Elucidating the strengthening efficiency of concrete cylinders confined by integrated CFRP sandwich structures with auxetic lattice cores

ABSTRACT This study investigates a novel hybrid confinement strategy for concrete cylinders that combines carbon fiber-reinforced polymer (CFRP) wraps with re-entrant auxetic honeycomb structures to simultaneously enhance strength and ductility. Finite element analyses were conducted to evaluate the individual and combined effects of two key parameters: the number of auxetic core layers and the percentage of continuous carbon fibers (CCF) embedded within the auxetic geometry. Parametric simulations revealed that CFRP wraps significantly improve compressive strength, while auxetic cores, especially those with higher CCF content, enhance both strength and strain capacity. Hybrid configurations integrating CFRP wraps with auxetic cores outperformed both standalone systems. Notably, the configuration consisting of a re-entrant auxetic core sandwiched between two inner and two outer CFRP layers achieved a 221% increase in compressive strength and an 80% increase in strain capacity compared to unconfined concrete, along with a 50% improvement in strain capacity over a four-layer CFRP-wrapped specimen. While compressive strength gains plateaued at higher CFRP wrap densities, strain capacity continued to improve with auxetic core inclusion. This enhancement is attributed to the auxetic core’s unique lateral contraction behavior under axial load, which intensifies confinement pressure and delays tensile cracking. When combined with CFRP, the auxetic core acts as an adaptive interlayer that maintains continuous contact with concrete substrate, mitigating debonding and promoting more effective stress transfer. These findings demonstrate the effectiveness of auxetic–CFRP hybrid confinement in enhancing both strength and ductility, offering a promising approach for the development of resilient, damage-tolerant concrete systems for structural applications.