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Formwork is the mould to shape the concrete structure. Good concrete cannot be made from a poor mould; thus formwork is very important in producing good concrete. Besides that, it has to be designed to take the concrete load and the working load generated from workers and plant. An excessive deflection or leaky and not well-fitted mould which loses grout and weakens the concrete can destroy the appearance of the concrete product. Moreover a leaky joint can cause honeycombing and joint marks on the face of concrete.
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, 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. 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.