| Summary: | The addition of nano-silica to ultra-high-performance concrete (UHPC) to increase its toughness has been proposed to obtain ultra-high-performance nano-concrete (UHPNC). This work mainly studies the reinforcement effect of UHPNC on concrete filled steel tube (CFST) columns under long-term load. Ten CFST columns strengthened with UHPNC were selected and reinforced with UHPNC. The influences of different thicknesses of UHPNC reinforcement layer and different nano-silica contents on the axial compression properties of specimens were mainly studied, by loading specimens in two steps: long-term load and ultimate load. This study discussed the failure modes, compressive toughness, ultimate bearing capacity, initial stiffness, and ductility coefficient of the specimens. The results show that the outsourced UHPNC reinforcement method is an effective method to improve the performance of CFST columns during service period. With the increase in the thickness of UHPNC reinforced layer, the ultimate bearing capacity of CFST column increases greatly. The compression toughness is increased with the increase in nano-silica content and UHPNC reinforcement layer thickness. The decrease rate of initial stiffness increases with the increase in nano-silica content.
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