Experimental Study on the Axial Compressive Behavior of Flax and Basalt Fiber Hybrid <scp>FRP</scp> Confined Concrete Columns With Fiber Layup and Orientation

ABSTRACT Natural fiber reinforced polymer composites (NFRPs) are increasingly utilized in civil engineering applications due to their sustainability and renewability. This study investigates the mechanical performance of concrete cylinders confined with a novel hybrid flax/basalt fiber‐reinforced polymer (HFRP) composite. Axial compression tests were conducted on both flax fiber‐reinforced polymer (FFRP) and HFRP‐confined concrete columns. The influence of different layering configurations—specifically sandwich and intermediate‐layer mixing—and fiber orientations ([0°/90°], [45°/−45°], and [0°/90°, 45°/−45°]) on mechanical behavior was systematically evaluated under identical mix proportions. Experimental results indicate that incorporating basalt fibers significantly enhances the confinement effectiveness of FFRP. The BBFFBB sandwich configuration achieved the highest axial compressive strength, showing a 161% increase compared to unconfined concrete, whereas the FB4F mixed‐layer specimen exhibited superior ductility, with an effective hoop strain ratio reaching 57%. As the fiber orientation angle approaches the circumferential direction, the load‐bearing capacity of HFRP decreases, but ductility improves correspondingly. Furthermore, considering the effect of fiber orientation, an improved analytical model for predicting the compressive strength of HFRP‐confined concrete is proposed. The model predictions show good agreement with experimental data, offering a reliable theoretical foundation for the practical application of HFRP in concrete confinement.