Interest in strengthening metallic structures was first led by the off shore industry. The civil engineering industry soon realised the opportunity for its use and major bridge owners such as London Underground Limited and Network Rail expressed interest.
In 1996 a DETR/PiT program was set up to develop the use of composite materials in strengthening metallic structures. Detailed areas for investigation included design, specification, materials performance and installation best practise. The main output from this program was the Institution of Civil Engineers design and practise guide ‘ FRP Composites Life Extension and Strengthening of Metallic Structures’, which was published in 2001.
Both cast iron and steel structures have been strengthened with bonded composite plates. Wrought iron is also a material being considered for strengthening.
Another major part of the DETR/PiT program was to carry out full-scale validation of the technique. A London Underground Ltd bridge called D65A was selected for this trial. The bridge consisted of 3 main longitudinal beams with transverse beams spanning between them. Two of the transverse beams were selected for the trial. A target of a reduction in peak strains in the beams of 25% was chosen and designed for. A known load was pasted over the bridge both before and after strengthening and a reduction of peak strains of 23% was recorded. The discrepancy between the target and the actual reduction in strain was investigated and found to be due to the stiffened beams attracting additional load from other parts of the structure.
The plates for strengthening metallic structures are generally considerably larger than those used for strengthening reinforced concrete. The requirement for increased stiffness of the strengthened element means larger cross section areas are required and the fibres used within the plates, although still carbon, have a higher stiffness than those used for strengthening reinforced concrete. These requirements mean that plates are generally manufactured to the exact dimensions and properties required for the project, making them bespoke plates.
To provide plates that have a high level of control of fibre alignment, and hence final plate properties, a pre-preg method of manufacturing is adopted. This involves unidirectional fibres being pre-impregnated with an epoxy resin to form a sheet. These sheets are then laid up individually in the required direction and a vacuum used to consolidate the layers and obtain the maximum fibre fraction content. The whole component is then cured in an autoclave. This method of production also allows tapers to be built into the plate, reducing the thickness at the end of the plates to approximately 1mm and in turn reducing forces on the bond line.
The finished plates are then delivered to site flat in a similar way to steel plates. However, they are considerably lighter than the equivalent steel plate. The substrate needs to be prepared to remove all contamination and corrosion products and provide the maximum levels of adhesion of the adhesive. Although the plates are lighter than steel plates they are too heavy for the adhesive to hold in place whilst still curing. Temporary supports are required to maintain pressure between the plate and the substrate while curing takes place. Once cured a final protective coating is applied over the plate and on to the metallic substrate to prevent corrosion of the substrate.
In 2002 the LUL bridge MR46A was strengthened using the Sika CarboDur DML System. The bridge is a single 28m span bridge consisting of mild steel main and transverse beams. Strengthening was required for the bridge to carry the full LUL load requirements. Various Sika CarboDur UHM plates up to 13m in length, 30mm thick and 250mm wide were installed.
In more recent times production of plates using aramid fibres has been carried out. The aramid fibre is less conductive than carbon fibres and this is believed to be beneficial in reducing induction forces when strengthening bridges over high voltage cable, a common situation on electrified railways.