Formwork

Molds for cast

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Animation depicting construction of multi-story building using aluminum handset formwork. Modular steel frame formwork for a foundation. Rebar has been stubbed up out of the concrete slab to form the base of future columns Timber formwork for a concrete column. Adjustable metal screw jacks both stabilize and plumb the form Aluminum formwork system Sketch of the side view of traditional timber formwork used to form a flight of stairs Placing a wall form. A matching form will be placed on the opposite side to create the space to pour concrete into

Formwork is molds into which concrete or similar materials are either precast or cast-in-place. In the context of concrete construction, the falsework 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

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Formwork may be made of wood, metal, plastic, or composite materials:

1. 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.
2. 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). Metal formwork systems are better protected against rot and fire than traditional timber formwork.
3. 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.[

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4. 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.
5. 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.
6. 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,[1] thereby offering the potential for significant embodied energy savings in new concrete structures.

Slab formwork (deck formwork)

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Schematic sketch of traditional formwork Modular formwork with deck for housing project in Chile Steel and plywood formwork for poured-in-place concrete foundation

History

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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 compression-resistant 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 construction. Because of the complexity and the limited production capacity of the building material[citation needed], concrete's rise as a favored building material did not occur until the invention of Portland cement and reinforced concrete.

Timber beam slab formwork

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Similar to the traditional method, but stringers and joists are typically replaced with engineered wood beams and supports are replaced with adjustable metal props. This makes this method more systematic and reusable.

Traditional slab formwork

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Traditional timber formwork on a jetty in Bangkok

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 cm) 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 3⁄4 inch and the most common metric thickness is 18 mm.

Metal beam slab formwork

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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.

Hand setting modular aluminum deck formwork Handset modular aluminum formwork

Modular slab formwork

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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

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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.

Flying formwork tables with aluminium and timber joists. In this system the tables are supported by adjustable shoes attached to previously poured columns and walls

Structure

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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 used. The joists are either made from timber, engineered lumber (often in the form of 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

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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

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The size of these tables can vary from 70 to 1,500 square feet (6.5 to 140 m2). There are two general approaches in this system:

1. 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 is cured, the decks are lowered and moved with rollers or trolleys 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 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.

Formwork tables in use at a building site with more complicated structural features

2. Crane fork or elevator handled:

By this approach the tables are limited in size and weight. Typical widths are between 6 and 10 feet (1.8 and 3.0 m), typical lengths are between 12 and 20 feet (3.7 and 6.1 m), 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

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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

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The main purpose of concrete-form oil is to reduce the adhesion between the foundation structure and the concrete mixture poured into it.[2] 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.[3]

Climbing formwork

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Climbing formwork, also known as jumpform, is a special type formwork for vertical concrete structures that rises with the building process. While relatively complicated and costly, it can be an effective solution for buildings that are either very repetitive in form (such as towers or skyscrapers) or that require a seamless wall structure (using gliding formwork, a special type of climbing formwork).

Various types of climbing formwork exist, which are either relocated from time to time, or can even move on their own (usually on hydraulic jacks, required for self-climbing and gliding formworks).

Climbing forms are commonly used on:

Flexible formwork

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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.[5]

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. 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.[clarification needed]

Simple optimisation methods[6][7][8] 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.[clarification needed]

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.[9] 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.[10]

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.[11]

Iron sheet formwork

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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

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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 requires the forms to be in place for 1–7 days after pouring,[12] while the Washington State Department of Transportation requires the forms to stay in place for 3 days with a damp blanket on the outside.[13]

Formwork stripped exposing the set concrete

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). 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 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 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

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• The concrete structure of the Royal National Theatre carries the impression of the timber shuttering

• Plastic concrete formwork for cross wall

• Coal tunnel constructed using handset aluminum concrete forms

• Concrete pool construction using aluminum concrete forms

• Soffit formwork to a flight of concrete stairs

• Stair formwork showing the use of strongbacks to support the riser shutters

• Sketch showing the use timber props for beam forms

• Coal silo construction using radius concrete formwork

• Twin steel walers and tie bolts used to secure wall forms

• Column poured using spiral ducting

• Concrete slab poured on roll formed galvanized steel with the form as a permanent part of the structure

• Concrete fence construction using aluminum forms to impart an ashlar stone impression in the finished product

• Concrete housing construction in Venezuela using aluminum concrete formwork

• Concrete construction in Brazil using handset aluminum concrete formwork

• Concrete construction in Moscow metro using special aluminum concrete formwork

• Engineered formwork system in Moscow metro

• Engineered formwork system in Moscow metro

• Pool construction using "Proster 21" flexible formwork.

See also

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• Cast in place concrete
• Climbing formwork, formwork that climbs up the rising building during the construction
• Concrete cover, depth of the concrete between reinforcing steel and outer surface
• Precast concrete
• Slip forming, construction method in which concrete is poured into a continuously moving form

Literature

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• Matthias Dupke: Einsatzgebiete der Gleitschalung und der Kletter-Umsetz-Schalung: Ein Vergleich der Systeme. 2010, Verlag Diplomarbeiten Agentur, Hamburg, ISBN 978-3-8386-0295-0.
• The Concrete Society, Formwork: A guide to good practice

References

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Concrete formwork is used in a wide variety of construction projects where pouring concrete is involved. Pouring the foundation of a building or structure, such as a house or a bridge, requires concrete formwork. Walls, columns, slabs, and floors made of concrete all utilize concrete formwork for pouring and casting. Beams, girders, and other support structures for buildings and bridges also depend on reliable concrete formwork during the pouring and curing process. 

Whether you own or rent your equipment, Concrete formwork plays a critical role in ensuring poured concrete takes the desired shape, meets the right dimensions, and has the strength to support intended structural loads.

What Is Concrete Formwork?

Concrete formwork refers to the molds or structures used to shape and support freshly poured concrete. Concrete formwork can be temporary or permanent and provide the shape and finish necessary for a concrete structure. When using formwork on a job site, it is important to understand the different types of concrete formwork to ensure the concrete is poured in the correct dimensions and shape for the intended outcome. Utilize concrete formwork to help the project gain sufficient strength and allow the concrete to support its own weight and any loads that may be placed on it. 

Concrete formwork can be made from a variety of materials, including: 

• Wood

• Metal

• Plastic

• Composite

When choosing the right concrete formwork for a project, the formwork must be able to withstand the weight of the wet concrete and the pressure as it forms. Learn more about the different types of concrete, including their pros and cons, to determine the best choice for a quality, strong, and durable finished concrete structure.   

Pros and Cons of Each Type of Concrete Formwork

Each type of concrete formwork has its own advantages and disadvantages based on the materials and intended usage. Here are a variety of types of concrete formwork, their uses, and the pros and cons for each.

Wooden Formwork

Wooden formwork is inexpensive and readily available, which makes it a common choice for a variety of construction projects. Wooden formwork is also easy to work with and can be easily cut to the required shape and size for the specific project. The two main types of wooden formwork are timber and plywood.

Timber Formwork Material

Timber formwork material is made from solid wood, like pine or spruce. Typically used for small to medium-sized projects, timber formwork is easy to work with and a cost-effective option in a variety of contexts. Timber formwork also provides good insulation and helps to regulate temperature and humidity during the concrete curing process. However, timber formwork is more prone to warping or splitting and will require regular maintenance to prevent rot or decay.

Plywood Formwork Material

Plywood formwork material is made from thin layers of wood veneer that get bonded together with adhesive before use. Wooden formwork made of plywood is more common for large or complex projects because of how easily plywood can be cut to specific shapes and sizes to meet design standards. Plywood can be reused multiple times and is also lightweight and easy to handle. However, plywood may require additional support or bracing to help prevent deformation during the concrete pouring and curing process. Plywood formwork materials are typically more expensive than timber formwork materials.

Metal Concrete Formwork

Metal for concrete formwork is known for its durability and ability to withstand the weight and pressure of wet concrete. This type of formwork can be reused many times and involves easy assembly and disassembly. The two main types of metal concrete formwork are aluminum and steel.

Aluminum Formwork Material

Aluminum formwork material is lightweight and easy to use in a variety of contexts, including large-scale products. This type of formwork material can help reduce construction time and labor costs because of how easy it is to assemble and disassemble. Aluminum formwork also provides a high-quality surface finish and is resistant to corrosion and weathering. However, aluminum formwork can be more expensive than other materials, like wooden formwork, and is not as strong as steel formwork.

Steel Formwork Material

Steel formwork material is very strong and durable, which makes it suitable for heavy loads like high-rise structures. Steel formwork provides good dimensional stability and can be adjusted to fit the required dimensions of a project. However, steel formwork is heavy and may require specialized training or machinery to move around. Steel formwork will also require protective measures to help prevent rust and corrosion.

Plastic Formwork

Plastic formwork is lightweight and easy to move around, which makes it a great choice for projects that require reusable forms and minimal maintenance. The durability of plastic formwork makes it ideal for small to medium-sized projects. Plastic formwork typically does not require additional forming work because it provides a smooth finish to the concrete. Plastic is also resistant to moisture and chemicals, making plastic formwork suitable for use in harsh environments. However, plastic formwork is not as strong as other types of concrete formwork materials like steel or aluminum or projects that require heavy loads.

Insulated Concrete Formwork

Insulated concrete formwork is made up of two layers of foam insulation that has a hollow space in between where the concrete gets poured. This type of concrete formwork offers excellent insulation for concrete during the pouring and curing process, which can also reduce energy consumption. Insulated concrete formwork, or ICF, is known for its strength and durability for a wide range of structures, including small home projects and large commercial builds. However, ICF can be more expensive than other materials and may require specialized equipment and labor for proper installation.

Stay-in-Place Formwork

Stay-in-place formwork, also known as permanent formwork, is a type of concrete formwork that does not get removed after the concrete has been poured and cured. This type of formwork is commonly used in applications where a smooth, uniform finish to the concrete is necessary or desired for both sides of the concrete structure. Some stay-in-place formwork, like precast concrete formwork, may be reinforced with steel. Utilizing stay-in-place formwork eliminates the need for additional formwork removal and reduces waste. However, it is not suitable for all applications.

Foam Concrete Formwork

Foam concrete formwork is made of a lightweight foam material that gets coated with a layer of plaster or stucco to create a smooth, uniform finish on both sides of a concrete structure. This type of concrete formwork molds the foam material into the desired shape and size for the concrete structure. Foam concrete formwork is easy to handle and can be molded into a variety of shapes and sizes, making this a popular choice for custom designs. This type of material is also lightweight and easy to transport, which can reduce costs and time. However, foam concrete is typically only recommended for smaller projects and may require additional finishing work compared to other types of concrete formwork.

Fabric Formwork Material

Fabric formwork is a type of formwork that involves stretching a fabric material over a mold or frame. Then concrete is poured into the fabric to create the desired shape or structure. Fabric for formwork material can be made from a variety of materials, including canvas, PVC, and woven or non-woven polypropylene. Fabric formwork is lightweight and easy to handle, especially for complex and irregular shapes. It also provides a unique aesthetic appeal because of how the fabric imprints onto the surface of the concrete. However, fabric formwork does have reduced durability compared to traditional concrete formwork materials and has limitations in the size and shape of the project.

Start Your Concrete Forming Project with Reliable Equipment Today

Forming America has everything you need for your next concrete forming project, with options available for both equipment rental and purchase. Speak with a representative to learn more about our concrete formwork products and discuss the different types of concrete formwork that would work best for your particular construction project.

For more information, please visit concrete slab formwork systems.