As a result, Docklands Railway Management Ltd recently approved the installation of carbon fibre reinforcing plates as the most cost-effective, practicable and least disruptive strengthening option. The £220k bridge refurbishment project was funded by the Dockland Light Railway Ltd, the holding company for the franchised railway. The two sections of the original bridge deck that still exist are a mixture of brick jack arches, or steel plates, supported by cast iron beams.

All lie beneath the footpaths either side of the carriageway. The bridge, originally constructed in 1850 as part of the north London railway, is now responsible for the conveyance of vehicles on the four-lane A11 trunk road that traverses the Docklands Light Railway.

As such, and in common with many other structures, it is currently being asked to accommodate vehicles with significantly increased wheel loading to those envisaged when first designed. In the event it came as little surprise that due to the numerous services that lay within its footpaths and the enormous volume of daily traffic dependant upon the bridge, the installation of carbon fibre reinforced polymer plates (CFRP) proved to be the preferred option.

For the initial assessment, CWP assembled a team comprising the material suppliers - Devonport Management Ltd. (DML) and Mitcham based specialist contractor Concrete Repairs Ltd (CRL). Extensive pre-contract testing was carried out, which included a full-scale mock-up of the works. Initial trials using pre-stressed standard CFRP plates showed that the technique was unlikely to be viable in the short railway possession that was allowed.

The high tensile capacity and lightweight nature of carbon fibre reinforced plating, together with it ease of application to the existing substrate, however, meant that CWP were keen to explore alternative carbon fibre methods. In order to assess the tensile capacity of the CFRP plate, its adhesive and the practicalities of implementation, trials were carried out on similar beams removed from Poplar High Street Bridge.

Discussion with the product manufacturer resulted in the adoption of the unstressed, custom-designed ultra high modulus carbon fibre plates, which together with the adhesive, also became the subject of validation trials to ensure compatibility and suitability.

CRL, originally contracted to carry out refurbishment work on the structure in early 1999 were awarded an extended contract to install the eleven 170mm wide, 20mm thick and 5 metre long plates, all of identical plan dimensions, to strengthen seven of the cast iron beams.

Subsequently, the company took possession of the bridge for an intense 24-hour period in Mid-December 1999 and since no over-run possession was sanctioned it was essential that all project requirements, including heating to an ambient temperature of above 5°C, were provided for.

CRL managed its trained operatives in a shift pattern over the 24-hour period. All cast iron beams were prepared by grit blasting prior to the eleven plates being successfully bonded to the bridge beams before being painted. The entire contract was successfully completed with 2 hours to spare and within budget.