Construction - the year is year-round, and, in order to avoid large losses, should not depend on weather conditions. The main criterion for high-quality concreting in winter is to warm the concrete.

Why is this done?

According to SNiP, the technological warming of concrete is regulated if the minimum daily air temperature drops below 0 ° C. Its purpose is to prevent the freezing of the raw concrete mix, which entails the formation of ice films in the thickness of the material and around the reinforcement.

Water is directly involved in the preparation of concrete, but, turning into ice, ceases to be part of the chemical hydration, preventing the mixture. In addition, expanding, the ice creates internal pressure and destroys the connection in the coarse concrete. After thawing fluid, the hydration process may resume, but some compounds are lost forever, which leads to a decrease in the quality of the material and the durability of the structure.

Methods of warming up concrete

The choice of a method of heating depends not only on the type of construction and weather conditions, but also from the economic feasibility and urgent framework upon completion of concreting. There are types of warm-up:

• preliminary;
• thermos;
• electrode;
• warming formwork;
• infrared;
• heating loops;
• induction.

Preheating

It implies the warming up of a concrete mix to a temperature of about 50 ° C using an electric current with a voltage supply of 220-380 V, for 5-10 minutes. After the hot concrete is flooded, its cooling occurs according to the thermos method.

For pre-heating, the site requires an electrical power of more than 1000 kW per 3-5 cubic meters of concrete mix.

Holding a concrete mix of thermos

The most economical and simple of all, this method has become widespread in construction. The mixture, temperature of 25-45 ° C, is delivered to the site and laid into the formwork. If you warm it up to a larger temperature, then during transportation there is a risk of solid.

Immediately after filling, the design from all sides is covered with thermal insulating material. As a result, concrete is hardening due to insulation from cold air, the heat of the mixture itself, as well as as a result of the exothermic cement reaction.

The amount of heat that receives concrete from these sources can be calculated, and in accordance with the value, select the desired layer of insulation. It should be enough to withstand concrete in the plus temperature up to its hardening and dismantling formwork, regardless of external temperature conditions.

However, not all designs can be warmed by thermos. The most suitable are those whose cooling area is relatively small. That is, if the mixture is prepared from portland cements of medium activity, the thermal maintenance is suitable if the surface module is not higher than 8.

In winter, it is recommended to use fast-hardening high-active cements, as well as introduce special additives in them - chemical hardening accelerators. The use of additives, as part of which is urea, is not allowed, since at temperatures above 40 ° C, it is decomposition and the shortage of concrete strength of up to 30%, which is expressed in low frost resistance and water permeability. Such measures allow the use of a thermos method on surfaces with a module from 10 to 15.

In accordance with the thermal calculation, which is produced in the design of thermal shelter, the amount of heat in the concrete mix should not be lower than the amount of heat loss when cooled over the entire period required for the formation of concrete hardness.

As a heater, boards and phanels with a layer of foam, sawdust, cardboard, mineral wool, etc. are used. Especially carefully, it is especially thorough to warm the designs with a drop of levels, angles and subtle elements. The formwork and heat shield are removed when the outer layer of concrete reaches 0 ° C.

Electric heating method

A method for accelerating the pouring of concrete by passing an electric current into it. It is widely used in the construction of monolithic structures from concrete and reinforced concrete in winter, as well as in the production of modular elements. Among the advantages are the reliability and simplicity of the method, a quick heating of the mixture. The disadvantages include the need for a high power source at the site: from 1000 kW per 5 m³ of concrete and a constant increase in heating temperature as material hardening.

Electrode Winter Warming Concrete is peripheral, end-to-end and using fittings as transmitting electrodes. The most commonly used when working with poor structures: foundations, walls, partitions, columns, overlaps. It can often be combined with concrete pre-warming and thermal method using chemical hardeners.

When entering concrete for a certain period of time, the current warms it evenly throughout the plane, regardless of the thickness of the segment. This is especially important when working with light concrete, difficult to warming up. The impact of current on the rejection of the mass is due to an increase in the temperature inside the material and the electrolysis of water, and the specific resistance of the concrete changes at different stages of its formation.

Heating concrete electrodes occurs with the use of at least two pins of metal. Connected to antiphase wires, they transmit a current to each other. It is very important at the same time a given voltage: it can be elevated (220-380 V) or reduced (60-128 V). Electric strokes Over 127 V applied only for unarmed structures and with strict safety compliance. In the reinforced concrete in the event of an increased voltage, local overheating may occur, causing evaporation of moisture and closure.

After filling, in the walls or columns, metal rods are stuck, on which a reduced voltage is supplied from the transformer. Electrodes are metal rods or strings, whose length is determined depending on the place of use. Their diameter ranges from 6 to 10 mm. Depending on the weather, the step between the electrodes can be from 0.6 m to 1 m.

If a three-phase transformer, one electrode will be enough for one column. Fast installation and effective heating on one side, with the other turns around high-cost disposable katan electrodes and energy consumption.

Method of heating formwork

The immediate contact of the electrodes with concrete is useful when warming up vertical structures, while the heat formwork method is more suitable for the fuses, but the essence of the procedure does not change.

The principle of electrode heating of the monolithic structure consists in entering heat from the formwork surface inside the concrete due to its thermal conductivity. The heat transmitters are used by the heat, cargophyte fiber, mica and mesh heaters.

To create a uniform temperature circuit, all open surfaces and ends should be insulated. Pulling the concrete mixture is preferably a warm formwork: this reduces the timing of concrete and reinforcement, and prevents the deformation of the form.

Before starting the mixture, the formwork should be turned off. Electricity feed mode to all shields should be the same, and this is manually set. The temperature of the pre-heated concrete should not exceed 60 ° C, since the moisture may begin to evaporate, which will increase the viscosity of the mass.

The mixture is placed by layers and immediately covered with heat insulating materials. Before turning on the electrodes, the concrete is withstanding for some time for a uniform temperature distribution. Then, carefully, one by one, shields are connected.

To achieve 80% of the strength, the total heating time of concrete at a temperature of 80 ° C is 13-15. In order to save, (almost one and a half times), the temperature can be lowered to 60 ° C, but the time of frozen will be equal to 20-23 hours.

Concrete warm-up scheme:

1. The control panel is installed and connected, the connecting cables are unplugged.
2. On the entire perimeter of the formwork and the plug connectors are connected to the temperature sensors.
3. Signal lights are connected to the console. After turning on the chopping on, the voltage will be supplied both on the power and signal circuit on which the presence of voltage in the phases is monitored. The network current is monitored over the voltmeter on the remote control panel.
4. Installation starts. With the help of switches, sensors are connected in formwork shields with an electronic temperature regulator.
5. If one of the shields overheats, the energy supply is terminated, as evidenced by the signal of the corresponding lamp.
6. When the heating is over, the installation automatically turns off.

Infrared heated

This method involves the principle of peripheral use of thermal energy obtained from the infrared emitter. They may be both metal (beans) and carborund emitters. Infrared transmitters in combination with reflectors and other devices are an infrared installation.

The optimal distance from the emitter to the heated surface is 1.2 m. For better heat absorption, formwork can be covered with black matte paint. In order to avoid evaporation of moisture from the surface, the construction is covered with polyethylene film, rubberoid or pergamine.

The process of warm-up concrete infrared rays are divided into three stages: exposure of the mixture and its heating, active warming, cooling.

Approximate electricity consumption for heating 1 m³ is 120-200 kW / h.

Infrared heat is sent to the external sections of the heated design and contributes to such processes:

• heating frostbed soil and layers of concrete, mortgage, reinforcement, cleaning them from nondes and snow;
• acceleration of the process of rejection of floors, monolithic structures, inclined and vertical structures;
• pre-heating zones of docking frozen and fresh mixtures;
• heating is inaccessible for insulation of places.

Use heating loops

The method with heating wires is that on the frame of the reinforcement in the formwork, the desired number of heating wires (PNSV) are laid out. Their quantity is calculated depending on the heat transfer and fill area.

Then the concrete mass is layered on top, and when the current is allowed along the wires, it, due to its thermal conductivity, is heated to 40-50 ° C. The wires for the concrete PNSV with insulation from PVC and a galvanized steel residential diameter of 1.2 mm are used as heating. You can also use PTPG in polyethylene insulation with two veins of 1.2 mm.

The supply of electricity is carried out through lowering transformers of the KTP-63 / OB or KTP-80/86, where the heating power can be adjusted depending on the changes in the external temperature. Within times, a substation is enough for heating to 30 cubic meters of concrete at air temperature to -30 ° C.

For heating 1 m³, an average of 60m heating wire is required.

Induction warming

At the heart of this method of warming up concrete in winter, it is the use of a magnetic component in an alternating electromagnetic field, where an electric current is formed as a result of induction. With this warming, the energy of a magnetic field aimed at the metal is converted to the thermal, from where is transmitted to concrete. The intensity of heating depends on the magnetic and electrical properties of the heat source (metal) and the voltage of the magnetic field.

The induction method is applied to constructions with a closed loop, where its length is greater than the size of the section, to reinforced concrete with thick reinforcement or structures with a metal formwork. In accordance with the safety technique, heating leads on a reduced voltage of 36-12 V.

Before pouring the mixture, along the contour of the design, a template is laid out where the inductor turns will be placed. Next in the grooves the insulated wire is stacked, where concrete is poured. As with any heating method, it is first withstanding 2-3 hours at a minimum temperature of about 7 ° C, for this, the inductor is activated by 5-10 minutes each hour. The temperature of concrete reads grow at a speed of 5-15 ° C and upon reaching the limit mark, the inductor can be turned off, then further heating is performed by the thermosis method or goes to the pulse mode, periodically supporting the desired level of heat.

The advantages of this method include uniform heating over the entire length and cross section of the structure, the possibility of heating reinforcement and savings on the electrodes.

The approximate energy consumption per 1 m³ is about 120-150 kW / h.

Calculation of the warm-up of concrete

As for the definition of the wire length per section and the number of such sections in the design, it depends on the characteristics of the wire and the transformer voltage.

For example, when flowing current 220V, the length of the PNSV section 1.2 mm is 110 m. If the voltage decreases, the wire length in the segment is proportionally reduced.

The heat obtained from the heating section with an average wire flow rate is 50-60 m³, able to heat the flooded concrete to 80 ° C.

To obtain an average concrete temperature during cooling, an empirical dependence is used. Approximate cooling calculation is determined as follows:

1. Based on a meteorological weather forecast for the entire winter period in the required area, the expected average temperature indicator of the outer air is established.
2. The surface module is determined, according to which the suitable thermal maintenance is calculated.
3. With the help of the formula, the average temperature of the concrete for all the time of cooling is calculated.
4. The cement supplier receives data on the premium mixture of which temperature will be delivered and what is it exothermic characteristics.
5. The formulas are calculated by heat loss during delivery and unloading.
6. The initial temperature of the concrete is determined since the laying time, given the return of its heat to heating the reinforcement and formwork.
7. Based on the requirements of the strength, the duration of the cooled of the concrete mixture is determined.

This calculation method is used to predict the timing of the establishment of concrete, taking into account the heat loss when pouring, as well as thermal radiation from the surface, but it should be remembered that the data is approximate.