Debonding failure of RC beams retrofitted with Near Surface Mounted FRP Reinforcement

Near Surface Mounted (NSM) Fibre Reinforced Polymer (FRP) reinforcement is an effective rehabilitation solution to strengthen RC structures, as it can enable higher load carrying capacity and ductility than conventional Externally Bonded Reinforcement (EBR). However, the performance of elements strengthened in flexure is still controlled by bond failure between the NSMR system and the concrete substrate. This can reduce both the effectiveness and safety of NSMR systems and should be accounted for in design. The development of high stresses due to the abrupt termination of NSMR is the main cause for the dominant end debonding failure. This type of failure is not well understood and needs to be examined in detail. The aim of this work is to achieve a comprehensive understanding of bond behaviour and debonding mechanisms of the NSMR in flexural strengthening applications both experimentally and theoretically so as to enable the development of practical and reliable design methods for RC beams strengthened in flexure with NSMR. An analytical elastic model is developed to facilitate a fundamental understanding of the distribution of bond stresses along the NSMR, especially in the region around the termination point. The model identifies differences of stress states between EBR and NSMR. However, since it is based on elastic analysis and continuum mechanics, it is unable to represent bond behaviour of NSMR at high load levels. This issue can only be examined in detail via specially developed experimental work. A total of ten RC beams, including two control beams and eight beams strengthened in flexure with CFRP and BFRP bars or strips, are tested to examine the overall structural behaviour of RC beams retrofitted with NSMR of different embedment lengths. Tested beams are heavily instrumented to examine the influence of yield penetration along the internal steel bars on the bond behaviour of the NSM reinforcement within the shear spans and the resulting debonding mechanisms. NSMR enhanced flexural capacity by up to 50% and the dominant failure mode was end debonding after yielding. The experimental results show that yielding of the steel reinforcement penetrates in the shear span much further than predicted through classic section analysis, reaching near or even beyond the termination point. The experimental evidence is used to develop a new simple, yet effective methodology to estimate the minimum embedment length. The new design method is validated against an extensive database collected from literature and is found to provide more accurate and reliable results.