Concrete heating: why it is needed and how to organize it

When performing concreting foundations and pouring monolithic structures at low temperatures (minimum below 0⁰C and daily average below 5⁰C) the heating of concrete according to the SNIP "Bearing and enclosing structures" must be carried out without fail. To ensure the warm-up can be used a variety of techniques, and we will describe the most popular ones in our article.

The need for heating the solution

Processes occurring in concrete

The process of heating the concrete during its hardening is quite expensive. It takes quite a lot of energy to maintain the temperature for a long time, but this is exactly the case when it’s not worth saving.

 The need for heating is directly related to the processes that occur in the solution:

• In order for concrete to gain strength, complete hydration of all the cement is necessary. The speed of this process is directly dependent on temperature, and therefore, when water freezes, hardening stops.
• In addition, freezing water increases in volume by about 15%. This leads to the destruction of the edges of the pores, and the material becomes loose.
• No less dangerous would be icing valves. Even a thin ice film breaks the bond in the metal-cement system, and the mechanical characteristics of concrete deteriorate.

It is for this reason that the instruction recommends that in no case should the solution freeze. And a variety of techniques can be used for this.

Passive and Superficial Cold Fighting Techniques

Situations in which additional heating of the poured structure may be required are conventionally divided into two types: planned and sudden. And if there are many methods to solve the planned problems, then during sudden frosts it is necessary to apply emergency measures.

What can be done:

• Firstly, if we know that a material can be exposed to frost, it is worthwhile to add special additives to it at the mixing stage.. They saturate the moisture in the solution with calcium and sodium salts (nitrites, bicarbonates), and the water does not freeze.

Note! Oversaturation of the solution with mineral salts can provoke efflorescence of white stains on the surface. That is why this technique is rarely used in cases where no masking of concrete is planned.

• Secondly, with small frosts, it is quite possible to do with high-quality thermal insulation of formwork.. And if to fill in the fixed heat-insulating contour warmed to 70-80⁰With the solution, and close the structure on top with foil film, working as a thermal mirror, it is quite possible to achieve an acceptable result.
• Finally, there is an option when you need to raise the temperature pretty quickly. In this case, infrared heating is applied using heat guns.. Of course, these plants are quite energy-intensive, and they are effective only when processing a layer that is not too thick, but in some situations they simply cannot find a worthy alternative.

Note! The main disadvantage of such a technique is simply gigantic heat loss: we mostly heat the air, and only a small part of the energy goes to concrete.

And yet, having learned how much it costs to rent a radiator, and how much energy will have to be spent, specialists usually stop at more advanced techniques. We will describe them in the next section.

Internal heating methods

Electrode heating

As a rule, for works that will be carried out in winter, the heating method is thought out in advance. And here either conductive or electrode systems are used.

Electrode heating цементного раствора осуществляется таким способом:

• At the stage of installation of the formwork, current-conducting elements - electrodes are laid into the construction. They can be placed both in the solution body (rod, string) and on its surface (strip, plate).

• In most cases, the electrodes are fragments of reinforcement, to which is connected the contact wire. Sometimes the instruction recommends using reusable special plates for warming up.
• After installing the electrodes, the solution is poured into the formwork. Then a current is applied to the contact wires, and an electromagnetic field is formed in the thickness of the wet concrete.

Note! A typical technical card for connecting heating electrodes allows the use of either a step-down transformer or a welding machine. The optimal voltage is from 60 to 127 volts.

• Part of the energy of this field is transferred to the liquid in the solution, which heats up and prevents freezing.

It should be noted that as the solution dries, the treatment efficiency decreases due to deterioration of conductivity. In this case, heating is usually accompanied by a smooth increase in voltage.

In order to minimize heat loss, they usually try to provide high-quality thermal insulation of the structure, covering its surface with a layer of sawdust or foil film. If this is not possible, the treatment time should be increased to 4-5 weeks.

Cable use

Another method involves the use of heat-carrying cables, which are laid in the formwork and when the current passes through it, the solution is heated:

• For work we take PNSV conductors in polyethylene or polyvinyl chloride insulation. The second option is preferable for use in reinforced structures, since PVC does not melt, which means that the risk of a short to reinforcement will be minimal.

Note! PVC in the cold loses its elasticity, so when laying the wire you need to be careful not to damage the insulation layer on the bend.

• Usually, heating is carried out with PNSV wires with a diameter of 1.2 or 1.4 mm. The material is cut into standard fragments (17 or 28 m, depending on the configuration) and is twisted into a spiral with a diameter of about 30 mm for a more compact installation.

• Then the spirals are connected into several “triangles” or “stars” (the diagrams are shown in the figures), and are assembled into several common tires.
• Since the PNSV cable under voltage quickly blows in the air due to low heat dissipation, the heating circuits inside the formwork are connected to the current source using thick aluminum wires - the so-called “cold ends”.

• "Cold ends" connect to the terminals of the step-down transformer. For work, it is best to use systems like SPB-40, KTPTO 80 and their analogues, since they provide adjustment of the activity of the entire heating system.

The heating process itself is divided into several phases:

During this time, the transformer regulates the strength of the current flowing to the conductors. Upon completion of the heating, the contact conductors are dismantled, and the PNSV wire remains in the thickness of the concrete.

Cables without the transformer

The main disadvantage of the above method is the use of a transformer. Even the rental price of this device will be substantial, and it does not make sense to pay for it to fill one design.

In this case, it is easier to use cables that operate from a 220-volt electrical network:

• Conductors of type КДБС (Russia) or ВЕТ (Finland) provide the power of heating up about 40 W per 1 meter of length. They can be used at temperatures up to -30-40⁰C, and therefore quite effective in private construction, even in the northern regions.

• Since these conductors are connected directly to the network, they are not required to be trimmed to the size of the structure. Manufacturers produce sections of heating cable of different lengths (from 3 to 150 m) and therefore you can easily dial the required number of segments.

Note! Calculation of power for heating concrete should be done on the basis of a standard of approximately 0.5 - 1.5 kW / m³. When casting conventional overlap or screed stack about 4 m per 1m² floor.

• Installation of the system is easily done by hand. The conductors are laid out in the formwork, fastening them to the fixture with wire or plastic clamps. The end couplings are outputted and connected to the power cable, which is included in the network.
• After pouring the solution, we perform a vibration seal. Ensuring that the conductors are not shifted.

• Then we give the constructions to stand for about an hour and turn on the cables for warming up, maintaining the voltage until the cement gains the necessary strength.

Of course, such cables are more expensive than the PNS wire, but it is much easier to use them. Due to the absence of a transformer, this work can be done even by a non-professional.

Conclusion

Heating concrete with heat guns, electrodes and immersion cables allows you to expand the climatic parameters of construction work. Now we do not need to limit ourselves to the warm season: of course, the cost of heating will be significant, but we guarantee the quality of concrete hardening, even in hard frost.

The technologies described in more detail are shown in the video in this article.