Formwork is
molds into which
concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...
or similar materials are either
precast
Precast concrete is a construction product produced by casting concrete in a reusable mold or "form" which is then cured in a controlled environment, transported to the construction site and maneuvered into place; examples include precast beam ...
or
cast-in-place. In the context of concrete construction, the
falsework
Falsework consists of temporary structures used in construction to support a permanent structure until its construction is sufficiently advanced to support itself. For arches, this is specifically called centering. Falsework includes temporary su ...
supports the shuttering molds. In specialty applications formwork may be permanently incorporated into the final structure, adding insulation or helping reinforce the finished structure.
Types
Formwork may be made of wood, metal, plastic, or composite materials:
#''Traditional timber formwork''. The formwork is built on site out of
timber
Lumber is wood that has been processed into dimensional lumber, including beams and planks or boards, a stage in the process of wood production. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, wi ...
and
plywood
Plywood is a material manufactured from thin layers or "plies" of wood veneer that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another. It is an engineered wood from the family of manufactured ...
or moisture-resistant
particleboard
Particle board, also known as chipboard or low-density fiberboard, is an engineered wood product manufactured from wood chips and a synthetic resin or other suitable binder, which is pressed and extruded. Particle board is often confused with ori ...
. 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
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
) and covered on the application (
concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...
) side with material having the wanted surface structure (steel,
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
, 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). Metal formwork systems are better protected against rot and fire than traditional timber formwork.
#''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. To get an added layer of protection against destructive weather, galvanized roofs will help by eliminating the risk of corrosion and rust. These types of modular enclosures can have load-bearing roofs to maximize space by stacking on top of one another. They can either be mounted on an existing roof, or constructed without a floor and lifted onto existing enclosures using a crane.
#''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
Fibre-reinforced plastic (FRP; also called fibre-reinforced polymer, or in American English ''fiber'') is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass fibre, glass (in fibreglass), Carbon fib ...
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
Shear may refer to:
Textile production
*Animal shearing, the collection of wool from various species
**Sheep shearing
*The removal of nap during wool cloth production
Science and technology Engineering
*Shear strength (soil), the shear strength ...
reinforcement, as well as serving to confine the concrete and prevent against environmental effects, such as
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
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.
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
In physics, tension is described as the pulling force transmitted axially by the means of a string, a rope, chain, or similar object, or by each end of a rod, truss member, or similar three-dimensional object; tension might also be described as t ...
or
torsional strength, these early structures consisted of compression-resistant
arch
An arch is a vertical curved structure that spans an elevated space and may or may not support the weight above it, or in case of a horizontal arch like an arch dam, the hydrostatic pressure against it.
Arches may be synonymous with vaul ...
es,
vaults and
dome
A dome () is an architectural element similar to the hollow upper half of a sphere. There is significant overlap with the term cupola, which may also refer to a dome or a structure on top of a dome. The precise definition of a dome has been a m ...
s. The most notable concrete structure from this period is the
Pantheon
Pantheon may refer to:
* Pantheon (religion), a set of gods belonging to a particular religion or tradition, and a temple or sacred building
Arts and entertainment Comics
*Pantheon (Marvel Comics), a fictional organization
* ''Pantheon'' (Lone St ...
in Rome. To mould this structure, temporary
scaffolding and formwork or
falsework
Falsework consists of temporary structures used in construction to support a permanent structure until its construction is sufficiently advanced to support itself. For arches, this is specifically called centering. Falsework includes temporary su ...
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
Masonry is the building of structures from individual units, which are often laid in and bound together by mortar; the term ''masonry'' can also refer to the units themselves. The common materials of masonry construction are bricks, building ...
construction. 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
Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th c ...
and
reinforced concrete
Reinforced concrete (RC), also called reinforced cement concrete (RCC) and ferroconcrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having hig ...
.
Timber beam slab formwork
Similar to the traditional method, but
stringers and
joists are typically replaced with
engineered wood
Engineered wood, also called mass timber, composite wood, man-made wood, or manufactured board, includes a range of derivative wood products which are manufactured by binding or fixing the strands, particles, fibres, or veneers or boards of woo ...
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
Masonry is the building of structures from individual units, which are often laid in and bound together by mortar; the term ''masonry'' can also refer to the units themselves. The common materials of masonry construction are bricks, building ...
and
carpentry
Carpentry is a skilled trade and a craft in which the primary work performed is the cutting, shaping and installation of building materials during the construction of buildings, ships, timber bridges, concrete formwork, etc. Carpenters tr ...
. 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 apart, upon which boards or
plywood
Plywood is a material manufactured from thin layers or "plies" of wood veneer that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another. It is an engineered wood from the family of manufactured ...
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 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
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
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 temporary formwork. Table forms vary in shape and size as well as their building material, with some supported by integral trusses. The use of these systems can greatly reduce the time and manual labor involved in setting and striking (or "stripping") 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
Plywood is a material manufactured from thin layers or "plies" of wood veneer that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another. It is an engineered wood from the family of manufactured ...
, but steel and
fiberglass
Fiberglass (American English) or fibreglass (Commonwealth English) is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cloth ...
are used. The joists are either made from timber,
engineered lumber (often in the form of
I-beam
An I-beam, also known as H-beam (for universal column, UC), w-beam (for "wide flange"), universal beam (UB), rolled steel joist (RSJ), or double-T (especially in Polish language, Polish, Bulgarian language, Bulgarian, Spanish language, Spanish ...
s), 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
truss
A truss is an assembly of ''members'' such as beams, connected by ''nodes'', that creates a rigid structure.
In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so that the assembl ...
es. 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:
#Crane handled: this approach consists of assembling or producing the tables with a large formwork area that can only be moved up a level by crane. Typical widths can be 15, 18 or 20 feet, or 5 to 7 metres, but their width can be limited, so that it is possible to transport them assembled, without having to pay for an oversize load. The length might vary and can be up to 100 feet (or more) depending on the crane capacity. After the
concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...
is
cured, the
decks are lowered and moved with rollers or
trolley
Trolley may refer to:
Vehicles and components
* Tram, or trolley or streetcar, a rail vehicle that runs on tramway tracks
* Trolleybus, or trolley, an electric bus drawing power from overhead wires using trolley poles
** Trolleytruck, a trolleyb ...
s to the edge of the building. From then on the protruding side of the table is lifted by crane while the rest of the table is rolled out of the building. After the
centre of gravity
In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
is outside of the building the table is attached to another
crane and flown to the next level or position.
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.
- Crane fork or elevator handled:
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.Tunnel formwork saves the time and the cost.
See structural
coffer.
Concrete-form oil
The main purpose of concrete-form oil is to reduce the adhesion between the foundation structure and the concrete mixture poured into it. It also reduces the possibility of cracks and chips occurring due to drying out or concrete overstressing. Without concrete-form oil, which reduces the adhesion between surfaces, it becomes virtually impossible to remove the structure without damaging the foundation, wall or bulkhead. The risk also increases with the size of the tier.
Climbing formwork
;Climbing forms are commonly used on
:
*
Skyscraper
A skyscraper is a tall continuously habitable building having multiple floors. Modern sources currently define skyscrapers as being at least or in height, though there is no universally accepted definition. Skyscrapers are very tall high-ris ...
s
* Bridge pylons
* Concrete columns
*
Airport control towers
*
High rise buildings
*
Elevator shafts
* Silos
Flexible formwork
There is an increasing focus on sustainability in design, backed up by
carbon dioxide emissions
Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and lar ...
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 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. The result, however, is high material use structures with large carbon footprints.
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
One way of attributing greenhouse gas (GHG) emissions is to measure the embedded emissions of goods that are being consumed (also referred to as "embodied emissions", "embodied carbon emissions", or "embodied carbon"). This is different from the ...
, 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.
Fabric formwork is a small niche in concrete technology. It uses soft, flexible materials as formwork against the fresh concrete, normally with some sort of strong tension textile or plastic material. The International Society of Fabric Forming conducts research on fabric formwork.
Iron sheet formwork
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.
* A circle can be made from a single sheet of "''21''" formwork, allowing cylindrical columns to be poured.
Usage
For removable forms, once the concrete has been poured into formwork and has set (or ''cured''), the formwork is ''struck'' or ''stripped'' to expose the finished concrete. The time between pouring and stripping depends on the job specifications, which include the cure required, and whether the form is supporting any weight; it is usually at least 24 hours after the pour is completed. For example, the
California Department of Transportation
The California Department of Transportation (Caltrans) is an executive department of the U.S. state of California. The department is part of the cabinet-level California State Transportation Agency (CalSTA). Caltrans is headquartered in Sacrame ...
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.
[ ection 6-02.3(11)from the WSDOT Standard Specifications, 2006]
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. "Form blowouts" also occur when under-designed formwork bends or breaks during the concrete pour (especially if filled with a high-pressure
concrete pump
A concrete pump is a machine used for transferring liquid concrete by pumping. There are different types of concrete pumps.
A common type of concrete pump for large scale construction projects is known as a boom concrete pump, because it uses a ...
). Consequences can vary from minor leaks, easily patched during the pour, to catastrophic form failure, even death.
Concrete exerts less pressure against the forms as it hardens. The hardening is an
asymptotic
In analytic geometry, an asymptote () of a curve is a line such that the distance between the curve and the line approaches zero as one or both of the ''x'' or ''y'' coordinates tends to infinity. In projective geometry and related contexts, ...
process, meaning that most of the final strength will be achieved after a short time, with further hardening over time reflecting the cement type, admixtures, and pour conditions such as temperature and ambient moisture.
Wet concrete also applies
hydrostatic
Fluid statics or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an imme ...
pressure to formwork. The pressure at the bottom of the form is therefore greater than at the top, causing most blowouts to occur low in the formwork. In the illustration of the column formwork above, 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 "permanent formwork" also can serve as extra reinforcement of the structure.
Gallery
File:National Theatre - detail of shuttered concrete.jpg, The concrete structure of the Royal National Theatre
The Royal National Theatre in London, commonly known as the National Theatre (NT), is one of the United Kingdom's three most prominent publicly funded performing arts venues, alongside the Royal Shakespeare Company and the Royal Opera House. I ...
carries the impression of the timber
Lumber is wood that has been processed into dimensional lumber, including beams and planks or boards, a stage in the process of wood production. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, wi ...
shuttering
File:Plastic concrete formwork for cross wall.jpg, Plastic concrete formwork for cross wall
File:Tunnel formwork.jpg, Coal tunnel constructed using handset aluminum concrete forms
File:Pool formwork.JPG, Concrete pool construction using aluminum concrete forms
File:Stairs-forms.jpg, Soffit
A soffit is an exterior or interior architectural feature, generally the horizontal, aloft underside of any construction element. Its archetypal form, sometimes incorporating or implying the projection of beams, is the underside of eaves (to ...
formwork to a flight of concrete stairs
File:Strongbacks.jpg, Stair formwork showing the use of strongbacks to support the riser shutters
File:Formwork-trad-beam.png, Sketch showing the use timber props for beam
Beam may refer to:
Streams of particles or energy
*Light beam, or beam of light, a directional projection of light energy
**Laser beam
*Particle beam, a stream of charged or neutral particles
**Charged particle beam, a spatially localized grou ...
forms
File:Silo Formwork.JPG, Coal silo construction using radius concrete formwork
File:Formwork-tie-bolts.jpg, Twin steel walers and tie bolts used to secure wall forms
File:Permanent-column-form.jpg, Column poured using spiral ducting
File:Bondek.jpg, Concrete slab poured on roll formed galvanized steel with the form as a permanent part of the structure
File:Fence Formwork.JPG, Concrete fence construction using aluminum forms to impart an ashlar stone impression in the finished product
File:Concrete Housing.jpg, Concrete housing construction in Venezuela using aluminum concrete formwork
File:Brazil Formwork.JPG, Concrete construction in Brazil using handset aluminum concrete formwork
File:Construction of formwork at the Volokolamskaya metro station of the Moscow metro system (Russia) - Опалубочная система для станции метро Волоколамская.jpg, Concrete construction in Moscow metro using special aluminum concrete formwork
File: Engineered Formwork System Специальная опалубка для бетонирования сводов с кессонами.jpg, Engineered formwork system in Moscow metro
File:Monolithic works at the Volokolamskaya metro station in Moscow. The process of rolling the formwork modules to a new bay. Перекатка опалубки сводов на новую захватку (Россия, Москва)..jpg, Engineered formwork system in Moscow metro
File:Wiki_proster21_pool_1.png, Pool construction using "Proster 21" flexible formwork.
See also
*
Climbing formwork (formwork that climbs up the rising building during the construction)
*
Concrete cover Concrete cover, in reinforced concrete, is the least distance between the surface of embedded reinforcement and the outer surface of the concrete ( ACI 130). The concrete cover depth can be measured with a cover meter.
Purpose of provision of conc ...
(depth of the concrete between reinforcing steel and outer surface)
*
Slip forming
Slip forming, continuous poured, continuously formed, or slipform construction is a construction method in which concrete is poured into a continuously moving form.Nawy, ''Concrete Construction Engineering Handbook,'' 2008, p. 10—33. Slip formi ...
(construction method in which concrete is poured into a continuously moving form)
*
Cast in place concrete
*
Precast concrete
Precast concrete is a construction product produced by casting concrete in a reusable molding (process), mold or "form" which is then cured in a controlled environment, transported to the construction site and maneuvered into place; examples i ...
Literature
* Matthias Dupke: ''Einsatzgebiete der Gleitschalung und der Kletter-Umsetz-Schalung: Ein Vergleich der Systeme.'' 2010, Verlag Diplomarbeiten Agentur, Hamburg, .
*
The Concrete Society, ''Formwork: A guide to good practice''
References
External links
Stripping time of form work as per india standards
{{Authority control
Concrete
Concrete buildings and structures
Building materials
Building engineering
Articles containing video clips