The use of locally sourced materials from remote offshore island and reef regions can effectively address challenges in island engineering construction, such as shortages of conventional building materials, poor resistance to salt-induced corrosion, and high transportation costs. However, the applicability of conventional structural design methods to such materials remains uncertain. In this study, nine beam specimens were designed and fabricated considering parameters including the presence or absence of web reinforcement, shear span ratio, longitudinal reinforcement ratio, and stirrup spacing. Experimental investigations were conducted on the shear behavior of coral seawater sea-sand concrete (CSSC) beams reinforced with basalt fiber-reinforced polymer (BFRP)-steel composite longitudinal bars and BFRP stirrups. A shear capacity prediction model incorporating the generalized dowel action of longitudinal reinforcement was proposed for such beams, both with and without web reinforcement. The results indicate that providing stirrups significantly enhances the shear capacity of the beams. Depending on the shear span ratio, the failure modes can be classified as diagonal compression failure, shear-compression failure, or diagonal tension failure. The shear capacity increases with decreasing shear span ratio and increasing longitudinal reinforcement ratio, while it decreases with increasing stirrup spacing. Comparisons with existing design codes and relevant research results demonstrate that the proposed calculation method agrees well with the experimental data. These findings provide a useful reference for the structural design of CSSC beams using locally sourced materials in tropical island and reef engineering construction.

The early-stage service performance evolution of glass fiber reinforced polymer (GFRP) bars in tropical marine environments remains insufficiently understood. To address this, a comparative study was conducted using in-situ exposure tests (atmospheric, tidal, and seawater immersion) alongside accelerated aging tests (ultraviolet radiation, salt spray exposure, UV+salt spray coupling, and UV+condensation cycling). Macroscopic mechanical testing combined with microstructural characterization was employed to reveal the degradation behavior and deterioration mechanisms of the fiber-resin interfacial bonding under various environmental conditions. The results indicate that in environments involving UV radiation, the interlaminar shear strength (ILSS) of GFRP bars initially increase and then decrease. The UV+salt spray coupled environment has the most pronounced effect on ILSS, leading to an 18.19% reduction after 40 days of exposure. Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscope (SEM) analyses verify that, due to the synergistic effects of post-curing, hydrolysis, and molecular chain scission of the resin matrix, the fiber-resin interface gradually transitions from a dense state to a rough structure accompanied by debonding.