Quasi-static Experimental Study On Prefabricated Composite Reinforced Concrete Columns

Prefabricated concrete columns are widely used in sea crossing bridges,urban interchanges and house engineering,but its durability is greatly reduced by the corrosion of steel bars.Steel-FRP composite bars（SFCB）effectively solves this problem.The dense distribution of reinforcement in the component brings great inconvenience to the construction.To reduce the construction difficulty and improve the efficiency,the bundle configuration of reinforcement is adopted.The application of SFCB and bundled reinforcement in prefabricated concrete columns is of great significance to further improve the construction convenience and durability of components,and it also needs more in-depth theoretical research.Therefore,based on the connection mode of prefabricated concrete columns,the application of FRP and high efficient bundled reinforcement,this paper designs eight prefabricated composite reinforced concrete columns and one cast-in-place specimen and carries out the quasi-static experiment study on them.Then the failure phenomena and hysteretic curves are analyzed,and the skeleton curves,ductility performances,stiffness degradation,energy dissipation capacity and residual deformation are quantitatively studied.The results show that:（1）The failure modes of different specimens are basically the same.The transverse cracks appear first,then the inclined cracks and finally the longitudinal cracks.They develop from bottom to top,and finally the reinforcement is damaged due to concrete collapse.（2）The hysteretic curves show that the residual displacement of the steel specimen is larger than that of the corresponding SFCB specimen.The residual displacement of the SFCB specimen is always smaller before reaching the maximum bearing capacity.Once the peak load is reached,the bearing capacity decreases rapidly and the residual displacement also increases rapidly.（3）The SFCB specimen shows obvious post-yielding stiffness and the bearing capacity is still rising steadily after yielding,which means SFCB can effectively improve the bearing capacity.However,the number of bundled bars has little effect on the bearing capacity when is 3 or less.（4）The specimens with bundled bars show excellent ductility,which can effectively delay the failure and slow down the stiffness degradation.The ductility of SFCB specimens is relatively poor,and it declines rapidly soon after reaching the maximum bearing capacity.（5）The initial stiffness of prefabricated specimens is higher than that of the cast-in-place specimen.The initial stiffness of single-bar specimens is higher than that of specimens with bundled bars,but the stiffness degradation is faster in the plastic stage.（6）The cumulative energy dissipation capacity of the cast-in-place specimen is excellent.The capacity of bundled reinforcement specimens is lower than that of single reinforcement specimens in the early stage of loading,but it turns the tide in the later stage.The capacity of SFCB specimens is better than that of steel reinforcement specimens.（7）BFRP can greatly reduce the residual displacement and increase the self-centering ability of the specimen.（8）The 3-bar or 2-bar bundled reinforcement reduces the crack spacing and the width,but increases the number of cracks,while FRP can expand the crack spacing and increase the number of cracks.On the basis of summarizing the previous theoretical formulas,the crack spacing and crack width of composite reinforced concrete columns are checked.It is suggested that the average crack spacing of cluster reinforced concrete specimens should still be calculated according to the scattered reinforcement,and the deviation of reinforcement specimens is very small.For SFCB specimens,the formula GB 50608-2010 should be used.On this basis,the influence coefficient of crack developmentα_c,non-uniformity coefficient of longitudinal tensile strainψ,tensile stressσ_sf,t and maximum crack width expansion coefficientτ_s in the calculation formula of crack width are explored and modified,and the calculation formula which is more consistent with the actual calculation results is obtained.Suggestions on crack spacing design considering the effect of bundled reinforcement are put forward.Based on the design parameters and experimental data of this experiment,the bending moment-curvature analysis of SFCB concrete columns is conducted.For SFCB concrete columns,increasing the reinforcement ratio will increase the yield load and the maximum bearing capacity,but the curvature of the section corresponding to the maximum bearing capacity gradually decreases,leading to the of the peak curvature.Increasing the axial compression ratio will also increase the yield load.The pushover skeleton curve of SFCB concrete column is deduced,and the theoretical calculation model is obtained with the load displacement calculation model considering the bond slip weakening of bundle reinforcement proposed.