Experimental Study On The Flexural Performance Of Hybrid Steel-BFRP Reinforced Concrete Beams

The deterioration of mechanical properties and durability of concrete structures caused by the corrosion of steel bar not only costs huge maintenance costs,but also has a negative impact on the use safety of building structures.It is a feasible solution to use FRP bars to replace steel bars as reinforcement materials in concrete structures,which have the advantages of strong corrosion resistance,light weight and high strength.However,due to the low elastic modulus of FRP bars,the flexural stiffness of FRP bars in concrete members is significantly lower than that in reinforced concrete structures,resulting in a substantial increase in structural deformation.Therefore,In this paper,the flexural behavior of the normal section of the hybrid Steel-BFRP reinforced concrete beams is further studied by carrying out the flexural test of the hybrid reinforced concrete beams.The main research contents and conclusions are as follows:(1)Four normal anchors,one BFRP bar loading point reinforced anchors with casing and four BFRP bar end casing reinforced anchoring Steel-BFRP bar hybrid reinforcement beams were designed and fabricated to carry out the flexural test of normal section under different concrete strength and different nominal reinforcement ratio.The results show that the deformation of the normal section of the hybrid Steel-BFRP reinforced concrete beams basically conforms to the assumption of the plane section,The flexural deformation of the hybrid reinforced beam is similar to that of the reinforced concrete beam,and it has good ductility.For the same area of tensile reinforcement,the larger the nominal reinforcement ratio is,the smaller the bending deformation and crack width of the member will be.Increasing the strength grade of concrete can increase the flexural stiffness and ultimate bending moment,but has no obvious effect on the crack width.(2)Under the same conditions of concrete strength and reinforcement,the deflection and deformation of the normal and end cased reinforced beams are basically the same,the maximum crack width and deflection of the cased reinforced beams at the loading point is smaller,the ultimate load is bigger.The bond performance of BFRP bars and concrete can meet the joint working needs of the hybrid reinforced members when they are flexural.The load-deflection curve of the mixed reinforced beam shows three linear characteristics,and two turning points in the curve correspond to the cracking of concrete and the yielding of the tensile reinforcement respectively.(3)Two kinds of boundary failure states and corresponding expressions of comprehensive reinforcement ratio are defined in this paper.According to the deformation coordination and moment balance conditions of the plane section,the formulas for calculating the moment of inertia before cracking,the height of the compression zone,the cracking moment and the bending capacity of the hybrid reinforced concrete beam are derived.Based on the bond-slip constitutive model of BFRP bars and concrete,the formula for calculating the anchorage length of BFRP bars was deduced.(4)The effective moment of inertia method and stiffness analytical method were used to establish the calculation formula of the bending stiffness of the section of the hybrid Steel-BFRP reinforced concrete beams,The calculated values of the flexural stiffness of the members were compared with the experimental values.The results show that the calculated values of the two theoretical formulas are in good agreement with the experimental value,and the stiffness analytical method is simpler and the calculated value has a certain safety reserve.The expressions of average crack spacing and maximum crack width under short-term load with the ratio of the area of BFRP bars to steel bars as parameters are given.Compared with the test results,the theoretical calculation value of maximum crack width under normal service condition has better applicability.(5)The nonlinear finite element simulation analysis of the bending performance of nine hybrid reinforced beams was carried out by Using ABAQUS software,and the deflection changes and failure process of the nine beams were analyzed by the model calculation results,and the experimental values of the ultimate bending capacity were compared with the finite element simulation values.The results show that the finite element simulation results are close to the actual test values,and the established finite element numerical model can reasonably reflect the actual flexural state of the hybrid reinforced beam.