Experimental assessment on axial compressive behaviour of FRP confined biochar concrete

Biochar, as a lightweight aggregate, can be partially incorporated into concrete to exert filling and internal curing effects, thereby improving the mechanical properties of biochar concrete to some degree, and becoming a potential carbon capture and storage technology. However, due to the inherently high porosity of biochar's microstructure, biochar concrete confronts with certain challenges such as low strength, poor corrosion resistance, instability and etc. Thereby, the FRP tube is proposed to enhance the biochar concrete in this study, then FRP confined biochar concrete is formed. The axial compressive test for FRP confined biochar concrete column specimens, with various parameters including FRP tube thickness, the volume content and water absorption rate of biochar, was performed. The axial load-strain curves, transverse strain versus axial strain responses, ultimate stress, ultimate strain, development of transverse strain, and etc. of specimens were emphatically analyzed. The results showed that under the same biochar volume content and water absorption, the ultimate compressive strength of FRP confined biochar concrete specimens increased by 490.4% to 563.3% compared to unconfined counterparts. With increasing volume content of biochar in biochar concrete, the ultimate strength of confined specimens decreased while the ultimate strain increased. Besides, an increase in water absorption rate of biochar led to an increase in the ultimate strength of confined specimens but a decrease in the ultimate strain. Additionally, an increase in the layers of FRP tube improved the secondary stiffness of confined specimens. The transverse strain-axial strain curve exhibited no obvious transition point between the elastic segment and the linear segment, indicating good synergy between FRP tubes and biochar concrete. Finally, some existing FRP confined concrete ultimate strength and ultimate strain models were used to evaluate the ultimate strength and ultimate strain of FRP confined biochar concrete.