Uniaxial behaviour of biomass bamboo coarse aggregates concrete-filled steel tube columns with different cross-section shapes

This study proposes combining bamboo aggregate concrete (BAC) with concrete-filled steel tube columns (CFSTs), leveraging bamboo's resource advantages while compensating for its lower strength and durability through steel tube confinement to achieve resource recycling and emission reduction goals. Monotonic axial compression tests were conducted on 38 CFSTs. Key parameters included the bamboo coarse aggregate replacement rate (r), cross-sectional shapes, and steel ratios (α). The research revealed that the steel tube's confinement effect on BAC substantially improves both its load-bearing capacity and ductility. Circular specimens primarily exhibited waist drum-shaped failure, whereas square specimens were dominated by shear slip failure. Under different confinement coefficients, the axial load-strain curves can be categorized into medium and strong confinement. The strength index (SI) of all specimens exceeded 1, demonstrating a "1 + 1 > 2" effect. Peak stress (f_cc) and SI showed positive correlations with α. When the steel tube parameters were consistent, an increase in r led to a reduction in the f_cc of BACFSTs but significantly improved their ductility index, with the maximum increase reaching 70.9 %. For core concrete with r = 0 % and 25 %, α≥ 0.170 is preliminarily recommended; for core concrete with r = 45 %, α≥ 0.130 is initially advised. Finite element simulations were conducted based on the modified uniaxial stress-strain model of core BAC. The simulations reproduced the failure modes and response curves consistent with the test, and the contributions of the steel tube and the concrete to the load-bearing capacity were analyzed separately. Finally, through regression analysis of test data, a model capable of predicting the axial bearing capacity of circular BACFSTs was proposed.