Formwork
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. Types. Formwork comes in several types: Slab formwork (deck formwork). History. Some of the earliest examples of concrete slabs were built by Roman engineers. Because concrete is quite strong in resisting compressive loads, but has relatively poor tensile or torsional strength, these early structures consisted of arches, vaults and domes. The most notable concrete structure from this period is the Pantheon in Rome. To mould this structure, temporary scaffolding and formwork or falsework  was built in the future shape of the structure. These building techniques were not isolated to pouring concrete, but were and are widely used in masonry. Because of the complexity and the limited production capacity of the building material, concrete’s rise as a favored building material did not occur until the invention of Portland cement (and developments by the Edison Portland Cement Company) and reinforced concrete. Timber beam slab formwork. Similar to the traditional method, but stringers and joist are replaced with engineered wood beams and supports are replaced with adjustable metal props. This makes this method more systematic  and reusable. Traditional slab formwork. On the dawn of the revival of concrete in slab structures, building techniques for the temporary structures were derived again from masonry and carpentry. The traditional slab formwork technique consists of supports out of lumber or young tree trunks, that support rows of stringers assembled roughly 3 to 6 feet or 1 to 2 metres apart, depending on thickness of slab. Between these stringers, joists are positioned roughly 12 inches, 30 centimeters apart upon which boards or plywood are placed. The stringers and joists are usually 4 by 4 inch or 4 by 6 inch  lumber. The most common imperial plywood thickness is ¾ inch and the most common metric thickness is 18 mm. Metal beam slab formwork. Similar to the traditional method, but stringers and joist are replaced with aluminium forming systems or steel beams and supports are replaced with metal props. This also makes this method more systematic and reusable. Aluminum beams are fabricated as telescoping units which allows them to span supports that are located at varying distances apart. Telescoping aluminium beams can be used and reused in the construction of structures of varying size. Modular slab formwork. These systems consist of  prefabricated timber, steel or aluminum beams and formwork modules. Modules are often no larger than 3 to 6 feet or 1 to 2 metres in size. The beams and formwork are typically set by hand and pinned, clipped, or screwed together. The advantages of a modular system are: does not require a crane to place the formwork, speed of construction with unskilled labor, formwork modules can be removed after concrete sets leaving only beams in place prior to achieving design strength. Table or flying form systems. These systems consist of slab formwork "tables" that are reused on multiple stories of  a building without being dismantled. The assembled sections are either lifted per elevator or "flown" by crane from one story to the next. Once in position the gaps between the tables or table and wall are filled with "fillers". They vary in shape and size as well as their building material. The use of these systems can greatly reduce the time and manual labor involved in setting and striking the formwork. Their advantages are best used by large area and simple structures. It is also common for architects and engineers to design building around one of these systems. Structure. A  table is built pretty much the same way as a beam formwork but the single parts of this system are connected together in a way that makes them transportable. The most common sheathing is plywood, but steel and fiberglass are also in use. The joists are either made from timber, wood I-beams, aluminium or steel. The stringers are sometimes made of wood I-beams but usually from steel channels. These are fastened together (screwed, weld or bolted) to become a "deck". These decks are usually rectangular but can also be other shapes. Support. All support systems have to be  height adjustable to allow the formwork to be placed at the correct height and to be removed after the concrete is cured. Normally adjustable metal props similar to (or the same as) those used by beam slab formwork are used to support these systems. Some systems combine stringers and supports into steel or aluminum trusses. Yet other systems use metal frame shoring towers, which the decks are attached to. Another common method is to attach the formwork decks to previously cast walls or columns, thus eradicating the use of vertical props altogether. In this method, adjustable support shoes are bolted  through holes (sometimes tie holes) or attached to cast anchors. Size. The size of these tables can vary from . There are two general approaches in this system: This technique is fairly common in the United States and east Asian countries. The advantages of this approach are the further reduction of manual labour time and cost per unit area of slab and a simple and systematic building technique. The disadvantages of this approach are the necessary high lifting capacity of building site cranes, additional expensive crane time, higher material costs and little flexibility. By this approach the tables are limited in  size and weight. Typical widths are between , typical lengths are between , though table sizes may vary in size and form. The major distinction of this approach is that the tables are lifted either with a crane transport fork or by material platform elevators attached to the side of the building. They are usually transported horizontally to the elevator or crane lifting platform singlehandedly with shifting trolleys depending on their size and construction. Final positioning adjustments can be made by trolley. This technique enjoys popularity in the US, Europe and generally in high labor cost countries. The advantages of this approach  in comparison to beam formwork or modular formwork is a further reduction of labor time and cost. Smaller tables are generally easier to customize around geometrically complicated buildings, (round or non rectangular) or to form around columns in comparison to their large counterparts. The disadvantages of this approach are the higher material costs and increased crane time (if lifted with crane fork). Tunnel forms. Tunnel forms are large, room size forms that allows walls and floors to be cast in a single pour. With multiple forms, the entire floor of a building can be done in a single pour. Tunnel  forms require sufficient space exterior to the building for the entire form to be slipped out and hoisted up to the next level. A section of the walls is left uncasted to remove the forms. Typically castings are done with a frequency of 4 days. Tunnel forms are most suited for buildings that have the same or similar cells to allow re-use of the forms within the floor and from one floor to the next, in regions which have high labor prices. See structural coffer. 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 - concrete can be poured into a mould of almost any shape. This fluidity is seldom used, 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. Its potential is further demonstrated in work by Lee. Fabric formwork is a small niche in concrete technology. It uses soft, flexible materials against the fresh concrete as formwork, normally with some sort of strong tension textile or plastic material. The International Society of Fabric Forming conducts research on fabric formwork. Fence-like design. A design from Russian NPO-22 factory (trademarked as "Proster", with model "21" designed to serve as formwork) uses  iron "sheets" (with perforations) which, if necessary, can be bent to form a curve. The sheet-based formwork with V-shaped rails keeps shape in one direction (vertically) but, before being reinforced with steel beams, can be bent. Multiple sheets can be fixed together in same manner fences made of iron "sheets" can be. 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. 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. Some models of "unremovable formwork" also can serve as extra reinforcement of the structure.