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Generally formwork is removed when the concrete has gained adequate strength. Permanent Formwork is not removed but left in place. It may contribute to the load carrying capacity of the structure (‘participating’ formwork). ´Composite construction´ refers to formwork and in-situ concrete acting together in the permanent structure. Alternatively permanent formwork may simply contain the concrete while it is being cast and gaining its initial strength but not contribute to the load carrying capacity of the final structure (´non-participating’ formwork).
Permanent formwork is used to speed construction by removing the need for extensive formwork and falsework. Economies can be achieved by the use of composite construction where the formwork becomes an integral part of the completed structure. Sometimes permanent formwork is used in localities where it is difficult or impossible to remove the formwork.
Permanent formwork is not removed so the formed concrete surface is not exposed, and therefore defects and faults are hidden. Care is needed during construction to reduce the risk of hidden defects. Some problems have been found (i.e. voids and exposed reinforcement) in buildings where permanent formwork has been subsequently removed.
Acknowledgement: The Concrete Society
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