Strengthening of an artificially degraded steel beam utilising a carbon/glass composite system

To rehabilitate damaged or sub-standard steel structures, techniques utilising the lightweight, high strength and corrosion resistance of fibre reinforced polymer (FRP) composites have been proposed. The flexural load carrying capacity of a steel girder can be increased significantly by adhesively bonding carbon fibre polymer (CFRP) composites to its tension flange. This paper discusses the experimental results to investigate the effectiveness of an ultra-high modulus, and a high modulus, CFRP prepreg in strengthening an artificially degraded steel beam of rectangular cross-section under four-point loading. Four beams were upgraded, two utilising U-shaped prepreg units, which extended up the vertical sides of the beam to the neutral axis height, whereas the other two beams used a flat plate prepreg. All beams had an identical hybrid lay-up of CFRP and glass fibre reinforced polymer (GFRP) composite but for each of the geometrical shapes either an ultra-high modulus or a high modulus CFRP was used. Fabrication of the prepreg material was undertaken in situ and all the prepregs were bonded to the steel substrate utilising an adhesive film. The composite containing the ultra-high modulus CFRP failed when the ultimate strain of the carbon fibre was reached in the pure moment region. The failure load exceeded the plastic collapse load of the undamaged beam, thus demonstrating the effectiveness of the proposed upgrading scheme. On re-loading the failed beams, the U-shaped hybrid upgrade continued to act compositely with the steel beam outside of a well confined region close to the original failure location, whereas the beams with the flat plate upgrade exhibited the typical response of a steel beam, owing to debonding having taken place over practically the entire length of the prepreg. The beams using the high modulus CFRP reached even higher ultimate loads and exhibited ductile response leading to very high deflections; neither fibre breakage nor adhesive failure was observed in either the U-shaped or the flat plate strengthened beam.