Concreting technology in winter conditions. 5. Electrical heating of the concrete mix in the structures.

The method of electric heating of concrete in constructions is based on the use of the heat released during the passage of electric current through it. To apply voltage, electrodes of various designs and shapes are used. Depending on the location of the electrodes, heating is divided into end-to-end and peripheral. During continuous heating, the electrodes are placed over the entire cross section, and for peripheral heating, they are placed along the outer surface of the structures. To avoid salt deposits on the electrodes, direct current is prohibited.

For through heating of columns, beams, walls and other structures erected in wooden formwork, rod electrodes are used, which are made from pieces of reinforcing steel with a diameter of up to 6 mm with a pointed end. To install the electrodes, drill holes in one of the formwork shields so that the electrodes do not come into contact with the frame reinforcement. Then insert the electrode and blow the hammer fix it in the opposite shield. The distance between the electrodes horizontally and vertically take the calculation. Then carry out their switching.

For peripheral heating with weak reinforcement and when contact is excluded by fittings, floating electrodes in the form of a closed loop are used. When heating flat structures (for example, preparation for underfloors, pavements, ribbed plates) plate electrodes are used.

Strip steel with a thickness of 3 ... 5 and a width of 30 ... 50 mm is used as floating electrodes. The distance between them is determined by calculation. Electrodes must be in contact with the concrete and may be slightly recessed into it. There should be no gap between them and the concrete. To do this, they are loaded with non-conductive materials (boards, bricks), the electrodes themselves must be free of curvature and kinks.

Nashivny electrodes, as well as floating, belong to the elements of peripheral heating. They are made from round reinforcing steel or metal plates with a thickness of 2 ... 3 mm. Electrodes are sewn onto the formwork shields, and the ends are folded at an angle of 90 0 and brought out. After installing the formwork produce switching electrodes. It must be remembered that the electrodes should not have contact with the reinforcement of the structure in order to avoid short circuits. Therefore, when installing reinforcement cages, plastic gaskets and clamps are used, which provide a predetermined thickness of the protective layer and prevent contact with the electrodes.

In the manufacture of lengthy structures (columns, beams, beams, piles) using string electrodes. They are made of smooth reinforcing steel with a diameter of 4 ... 6 mm. Located in the central part of the cross section of the structure. The ends of the electrodes are bent at an angle of 900 and lead out through the holes in the formwork for connecting commutating wires.

For peripheral heating of massive structures, as well as elements of buildings of low massiveness (walls, tanks, strip foundations), metal formwork panels and structural reinforcement are used as electrodes. In the first case, single-phase current is used: the first phase is connected to the formwork panels, and zero to the reinforcement cage. In the second case, the reinforcement cage is not connected to the network, and each formwork element is connected to one of the three phases. Insulators between the formwork shields are wooden bars.

The uniformity of the field temperature depends on the layout of the electrodes and the distance between them. The closer the electrodes are to each other and the stronger the reinforcement of the structure, the greater will be the temperature drops in hardening concrete, as a result of which the curing mode will be non-uniform and the quality of concrete will deteriorate. Therefore, in each case, calculate the layout of the electrodes based on the degree of reinforcement of the structure. When the voltage on the electrodes is 50 ... 60 V, the distance between the electrodes and the reinforcement should be at least 25 mm, and at 70 ... 85 V - at least 40 mm.

Rod electrodes are used, as a rule, in the form of flat groups that are connected to a single phase. With a large length of structures instead of one electrode install two or three in length. The permissible length of the strip, rod or string electrodes is taken by calculating the minimum voltage loss along its length.

Ways to install the electrodes and their application.

Table 10.3.

To obtain high quality reinforced concrete, the temperature regime of heating is strictly observed, which is divided into three stages:

1. Raising the temperature of the concrete. Lifting speed depends on surface modulus:

М ………………………… 2 ... 6 6 ... 9 9 ... 15

Lifting speed С0 / h 8 10 15

2. Isothermal heating. At this stage, the concrete is maintained at a given temperature. The duration of the stage depends on the type of construction (heated to obtain the necessary strength of concrete). Most often, the critical strength of concrete is reached at the isothermal heating stage.

3. Cooling designs. When cooled to 0 ° C, concrete continues to gain strength, which is especially important when concreting massive structures.

For structures with Mn = 6 ... 9, a mode is used in which, by the time of cooling, concrete must gain strength no less than critical. For structures with Mn = 9 ... 15, the mode is the same, but at the end of isothermal heating, the concrete must gain at least 50% strength. This circumstance determines the time of isothermal heating. In the manufacture of prestressed structures by the end of isothermal heating, the strength of concrete must be at least 80%.

Violation of the technological mode of electrical heating can lead to concrete overheating as a result of overheating of the concrete mix above 100 ° C, insufficient strength, cracking due to heterogeneity of the temperature field.

The heating temperature of concrete depends on the design and type of cement.

The maximum allowable temperature of concrete, 0C, with electrical heating.

Table 3.3.2

The maximum temperature for heating more massive structures is prescribed from the condition of obtaining a uniform temperature field and exclusion of high thermal stresses in them.

The required temperature for heating the concrete is obtained by changing the voltage, periodically disconnecting and turning on the entire heating or part of the electrodes. When electric heating of concrete structures with the help of instrumentation constantly monitor the voltage, current and temperature of concrete. During the first 3 hours of heating, the temperature is measured every hour, and then after 2 ... 3 hours.

The rate of cooling of the concrete is also regulated.

Allowable cooling rate of concrete structures.

Table 10.4

If the cooling rate exceeds the permissible, thermal stresses occur in the concrete mixture, which can destroy the structure of the concrete or form cracks in it. Regulate the cooling rate by proper selection of thermal insulation formwork.

Before the start of concreting, the correctness of the installation of the electrodes and their switching, the quality of the insulation of the formwork are checked, the reliability of the contacts of the electrodes with conductive wires is determined.

When electrical heating must carefully comply with the requirements of electrical safety and labor protection.