How Does Formwork Systems Work?

You drive past a high-rise office building in the midst of construction, a skeleton slowly filling out with hard-hat-wearing workers pounding away. You notice a hose extending from a cement truck into a stack of panels that towers high and almost resembles a giant window pane, and you might scratch your head. What are those panels for? It turns out they're crucial components of construction called formwork.

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Formwork is the name for the molds used to create walls, columns, slabs, staircases and other concrete structures. Freshly poured concrete -- a combination of sand, gravel, cement, and water -- is wet, so it can't support its own weight or hold its shape. Formwork supports the weight of the concrete until it has dried into a specified shape and acquired the strength to support itself. "It's like when you bake a cake," says Harry Stamaty, owner of the formwork design and consulting business Detail By Design. "The pan that you put the cake in is the form, and the cake mix is the concrete" [source: Stamaty].

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The concept of formwork is nothing new. The Pantheon, a domed icon of Roman architecture built around 125 AD, was one of the earliest structures to use concrete formwork in its construction. But formwork remained a fairly uncommon tool and technique for the next several centuries – cement and concrete were rare building materials until the inventions of Portland cement and reinforced concrete in the 19th century [source: Stewart].

Today, formwork is used to build everything from office buildings to single-family homes, from driveways to sports stadiums -- basically, any structure that incorporates concrete. But formwork is rarely a part of the final design. Forms are temporary structures, a variety of construction agents that provide access and support during the creation of the project's permanent features before being removed or discarded [source: Nemati]. But as we'll see, even though formwork is intended to come and go without a trace, it's an important facet of the building process -- one that threatens grave consequences for carelessness.

Let's take a look at some of the materials used to make formwork.

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Forms, or the molds used to build formwork, have countless permutations. They can be derived from timber, plywood, steel, plastic, fiberglass and a variety of other materials. The way they're erected on a job site can vary, too. Sometimes formwork can arrive as a series of panels, along with hardware like ties, wedges, clamps, braces and brackets, which workers assemble by hand. On other occasions, prefabricated forms designed and built in a factory can be shipped to a job site, hoisted to the right position with a crane, and connected with simple locking mechanisms. Collaboration between the project's architect-engineer and the concrete contractor often determines the best formwork setup. Maybe plywood better accommodates crucial details of the concrete structure; maybe it would prove more economical to reuse forms that the contractor has stockpiled from a previous job.

All formwork must account for two key factors: the rate of pour and lateral pressure. The rate of pour is literally the speed at which concrete is poured into the vacancy of the form. Wet concrete is heavy, weighing roughly 150 pounds (68 kilograms) per cubic foot. As the vertical height of the poured concrete increases, the substance exerts lateral pressure as it pushes against the interior faces of the form. If too much concrete is poured at once, and the connecting devices aren't strong enough to contain the pressure, the wet concrete can burst through the form.

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The concrete poured at the bottom needs time to set and gain adequate strength before additional concrete is poured on top. Stamaty says that a rate of pour of 4 to 5 feet (1.2 to 1.5 meters) per hour is a good guideline to follow for virtually any formwork system, meaning a 12-foot-tall (3.7-meter) wall would take up to three hours for the full pour [source: Stamaty].

How exactly is formwork put into action on a construction job? Turn the page to find out.

Watch the Weather

The rate of pour can change depending on the season: Ambient temperature affects the speed at which concrete sets, so the rate of pour is likely to be higher in the summer and lower in the winter.

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Formwork accounts for roughly 40 to 60 percent of the concrete budget for a given project [source: Lab]. Materials actually comprise a small part of the expense of formwork: Most of the money is spent on the labor to assemble and dismantle forms. Here are the basics of how formwork is used on the job site.

After the contractor and architect-engineer have looked at the structural and architectural drawings of a project and considered the form materials available to them, they'll select a formwork system, which, as mentioned earlier, either arrives in component pieces or already assembled. Once the formwork has been erected, workers must treat the interior of the molds with a release agent -- a compound made from wax, oil or plastic that prevents the form from sticking to the concrete -- because untreated forms can cause the surface of the concrete structure to become discolored or misshapen, which just requires more work to fix it.

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Once the form is in place and the concrete mixture is approved, workers begin pouring the concrete into the form's vacant space. (Representatives from the company supplying the concrete are usually on-site during larger commercial projects.) The concrete is distributed by hand or through a hose connected to a concrete truck, with a pump helping the concrete reach portions of the project where the truck can't travel. After the pour, workers often use industrial vibrating devices, which can either attach to the exterior of the form or be placed in the thick of the concrete. Vibration helps consolidate the elements of the concrete and eliminate air pockets. If necessary, the construction works can treat the concrete with water or steam.

After the concrete has dried, the forms are disassembled and removed, or "stripped" in construction parlance. The form can be repositioned in another area of the job site, stockpiled for later use or, if necessary, destroyed. These days, most forms are designed and built to be reused dozens of times.

Effective formwork has to balance several competing interests. Click ahead to find out what they are.

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A common sentiment in the concrete industry is that formwork should balance quality, cost and safety. In terms of quality, forms should accurately create concrete structures to the architect-engineer's specifications, without causing bulges or other defects to the surface of the concrete. In terms of cost, we already mentioned that formwork comprises a good chunk of the project's concrete budget. Failure to finish formwork in a timely fashion can also delay the rest of the construction schedule and lead to additional labor expenses, giving concrete workers an incentive to work efficiently.

But safety trumps all. About 25 percent of all construction failures result from collapses and failure of concrete structures, and formwork failures account for more than half of the figure [source: Hanna]. In December , a slab collapsed during the pouring of concrete during a casino construction project in Cincinnati, Ohio, resulting in several injuries to workers on the job site. The following month, another slab at different casino construction site collapsed during the concrete pour in Cleveland, Ohio [source: Feran].

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Formwork has also caused hundreds of deaths on construction sites [source: Nemati]. In Bailey's Crossroads, Va., in , workers removed the shores -- the supports that hold up horizontal slabs while the concrete sets -- from the 24th floor of an apartment building too early, triggering the collapse of the entire building, killing 14 workers and maiming many others [source: Hurd]. Forms can collapse from overloading with concrete, inadequate bracing, inadequate shoring, insufficient strength in the concrete before removal of the formwork, improper stripping and a host of other reasons.

Organizations like the Occupational Safety and Health Administration issue guidelines for designing and working with formwork. Having supervisors on site during the erection of the forms and the pouring of concrete can help minimize the hazards of using formwork. If the formwork design needs to be altered, contractors should consult the form designer ahead of time. Formwork might be a temporary structure, but its consequences to the structure and those who build it can be permanent.

Contact us to discuss your requirements of Formwork Systems. Our experienced sales team can help you identify the options that best suit your needs.

For lots more information on the construction process, see the links on the next page.

Slump

To measure the relative strength of a batch of concrete, contractors will gauge its slump. Basically, they'll scoop a sample of concrete from the batch in a one-foot-tall (3.7-meter) cone, lift the cone and let the concrete to fall into a pile, and then measure the difference between the height of the cone and the height of the concrete pile. A difference in height of 3 to 4 inches (7.6 to 10.2 centimeters) is ideal, Stamaty says. "If it's a 10-inch slump, it means there's too much water in the concrete mix, which means it's really weak" [source: Stamaty].

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What is Formwork?

In a nutshell, formwork is the mold wherein fresh concrete is poured and formed. The mold, which can be permanent or temporary, holds the poured concrete and shapes it until it solidifies and becomes strong enough to support itself and other loads.

Formwork is also known as “shuttering.” However, in certain parts of the world, shuttering refers to using plywood or timber to create the mold.

Formwork systems can be classified according to the following parameters:

• Types of material used to create the mold (plastic, steel, timber, aluminum)
• Types of concrete structure it supports (slab, column, wall)

Construction costs usually consist of building materials (ca. 45%), labor (ca. 45%), and operating expenses (ca. 10%). Formwork material comprises 15% of the total building material and contributes roughly 44% of the total labor. Meanwhile, formwork construction accounts for up to 25% of the total cost of building the structure. For this reason, many builders and construction companies prefer reusable forms (panel forms). The process of removing formwork is called “stripping.”

2. Column

As the name suggests, column formwork is used in the construction of circular or rectangular columns. Columns have formwork sections with a “closed load transmission,” which is supported by the formwork’s design and the guaranteed tensile strength rather than by using ties.

Steel formwork is often used to form circular columns as it is economical and available in incremental dimensions. Single-use forms, which are destroyed and disposed of during stripping, can be used as an alternative. However, using single-use formwork is not advisable if you need to construct multiple circular columns.

Meanwhile, rectangular columns are formed using three systems depending on their specifications. For instance, classic timber girders with steel walers and a plywood facing are often used when builders need to construct a significant number of columns with special dimensions. The windmill system, which is composed of columns or multipurpose panels of frame formwork systems, is suitable for building single columns with a standard dimension.

Finally, a foldable column formwork is highly suitable for columns with great heights. Because all the essential components (panels, connecting devices, ladders, and access platforms) are integrated into this type of column formwork, it helps builders reduce their construction time and costs.

3. Wall

Wall formwork comes in different types and classifications, as outlined below.

Conventional
Conventional wall formwork consists of boards or sheets and squared timber. It is flexible, but it can be costly and time-consuming as each component must be assembled on-site. In addition, all of its parts should be made according to the project’s specifications, and they must be nailed together and dismantled again after concreting.

Girder

This is a better version of the conventional formwork. Its components, which usually consist of dimensionally stable girders with two chords and one web, have been standardized to facilitate the assembly of identical and ready-to-use panels. The connection of the panels has also been systematized.

Frame
This type of wall formwork helps reduce labor time since its essential components (forming face, support for forming face, and steel walers) are assembled as one panel. The profile nose of the frames protects the edges of the forming face, thereby extending its lifespan. Connecting devices are used when assembling the frame panels to large-sized units, which are then usually transported by crane.

Crane-independent (hand-set)

This type of formwork can be moved by hand. Because of weight considerations, it is usually made of aluminum or plastic. It can take less concrete pressure than crane-dependent formwork, and is often used in housing and municipal construction projects.

Crane-dependent 

Crane-dependent formwork systems feature a large frame and formwork panels, usually made of steel. As a result, they cannot be moved manually. Since they can resist more fresh concrete pressures than crane-independent forms, they are suitable for the construction of commercial buildings and other extensive infrastructure projects.

Two-sided
As the name implies, two-sided formwork is erected on both sides of the wall. Its formwork ties, which are usually sleeved by spacing plastic tubes so they can be reused, take up the fresh concrete pressure. Push-pull props or large heavy-duty braces are attached to the formwork to align and secure it against wind loads during operation.

Single-sided
Single-sided formwork is used when the concrete has to be poured against existing structures or when builders need to do concreting against a hill or soil. This is why it is most suitable for reconstruction jobs. With this type of formwork system, concrete pressure is transferred from the formwork to the base plates through a support structure. 

Prefabricated
This formwork consists of two prefabricated concrete panels which are assembled in advance, and then transported and filled with concrete on-site. Braces and push-pull props are often used to secure the walls, while working and safety scaffolds are installed with the help of special adapters to make the construction process more cost-effective. Prefabricated formwork helps minimize project duration and labor costs. However, pre-planning is required to ensure that it is transported safely to the site.

Circular
Circular formwork is designed for the construction of curved and polygonal walls. It is also quite useful in the construction of specific concrete structures, such as septic tanks and car park ramps. This formwork system comes in three different types:  

• Round girder – timber spacers are added between the timber girders and steel walers so the formwork can be adjusted to the required radius. 
• Flexible girder – ideal for the construction of curved walls with different radii. It comprises timber/steel girders and a spindle, which allows builders to adjust the formwork to the required radius without having to reassemble the panels.
• Polygonal – existing “flat” frame formwork panels can be refurbished as polygonal formwork by adding supplementary radius panels and rails, allowing builders to minimize their project costs.

Climbing
Climbing formwork is quite useful in the construction of high-rise concrete structures, such as control towers and skyscrapers, because it climbs with the wall. It comprises large wall formwork mounted to a climbing scaffold. There are three different types of climbing formwork:

• Crane-dependent – the climbing unit (scaffold and formwork) requires the use of a crane to reach the next cycle.
• Self-climbing – an automatic climbing system that does not require the use of a crane to reach the next suspension point. Hydraulic rams/pumps lift the scaffold, secondary platform, and formwork to the next pouring cycle. It is ideal for the construction of very tall concrete structures.
• Slipform – a two-sided formwork that slides upwards along the structure being built at a rate of 20–25cm per hour. A system of pipe rods, which is integrated into the already-set concrete, supports the slipform.
   

MEVA is an industry leader offering a wide variety of wall formwork systems that provide highly efficient shuttering, flexibility, and reliability. Our products are lightweight and easy to assemble, and therefore will facilitate the rapid completion of your construction projects. Click here to learn more.

Want more information on Bespoke Formwork Systems? Feel free to contact us.