Thermoactive (heating) formwork are called multilayer shields, which are equipped with heating elements and are insulated. Heat through the deck of the shield is transferred to the surface layer of concrete, and then distributed throughout its thickness. Heating of concrete in this way does not depend on the outside temperature. Heating formwork is used in the erection of thin-walled and medium-mass structures, as well as in the monolithing of joints and seams at ambient temperatures up to –40 ° C.
The designs of the heating formwork are diverse. The main requirement for them is the uniform distribution of temperature over the formwork of the shield.
Tubular electric heaters (heating elements), heating wires and cables, flexible fabric tapes, as well as heaters made of nichrome wire, polymeric materials with graphite (carbon band heaters) and conductive elements, etc. are used as heating elements.
Tubular electric heaters consist of pipes (steel, copper, brass) with a diameter of 9-18mm, inside which is a nichrome spiral. The space between the spiral and the walls of the tube is filled with crystalline magnesium oxide. The heating temperature of the heating elements is 300-6000С, so they should not be in contact with the formwork surface adjacent to the concrete, but located 15… 20 away from it.
Wire heating elements are made of nichrome wire with a diameter of 0.8 ... 3 mm, which is wound on a framework of insulating material and insulated with asbestos. Such heaters are less reliable, as they are subject to deformations during loading and unloading operations, therefore they require careful handling.
As the heating cables used cables type KSOP or KVMS. They consist of a 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.
Place the heaters on the formwork panel, depending on the heating modes and power: heating wires and cables are installed close to the deck, heating elements - at a short distance from it.
In the plywood heating formwork, heating cables and wires are pressed into protective coatings consisting of a package of thin polymer films.
Carbon tape heaters are glued with special adhesives on the shield deck. To ensure strong contact with the commutating wires, the ends of the tapes are subjected to copper coating.
Before installing thermoactive panel formwork, check the integrity of the insulation and electrical wiring by inspection. The formwork is installed in the block of concreting with individual shields manually or enlarged panels using cranes. After mounting the boards and panels are connected to the electrical network. Installations for powering thermoactive formwork and controlling the mode of heating concrete consist of a step-down transformer, a wiring system, a control panel, and a room for the electrician or operator on duty. The installation provides power 100 ... 150 m2 formwork.
Connect the formwork to special terminal boxes, which are located above the surface of the formwork not less than 0.5 m. When heating the frame elements (columns, girders, beams), terminal boxes are suspended on sliding clamps installed at a distance of 50 ... 70 cm from the heated element.
Before concreting, the reinforcement and previously laid concrete are heated. To do this, for a short time include thermo-active formwork, pre-covering the concreting unit with a tarpaulin or plastic wrap from above.
The minimum temperature of the laid concrete mix 50C. They put it in the usual way, while taking care not to damage the electric cable and not to moisten the insulation. With a wind speed of more than 12m / s, the form forms are covered with a tarpaulin or plastic sheeting.
Observance of the technological mode of heating allows to obtain concrete with the required physical and mechanical characteristics. Controlled parameters of heating are the rate of heating of concrete, the temperature on the deck of the boards and the duration of heating.
In winter, thermoactive flexible coatings (TAGPs) are used for heating monolithic concrete for coatings and road foundations, for under-floor preparation, for joints between prefabricated structures - lightweight, flexible devices with carbon tape heaters and wires that provide heating up to 500C. A coating is produced by hot pressing of a package consisting of a layer of sheet unvulcanized rubber, reinforced glass cloth gaskets, carbon fabric electric heaters or wires and insulation. Thermoactive flexible coatings can be manufactured in various sizes, which allows them to be used as heaters of thermoactive formwork. The coating can be placed on vertical, horizontal and inclined structures. The power supply TAHP is carried out from step-down transformers with voltage 36 ... 120V. Like thermo-fencing shields, TAGPs are equipped with temperature sensors with output to the control panel. This allows you to quickly control the warm-up mode.
Thermoactive flexible coating is convenient to use, compact and reliable in operation. At the end of the work, it is rolled up and placed in a special two-piece cabinet. In one section there is a transformer with a control panel, and in the other - compartments for storing the coating. Apply special mobile units equipped with transformers, compartments for storing cabling and a set of TAGP.
Before starting work, check the state and performance of the heating equipment and temperature control automation. The general scheme of laying the coating on the concreted structure, its switching and warm-up modes should be given in the design of the work. To comply with the technological mode of heating the concrete, it is necessary to measure the temperature of the concrete at least once and at least once to measure the temperature of the outside air.
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