Study On The Compressive Behavior Of Concrete Columns Confined With Close-type CFRP Strip Ties

Over the past two decades,Fiber Reinforced Polymers(FRP)have been widely used in the retrofit and reconstruction field of reinforced concrete structures due to their lightweight,high tensile strength,good corrosion resistance,and fatigue performance.In recent years,the use of FRP bars and ties as a replacement of conventional steel reinforcement in the construction of new reinforced concrete structures has got more and more attention.FRP materials could effectively avoid the corrosion issue of traditional steel rebars and therefore improve the durability and serviceable life of concrete structures.Especially,it has a good application prospect in ocean and bridge structures.Conventional pultruded FRP ties are fabricated by bending FRP bars prior to the polymerization of resin in order to make different shapes.Bending of bars results in kinking of internal fibers and stress concentration of external fibers at the bend parts,resulting in lower bending tensile strength.On the other hand,pultruded FRP ties have bond-slip failure risk at the overlapping regions after crushing of concrete cover,which results in low confinement efficiency for core concrete.Furthermore,the FRP ties are fabricated in the factory and have non-plastic nature to accommodate any on-site modification in size and shape,unlike steel reinforcement,resulting in extra cost of refabrication to the project.To overcome the aforementioned shortcoming of pultruded FRP ties and to suit for on-site fabrication,this study proposed closed-type rectangular cross-section FRP strip ties fabricated by directly winding epoxy impregnated FRP strips on longitudinal bars of the structural member through wet-layup technique.The proposed ties were aiming to make the on-site fabrication of FRP ties possible,improve the tensile strength at bending parts,completely overcome the bond-slip failure,and increase the confinement efficiency of FRP ties.Carbon-FRP(CFRP)was selected as confining reinforcement material due to its elastic modulus comparable with steel,unlike other FRPs such as Glass-FRP and Basalt-FRP,which have significantly low elastic modulus.In this study,steel bars and CFRP strip ties were used as hybrid reinforcement in square concrete columns/piers.The durability and serviceable life could be improved due to the better corrosion resistance of CFRP ties and the addition of extra concrete cover to steel re-bars.The concrete columns are expected to have better confined concrete strength and ductility due to the excellent mechanical properties of CFRP strip ties.By means of experimental research and finite element analysis,the basic properties of CFRP strip ties and confining performance of CFRP strip ties were studied;then a stress-strain model for concrete confined with CFRP strip ties was developed,which could be a helpful tool for seismic analysis of hybrid-reinforced concrete columns;finally,the axial load-bending moment diagrams were developed for FRP-reinforced columns(FRP-RC)using finite element and theoretical analysis.The findings of this study could be of great significance for the improvement of durability and performance of reinforced concrete columns and for the design of hybrid-reinforced piers.The research methodology and main findings of the research are as follows:(1)The experimental evaluation of bend strength ratio of CFRP strips ties in comparison with conventional pultruded FRP bar ties indicated the higher bend strength ratio for strip ties compared to pultruded ties.The confining performance of strip ties in concrete columns was evaluated through axial compression testing of concrete columns.Tested columns consist of longitudinal steel bars and lateral steel or CFRP ties.Test results indicated that the columns with strip ties provided significantly higher ductility compared to their counterparts having steel or pultruded CFRP ties.(2)A finite element model is developed for concrete columns internally confined with CFRP strip ties and the validity of this model was verified by comparison with the experimental results.A comprehensive parametric analysis was performed to evaluate the effect of the confinement efficiency of strip ties and the parameters related to ties on the compressive performance of concrete columns.From parametric study,it was concluded that the effectively confined core was closely related to the configuration of CFRP strip ties.Octagon-shaped ties have higher confinement efficiency than quadrilateral-shaped ties.The tie spacing and the volumetric ratio of strip ties lightly influence the peak load but significantly influence the confinement efficiency.(3)Based on the FEM and experimental results of this study,the defining principles for well confined,moderate confined,and low confined columns with CFRP strip ties were proposed,the effectively confined core was determined,and the calculation formulas for the peak and the ultimate load were established.A uniaxial stress-strain relationship for concrete columns reinforced with CFRP strip ties was proposed,which fits better with the experimental results than other stress-strain models.(4)Finite element analysis on CFRP strip stirrups reinforced concrete columns with steel and FRP bars is performed,and the contribution of FRP bars to compressive behavior was figured out.The theoretical prediction model for the capacity of eccentric compressive behavior of concrete columns was established based on the sectional analytical analysis,the N-M diagrams of the FRP reinforced columns were proposed,and the effects of concrete strength,longitudinal ratio,and slenderness on the N-M diagram are investigated.The capacity prediction model of steel reinforced concrete columns was found suitable for hybrid-reinforced columns.Ignoring the contribution of FRP bars under compression at low level of eccentricity was found to be highly conservative.The limiting values of slenderness ratio for FRP-RC columns bent in single curvature are lower than that of steel-RC columns.