The development and use of alternative materials has been a constant process almost since the first use of steel. The installation problems associated with the weight of the steel plates and the potential for corrosion to reduce the durability of the system led to composite materials being considered. In the early 90’s much of the research was carried out at EMPA in Switzerland. In the UK a Dti Link project called ROBUST was established to investigate the use of composite materials for strengthening structures. In 2000 the Concrete Society launched Technical Report 55 ‘Design Guidance for strengthening concrete structures using fibre composite materials’.
The first UK strengthening scheme using composite materials was completed in 1996 at Kings Collage Hospital in London. The addition of a new floor to a building changed the loading requirements of the existing roof to new floor loadings. 1.3Km of Sika CarboDur plates were installed to the soffit of the longitudinal ribs under the slab.
Preparation of the concrete surface for either steel or composite plate bonding is identical. The composite plate is delivered to site in a roll with a diameter of approximately 1.5m. The lightweight nature of the composite material means a roll containing 250m can be easily lifted and moved by a single operative. The roll is cut on site to give the required plate lengths.
The plates are applied to the concrete surface using a similar adhesive to the one used for steel plate bonding. The initial grab of the adhesive is enough to hold the lightweight plate in place during the full cure period of the adhesive, eliminating the requirement for temporary works.
The composite plates are 1.2-1.4mm thick. This means that any residual longitudinal forces in the end of the plate have a much smaller eccentricity to the concrete surface compared to steel plates. In turn this means that peeling forces are lower which generally removes the requirement for anti peel bolts.
As composite materials do not corrode, corrosion protection systems are not required. A decorative coating can be applied to help conceal the strengthening.