Fatigue performance degradation of steel-FRP composite bar reinforced seawater sea-sand concrete beams: Coupled effects of seawater immersion and sustained load

This study aims to explore the coupled effects of environmental and loading conditions on the fatigue degradation of steel-FRP composite bar (SFCB) reinforced seawater sea-sand concrete (SWSSC) beams. Here, the failure modes, surface conditions of SFCBs, fatigue lives, loaddeflection curves, crack propagation, and maximum crack width were investigated. The degradation mechanism of fatigue performance of specimens was further revealed. Experimental results indicate that the fatigue lives of unconditioned beams dropped from over 2,000,000 to 124,851 cycles as the stress level increased from 0.2 to 0.4. After 6 months of exposure, the fatigue life of the beam under the single effect of sustained load was over 2 million cycles at a stress level of 0.2, whereas the ultimate capacity after 2 million cycles decreased by 45.2 %. The coupled actions of high-temperature seawater immersion and sustained load degraded fatigue properties in the first 3 months, but the deflection and crack width reduced due to the enhanced flexural stiffness after 6 months. Compared to the sustained load, the high-temperature seawater dominates the coupled effects. Surface conditions and microscale observation of SFCBs indicate that the weakening in fiber-resin interface and the corrosion of steel core lead to the fatigue degradation of beams.