Formwork for monolithic construction

Recommendation points

• Characteristics of monolithic buildings
• Classification of formwork used in monolithic construction
• How to choose formwork
• At the end

In this article: the pros and cons of monolithic housing construction; classification of formwork used in monolithic technology; removable and non-removable formwork; characteristics of wood, plastic, aluminum, steel and expanded polystyrene formwork; how to choose formwork; cleaning and repair of formwork elements.

In the late 1980s, the technology of monolithic construction of buildings was developed in Russia, gradually replacing the previously popular prefabricated construction of high-rise buildings from reinforced concrete panels. Monolithic construction makes it possible to construct buildings in a short time without restrictions on the number of storeys, and the use of specialized construction equipment during the construction of an object – formwork designed at the plant – allows achieving high accuracy of enclosing structures, the surfaces of which have practically no mating seams. What is the technology of monolithic construction of buildings and what kind of formwork is used during such construction?

Characteristics of monolithic buildings

Monolithic construction takes place in several stages – preparation and delivery of concrete grade 200-400 to the construction site, installation of formwork, filling it with ready-mixed concrete, dismantling of removable formwork after concrete hardening. In addition to the creation of completely monolithic houses, this technology is used in the construction of prefabricated monolithic buildings that combine monolithic load-bearing elements and external walls from traditional building materials (bricks, reinforced concrete panels, etc.).

Consider the characteristics of high-quality monolithic housing construction, previously positive:

• the minimum service life of such a building is 200 years, while panel houses cannot be used for more than 50 years due to corrosion of the reinforcing metal elements in the composition of reinforced concrete slabs. And the metal reinforcement used to create a monolithic building is completely embedded in the thickness of concrete, which becomes only stronger every year;
• high, permissible norms, load on floors – up to 600 kg per m², and for the floors of panel buildings, only 200 kg per m is allowed². Accordingly, on the floors of monolithic buildings, it is allowed to place massive household equipment, such as a jacuzzi, a swimming pool and a sauna;
• construction using monolithic technology does not require a construction site of significant size – high-rise buildings can be erected on a small area, which facilitates construction work in dense urban areas;
• the construction time for buildings is more than halved compared to the construction of brick houses;
• the construction site does not depend on the proximity of factories for the production of reinforced concrete slabs, bricks, and even from suppliers of ready-mixed concrete – a mobile concrete plant occupying a small area is located right at the construction site;
• construction of high-rise residential buildings with open-plan premises (subject to the construction of buildings with load-bearing columns and without monolithic walls). If high-quality floor formwork is used, then the internal surfaces in the premises of such buildings require minimal preparation for finishing;
• seismic resistance of monolithic buildings is 8 points on the Richter scale, which allows them to be erected in seismic areas.

There are not many negative characteristics, but they are:

• each stage of construction of a building using monolithic technology requires careful control, especially when exposing the formwork;
• construction is allowed in the cold season, however, it will not be cheap – special additives to concrete that increase its solidification and cement brands that release heat during hydration will significantly increase the cost of building a building.

Classification of formwork used in monolithic construction

Formwork, which is a box-like structure for casting monolithic walls, is classified according to its area of ​​application during construction work. Formwork kits are produced for creating walls, floors, supporting elements in the form of columns, ring walls, creating tunnel vaults, etc. In addition, the formwork is subdivided: by type of construction – into girder and frame; depending on the method of installation – on a stationary, hydraulic self-lifting, lifting and lifting-and-moving; by overall dimensions – into small-piece and large-panel; by way of use – removable (reusable) and non-removable.

The frame formwork system consists of frame panels, supports and fixings. The main elements of frame panels are a supporting steel frame, transverse steel stiffeners and a formwork plate. The frame design uses a closed hollow profile, on the outer surface of which a shaped corrugation is applied, which performs two functions – it facilitates the joining of adjacent frames to each other and ensures the safety of the end sections of the formwork board from damage. Thanks to the steel frame construction, on-site assembly of the frame formwork is quick and easy. With its help, concreting of any horizontal and vertical elements of the constructed object is carried out, for example, the formwork of columns and ceilings.

In addition to wooden I-beams, the girder formwork kit includes supports, girders, formwork panels, scaffolding and scaffolding for concrete workers. To preserve wooden beams, plastic or steel tips are put on their ends, the length of the beams is strictly standardized. When placing the girder formwork on the construction site, a verified pitch is observed between the girders connected by steel fasteners.

Structurally, the tunnel formwork consists of non-separable half-sections, including horizontal and vertical formwork panels. With its help, walls and ceilings are simultaneously created in typical buildings during their construction – each half-section is exposed by a crane.

When erecting monolithic buildings, removable formwork is most often used, consisting of wooden, plastic, steel and aluminum elements; for permanent formwork, panels of extruded polystyrene foam and chipboard are used.

Probably the only advantage of wooden formwork is its low cost. There are much more disadvantages – high hygroscopicity, poor resistance to mechanical damage. The number of cycles of using wooden formwork does not exceed 30, while it is required to replace its damaged elements with new ones. Due to rapid wear, the most critical elements of such formwork are made of glued wood and plastic.

Plastic formwork is an optimal balance of strength and a smooth surface that does not change shape under local mechanical influences. An important advantage of plastic is its absolute inertness to moisture, which allows concrete to harden qualitatively, and the formwork itself does not change its shape in any way. The plastic is lightweight, which makes it easy to transport the formwork made of this material. Disadvantages of plastic formwork – no more than 200 cycles of use, insufficient mechanical strength in monolithic multi-storey construction. For the latter reason, plastic formwork is used mainly in low-rise housing construction..

Steel formwork is made of galvanized or galvanized, powder coated steel sheet with a thickness of 0.7 mm and more (depends on the specification of this steel formwork kit). Compared to any other materials used in the creation of formwork for monolithic housing construction, steel formwork has the highest characteristics – the resistance of steel to mechanical stress allows the use of the same set more than 500 times, which explains its popularity in multi-storey construction, especially often with its with the help of wall formwork. Lack of steel formwork – significant weight.

The advantage of aluminum formwork is its low weight – compared to steel, aluminum weighs less, respectively, such a formwork is easier to transport to the construction site and carry out its installation on site. However, its structural elements lose the geometric correctness of the lines after 300 cycles of use, and it is almost impossible to restore them. Another disadvantage of aluminum formwork is its low resistance to corrosion in contact with liquid concrete, which requires special anti-corrosion treatment..

Permanent formwork is used during the construction of only one building object – when concrete is cured, its connection with it becomes inseparable, i.e. permanent formwork elements form the walls of the future house. The assembly of the fixed formwork is carried out using multiple intermediate partitions connecting its opposite wall-blocks to each other and resembles the well-known Lego constructor. Reinforcement is passed into the cavities formed by the walls and concrete is poured, after hardening of which the permanent formwork becomes the outer sides of the concrete casting. The advantages of fixed formwork are low weight (in the case of expanded polystyrene), increased sound and heat retention characteristics of concrete walls. Disadvantage – such formwork can be used no more than once..

How to choose formwork

Formwork systems designed for multiple use on various projects under construction are assessed according to four main criteria – quality of performance, number of cycles of use, versatility and the period for which this formwork system pays for itself.

The assessment according to the first two criteria includes the strength characteristics of the formwork panels, the strength and reliability of the fasteners, the resistance of the formwork system to loads when assembled. In addition to the maximum number of cycles of use, the ability of formwork elements to maintain performance during their storage period is assessed. If the need for a formwork system arises quite often, then it makes sense to purchase steel formwork, as the most durable among other types.

The universal formwork system will allow it to be used in the construction of buildings of almost any architecture – with its help it will be possible to create both rectilinear and angular, as well as rounded surfaces, without attracting additional formwork systems of other types. When choosing a universal formwork, you should pay attention not only to its structural capabilities, but also to the permissible methods of installation – ideally, its installation should be carried out both manually and using lifting equipment. It is equally important to consider the possibility of interchangeability and ease of repair of the main elements of the formwork.

Often, different types of formwork are used at the same construction site, which must be connected to each other. Therefore, even at the stage of choosing a supplier, it is necessary to find out which mechanisms for connecting different formwork systems are available in the structures he offers. If such mechanisms do not exist, then it is necessary to purchase additional fasteners, which will significantly speed up the progress of construction in the future..

Evaluation of offers on the market of formwork systems only from the point of view of price is fundamentally wrong – it is necessary to choose a system that is optimal according to most of the above criteria.

At the end

Upon completion of the construction of the next object using monolithic technology, it is required to perform a complete cleaning and restoration of the geometry of the formwork elements, which will allow them to return to their original performance characteristics. It is most often impossible to carry out these works independently – cleaning and restoration are carried out on special equipment in a workshop. In the course of such work, repair and replacement of worn-out formwork elements are also carried out – since different materials are used in its creation, the degree of their working wear is also not the same. The outer coating of formwork panels becomes unusable especially quickly – it is easier and cheaper to restore it than to purchase new panels.