Technology of monolithic concrete and reinforced concrete. Formwork classification

For the manufacture of concrete and reinforced concrete structures of a certain size and configuration, it is necessary to put the concrete mix and reinforcement into a pre-prepared form called formwork. The formwork is at a height and maintained in the project position by scaffolding. Formwork and scaffolding must be rigid, durable and immutable, easy to manufacture, assemble and disassemble. The side of the formwork, adjacent to the concrete, should be smooth, the joints of boards and shields should not skip cement milk during concreting. To reduce the cost of concrete and reinforced concrete structures, shields and other formwork elements are made taking into account their multiple use. The formwork cost is 20-30% of the total value of concrete and reinforced concrete structures.

Classification of formwork by material. According to the basic material, the formwork of monolithic concrete and reinforced concrete structures is divided into wood, metal, plywood, reinforced concrete and combined. The wooden formwork is usually made on the formwork yard or in the carpentry shop of the woodworking plant of construction. Coniferous wood with a wood moisture content of up to 25% is used to make wooden formwork. The formwork elements are harvested on machines. The quality of the erected structures largely depends on the accuracy of manufacturing the formwork elements, therefore deviations from the design dimensions in the manufactured elements should be minimal.

Wooden formwork has a low thermal conductivity compared to metal and reinforced concrete, which is of great importance when working in low temperature conditions. It is easy to attach various elements of warming to it in the winter, moisture-absorbing cladding and other devices. The main disadvantages of wooden formwork are its relatively low strength and tendency to deform when wet, dry and transport, resulting in warping, cracking of the board, opening of the seams between them. Despite these drawbacks, wooden formwork is still widely used in the construction of monolithic concrete and reinforced concrete structures and structures.

Metal formwork and tooling for it are made in mechanical workshops or shops of metal structures. Details of metal formwork are made of steel grade St.0. Formwork blanks are processed with a fairly high accuracy class. Allowable deviations from the design dimensions in the length and width per 1 meter of shields of metal panel formwork should not exceed 2 mm, deviations in the arrangement of holes for connecting elements (wedges, bolts, etc.) - 0.5 mm.

Tolerances in the dimensions of the elements of the movable, rolling and lifting-adjustable formwork should be taken in each case in accordance with the instructions given in the draft formwork.

Metal formwork passes the control assembly. Its parts in contact with the concrete are coated with a lubricant, and the rest are painted, after which all formwork elements are marked.

Metal formwork provides a smooth, smooth surface of concrete and as a type of multi-turnable inventory formwork has many advantages. It is much more expensive than wood, but practically has boundless turnover. It is considered economically feasible to use metal formwork with its turnover at least 50 times. In addition, the metal formwork has the following positive qualities, namely: rigidity, ease of form removal (with appropriate lubrication of the surfaces of the formwork), the absence of deformations under different humidity conditions. The disadvantages of metal formwork are its high cost, thermal conductivity, the difficulty of attaching various elements to the formwork.

Plywood formwork along with metal can be attributed to the number of high-turn, inventory types of formwork. Plywood is usually used only for plating, carrying the same floor formwork formwork is made of wood or metal.

Plywood formwork has a lower thermal conductivity than metal, to it is easier to attach various elements. Compared with wood and metal, it has less weight.

It is especially advisable to use plywood formwork for curved surfaces. But relatively high demands are made on plywood used for formwork, for example, it must be waterproof.

The scarcity and relatively high cost of this sort of plywood limit its widespread use as a material for formwork. Therefore, the use of plywood formwork is still limited.

Reinforced concrete formwork during the period of concreting plays the role of formwork, and subsequently is a permanent structural element of the structure.

The advantage of reinforced concrete formwork is the elimination of the process of stripping. In this regard, its mounting is greatly simplified. The disadvantages of reinforced concrete formwork are high thermal conductivity and relatively large weight.

It is used mainly in the construction of hydraulic structures, where it is a permanent external protective lining of the structure.

Combined formwork is arranged in order to make the best use of the positive qualities of various materials. This formwork is often combined from wood and metal.

Classification of formwork for constructive methods. According to the design features in construction, the following types of formwork are used: stationary; dismountable; sliding, lifting and shifting; rolling; concrete and reinforced concrete blocks and shell plates; reinforcement and metal plates; formless concreting (mesh form).

The use of stationary (non-rotatable) formwork is allowed in exceptional cases for non-standard structures and structures that do not have repeating elements. For scaffolding, round and sawn forest, mainly coniferous, varietal steel and pipes are used. All the supporting parts of the forest should be installed on a solid foundation with a sufficient bearing area in order to avoid unacceptable sediment of concrete structures and preserve the design marks of the structures during freezing and thawing of the soil.

Collapsible formwork is widely used in construction practice, consisting of individual shields, installed manually or with the help of cranes, and their supporting parts - round, ribs, fights, screeds, clamps.

Sliding or mobile formwork is widely used in the construction of silos, cement warehouses, grain elevators, tanks, water towers and other structures having a large height and relatively small cross-section. The formwork consists of metal walls or solid wooden shields covering the structure along the entire contour from the inner and outer sides. The formwork is raised to the next working position during concreting with the help of a jacking frame. The filling of continuously raised formwork with concrete is carried out in layers of 10-15 cm without interruption, and the level of the concrete mix is ​​not brought to the top of the forms by 15-20 cm. Breaks in concreting more than 2-3 hours are not recommended. Concrete is compacted by conventional methods with a flexible shaft shaft vibrator.

The use of sliding formwork eliminates the need to organize forests and multiple assembly and disassembly of formwork.

Rolling (mobile) formwork is used for concreting large- scale linear structures with a constant cross-section. Combined roller formwork moves on rollers or wheels on the track.

Formwork cladding is used as formwork slab shells and blocks. Such formwork, which is firmly connected to the concreted part of the structure with the help of reinforcement, remains in the building as a lining. When erecting massive concrete and reinforced concrete structures, in addition to the above, vacuum formwork and absorbent formwork are used.