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Externally bonded fibre composite materials (FRPs), consisting of carbon, glass or aramid fibres combined with a suitable resin, are widely used for strengthening concrete structures. The materials may be preformed into composite plates or shells, which are then bonded to the concrete surface. Alternatively, the composite may be formed in situ, with the fibres in the form of fabrics impregnated with resin and applied to the concrete surface.
The main advantages of FRP materials are their high strength (up to 3000 N/mm2) and light weight (one-third that of steel). Minimal surface preparation is required and installation is simple and quick. Generally temporary support while the adhesive gains strength is unnecessary. Speed is particularly important for applications such as bridges, because of the high costs of possession times. The weight of the structure and the dimensions of the member are not significantly increased. One disadvantage of FRP strengthening is the risk of fire, vandalism or accidental damage.
There are many concrete structures around the world that have been externally strengthened with FRP, including bridges, retail and industrial building, multi-storey car-parks, lighthouses and cooling towers.
Design guidance is given in Concrete Society Technical Report 55, along with guidance on site operations, surface preparation, quality control etc. Guidance on some aspects of bridge strengthening is given in Highways England documents.
Acknowledgement: The Concrete Society
Other references:FIB (Federation internationale du beton), ŽExternally bonded FRP reinforcement for concreteŽ, Bulletin 14.
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