Concrete warming up with a pnsv wire: objectives and

In this article, we will study the technology of heating the concrete with the PNSV wire. In addition, we will touch on the goals pursued by this operation, the method of calculating the heating and the features of the wire itself, which must be taken into account by the technological map for the heating process. So go.

Goals

Why heat concrete?

All sorts of ways to increase the temperature of the mixture laid in the formwork pursue one of two goals:

1. Providing strength in the winter. When the temperature of the monolith drops below zero degrees, the water in it crystallizes, which completely stops the process of cement hydration.

In addition: the crystallization of unbound water destroys the concrete, expanding its pores.

1. Acceleration of setting and strength in other seasons. Increasing the temperature of the mixture dramatically accelerates all processes occurring in it.

Since in the process of setting the cement emits a fair amount of heat, additional heating in the winter does not always need mixtures.

PNSV wire for heating concrete during the cold season is used when:

• The temperature of the outside air is well below zero.
• There is no possibility to provide high-quality thermal insulation of the formwork.
• The modulus of the monolith surface (the ratio of its area to volume) exceeds the value of 10 m ^ -1.

The wire

What is a PNSV wire?

Specifications

Оптовая цена провода диаметром 1,2 мм составляет 1,8 - 2 рубля/метр, что существенно дешевле медных аналогов (читайте также статью «Бетонные столбики – важный элемент строительства»).

Features of use

Technological map of concrete heating with a PNSV wire must take into account a number of its features.

• Steel has a relatively high resistivity, which leads to much stronger than copper or aluminum heating of the conductor at moderate currents. 14-16 amperes are considered the norm for wires laid in concrete; however, in the air such an amperage will melt the insulation.

Practical consequence: PNSV is connected to the transformer or other current source with a wire with a lower resistivity. Alternatively, the connection can be made with a cable of the PNSV of the same section, but doubled.

• Overlaps and laying of adjacent wires at a distance of less than 15 millimeters are unacceptable because of the possibility of overheating with melting insulation and short circuit.
• Since steel does not have high flexibility, the wire is laid with smooth bends with a radius of at least 25 mm.
• Laying is permissible at an ambient temperature not lower than -15 degrees. The instruction is related to the fact that plastic insulation at lower temperatures loses elasticity and can be broken during bending.
• For a more uniform heating of the concrete mix, the laid wire is recommended to be covered with metal foil with a thickness of 0.2 - 0.5 mm.
• The heating section can be assembled from several segments; at the same time connection is allowed not only through blocks, but also twists. Warming up is a one-time event, and the contacting surfaces simply do not have time to significantly oxidize.

А вот соединение так называемого «холодного» конца (провода, ведущего к трансформатору) с ПНСВ рекомендуется выполнять пайкой или через клеммную коробку.

• The method of heating a concrete with a PNSV wire is due to its mechanical properties; in particular, the coefficient of thermal expansion. When heated, the temperature of the monolith rises at a speed of no more than 10 ° C / hour and drops at a speed of no more than 5 ° C / hour.

Exceeding the speed can lead to an increase in internal stresses and cracking. The adjustment is performed by gradually increasing and decreasing the voltage on the transformer.

• When using power from 380V through a step-down transformer, the main limiting current factor is the danger of overheating of the PNSV itself. The problem is solved simply and elegantly: the laying scheme for heating concrete with a PSNV wire, if it is necessary to increase power, includes several sections connected in parallel.

Length calculation

The calculation of the concrete heating by the PNSV wire is based on two variables:

1. The need of the object is warm. It, in turn, depends on temperature, wind strength, degree of thermal insulation, brand of cement and the shape of the monolith.

1. The maximum power density of the wire. For reinforced concrete it is taken equal to 30 - 35 W / m, for unreinforced - 35 - 40.

Next is simple arithmetic. So, to get 4 KW of heat in an unarmoured structure, we will need 4000/40 = 100 meters of wire.

It is somewhat harder to do your own hands the calculation of the maximum length of an individual section. Here you need to know the resistivity of the steel conductor for different sections.

Our goal is to get a current of 14-16 amps. Recall the relationship between voltage, current and resistance: U = IR, where U is the voltage, I is the power of the current, R is the total resistance of the circuit.

So, at U = 75 volts and the target current I = 15 amps, we need the circuit resistance to be 75/15 = 5 ohms. When the conductor has a cross section of 1.4 mm, it will be provided with a wire of 5 Ohm / 100 Ohm / km = 0.05 km, or 50 meters.

Attention: we gave an extremely simplified method of calculation. The actual resistance of the conductor changes with an increase in its temperature, which amends the result.

Concrete processing after warming up

Is it possible to cut or drill the concrete after it sets the minimum strength (usually at the time of stopping the heating, it is far from the mark)?

Yes. But without shock loads. The ideal option is to use diamond tools: diamond drilling of holes in concrete will make the edges ideal and will not cause cracks. In addition, drilling with a diamond bit (as well as cutting reinforced concrete with diamond circles) will allow reinforcement to pass without a tool change.

Conclusion

Повторимся: мы дали читателю лишь самую общую информацию о методах нагрева бетона  и расчета провода (см.также статью «Схватывание бетона – основные особенности данного процесса»).

Some amount of additional information will be offered to his attention by the video in this article. Successes!