The formwork is the casting mold into which fresh concrete is poured for the manufacture of concrete components. After the concrete has hardened, it is usually removed. The formwork is the corresponding hollow shape for the concrete component. The geometry of the formwork is shown in the formwork plan , the construction in the formwork plan . Here, formwork as such has an exclusively shaping character in accordance with DIN EN 12812. Load-bearing elements, even for industrially prefabricated parts, are shoring in the sense of the standard. The term formwork only covers flat cladding of the load-bearing structure (shoring).
Formwork must meet the requirements for shape, surface quality, evenness, etc. Your substructures (shoring) must on the one hand be stable in order to be able to carry the fresh concrete loads (vertically and horizontally) and on the other hand sufficiently rigid to obtain high dimensional accuracy and no undesirable deformations. The surface formation of the concrete component is determined by the structure of the formlining . Modern, industrially prefabricated auxiliary construction structures therefore consist of formwork (formlining) and its substructure (supporting structure). This is reflected, among other things, in the fact that these constructions are to be dimensioned in accordance with the technical regulations.
The formwork systems are construction aids that consist of both formwork and supporting scaffolding. Taking into account the actually decisive point of stability, the colloquially designated formwork is also shoring, which must meet the requirements of the relevant technical regulations. Fixed auxiliary structures (formwork or standing formwork), such as those used for. B. be used on ceilings, walls and columns. In addition, special movable formwork systems are used, which include climbing and sliding formwork. Such is used in particular in the construction of vertical components such. B. Walls and shafts.
After shuttering , reinforcement and concreting usually follows the stripping unless a "permanent formwork" is used. Stripping takes place after a certain time (stripping time), which depends on the position of the formwork, the temperature , the type of concrete and the load .
When concreting a road, the side formworks are pulled away after about five minutes. On the other hand, the supports under the suspended ceilings must remain in place for up to 28 days (longer at temperatures below 5 ° C).
The stability of the constructions must be guaranteed in every construction phase. In addition, the requirements for serviceability must also be observed, with the deformation behavior being of particular interest in order to comply with permissible structural tolerances.
When dimensioning these auxiliary structures, consisting of formwork and shoring, various load effects (such as dead weight, fresh concrete pressure , wind loads and effects from imperfection of the structure) must be taken into account (see DIN EN 12812 or ACI 347-04).
Types of formwork
Gmünder Einhorn-Tunnel , formwork carriage for open tunnel construction at the eastern portal
It consists of wooden panels on wooden beams or metal or plastic elements that form the supporting structure, i.e. the supporting structure. Mostly these are placed as a negative at a distance equal to the thickness of the wall and braced against each other with formwork anchors . One then speaks of a two-sided wall formwork.
A single-sided wall formwork is used when, for reasons of space, the auxiliary construction on one side of the wall cannot be used and z. As against the excavation shoring is concreted. If, as in this case, the anchoring of the formwork on the opposite side is not possible or is only possible with difficulty, the horizontal fresh concrete pressure on the one-sided supporting structure must be removed with support frames.
The fresh concrete pressure that acts on the scaffolding is approximately hydrostatic , provided that the rate of climb during concreting is chosen to be slow enough. When using self-compacting concrete (SVB, English: SCC: "self compacting concrete"), the full hydrostatic pressure must always be applied. The concrete pressure can be up to a maximum of 60 kN / m² with an aluminum frame construction (can be moved by hand) and about 80 kN / m² with a steel frame construction ( lifting gear required). A higher concrete pressure is possible with some of the system armaments brought in with the crane.
Single-sided (one-sided) formwork
The construction, known colloquially as single- sided formwork (also without anchors or one - hip formwork), consists of a heavy shoring structure that has to absorb the entire concrete pressure and the formlining that covers the supporting structure. This construct is used in the manufacture of reinforced concrete components that are particularly bulky or only accessible from one side, making it difficult or impossible to brace opposing elements.
In contrast to two-sided formwork, with single-sided formwork, the armor is only attached to one side of the component. An anchoring through the component, as is usual with double-sided formwork, is therefore not possible. The fresh concrete pressure acting on the formwork during concreting must be removed by a support frame (also called A- frame or support frame ), which is anchored to the subsurface. The support frame consists of a framework construction that transfers the forces from the fresh concrete pressure. The ground anchoring is necessary so that the support frame is not pushed to the side together with the formwork element. The number and spacing of the support frames depend on the size of the fresh concrete pressure. In order to avoid steps and joints, the support frames must be braced together, i.e. connected.
Examples of the use of single-sided formwork are large block foundations or cellar walls that are concreted against a construction pit wall. Single-sided formwork is less economical due to the increased time and effort involved.
Reinforced concrete columns are often created from industrially prefabricated frame structures. Multipurpose elements are used, which are arranged in a grid of z. B. 5 cm can be anchored. Round column formworks of various diameters (in steps of 5 cm), the formwork skin of which is made of steel or cardboard, are common. Round column formwork for one-time use is also available with styrofoam packing in order to be able to cast rectangular column cross-sections.
Beam formwork (beam formwork)
The formwork of horizontal beams can be carried out similar to the wall formwork. It is often made by carpenters for the creation of beams or coverings. Prefabricated frame constructions, which almost every manufacturer offers, are also used for beams.
System formwork elements are often too inflexible to be able to use them to create a staircase that is adapted to the building structure. For this reason, stairs that are inserted to save space are often formed with formwork created in a carpenter's manner. The use of precast concrete elements can be the more economical solution for straight stairs and for larger construction projects .
A distinction is made between crane-dependent and crane-independent climbing / moving formwork. The crane-independent climbing / repositioning formwork is called self-climbing formwork . The scaffolding is moved to the next concreting section on climbing rails by lifting devices. Today it is also possible to move all the formwork and scaffolding of an entire storey at the same time up to the next concreting section.
In the case of crane-dependent climbing and moving formwork, a further distinction is made between guided and unguided climbing and moving formwork. Guided means that the scaffolding on which the formwork is attached is pulled up on guide rails using a crane. An unguided climbing and repositioning formwork is completely separated from the structure and suspended in the next concreting section using a crane. The advantage of the guided climbing and moving formwork is that it can be moved safely even in windy conditions. In contrast to sliding formwork, a uniform surface quality can be maintained, but construction progress is slower.
Sliding formwork is used to erect tall, tower-like structures with a uniform wall structure in a continuous construction process. The sliding process of the formwork takes place with climbing poles which are arranged at intervals of approximately 1.80 m to 2.30 m and on which the entire formwork construction is continuously pushed up hydraulically . The recurring work sequence consists of the production of recesses, openings and the assembly of built-in parts, reinforcement and concreting.
The concrete that sets while the formwork continues to push itself up must develop sufficient strength early on in order to be stable after the automated formwork. The special concrete recipe must be adapted to the ambient temperature, among other things.
In contrast to climbing formwork, the manufacturing process in the sliding construction process runs continuously in 24-hour shifts. The so-called sliding or climbing formwork must be dimensioned for stability and usability, which is why they are to be assigned to the shoring .
The decisive advantages of sliding formwork compared to climbing formwork systems are the rapid construction progress, an anchor-free, homogeneous, uniform concrete surface without visible joints through construction joints and the economical installation of concrete and reinforcement. Disadvantages are the necessary 24-hour shift work, complex reinforcement installations, since the reinforcement cannot cross the formwork, and the limited surface quality of the concrete.
A "lost formwork" is formwork that is not removed after concreting and hardening of the concrete. There is no need for dismantling, cleaning or removal. Formwork work in areas such as foundations that are difficult to access can thus be rationalized. Lost formwork made of lightweight materials can also serve as component insulation.
Especially slab edge formwork, ring anchor formwork, beam formwork, lintel box formwork, foundation formwork and now also floor slab formwork for residential and industrial construction are offered as industrially manufactured series products.
In bridge construction in Germany, apart from the use of semi-prefabricated reinforced concrete, lost formwork is no longer used, as it has proven to be prone to damage. They were often indented or floated during concreting. Today it is the standard in bridge construction that all concrete surfaces must be inspectable in order to be able to detect damage caused by cracks and corrosion of the reinforcement early and reliably.
Lost polystyrene foam formwork
In building construction, lost formwork is experiencing a renaissance in the low-energy sector. A modular system, based on the Lego principle, enables easy assembly by snapping together the formwork, which is then filled with concrete. After the concrete has set, the formwork serves as thermal insulation and protects the load-bearing concrete core from the weather. The formation of cavities when pouring the liquid concrete can be problematic, since the outer and inner surfaces of the formwork bodies are connected by integrally formed webs that represent obstacles when filling the cavity.
Inflatable hollow bodies are used to produce compact, rounded cavities. Hoses are often used which have to be attached to the ground during concreting so as not to float. After the concrete has hardened, the air is let out of the hollow body and it can be pulled out. Inflatable formwork is rarely used due to its poor dimensional accuracy.
Frame formwork are wall formwork systems in which the formwork elements consist of frames that are welded at the factory, on which the formwork (plywood or plastic) is attached. Frame formwork is available with steel or aluminum frames.
The large-area formwork is a construction aid that is designed and dimensioned by an engineer and must therefore be assigned to the shoring . The large-area formwork consists of the formwork facing which is attached to a supporting structure. It is used to manufacture self-supporting or large-format walls, columns or ceilings. The shoring is often formed from wooden trusses , which are stiffened by steel girders running perpendicular to them.
In order to ensure that the formlining is properly detached from the concrete surface, the formwork is pretreated with release agents ("formwork oil "). The release agents are mostly oils that are sprayed on. This is particularly necessary with exposed concrete , but also important when the formlining is used several times.
Fair-faced concrete formwork
In the case of exposed concrete formwork, there are special surface requirements. In order to avoid discoloration of the concrete due to adhering production residues, the shuttering boards used should have already been used once. Alternatively, they can be cleaned thoroughly or pre- painted with cement milk . Formwork panels with absorbent coverings are also used to prevent the uneven accumulation of moisture.
Structural formwork is a special type of exposed concrete formwork for the production of structural concrete . In the simplest case , the formwork can consist of wooden boards , which then leave the wood grain as an imprint on the visible surface of the structure . More sophisticated structured concrete surfaces of any kind can also Formliners from polyurethane achieve. The formliners can be used in in- situ concrete construction and in precast concrete plants. The textured formliners are usually glued to a facing formwork. If it is only used once, the textured formliner can also be nailed onto the facing formwork. Once the exposed concrete has hardened, a correspondingly structured concrete surface is obtained after stripping.
The quality of the structural details shown in the exposed concrete after completion is subject to:
- the quality and compliance with the intended use cycles of the textured formliner (in accordance with the work and use instructions)
- working cleanly with the formwork with the textured formliner (sticking on, even application of release agent , matrix cleaning when used multiple times , etc.)
- the use of proven exposed concrete mixtures, which are recommended by the manufacturer for textured formliners
- the weather-related setting behavior of the concrete, the speed at which the formwork is backfilled and the duration of the vibration or ventilation
- the weather conditions during concreting and curing (dryness and consistently high temperatures from 18 ° C favor a high-quality result)
- the professional stripping.
Manufacturer of formwork systems
Climbing formwork ( Rügen Bridge )
- Peter Grupp: Formwork atlas (formwork systems and use in practice). Verlag Bau + Technik, Düsseldorf 2009, ISBN 978-3-7640-0484-2 .
- Christian Hofstadler: Formwork work, technological basics, exposed concrete, system selection, process planning, logistics and costing. Springer-Verlag Berlin, Heidelberg 2008, ISBN 978-3-540-85178-3 .
- Wolfgang Malpricht: Formwork planning . A textbook and exercise book. Fachbuchverlag Leipzig in Carl Hanser Verlag, Munich 2010, ISBN 978-3-446-42044-1 .
- Roland Schmitt: The formwork technology (systems, use and logistics). Verlag Ernst & Sohn, Berlin 2001, ISBN 3-433-01346-2 .
- Matthias Dupke: Areas of application of the sliding formwork and the climbing-moving formwork: A comparison of the systems. Publishing house diploma thesis agency, Hamburg 2010, ISBN 978-3-8386-0295-0
- Infoline formwork - online lexicon with basic knowledge, examples, dates, addresses, etc.