Evaluation of the flexural responses of solid timber beams strengthened with tension-side fibre-reinforced polymer (FRP) strips

This research investigates the flexural performance of timber beams strengthened with tension-side fibre-reinforced polymer (FRP) by external adhesively bonded reinforcement (EBR) and near-surface mounted (NSM) methods under four-point bending tests. It examines the effects of interfacial stress concentration at the end of EBR-FRP strips with various lengths and numbers of layers, and the influences of deep groove cuts in NSM-FRP strips. The results show that two-layer integral EBR‑CFRP strips achieve the highest peak load gain of 35.7%, but it can be slightly undermined by timber fracture at the end of reinforcement. Both beam groups with one-layer integral CFRP and long non‑integral GFRP fail in loading span at higher average displacements but deliver smaller peak load increases. In contrast, timber fracture at the end of two-layer short non-integral CFRP strips led to 50.8% and 77.2% reductions in the peak load and failure displacement, respectively. Peak load and displacement trends show more severe end fracture in strengthened beams of shorter non-integral CFRP strips with more layers, leading to larger overestimations of theoretical peak loads in the loading span. The slight decline in the flexural response of NSM-GFRP strips would be associated with deep groove cuts affecting timber fibre integrity. However, beams with two NSM-GFRP reinforcements exhibit noticeable post-peak load resistance, effectively arresting timber crack propagation. There is no premature debonding before timber fracture in the FRP-strengthened beams. Supplementary measures like end anchorage are recommended where the integral scheme of EBR-FRP reinforcements cannot mitigate interfacial stress concentration.