Lateral behavior of circular concrete columns reinforced with partially unbonded steel basalt-fiber composite bars and hybrid stirrups

Abstract

Utilizing steel basalt fiber reinforced polymer (FRP) composite bars (SBFCBs) in concrete structures offers numerous benefits, such as controllable post-yield stiffness and reduced residual displacement. The effects of the degree of confinement, post-yield stiffness and unbonded length on the SBFCBs reinforced columns have not been conclusively evaluated. First, the axial compressive test of short concrete columns confined by hybrid stirrup with a confine ratio of 0.28 (equal-strength stirrup ratio of 1.23 %) showed that the hybrid stirrup could increase the axial compressive strength of the plain concrete from 51.8 MPa to 89.8 MPa, with an increase of 73.3 %, and the corresponding ultimate strain is 0.0282. Dong's model (FRP confined concrete) for predicting the axial stress-strain curve of steel basalt fiber reinforced polymer (BFRP) hybrid stirrups (SBFHSs) confined concrete was validated, and damage assessment based on concrete damage plastic model (CDPM) was performed. Subsequently, 3D finite element analysis (FEA) models of partially unbonded SBFCBs and SBFHSs-reinforced concrete columns were established and verified. Variables include the post-yield modulus ratio (r_sf) of reinforcement, unbonded length (l_ub), and core concrete’s confined level (C_y). The results showed that, compared to column S12, the load capacity and post-yield stiffness ratio of column SUB-R0.5-C2-U0, with a post-yield modulus ratio of 0.5 and C2 grade core concrete (with confine ratio of 0.20, ε_cu=0.0147, f_cu=74.50 MPa), increased by 20.2 % and 325.0 %, respectively. The unbonded reinforcement has a positive impact on the post-yield stiffness ratios and the l_ub may significantly increase the r_c (ranging from 2.21 % to 6.54 %). The effects of design parameters (r_sf, l_ub and C_y) on the post-yield stiffness ratios (r_c) and ductility index (μ_δ) were evaluated. The results show that, compared to fully bonded column, the slip deformation of SUB-R0.5-(C1∼C5)-U450 increased by 89.3 %, 97.8 %, 56.2 %, 107.1 %, and 146.1 %, respectively, indicating that the significant impact of unbonded reinforcement on slip on deformation. An empirical formula for the post-yield stiffness ratio, considering the unbonded length and the confined level of the core concrete, is presented with high accuracy, exhibiting a root mean square error (RMSE) of only 4.27 %. The debonding treatment described here is expected to provide a performance improvement and design methodology for resilient bridge piers.