Technical ManagerDML Composites

BONDING plates made from carbon fibre reinforced polymers (CFRP) to concrete bridges is now becoming accepted practice. However, the application of this strengthening technique to metal structures has yet to be exploited on a similar scale.

London Underground has been active in this field, and has participated in a project sponsored by Britain’s Department of the Environment, Transport & the Regions. One important step was the demonstration on a steel underbridge at Acton of stress relief through gluing CFRP plates to the underside of steel beams. This was preceded in 1999 by the strengthening in-situ of cast iron struts and braces supporting retaining walls at Shadwell station.

A design guidance note on the use of composite materials for rehabilitating structures was due to be issued at the end of last month. It should encourage civil engineers to make wider use of carbon fibre reinforced materials in extending the life of iron and steel structures, which can be a far cheaper and less disruptive option than replacing them.

The consortium undertaking the project, funded from the LINK Inland Surface Transport programme, is led by DML Composites, part of Devonport Royal Dockyard Ltd. Other members are the University of Southampton, DERA, MSL Engineering and Structural Statics Ltd.

Steel needs thicker plates

The pre-formed high strength carbon fibre plates currently being used for concrete structures are typically 4mm thick. To strengthen steel beams, they would need to be at least 20mm thick, in order to achieve a significant increase in bending moment in the composite beam. DML has therefore developed stronger plates based on Mitsubishi Chemical’s ultra high modulus carbon fibre for use on metal structures. This reduces the thickness required to 7 mm.

The guidance notes will set out best practice on practical issues such as surface preparation, application of the adhesive, and cure times which range from 4 to 8 h. To benchmark the monitoring techniques used to confirm that a bridge carrying live loads had actually achieved the intended increase in strength, LU made available for trials last April a bridge carrying trains over the entrance to Acton works.

Data was collected for two weeks before two crossbeams were strengthened by gluing CFRP plates to the underside. Trains continued to run while this operation was carried out. Two further weeks of monitoring established that stress in the beams had indeed been reduced by 20%.

Cast iron

LU owns a large number of 19th century structures in which cast iron beams and struts are used, and need additional strength. The first task was to develop design techniques. Extensive testing at laboratory scale and full scale has shown that the theoretical design methods used are indeed valid.

When LU’s East London line was built for steam operation, cast iron struts spanning two tracks were used to support the retaining walls of deep cuttings that ventilated each end of Marc Brunel’s Thames Tunnel. Constrained by the walls from expanding during ambient temperature cycles, they are subject to varying stress leading to fatigue. The failure of a strut at the Rotherhithe end in August 1995 precipitated remedial action.

Having demonstrated at one-sixth scale that the deflection of specially-cast test struts subject to eccentric end loads was reduced by over 35% by adding carbon fibre reinforcement, one of the struts replaced at Rotherhithe was tested with similar results. This convinced LU that the new technique could safely be used at the Shadwell end of the tunnel.

The struts at Shadwell were both corroded and bowed, making it impossible to bond pre-formed CFRP plates. DML offered a patented resin infusion technique where the carbon fibres are applied to the substrate, and resin is introduced at a later stage under vacuum. This coped with the irregular surface, and 18 struts plus cross-bracings were successfully reinforced using 1·5 tonnes of CFRP.

Composite beams

LU’s sub-surface lines typically run in covered ways under streets, buildings and open spaces. The roof normally consists of cast iron beams with brick jack arches supported on the lower flange. Most sections under streets have been reinforced with additional steel beams in the jack arches since 1959, when a cast iron beam suffered a major fracture.

However, following successful strengthening works at Sloane Square in 1995, LU decided that UHM carbon fibre plates should be bonded to the underside of cast iron beams supporting the covered way carrying Circle line trains between High Street Kensington and Gloucester Road.

Between April and June 1999, a total of 176 plates each weighing 34 kg were bonded to the underside of beams during 2h periods on 35 nights. These plates met LU’s stringent fire protection standards regarding the flammability and toxicity of materials used in tunnels.

This project represents the largest use to date of carbon fibre composites to strengthen an existing structure. n

CAPTION: The bridge carrying London Underground trains over the entrance to Acton works was used in April for a trial to benchmark the effectiveness of using CFRP plates to relieve stresses in the steel girders


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