are inhibited and the hardening of concrete slows down.

Therefore, when concreting in winter conditions, it is necessary to create and maintain such temperature and humidity conditions under which concrete hardens to acquire either critical or specified strength in the shortest possible time with the lowest labor costs. For this, special methods of preparation, supply, laying and curing of concrete are used.

When preparing a concrete mixture in winter conditions, its temperature is increased to 35 ... 40C by heating aggregates and water. Aggregates are heated up to 60C by steam registers, in rotating drums, in blowdown plants flue gases through the filler layer, hot water. Water is heated in boilers or hot water boilers up to 90C. Heating of cement is prohibited.

When preparing a heated concrete mix, a different order of loading the components into the concrete mixer is used. In summer conditions, all dry components are loaded into the mixer drum, pre-filled with water, at the same time. In winter, in order to avoid “brewing” of cement, water is first poured into the mixer drum and coarse aggregate is loaded, and then, after several turns of the drum, sand and cement are loaded. The total duration of mixing in winter conditions is increased by 1.2 ... 1.5 times. The concrete mixture is transported in a closed container (buckets, car bodies), insulated and warmed up before starting work. Cars have a double bottom, into the cavity of which the exhaust gases of the engine enter, which prevents heat loss. The concrete mixture should be transported from the place of preparation to the place of laying as quickly as possible and without overloading. The places of loading and unloading must be protected from the wind, and the means of supplying the concrete mixture to the structure (trunks, vibrating hobs, etc.) are insulated.

The condition of the base on which the concrete mixture is laid, as well as the laying method, should exclude the possibility of its freezing at the junction with the base and deformation of the base when laying concrete on heaving pounds. To do this, the base is heated to positive temperatures and protected from freezing until the newly laid concrete acquires the required strength.

Formwork and reinforcement before concreting are cleaned of snow and ice, reinforcement with a diameter of more than 25 mm, as well as reinforcement from rigid rolled profiles and large metal embedded parts at temperatures below -10 ° C are heated to a positive temperature.

Concreting should be carried out continuously and at a high rate, while the previously laid layer of concrete should be covered before the temperature in it falls below the intended one.

The construction industry has an extensive arsenal of effective and economical methods of curing concrete in winter conditions, which make it possible to ensure high quality structures. These methods can be divided into three groups: a method that involves the use of the initial heat content introduced into the concrete mixture during its preparation or before laying in the structure, and the heat release of cement that accompanies concrete hardening - the so-called "thermos" method, methods based on artificial heating of concrete laid in the structure - electrical heating, contact, induction and infrared heating, convective heating, methods that use the effect of lowering the eutectic point of water in concrete with the help of special antifreeze chemical additives.

These methods can be combined. The choice of this or that method depends on the type and massiveness of the structure, the type, composition and required strength of concrete, the meteorological conditions of the work, the energy equipment of the construction site, etc.

Thermos method

The technological essence of the "thermos" method lies in the fact that having a positive temperature (usually in the range of 15 ... 30 ° C), the concrete mixture is placed in an insulated formwork. As a result, the concrete of the structure gains the specified strength due to the initial heat content and exothermic heat release of the cement during cooling down to 0°C.

In the process of concrete hardening, exothermic heat is released, which quantitatively depends on the type of cement used and the curing temperature.

High-quality and fast-hardening Portland cements have the highest exothermic heat release. The exotherm of concrete provides a significant contribution to the heat content of the structure, maintained by the "thermos" method.

Concreting by the method "Thermos with additives-accelerators"

Some chemicals (calcium chloride CaCl, potassium carbonate - potash K2CO3, sodium nitrate NaNO3, etc.), introduced into concrete in small quantities (up to 2% by weight of cement), have the following effect on the hardening process: these additives accelerate the hardening process in initial period curing concrete. So, concrete with the addition of 2% calcium chloride by weight of cement already on the third day reaches a strength 1.6 times greater than concrete of the same composition, but without the additive. The introduction of accelerator additives into concrete, which are also antifreeze additives, in the indicated quantities lowers the freezing point to -3 ° C, thereby increasing the duration of concrete cooling, which also contributes to the acquisition of greater concrete strength.

Concrete with accelerator additives is prepared on heated aggregates and hot water. At the same time, the temperature of the concrete mixture at the outlet of the mixer varies between 25...35°C, decreasing by the time of laying to 20°C. Such concretes are used at an outdoor temperature of -15 ... -20 ° C. They are laid in insulated formwork and covered with a layer of thermal insulation. Concrete hardening occurs as a result of thermos curing in combination with the positive effect of chemical additives. This method is simple and quite economical, allows you to use the "thermos" method for structures with Mn

Concreting "Hot thermos"

It consists in short-term heating of the concrete mixture to a temperature of 60 ... 80 ° C, compacting it in a hot state and thermos keeping or with additional heating.

In the conditions of a construction site, the heating of the concrete mixture is carried out, as a rule, by electric current. To do this, a portion of the concrete mix is ​​connected by means of electrodes to an alternating current electric circuit as a resistance.

Thus, both the released power and the amount of heat released over a period of time depend on the voltage supplied to the electrodes (direct proportionality) and the ohmic resistance of the concrete mixture being pierced (inverse proportionality).

In turn, the ohmic resistance is a function of the geometric parameters of flat electrodes, the distance between the electrodes and the specific ohmic resistance of the concrete mixture.

The electric heating of the concrete mixture is carried out at a voltage of 380 and less often 220 V. To organize electric heating, a post with a transformer (voltage on the low side 380 or 220 V), a control panel and a switchboard is equipped at the construction site.

The electrical heating of the concrete mixture is carried out mainly in tubs or in the bodies of dump trucks.

In the first case, the prepared mixture (at a concrete plant) having a temperature of 5...15°C is delivered by dump trucks to the construction site, unloaded into electric tubs, heated to 70...80°C and placed in the structure. Most often, ordinary tubs (shoes) are used with three electrodes made of steel 5 mm thick, to which wires (or cable cores) of the mains are connected using cable connectors. For uniform distribution of the concrete mixture between the electrodes when loading the bucket and better unloading of the heated mixture into the structure, a vibrator is installed on the body of the bucket.

In the second case, the mixture prepared at the concrete plant is delivered to the construction site in the back of a dump truck. The dump truck enters the heating station and stops under the frame with electrodes. When the vibrator is running, the electrodes are lowered into the concrete mixture and voltage is applied. Heating is carried out for 10 ... 15 minutes to the temperature of the mixture on quick-hardening Portland cements 60°C, on Portland cements 70°C, on slag Portland cements 80°C.

To heat the mixture to such high temperatures for short span time, large electrical power is required. So, for heating 1 m of the mixture to 60°C in 15 minutes, 240 kW is required, and in 10 minutes - 360 kW of installed power.

Artificial heating and heating of concrete

The essence of the method of artificial heating and heating is to increase the temperature of the laid concrete to the maximum allowable and maintain it for the time during which the concrete gains critical or specified strength.

Artificial warming up and heating of concrete is used when concreting structures with Mn> 10, as well as more massive ones, if in the latter it is impossible to obtain a given strength in a timely manner when cured only by the thermos method.

The physical essence of electrical heating(electrode heating) is identical to the method of electric heating of the concrete mixture discussed above, i.e., the heat released in the laid concrete when an electric current is passed through it is used.

The resulting heat is spent on heating the concrete and formwork to a predetermined temperature and compensating for heat losses to the environment that occur during the curing process. The temperature of concrete during electrical heating is determined by the amount of electric power released in concrete, which should be assigned depending on the selected heat treatment mode and the amount of heat loss that occurs during electrical heating in the cold.

Various electrodes are used to supply electrical energy to concrete: plate, strip, rod and string.

The following basic requirements are imposed on the designs of electrodes and their layouts: the power released in concrete during electrical heating must correspond to the power required by thermal calculation, electric and, therefore, temperature fields should be as uniform as possible, the electrodes should be located as far as possible outside the heated structure to ensure minimal metal consumption, the installation of electrodes and the connection of wires to them must be done before the concrete mix is ​​laid (when using external electrodes).

Plate electrodes satisfy the stated requirements to the greatest extent.

Plate electrodes belong to the category of surface electrodes and are plates made of roofing iron or steel, sewn onto the inner surface of the formwork adjacent to concrete and connected to opposite phases of the power supply network. As a result of the current exchange between the opposite electrodes, the entire volume of the structure is heated. With the help of plastic electrodes, weakly reinforced structures of the correct shape are heated small size(columns, beams, walls, etc.).

Strip electrodes are made from steel strips 20 ... 50 mm wide and, like plate electrodes, are sewn on inner surface formwork.

The current exchange depends on the connection scheme of the strip electrodes to the phases of the supply network. When opposite electrodes are connected to opposite phases of the supply network, current exchange occurs between opposite faces of the structure, and the entire mass of concrete is involved in heat release. When adjacent electrodes are connected to opposite phases, current exchange occurs between them. In this case, 90% of all input energy is dissipated in peripheral layers with a thickness equal to half the distance between the electrodes. As a result, the peripheral layers are heated due to the Joule heat. The central layers (the so-called “core” of concrete) harden due to the initial heat content, exotherm of cement, and partly due to the influx of heat from the heated peripheral layers. The first scheme is used for heating weakly reinforced structures with a thickness of not more than 50 cm. Peripheral electrical heating is used for structures of any massiveness.

Strip electrodes are installed on one side of the structure. In this case, adjacent electrodes are connected to opposite phases of the supply network. As a result, peripheral electrical heating is realized.

One-sided placement of strip electrodes is used for electrical heating of plates, walls, floors and other structures with a thickness of not more than 20 cm.

With a complex configuration of concreted structures, rod electrodes are used - reinforcing bars with a diameter of 6 ... 12 mm, installed in the body of concrete.

It is most expedient to use rod electrodes in the form of flat electrode groups. In this case, a more uniform temperature field in the concrete is provided.

For electrical heating of concrete elements of small cross section and considerable length (for example, concrete joints up to 3 ... 4 cm wide), single rod electrodes are used.

When concreting horizontally located concrete or having a large protective layer reinforced concrete structures use floating electrodes - reinforcing bars 6 ... 12 mm, sunk into the surface.

String electrodes are used to heat structures whose length is many times greater than their dimensions. cross section(columns, beams, girders, etc.). String electrodes are installed in the center of the structure and connected to one phase, and the metal formwork (or wooden formwork with roofing steel decking) to the other. In some cases, a working fitting can be used as another electrode.

The amount of energy released in concrete per unit time, and hence temperature regime electrical heating depends on the type and size of the electrodes, the scheme of their placement in the structure, the distances between them and the scheme of connection to the mains. In this case, the parameter that allows arbitrary variation is most often the input voltage. The generated electric power, depending on the parameters listed above, is calculated by the formulas.

The current to the electrodes from the power source is supplied through transformers and switchgears.

As main and switching wires, insulated wires with a copper or aluminum core are used, the cross section of which is selected from the condition of passing the calculated current through them.

Before turning on the voltage, the correct installation of the electrodes, the quality of the contacts on the electrodes and the absence of their short circuit to the armature are checked.

Electrical heating is carried out at low voltages within 50 ... 127 V. Average specific consumption electricity is 60 ... 80 kW / h per 1 m3 of reinforced concrete.

Contact (conductive) heating. This method uses the heat released in the conductor when an electric current passes through it. Then this heat is transferred by contact to the surfaces of the structure. The transfer of heat in the concrete of the structure itself occurs by thermal conduction. For contact heating of concrete, thermoactive (heating) formworks and thermoactive flexible coatings (TAGP) are mainly used.

Heating formwork has a deck made of metal sheet or waterproof plywood, with back side which are located electrical heating elements. In modern formwork, heating wires and cables, mesh heaters, carbon tape heaters, conductive coatings, etc. are used as heaters. The most effective use of cables, which consist of constantan wire with a diameter of 0.7 ... 0.8 mm, placed in heat-resistant insulation . The surface of the insulation is protected from mechanical damage by a metal protective stocking. To ensure uniform heat flow the cable is placed at a distance of 10 ... 15 cm branch from the branch.

Mesh heaters (metal mesh strip) are isolated from the deck with an asbestos sheet gasket, and on the back side of the formwork shield - also with asbestos sheet and covered with thermal insulation. For creating electrical circuit individual strips of the grid heater are interconnected by distributing tires.

Carbon tape heaters are glued with special adhesives to the deck of the shield. To ensure strong contact with the switching wires, the ends of the tapes are copper-plated.

Any inventory with a steel or plywood deck can be converted into a heating formwork. Depending on the specific conditions (rate of heating, ambient temperature, power of thermal protection of the rear part of the formwork), the required specific power can vary from 0.5 to 2 kV A/m2. Heating formwork is used in the construction of thin-walled and medium-massive structures, as well as in monolithic units of prefabricated reinforced concrete elements.

Thermoactive Coating (TRAC) is a lightweight, flexible device with carbon tape heaters or heating wires that provide heating up to 50°C. The basis of the coating is fiberglass, to which the heaters are attached. For thermal insulation, staple glass fiber is used with a shielding layer of foil. Rubberized fabric is used as waterproofing.

Flexible coating can be produced in various sizes. For fastening individual coatings to each other, holes are provided for the passage of braid or clips. The coating can be placed on vertical, horizontal and inclined surfaces of structures. At the end of work with the coating in one place, it is removed, cleaned and rolled into a roll for ease of transportation. It is most effective to use TRAPS in the construction of floor slabs and coatings, floor preparation, etc. TRAPS are manufactured with a specific electrical power of 0.25 ... 1 kV-A/m2.

Infrared heating uses the ability of infrared rays to be absorbed by the body and transformed into thermal energy, which increases the heat content of this body.

Infrared radiation is generated by heating solids. In industry, infrared rays with a wavelength of 0.76 ... 6 microns are used for these purposes, while bodies with a radiating surface temperature of 300 ... 2200 ° C have the maximum flux of waves of this spectrum.

Heat from the source of infrared rays to the heated body is transferred instantly, without the participation of any heat carrier. Absorbed by irradiation surfaces, infrared rays are converted into thermal energy. From the surface layers heated in this way, the body warms up due to its own thermal conductivity.

For concrete work, tubular metal and quartz emitters are used as generators of infrared radiation. To create a directed radiant flux, the emitters are enclosed in flat or parabolic reflectors (usually made of aluminum).

Infrared heating is used for the following technological processes: heating of reinforcement, frozen bases and concrete surfaces, thermal protection of laid concrete, acceleration of concrete hardening during installation floors, erection of walls and other elements in wooden, metal or structural formwork, high-rise structures in sliding formwork (elevators, silos, etc.).

Electricity for infrared installations usually comes from transformer substation, from which a low-voltage cable feeder is laid to the work site, supplying distribution board. From the latter, electricity is supplied via cable lines to separate infrared installations. Concrete is treated with infrared rays, if available. automatic devices, providing the specified temperature and time parameters by periodically turning on / off infrared installations.

During induction heating of concrete, the heat released in the reinforcement or steel formwork, which are in the electromagnetic field of the inductor coil, through which an alternating electric current flows, is used. To do this, an insulated wire-inductor is laid in successive turns on the outer surface of the formwork. An alternating electric current passing through an inductor creates an alternating electromagnetic field. Electromagnetic induction causes eddy currents in the metal (reinforcement, steel formwork) located in this field, as a result of which the reinforcement (steel formwork) heats up and the concrete heats up from it (conductively).

"Winter conditions" are created at a facility under construction, where a significant proportion of the work is associated with monolithic reinforced concrete, much earlier than winter comes according to the calendar. Construction becomes "winter" as soon as the average daily temperatures drop to +5 o C and at night the temperature drops below 0 o C.

At sub-zero temperatures, the water in the composition of completely uncured concrete stops reacting with cement and freezes, becoming ice. The intensity of hydration processes decreases sharply, the concrete stops hardening. At the same time, internal pressure builds up in the thickness of the concrete due to a 9% increase in the volume of water that has turned into ice. If freezing of concrete casting occurs at an early stage of work (immediately after concrete is laid), then the structure of reinforced concrete is completely broken, since it lacks the ability to withstand the processes of freezing of the internal volume of the liquid. In the case of concrete thawing, the ice becomes water again and the hydration process is activated, but the complete restoration of the concrete structure will not occur.

When freshly laid concrete is frozen, an ice crust is formed around its internal reinforcing "skeleton" and filler grains, which grows due to the incoming water from the internal zones of concrete with more high temperature. Each ice crust gradually increases the wall thickness and moves the cement paste away from the concrete filler and reinforcement, which reduces the strength characteristics of concrete and negatively affects its durability.

If the concrete has time to gain the minimum sufficient strength before freezing, then negative processes in its structure will not develop. The degree of strength of concrete, at which low temperatures do not pose a danger to it, is called "critical".

The standards for the critical strength of concrete are associated with its class, type and conditions in which this structure will operate. In the case of structures made of concrete and reinforced concrete (non-tensioned reinforcement), the critical strength should be at least 50% of the design strength for B7.5-B10, at least 40% for B12.5-B25, and 30% for more than B30. For concrete structures containing prestressed reinforcement, the critical strength must be at least 80% of the design strength. For concrete structures subject to alternating cycles of freeze and thaw, 70% strength must be achieved. Loaded structures are required to gain full, 100% strength from the design before being exposed to sub-zero temperatures.

The duration of the concrete curing period, during which a set of required strength characteristics is achieved, largely depends on the temperature conditions at the construction site. The higher the air temperature, the higher the activity of the water component of the concrete mixture - the reaction processes with cement clinker are faster, which accelerates internal coagulation and the formation of a crystalline structure. Accordingly, a decrease in temperature leads to a slowdown in these processes.

Concrete work in winter must be carried out under artificially created conditions in terms of temperature and humidity, achieving concrete hardening to critical or design strength in less time and at lower cost. In order to achieve the desired results, special mixing technologies, delivery to the location on site, as well as the subsequent curing of the concrete are used.

Preheating of the concrete mix

During the preparation of the concrete mixture at low temperatures, it is heated to 35-40 ° C, provided by the preheating of the components. hot water. It is absolutely impossible to heat cement.
Artificially heated concrete mix for a "winter" construction site is made differently than in the warm season. If in summer the dry components of the mixture are simultaneously loaded into the mixer hopper, where water was previously poured, then in winter the order is as follows - water is first poured and large aggregate fractions are poured. When the mixing drum makes several revolutions, cement and sand are loaded into it. Ignoring this sequence of actions will lead to the "brewing" of cement.

The duration of mixing the concrete mixture at negative temperatures must be increased by 1.2-1.5 times compared to the "summer" period of its mixing. Transportation of ready-made concrete is carried out in a heated, insulated and closed container, whether it is a tub or a car body. The heating of the vehicle body is provided in this way - it is made double, exhaust gases from the engine are directed into the cavity created in this way, which will reduce heat loss. Delivery of the concrete mixture should occur as quickly as possible and without any intermediate overloads. The areas where the concrete mixture is loaded and unloaded must be fenced off from the wind, and the means through which the concrete enters (trunks) must be insulated.

Preparing concrete work in winter

Concrete should be laid on a base, the state of which completely excludes the freezing of the mixture along the junction line with it, as well as the possibility of deformation due to the heaving of the soil. For these purposes, the base of the concreting area is heated until it reaches a positive temperature, and after laying the mixture, it is preserved from freezing until the concrete gains critical strength.

Immediately before the start of concreting, the formwork and reinforcement are cleaned of ice and snow masses. If the diameter of the reinforcement exceeds 25 mm, or it is made of rigid profiled rolled products or contains metal embedded elements of a significant size, then in conditions of negative temperatures less than -10 ° C, the reinforcement should be heated.

Concreting processes in winter conditions are carried out quickly and continuously - each underlying layer of concrete must be covered with a new one before its temperature drops below the calculated one.

Modern technologies for performing concrete work in the winter period make it possible to achieve High Quality building structures at the optimum level of costs. Conventionally, they are divided into three groups:

• "thermos" technology, based on the preservation of the initial heat of the mixture, heated during the preparation process or before laying on site, as well as on the use of heat emissions resulting from the reaction of cement with water during concrete curing;
• technology of artificial heating of the concrete mixture after it has been laid into the structure;
• technology for chemically lowering the freezing point of water in the concrete mixture and increasing the reaction rate of cement.

Depending on the situation at the construction site, the above methods of curing concrete at low temperatures can be used in combination. The final choice in favor of one of the technologies is based on the type of structure and its dimensions, on the type of concrete, its composition and design strength that it must gain, local climatic conditions at the time of work, energy capabilities at the construction site, etc.

Concrete work in winter and "thermos" technology

Its essence is in laying a mixture of concrete, having a temperature in the range from 15 to 30 ° C, in a formwork with insulation. This will ensure that the concrete builds up sufficient strength due to its initial thermal energy and the exothermic reaction of the cement, which will not allow the concrete structure to freeze prematurely. The amount of heat generated as a result of exothermic reactions depends on the holding temperature and the type of cement used in the preparation of the mixture.

The best heat dissipation data show Portland cements of high grades and with fast curing. The preservation of heat in concrete significantly depends on exotherm, therefore, concrete work using the "thermos" technology should be carried out on mixtures with fast-hardening and highly exothermic Portland cements, placed with an artificially raised initial temperature in a well-insulated structure.

Application of special chemical additives. Some chemicals - potash K 2 CO 3, calcium chloride CaCL, sodium nitrate NaNO 3, etc. - being introduced into the concrete in a small volume, usually not more than 2% of the amount of cement, increase the rate of concrete hardening by initial stage keeping. For example, with the introduction of calcium chloride in an amount of 2% by weight of cement, it provides 1.6 times the strength of concrete after 2.5 days from the moment of laying in the structure, compared with concrete of an identical composition, but not containing a special additive. Chemical additives also provide a shift in the freezing point of water to -3 ° C, which allows you to increase the cooling time of concrete and thereby provide it with a greater set of strength. More detailed information on the methods of chemical improvement of the characteristics of concrete for winter construction is disclosed.

The preparation of concrete mixtures, including chemical additives, is carried out using hot water and heated filler grains. When removed from the mixer, such concrete usually has a temperature of from 25 to 35 ° C, immediately before laying its temperature drops to about 20 ° C. Laying in the structure of chemically modified concrete is carried out at an external air temperature of -15 to -20 ° C, after placement in the insulated formwork, one or two layers of thermal insulation are laid on top. The curing of the concrete structure occurs due to the effect of "thermos" with the simultaneous action of dosed chemical components. The technology of "thermos" concreting, along with the use of chemicals, is simple and relatively inexpensive, it can be used to create a structure with a surface modulus (Mn) of less than five.

Concreting according to the "hot thermos" method. It is based on the rapid heating of concrete to 60-80 ° C and compaction of the mixture in the structure before it cools down. Further, the concrete mixture is aged according to the “thermos” technology, or it is additionally heated during the period of critical strength gain.

At the construction site, the concrete mixture is most often heated using electric current - electrodes are placed in it and alternating current is supplied, heating occurs due to the resistance of concrete. The power and amount of thermal energy generated per unit of time is directly proportional to the voltage on the electrodes and inversely proportional to the ohmic resistance of the mixture. In this case, the intensity of ohmic resistance depends on the planar dimensions of the electrodes, the distance between them and the specific ohmic resistance of the concrete mixture.

Electric heating of the concrete mixture is carried out under a current of 380V, in more rare cases - under 220V. To ensure this operation, construction site equipped with a transformer post, switchboard and control panel. The mixture is heated in a tub or in the body of a dump truck directly. The first method is carried out in the following sequence - the mixture compiled at the concrete plant is transported by road to the construction site, special buckets equipped with electrodes are reloaded, heated until its temperature is 70-80 ° C, and then placed in the formwork on the spot works. As a rule, tub shoes are used, equipped with three 5 mm steel electrodes, powered to the mains through cable connectors. In order for the concrete to be evenly distributed in the electric tub, as well as to simplify further unloading, a vibrator is mounted on the tub body.

Following the second method, a dump truck, which contains a concrete mixture in its body, arrives at the construction site and follows to the heating post - its body is located exactly under the electrode frame. The operation of the vibration unit is activated, then electrodes are inserted into the concrete contained in the body, and an electric current is supplied to them. The heating of the mixture is carried out for 10-15 minutes when it is heated to 60 ° C (true for fast-hardening Portland cements), up to 70 ° C for Portland cements and up to 80 ° C for Portland slag cements.

To quickly and extremely short term to heat the concrete to the required temperature, it is important to provide the site with high electrical power. For example, a 15-minute warm-up of a cubic meter of concrete mix to 60 ° C will take 240 kW, and a faster 10-minute warm-up to the same temperature - 360 kW.

The next part of the article, devoted to heating the mixture laid in the structure, is located.

Excerpts from SNiP related to concrete work in winter: transportation, laying concrete mix, how to pour concrete in winter at low temperatures.

SNiP. PRODUCTION OF CONCRETE WORKS AT NEGATIVE AIR TEMPERATURES

2.53. These rules are followed during the period of concrete work with the expected average daily temperature outside air 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 the calculation. It is allowed to use unheated dry aggregates that do not contain ice on grains and frozen clods. At the same time, 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 that the temperature of the concrete mixture does not drop below the 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 laying method, must exclude the possibility of freezing of the mixture in the zone of contact with the base. When keeping concrete in a structure by the 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-porous base or old concrete if, according to the calculation, in the contact zone during the estimated period of concrete curing, it does not freeze.

At air temperatures below minus 10 °C, concreting of densely reinforced structures with reinforcement with a diameter of more than 24 mm, reinforcement from rigid rolled profiles 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 vibrating 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 a rigid interface of nodes (supports), the need for gaps in the spans, depending on the heat treatment temperature, taking into account the resulting thermal stresses, should be agreed with the design organization. Unformed surfaces of structures should be covered with steam and thermal insulation materials immediately after concreting.

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

2.58. Before laying concrete (mortar) mix the surfaces of the joint cavities of prefabricated 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. The choice of concrete curing method winter concreting monolithic structures should be produced in accordance with the recommended appendix 9.

2.61. Concrete Strength Control should be carried out, as a rule, by testing samples made at the place of laying the concrete mixture. Samples stored in frost should 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. Requirements for the performance of work at negative air temperatures are set in the table. 6