Formwork is temporary or permanent molds into which concrete or similar materials are poured. In the context of concrete construction, the falsework supports the shuttering molds. The formwork sector is being influenced by the requirements of improved workplace safety and crane-independent systems that are fast and reliable. Working at heights has become less hazardous. Due to these reasons, concrete formwork is becoming popular.
In addition, concrete contributes to the creative modern architectural design. Therefore, modernization of formwork systems was essential to keep pace with concrete advancement. As tall structures have gradually emerged to be the modern trend, formwork systems have also developed from being simple timber-based to pre-engineered structures of steel, aluminum, timber, plywood, and plastics. The formwork technology has translated into rapid construction with less labor.
Formwork comes in several types:
- Traditional timber formwork. The formwork is built on site out of timber and plywood or moisture-resistant particleboard. It is easy to produce but time-consuming for larger structures, and the plywood facing has a relatively short lifespan. It is still used extensively where the labour costs are lower than the costs for procuring reusable formwork. It is also the most flexible type of formwork, so even where other systems are in use, complicated sections may use it.
- Engineered Formwork System. This formwork is built out of prefabricated modules with a metal frame (usually steel or aluminium) and covered on the application (concrete) side with material having the wanted surface structure (steel, aluminum, timber, etc.). The two major advantages of formwork systems, compared to traditional timber formwork, are speed of construction (modular systems pin, clip, or screw together quickly) and lower life-cycle costs (barring major force, the frame is almost indestructible, while the covering if made of wood; may have to be replaced after a few - or a few dozen - uses, but if the covering is made with steel or aluminium the form can achieve up to two thousand uses depending on care and the applications).
- Re-usable plastic formwork. These interlocking and modular systems are used to build widely variable, but relatively simple, concrete structures. The panels are lightweight and very robust. They are especially suited for similar structure projects and low-cost, mass housing schemes.
- Permanent Insulated Formwork. This formwork is assembled on site, usually out of insulating concrete forms (ICF). The formwork stays in place after the concrete has cured, and may provide advantages in terms of speed, strength, superior thermal and acoustic insulation, space to run utilities within the EPS layer, and integrated furring strip for cladding finishes.
- Stay-In-Place structural formwork systems. This formwork is assembled on site, usually out of prefabricated fiber-reinforced plastic forms. These are in the shape of hollow tubes, and are usually used for columns and piers. The formwork stays in place after the concrete has cured and acts as axial and shear reinforcement, as well as serving to confine the concrete and prevent against environmental effects, such as corrosion and freeze-thaw cycles.
- Flexible formwork. In contrast to the rigid moulds described above, flexible formwork is a system that uses lightweight, high strength sheets of fabric to take advantage of the fluidity of concrete and create highly optimised, architecturally interesting, building forms. Using flexible formwork it is possible to cast optimised structures that use significantly less concrete than an equivalent strength prismatic section, thereby offering the potential for significant embodied energy savings in new concrete structures.
Characteristics Of Self-climbing Formwork
Self-climbing formwork was conceived due to the huge construction projects that were required to be completed in tight schedules. Workplace safety is an important requirement that has also contributed in the development of self-climbing formwork techniques. These techniques have reduced the requirement of cranes.
Workers now function in an enclosed safe environment, and there is adequate space for the tools and equipment. A self climbing formwork presents an operational platform on which the concrete forms are placed, and the complete assembly is raised by hydraulic cylinders. Each construction project has dissimilar characteristics, due to which the assemblies normally are customized to ensure their effectiveness and safety. Enhanced workplace safety, reduced construction period, and better efficiency are available when self-climbing formwork techniques are used.
Suitability Of Self Climbing Formwork
This technique is appropriate for locations where availability of unskilled labor is restricted. In such systems, heavy machinery and high technology is not significantly involved. The method is technologically valid for nearly all kinds of buildings. Formwork is utilized as a mold where wet concrete is transferred temporarily. The temporary system functions as a provisional support for the structure.
This method is useful for the elimination and reduction of the conventional site based skills like timber formwork, masonry, and plastering. Carefully planned formwork can increase the production, speed, and precision of the construction. Usually, in main housing schemes the cost of formwork is approximately up to 10 per cent of the general project price, and the time involved is up to 60 per cent. However, engineered formwork is still gaining popularity.
Flexible formwork
There is an increasing focus on sustainability in design, backed up by carbon dioxide emissions reduction targets. The low embodied energy of concrete by volume is offset by its rate of consumption which make the manufacture of cement accountable for some 5% of global CO2 emissions.
Concrete is a fluid that offers the opportunity to economically create structures of almost any geometry - we can pour concrete into a mould of almost any shape. This fluidity is seldom utilise, with concrete instead being poured into rigid moulds to create high material use structures with large carbon footprints. The ubiquitous use of orthogonal moulds as concrete formwork has resulted in a well-established vocabulary of prismatic forms for concrete structures, yet such rigid formwork systems must resist considerable pressures and consume significant amounts of material. Moreover, the resulting member requires more material and has a greater self-weight than one cast with a variable cross section.
Simple optimisation methods may be used to design a variable cross section member in which the flexural and shear capacity at any point along the element length reflects the requirements of the loading envelope applied to it.
By replacing conventional moulds with a flexible system composed primarily of low cost fabric sheets, flexible formwork takes advantage of the fluidity of concrete to create highly optimised, architecturally interesting, building forms. Significant material savings can be achieved. The optimised section provides ultimate limit state capacity while reducing embodied carbon, thus improving the life cycle performance of the entire structure.
Control of the flexibly formed beam cross section is key to achieving low-material use design. The basic assumption is that a sheet of flexible, permeable fabric is held in a system of falsework before reinforcement and concrete are added. By varying the geometry of the fabric mould with distance along the beam, the optimised shape is created. Flexible formwork therefore has the potential to facilitate the change in design and construction philosophy that will be required for a move towards a less material intensive, more sustainable, construction industry.
Usage
For removable forms, once the concrete has been poured into formwork and has set (or cured), the formwork is struck or stripped (removed) to expose the finished concrete. The time between pouring and formwork stripping depends on the job specifications, the cure required, and whether the form is supporting any weight, but is usually at least 24 hours after the pour is completed. For example, the California Department of Transportation requires the forms to be in place for 1–7 days after pouring, while the Washington State Department of Transportation requires the forms to stay in place for 3 days with a damp blanket on the outside.
Spectacular accidents have occurred when the forms were either removed too soon or had been under-designed to carry the load imposed by the weight of the uncured concrete. Less critical and much more common (though no less embarrassing and often costly) are those cases in which under-designed formwork bends or breaks during the filling process (especially if filled with a high-pressure concrete pump). This then results in fresh concrete escaping out of the formwork in a form blowout, often in large quantities.
Concrete exerts less pressure against the forms as it hardens, so forms are usually designed to withstand a number of feet per hour of pour rate to give the concrete at the bottom time to firm up. For example, wall or column forms are commonly designed for a pour rate between 4–8 ft/hr. The hardening is an asymptotic process, meaning that most of the final strength will be achieved after a short time, though some further hardening can occur depending on the cement type and admixtures.
Wet concrete also applies hydrostatic pressure to formwork. The pressure at the bottom of the form is therefore greater than at the top. In the illustration of the column formwork to the right, the 'column clamps' are closer together at the bottom. Note that the column is braced with steel adjustable 'formwork props' and uses 20 mm 'through bolts' to further support the long side of the column.
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