Formwork
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Formwork is the term given to either temporary or permanent moulds into which concrete or similar materials are poured. In the context of concrete construction, the falsework supports the shuttering moulds.
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[edit] Formwork types
Formwork comes in three main 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 re-usable 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 systems. This formwork is built out of prefabricated modules with a metal frame (usually steel) and covered on the application (concrete) side with material having the wanted surface structure (steel, timber, etc.). The two major advantages of formwork systems, compared to traditional timber formwork, are speed of construction (mmodular systems clip or screw together quickly) and lower life-cycle costs (barring major force, the frame is almost indestructible, while the covering may have to be replaced after a few - or a few dozen - uses, depending on the applications).
- Stay-In-Place Formwork systems. This formwork is assembled on site, usually out of prefabricated insulating concrete forms. The formwork stays in place (or is simply covered with earth in case of buried structures) after the concrete has cured, and may provide thermal and acoustic insulation, space to run utilities within, or backing for finishes.
- Stay-In-Place Structural Formwork systems. This formwork is assembled on site, usually out of prefabricated fibre-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.
[edit] 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[1], while the Washington State Department of Transportation requires the forms to stay in place for 3 days with a damp blanket on the outside[2].
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 underdesigned formwork bends or breaks during the filling process (especially if filled with a high-pressure concrete pump). This then results in liquid 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.[citation needed] The hardening is an asymptotic process, meaning that most of the final strength will be achieved after a relatively short time, though some further hardening can occur depending on the cement type and admixtures.
[edit] See also
- PERI GmbH (one of the largest international modular / custom-made formwork manufacturers)
- Moladi (maker of reusable plastic mould formwork for low-cost housing)
- Concrete cover (depth of the concrete between reinforcing steel and outer surface)
- Climbing formwork (formwork that climbs up the rising building during the construction)
[edit] External links
- PERI scope - Formwork Engineering Magazine (from the PERI GmbH website)
- Doka International Forms (Doka formwork manufacturer website)
- Williams Form Engineering Corp. (Williams Form Engineering corporation website)
- EFCO Forms (EFCO corporation website)
- APA Concrete Forming (from the APA Engineered Wood Association website)
- Timber formwork for concrete stairsA non commercial site.
