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"Permanent formwork" is a term that refers to braces or molds that are put in place to contain concrete as it sets. Unlike other types of formwork that is removed after the concrete sets completely, permanent formwork is left in place as an additional support or stabilizer. The materials used for formwork that is left in place permanently can vary, though galvanized steel is a common choice for several reasons: it is resistant to water damage and corrosion, it is very durable, and it has enough flex in it to accommodate the subtle movements of concrete as the material freezes or thaws.
Very often the permanent formwork comes in large corrugated sheets. The sheets are corrugated, or bent into ridges, for added flexibility and adhesion to the concrete as it sets. The concrete and the formwork can bond more solidly through the corrugations, though the ridges will also provide enough flexibility that the likelihood of cracking is reduced. When the permanent formwork is being put in place during the concrete pouring process, it may be held by falsework, which is a series of posts or rods that hold the formwork in place as the concrete cures. Such falsework will generally be removed once the concrete sets.
One of the advantages to using permanent formwork becomes apparent when pouring concrete into the ground. Without a form, the concrete will seep into the soil, leading to excess waste of concrete. Forms are used to prevent such waste; temporary formwork can be used to create the mold, but if permanent formwork is used instead, two distinct advantages become apparent. First, less digging will be involved if the forms are permanent, since extra space within the hole will be necessary to remove the temporary forms once the concrete sets. Second, leaving the formwork in the hole once the concrete is set creates an extra barrier between the moisture in the soil and the concrete that can be damaged by moisture freezing and thawing.
In most cases, the permanent formwork is used in addition to rebar rather than in place of it, though using a permanent form can reduce the amount of rebar that is necessary in a structure. Rebar consists of steel bars that are placed within the void where the concrete will be poured to give the set concrete higher tensile strength. Without such rebar, the concrete would be exceptionally susceptible to cracking.