Axial compressive behavior and stress-strain model of CFRP-UHPC composite-strengthened concrete columns

To mitigate brittle failure in UHPC-strengthened concrete columns caused by core concrete expansion under axial compression, this study adopts a CFRP-UHPC composite strengthening approach. To investigate the axial compressive behavior and stress-strain models of CFRP-UHPC composite-strengthened concrete columns, uniaxial compression tests were conducted on 3 plain concrete columns, 9 CFRP-strengthened columns, 9 UHPC-strengthened columns, and 27 CFRP-UHPC composite-strengthened columns. The effects of CFRP layers and UHPC thickness on the axial compressive behavior of the core concrete were systematically examined. The results demonstrate that, compared to UHPC-only strengthening, composite-strengthened specimens exhibit a 10%–16.4% enhancement in initial stiffness due to the CFRP-UHPC synergistic confinement effect. The ultimate capacity increase significantly exceeds the additive effect of individual strengthening methods, with specimen CUF-20-4 achieving an ultimate capacity enhancement of 431.28%; Under identical UHPC thickness, the composite-strengthened specimens exhibited bearing capacity increases of 11.9%∼60.8% and ductility improvements of 4.18%∼16.73% with increasing CFRP layers, while the initial stiffness remained virtually constant. For specimens with identical CFRP layers, an increase in UHPC thickness enhanced the bearing capacity by 12.20%∼61.54% and initial stiffness by 3.43%∼7.64%, while ductility exhibited a marginal decline. Furthermore, the failure mechanisms of CFRP-UHPC composite-strengthened columns were proposed based on experimental phenomena and material strains. Based on the ultimate stress-strain formulation for confined concrete proposed by Xiao et al., coefficients in the formula were calibrated to develop an ultimate stress-strain model for CFRP-UHPC composite-strengthened columns. Stress-strain models for low and high confinement ratios were established, with predicted curves demonstrating strong agreement with experimental results. • CFRP-UHPC composite retrofitting mitigates brittle failure characteristics of UHPC-Strengthened columns. • Experimental investigations were conducted to analyze the failure modes of CFRP-UHPC composite systems. • Characterization of load-displacement and stress-strain curves for CFRP-UHPC composite systems. • The reinforcement mechanisms and failure mechanisms of CFRP-UHPC composite strengthening systems were discussed. • Peak stress-strain models and constitutive stress-strain relationships for CFRP-UHPC retrofitted columns were established