Bond performance of marine concrete with nano-particles to steel bars under the action of both Cl− erosion and freeze-thaw cycles

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Offshore structures serve in harsh environment here the combined effects of freeze-thaw cycles and Cl− erosion severely degrade the bond performance between steel bars and concrete, significantly reducing the structures' service life. This paper investigates the bond performance of marine concrete with nano-particles to steel bars under the simultaneous influence of Cl− erosion and freeze-thaw cycles. The bond-slip (τ-s) curves are derived from pull-out tests, in order to reveal the mechanism of nano-particles to improve the bond performance of reinforced concrete under the action of both Cl− erosion and freeze-thaw cycles, the microstructure of the bond interface between steel bar and concrete is examined using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and mercury impression test (MIP). The test results show that adding nano-SiO2 and nano-TiO2 enhance the cracking bond strength (τs), ultimate bond strength (τu), ultimate slip value (Su), and residual bond strength (τr) of reinforced concrete. The optimal content for both nano-SiO2 and nano-TiO2 are 2 %, with nano-SiO2 showing superior effect. When Cl− erosion and the number of freeze-thaw cycles are certain (N = 100), the τu values of NSC20 and NTC20 are increased by 26.25 % and 16.68 %, and the Su values are increased by 25.30 % and 22.89 %, respectively, compared with PC. The results of the SEM and EDS analyses indicate that nano-particles promote C–S–H gel formation, reduce porosity and optimize concrete's pore structure, enhance shear strength at the bonding surface, reduce Cl− content at the interface, slow down the corrosion of the steel bars. Thus, the addition of nano-particles significantly improves the bond performance of reinforced concrete under the action of both Cl− erosion and freeze-thaw cycles, which is vital for enhancing the load-bearing capacity and seismic performance of engineering structures.