Torsional strengthening of reinforced concrete beams using different configurations of NSM FRP with epoxy resins and cement-based adhesives

The strength of reinforced concrete (RC) members has been improved with a good deal of success using carbon fibre-reinforced polymers (CFRP) with two application techniques i.e. externally-bonded reinforcement (EBR) and the near-surface mounted (NSM) technique. However, to date no studies have been reported on torsional strengthening using NSM methods, as the research on torsional strengthening has been conducted using EBR techniques only. Thus far, NSM strengthening has been successfully employed with epoxy in improving the flexure and shear strength of RC beams. As toxic fumes and poor performance in high-temperature environments are major problems associated with using epoxy, some studies have used a new cement-based adhesive as an alternative to epoxy. This paper presents the results of experimental work on torsional strengthening using the NSM technique in a specialized torsional rig system. Eight RC members were tested to evaluate the strength enhancement provided by two different NSM FRP configurations. Six beams were strengthened with CFRP laminates embedded into pre-cut grooves. The laminates were applied to all four faces of two beams (full wrapping) and to only three faces of the other four beams (u-wrapping). An epoxy and a new cement-based adhesive were examined in this study. For each type of adhesive, one beam with full wrapping and two beams with u-wrapping were considered. Two control beams were initially tested and a comparison was made with the results of the strengthened beams in terms of the total performance and the ultimate strength of the beams. The results show that the u-wrapping configuration provides a reasonable improvement to the torsional behaviour of the beams when using epoxy resin as the bonding agent. On the other hand, cement-based adhesive is less efficient in increasing the torsional strength than epoxy. A theoretical design capacity method was adopted in this study to compare its prediction to the experimental data.