Experimental Investigation of the Axial Compression Behavior of Larch Timber Columns Strengthened by CFRP and BFRP

Timber is a natural and renewable construction material, so it is environmentally friendly. However, timber has natural defects and also deteriorates over time. These problems require structural reinforcement. The present study aims to systematically explore the compression performance of natural Larch circular columns reinforced with Carbon Fiber-Reinforced Polymer (CFRP) and Basalt Fiber-Reinforced Polymer (BFRP). Thirty specimens were tested in pure axial compression to investigate the influence of the number of wrapping layers (0–3 layers), the specimen height (150, 200 and 300 mm) and the type of FRP material. The strengthening mechanism primarily relies on the passive hoop confinement provided by the FRP, which restricts the transverse expansion of the timber under axial load. Because CFRP possesses a higher tensile strength and elastic modulus than BFRP, it activates confining stresses more rapidly and provides a stronger restraint, leading to distinct improvements in load-bearing performance. The experimental results show that the failure mode of the short columns changes from inherent brittle splitting to a more ductile failure pattern, characterized by FRP ruptures and crushing of the timber as a result of external FRP wrapping. The axial compressive performance of the timber columns has been improved with both FRP materials. Given the same conditions, the CFRP caused increases in load-bearing capacity and stiffness, as a result of its higher tensile strength and elastic modulus, which gave rise to peak loads that were 4.9% to 7.8% greater than the BFRP-strengthened groups. There was a tendency for the reinforcement efficiency to increase with the number of layers of CFRP wrapping, and 2–3 layers of CFRP was found to be the optimal number of layers based on the aspect of material efficiency. In addition, FRP confinement was able to prevent premature failure and improve the ultimate transverse strain by as much as 2.1 times, significantly increasing ductility and energy dissipation. Finally, a theoretical ultimate strength prediction model was developed based on the passive confinement theory with the introduction of a height correction factor to consider the slenderness effects. The proposed model showed an overall coefficient of determination R2 of 0.8027, which was good for reference for designing the reinforcement and evaluation of the performance of sustainable timber structure.