Challenges for strengthening concrete structures in flexure, shear and compression using fibre reinforced polymers

The strengthening of reinforced concrete structures using fibre reinforced polymers (FRPs) has become commonplace. However, despite this, there are many aspects of behaviour of such strengthened structures which are still poorly understood. This paper presents investigations into three critical situations which, despite being commonplace in real structures, have been largely overlooked. The first is strengthening continuous beams or slabs in flexure and the ability to redistribute moments. FRP is a linear elastic material whose application reduces the ductility of a section, so redistribution of moments is potentially limited. The research has shown that redistribution from an unstrengthened zone to a strengthened zone can be achieved without limitation, but that redistribution is severely limited in other strengthening configurations. Secondly effectiveness of shear strengthening is examined. Previous research has focussed on shear near the ends of simply supported beams, but in many situations, shear is accompanied by large moments. A method called deep embedment strengthening has been developed which is effective at increasing shear capacity when shear forces are accompanied by moments and which can reliably make use of plasticity based analysis methods. Lastly column strengthening is considered, specifically square columns. Just like with beams, most concrete columns typically carry large moments in addition to axial load. The research investigating this scenario has shown that when large moments are applied, the strengthening which can be achieved by confining the column is significantly affected by P-Delta effects, non-uniform confinement and FRP debonding. The results of this work are currently being incorporated into the Eurocode, EN1992, for the design of concrete structures.