Performance of UHP-SWSSC-Filled CFRP and GFRP Tubes under Combined Sustained Load and Seawater Exposure

Abstract

This experimental investigation is the first of its kind to evaluate the effects of a combination of long-term loading and marine exposure on the structural response of carbon fiber–reinforced polymer (CFRP) and glass fiber–reinforced polymer (GFRP) tubular composites infilled with seawater and sea sand-based ultrahigh-performance concrete (UHP-SWSSC). The specimens were conditioned under natural seawater exposure at 40°C, 3,000 h of sustained loading, and a combination of the two. Long-term strains indicate that the composite tubes (especially GFRP) experienced swelling due to marine exposure, even under sustained load, suggesting gradual detachment of the tubes from the ultrahigh-performance concrete (UHPC) core. Considerable hoop strength and stiffness of the confining fiber-reinforced polymer (FRP) were necessary to restrain the dilation of UHPC, which was mechanically improved by sustained loading and seawater exposure. Lateral confining pressure and stiffness of FRP correlated with improved confined UHPC properties. The FRP tubes utilized their full material capacity only at a sufficiently high level of confinement to efficiently restrain the UHPC core. Despite the increase in compressive strength, marine exposure reduced the ductility of the FRP–UHPC composites, particularly in those containing GFRP. The gradual stress redistribution from the inner concrete to the outer FRP tubes facilitated better utilization of FRP confinements after creep (with or without seawater exposure).