Experimental Study On The Flexural Behavior Of ECC-concrete Composite Beams Hybrid Reinforced With Steel And FRP Bars

ECC（Engineered cementitious composites）is a highly ductile fiber cementitious composite material,which has the characteristics of high ductility,toughness as well as the steady cracking development of multiple fractures.FRP（Fiber Reinforced Polymer）is a kind of fiber reinforced composite material with many kinds of fibers,different properties and strong design ability.It is the best choice to solve the corrosion of major engineering structures and to realize the service life and high performance.Due to the advantages of good ductility of steel bars and low density,high strength and corrosion resistance of fiber-reinforced polymer（FRP）,concrete beams hybrid reinforced with steel and FRP bars have better ductility,smaller crack width and lower deflection compared with FRP reinforced concrete beams and higher load carrying capacity and better corrosion resistance compared with steel reinforced concrete（RC）structures.However,hybrid FRP/steel bars deformed incompatible with concrete,which leads to larger crack width and therefore corrosion to steel reinforcement and accelerated fracture of FRP reinforcement due to stress concentration.In this paper,a new type of FRP bars and steel bars reinforced ECC-concrete composite beam members and U-shaped ECC-FRP bars prefabricated template composite beam members are proposed.The FRP bars are placed at the bottom of the beam members,and ECC partially replaces concrete,so as to maximize the effect of FRP bars and ECC on the basis of cost control.This study tested flexural behavior of six hybrid reinforced ECC and ECC-concrete composite beam with various reinforcement ratios,matrix types and ECC pouring positions.On the basis of the experiments,the whole process of bending moment and curvature of beam members was numerically calculated by MATLAB software,and the accuracy of the model was verified by comparing with the experiments.Then,the influence of section size,ECC replacement height,concrete compressive strength,ECC material parameters and reinforcement ratio on the flexural performance is investigated by parameter analysis.A simplified calculation method of flexural bearing capacity is proposed,which provides a basis for engineering application.Finally,the concept of U-shaped ECC-FRP formwork composite beams and the simplified calculation method of bearing capacity are put forward.The specific research contents are as follows:（1）Static load bending test is carried out on 6 beam members.The results show that the increase of reinforcement ratio improves the flexural bearing capacity and section stiffness of ECC beams with composite bars;The flexural bearing capacity and section stiffness of composite beams are higher than those of concrete beams.In addition,the addition of ECC can effectively avoid excessive deformation and control excessive crack of the beam,and can significantly improve the bending capacity of the beam.In conclusion,ECC-concrete composite beam members reinforced by composite bars have superior flexural performance,which can not only obtain higher bearing capacity and effectively reduce excessive beam deformation,but also inhibit fracture development.（2）The moment-curvature curve of beam member is calculated based on material constitutive model of concrete and ECC and numerical analysis method.The curves are in good agreement with the experimental moment-curvature curves,and parameter analysis is carried out.The results show that the height of beam section has more influence on the flexural performance than the width of the beam section;The addition of ECC materials can significantly improve the bearing capacity of the beam component,but the ECC replacement level reaches a certain height is little influence on the performance of the bending.Therefore,a third of the ECC height can improves the bearing capacity of beam components and achieve the effect of corrosion resistance,which both control tensile zone area of crack width and reinforcement material improves the bending performance of the component,and can control the cost and reduce the use of ECC;Moreover,the larger the compressive strength of concrete is,the larger the bearing capacity of the beam member is,and the smaller the curvature is.The ultimate tensile strain has little effect on the bearing capacity of beam members.The larger the reinforcement ratio,the greater the bearing capacity of the beam members,and the smaller the curvature.On the basis of parameter analysis,a simplified calculation method for the flexural capacity of ECC/composite beams reinforced by composite reinforcement is proposed.The simplified calculation results are in good agreement with the experimental and theoretical results,which verifies the accuracy of the calculation method.（3）A new type of U-shaped ECC-FRP bar prefabricated template composite beam is put forward,which uses U-shaped ECC-FRP bar prefabricated template as a template of RC beam.Then MATLAB software is used to carry out numerical calculation of the whole process of bending moment-curvature of the beam member,and the accuracy of the model is verified by comparing with the experiment.In addition,the parameter analysis shows that the section size and reinforcement ratio have great influence on the bending performance of the beam;Among the four FRP bars,CFRP bars with best ductility have the greatest influence on the flexural performance.The side plate width of ECC prefabricated formwork has little effect on bending performance.Finally,a simplified calculation method for the flexural capacity of the U-shaped ECC-FRP prefabricated formwork composite beam is proposed.The calculated results are in good agreement with the experimental and theoretical results,which verifies the accuracy of the calculation method.