Durability Assessment of Hybrid Double-Skin Tubular Columns under Wet-Dry Cyclic Environments

Hybrid fiber-reinforced polymer (FRP)-concrete-steel hybrid double-skin tubular columns (DSTCs) consist of an outer FRP tube, an inner steel tube, and a layer of concrete filled between the two tubes. Previous studies have demonstrated their structural performance compared with conventional concrete columns. However, their durability performance, particularly in aggressive environments, is not well understood. Therefore, this study aims to investigate the durability of hybrid DSTCs subjected to a wet-dry cyclic environment for up to 2 years. The time-dependent behaviors, including axial load-strain (axial and hoop) curves, ultimate load, and ultimate axial and hoop strains, are tested and discussed with regard to the aging time. The test results indicate that the ultimate load of hybrid DSTCs with a 6-mm glass fiber-reinforced polymer (GFRP) tube continuously increased by 15.1% after 2 years of exposure primarily owing to the increase in concrete strength. By contrast, the ultimate load of hybrid DSTCs with a 3-mm GFRP tube increased by 15.2% after 1 year of exposure and increased by 11.6% after 2 years of exposure. The results indicate that the reductions can be attributed to the degradation of the GFRP tubes, especially for a thinner GFRP tube. In addition, a design-oriented stress-strain model for concrete in DSTCs is verified against the test results.