Experimental investigation on the fatigue behavior of hybrid (steel+GFRP bars) RC beams after four-year simulated seawater exposure

In offshore infrastructures and structural components exposed to de-icing salts, FRP bars serve as promising alternatives to steel reinforcement. In contrast to conventional accelerated aging tests, this study investigates the fatigue behavior of concrete beams reinforced with hybrid steel‑FRP bars after four‑year natural immersion in simulated seawater. Chloride ion diffusion, static and fatigue failure modes, fatigue life, fatigue deflection, crack width and post-fatigue concrete properties were investigated. After four-year simulated seawater exposure, the chloride ion content at a depth of 2 mm from the concrete surface was 1.2%. Experimental results revealed that the static failure mode remained a typical flexural failure mode after four years of immersion while the fatigue failure mode of the beam may shift from flexural fatigue failure to shear fatigue failure. The stress range in GFRP stirrups remained low. GFRP stirrups at the diagonal crack were subjected to a combined tension-shear stress state and failed prematurely. The compressive strength of concrete increased from 37.1 MPa to 43.8 MPa after immersion. According to MIP test results, long-term immersion promoted continued hydration reactions and pore size refinement in concrete, with the dominant pore size decreasing from 1000 nm to 500 nm, while the total porosity remained nearly constant. Furthermore, fatigue loading had a beneficial effect due to the reduction in total porosity from 15.53% to 11.61%.