Axial compression performance of underwater columns reinforced with stainless steel tube-FRP grids

Stainless steel tubes have excellent plasticity and ductility, and FRP grids have favorable advantages for application in specific structural reinforcements. For systems including underwater construction without drainage, underwater columns reinforced with stainless steel tube-FRP grids are proposed using nondispersive mortar (NDM) as the reinforcement layer to best leverage the material properties of all elements. This paper carried out axial compression tests on 18 underwater columns reinforced with stainless steel tube-FRP grids (SS-FGWCs), 6 columns reinforced with stainless steel tube-FRP grids on land (SS-FGLCs), 3 underwater columns reinforced with stainless steel tubes (SSWCs), and 3 unreinforced concrete columns. The research parameters include the number of FRP grid layers, the types of FRP grids, and the reinforcement layer mortar pouring environments. The test results showed that FRP grids can be combined with stainless steel tubes (SSs) and nondispersive mortar (NDM) to reinforce concrete columns in underwater construction without drainage. In addition, compared with those of the SSWC specimens, after the BFRP grid and CFRP grid were built in, the ultimate stresses of the specimens increased by 13.4% ∼ 20.8% and 19.5% ∼ 25.4%, respectively, and the ultimate strains increased by 6.8% ∼ 31.7% and 21.8% ∼ 59.1%, respectively. Compared to BFRP grids, CFRP grids provide better reinforcement of the underwater columns. Existing prediction models were evaluated for ultimate stress calculations, the calculation methods for the ultimate stress and ultimate strain applicable to the SS-FGWCs and SS-FGLCs were revised, and a calculation model for the FRP grid failure point was proposed. Finally, a corresponding full stress–strain curve model was established.