Investigation of flexural behaviour of structural timber beams strengthened with NSM basalt and glass FRP bars

Abstract

Near Surface Mounted (NSM) fibre reinforced polymer (FRP) bars is an effective technique of improving the performance of timber beams. Despite the potential of NSM technique, literature remains limited, whilst there is not any standard method for the prediction of the NSM FRP timber flexural capacity. Extending the pool of experimental data and thus the understanding of FRP strengthened timber structures, the present paper reports an experimental study of timber beams reinforced with glass and basalt FRP bars. Moreover, the paper presents a general theoretical model in order to estimate the moment resistance of the NSM FRP timber beams. The experimental study examined white spruce timber specimens in two reinforcement configurations; one with reinforcement bars only on the tension zone and one with reinforcement bars both on the tension and the compression zone. Control specimens were also included for comparison purposes. All beams had a rectangular cross-section of 70 × 215 mm and were loaded under four-point bending configuration with a 2.3 m span. A total of 20 specimens were tested under displacement control quasi-static monotonic loading. The main failure mechanism observed for both NSM FRP reinforced and unreinforced specimens was brittle tensile failure of the timber at the tensile zone. The load–deflection curves, the strain distribution profiles and the failure modes were discussed. It was observed that a significant increase on the ultimate load (33–69%) and the flexural stiffness (22–33%) of the timber beams can be achieved due to the NSM reinforcement. The proposed theoretical model for the ultimate strength of NSM FRP strengthened timber beams is assessed on the basis of the test results and collated data, showing a good comparison between the experimental and theoretical results.