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There may be situations where conventional timber formwork has to be left in place if it difficult or not possible to remove it. With time the timber rot and collapse. Thus timber should only be used as permanent formwork in situations in which failure of the timber would not constitute a hazard. The timber may act as a sponge, storing moisture and acting as a source for moisture ingress into the structure. This may lead to durability problems.
Oriented strand board, with a GRP (glass reinforced plastic) coating on one face, has been used as permanent formwork for the decks of bridges, spanning between the main precast concrete beams.
Timber permanent formwork should not be assumed to contribute to the strength of the completed structure.
Acknowledgement: The Concrete Society
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