| The bond-slip behavior of steel bars or fiber reinforced polymer(FRP)bars plays an important role in the performance of reinforced concrete(RC)structures,whether it is a new structure or an existing structure.Existing RC structural experiments have shown that the phenomenon of bond-slip of steel bars cannot be ignored,which affects the overall performance of the component,leading to a reduction in the stiffness and energy dissipation of the component.Whether or not considering the bond-slip in the static nonlinear analysis and seismic performance analysis will cause some difference.With an emphasis on strengthening of existing RC structures,this dissertation focuses on the bond slip issue of FRP bar and steel bar with concrete,and uses a combination of experiments,theory and numerical finite simulation to study the important role and mechanism of bond-slip effect on nonlinear analysis of FRP reinforced/retrofitted concrete structures.The main contributions are as follows:(1)A three-dimensional(3D)high resolution finite element(FE)model based on the rib scale was established for FRP bar and concrete.Considering the material nonlinearity and concrete size effects,a smeared crack model was adopted.With the contact model between FRP bar and resin,the mode l can intuitive ly demonstrates the mechanism of bond-slip and the whole process of debonding failure.The proposed FE model can not only predict the overall bonding strength of the joints,but a lso fully present the3 D deformation field of the bonding area and the dynamic development process of local damage.The nonlinear bond slip process of FRP bars is generally affected by the crack damage of resin and concrete as a whole,but in the later stage,the residual part of the bond slip curve is mainly controlled by the cracking and softening of the concrete.For how to consider the bond-slip effect in nonlinear analysis of the concrete structures,the local bond-slip curve and reasonable selection of parameters will become the key factors.(2)A comprehensive two-dimensional(2D)nonlinear FE analysis of RC beams strengthened with prestressed or non-prestressed near surface mounted(NSM)carbon fiber reinforce polymer(CFRP)bars was conducted.In this model,the bond-slip relationship between steel reinforcement and surrounding concrete as well as that between NSM CFRP reinforcement and concrete substrate are precise ly modeled with interface cohesive elements.Based on the smeared crack approach,the tensile and shear behavior for cracking of concrete are considered using appropriate mode ls.Comparisons between numerical and experimental results confirm that the proposed FE method is appropriate for predicting the load-deflection curves,cracking patterns as well as the debonding mechanism of the NSM CFRP bars.Furthermore,the proposed mode l is utilized to conduct a parametric study and can be used to offer help for realistic structural applications and optimization of the use of prestressed NSM bars.(3)A total of six reduced-scale specimen models of RC bridge columns were designed and manufactured to study the hybrid strengthening effect with a combination of NSM ba salt fiber-reinforced polymer(BFRP)bars and BFRP jackets.The two retrofit methods take advantage of each other's strength,and the strengthening effect is pretty good.At the same time,with the increase of the diameter of BFRP bars,BFRP bars tend to slip in the concrete cover.This slip phenomenon can improve the ductility of the specimen and avoid premature fracture of the BFRP bars.The experimental results show that,combining NSM BFRP bars and BFRP jackets can simultaneously improve the flexural ca pacity and ductility of the columns,which is an excellent seismic strengthening method.(4)To conduct the analysis of seismic performance of RC bridge columns retrofitted with NSM FRP bars and/or FRP jackets,an axial stress versus loaded-end displacement model of NSM FRP bar including both elastic elongation and bond-slip effect is proposed to account for the significant slippage of FRP bars in the plastic hinge region.The simplified FRP bar model is then converted into the equiva lent stress-strain relationship so as to be easily implemented in the fiber-based analysis.The proposed method avoids the nested iterations in the sectional analysis that requires the force equilibrium and deformation compatibility.Furthermore,the mechanism of the hybrid effect of combining NSM FRP bars with FRP jackets can be interpreted after the analytical study,that is,the bond-slip effect mitigates the premature fracture of NSM FRP bars and leads to a more effective utilization of the NSM reinforcement and a more ductile column behavior.(5)A systematic analysis of the effect of the bond-slip of steel bars on the nonlinear analysis of FRP confined RC columns is performed.A one-dimensiona l reinforcement axial stress-slippage model with confining effect is proposed.The comparison with the experimental data shows that the calculation method proposed in this study,which takes into account the bond slip effect of steel bar,better explains its significant role in nonlinear analysis and is in good agreement with the experimental results.For columns with lap spliced reinforcement in the plastic hinge zone,whether or not there is external FRP confinement,the bond-slip effect of the steel bar will significantly affect the entire nonlinear process,inc luding initia l stiffness,peak load,peak displacement,and post-peak behavior after the peak load.For columns with continuous longitudina l reinforcement in the plastic hinge zone,whether or not there is external FRP confinement,the bond slip effect of the reinforcement will significantly affect the first half of the nonlinear process,that is,the initia l stiffness,peak load,peak displacement,and ductility,while the later descending portion is mainly affected by the low cycle fatigue fracture or buckling of the steel bars.(6)Based on the research in previous chapters,a case study of two structures,FRP strengthened three-span RC bridge and 7-story RC frame,is performed.It is shown that if the bond-slip effect is not considered reasonably in the structural analysis,there could be huge errors under some load cases,and even the nonlinear analysis could fail completely.The FE model of the RC component considering bond-slip effect proposed in this study is suitable for the nonlinear analysis of FRP strengthened RC bridges and frame structures.Combining NSM FRP bars and FRP jackets can simultaneously improve the flexural capacity and ductility of bridge columns,thereby improving the vulnerability and fragility of the bridges.FRP confinement of weak frame structure columns can suppress the shear failure mode,and at the same time enhance the performance of the steel bars in the plastic hinge zone and the concrete ductility,thereby improving the ductility and bearing capacity of the strengthened RC frames.The proposed fiber-based model of the RC columns considering bond-slip effect and shear failure mode can be effectively used in the seismic performance evaluation and optimization calculation of weak frame structures. |
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