Ductility and deformability of fibre-reinforced polymer-strengthened reinforced concrete beams

Abstract

This paper discusses the definition of, and the distinction between, deformability and ductility of reinforced concrete (RC) beams that are strengthened by advanced composites. The study examines the suitability of a new, design-based method for the determination of deformability, as well as an energy-based process, which is found to be suitable for quantifying the ductility levels of fibre-reinforced polymer (FRP)-strengthened RC members. Ten FRP-strengthened RC beams and four slabs have been load-tested to ultimate failure in the current study. The test results, together with the load–deflection data of an additional 26 beams from literature, form the basis of analyses and discussions. The paper concludes that high deformability does not necessarily lead to good ductility, as very brittle failure modes of such beams have been observed in the experimental studies and reported in literature. It was found that a ductility index of between 2·0 and 2·5 reflects 25–33% of elastic energy stored in the strengthened system. This level of elastic energy is considered to be the maximum acceptable in FRP-strengthened concrete flexural elements for ductile behaviour. It was also found that for ductile failure modes the deformability and ductility indices tend to converge, whereas for brittle behaviour the deformability index could be up to 33% higher. The results presented in this paper provide a rationale for the ductility considerations to be incorporated into the development of design equations for FRP strengthening.