Fibre reinforced composites are becoming en vogue for strengthening existing structures. As your article 'Keeping damage under wraps' (NCE 20/27 August) suggests, there are some efficient uses for this type of retrofitting. However, the suggestion in 'M11 viaduct fails shear test' (NCE15 October) of using carbon fibre composites for shear strengthening is of some concern.
Plate bonding for shear is severely limited by the anchorage area available. Anchorage area is the area of plate required to be adhered to concrete on either side of possible shear crack. This is often further restricted by the poor tensile capacity of the concrete within the structure. By its very nature, the high tensile capacity of carbon fibre means that the material is unlikely to be efficiently utilised.
Tests on beams subjected to high shear loads have shown that the effectiveness of plate bonding is often limited by the plate anchorage available. In extreme cases this can lead to plate peeling at a low load.
At the University of Dundee, alternative anchorage methods have been developed utilising steel shear plates. Steel plates are attached on the vertical sides (web) of the beam by using bolt connections (see photograph). This bolted anchorage method transfers the plate force to the core of the existing section, utilising the compressive strength of the concrete and providing additional confinement.
Enhancements to the shear capacity of 160% have been achieved, with average shear stresses in excess of 15N/mm2. Yielding of the steel plate at failure increases ductility of the section and allows material optimisation. By specifying non-corrosive steel the problems with aggressive environments can be minimised and aesthetic appearance improved.
Dr NK Subedi (M) and Dr PS Baglin (G), Structural Engineering Research Group, Department of Civil Engineering, University of Dundee, DD1 4HN.