Axial compression behavior and analytical model of CFRP-ECC confined underwater-cast concrete columns for marine structures

This study proposed an underwater repair method for concrete columns based on carbon fiber-reinforced polymer (CFRP)/underwater-castable high-volume fly-ash eco-friendly engineered cementitious composite (UHVFA-ECO-ECC) composite strengthening. Through axial compression tests on concrete core columns, effects of casting condition, UHVFA-ECO-ECC interlayer thickness, and CFRP jacket layers on failure mode, stress-strain response, deformation capacity, toughness, and confinement mechanism of composite strengthened columns were systematically investigated, and a mechanical model for peak load-carrying capacity was established. The results showed that CFRP/UHVFA-ECO-ECC composite strengthening system significantly improved failure mode of concrete columns, transforming the specimens from brittle splitting failure to an integral failure mode characterized by multiple cracking and the synergistic action of hoop confinement. Although underwater casting had a certain adverse effect on composite-strengthened columns, their ultimate strength remained above 90% of that of corresponding land casting specimens. In addition, number of CFRP layers and UHVFA-ECO-ECC thickness showed clear positive effects on specimen mechanical performance. With increasing UHVFA-ECO-ECC interlayer thickness and CFRP jacket layers, the composite-strengthened columns exhibited higher ultimate strength, better post-peak load stability, and greater toughness. Further analysis revealed the composite confinement mechanism, based on which a peak-capacity prediction model was developed. The predicted values were in good agreement with test results.