Research On Flexural Performance Of Hybrid Beams Composed Of Prefabricated UHPC Permanent Formwork And Cast-in-situ Concrete Core Reinforced By GFRP Bars

Fiber Reinforced Polymer(FRP)bars have the advantages of light weight,high strength,good corrosion resistance,good fatigue resistance,good electrical insulation,and convenient molding.The tensile strength of FRP bars are much higher than that of steel bars,but the elastic modulus of FRP bars is usually only 20%-70% of the steel bars.The constitutive model has no yield platform,resulting in low bending stiffness,large crack width and deflection of FRP reinforced concrete members,and the high tensile strength of FRP bars cannot be fully developed.In this paper,from the perspective of improving the bending resistance of glass fiber reinforced polymer hybrid(GFRP)reinforced concrete beams,and put forward the prefabricated ultra high performance concrete(UHPC)permanent formwork-GFRP bars reinforced concrete hybrid beams and mixed reinforcement(GFRP bars,steel bars)concrete hybrid beams.It uses UHPC's excellent tensile and compressive resistance,good toughness and durability to improve the flexural stiffness,cracking load,ultimate load and durability of GFRP reinforced concrete beams and reduce crack width and deflection.At present,there are few relevant studies on prefabricated UHPC permanent formwork-GFRP reinforced concrete hybrid beams and mixed reinforced(GFRP bars,steel bars)concrete hybrid beams.In this paper,through the four-point bending test of five new composite beams,the effects of different reinforcement ratios and the area ratio of GFRP bars to steel bars on the bending performance of the test beams were studied;Using numerical simulation to parametrically analyze the new composite beams,the effects of different prefabricated UHPC permanent form forms,concrete strength,and bottom formwork thickness on the bending performance of the test beams were studied;Based on the test results of this paper,considering the contribution of UHPC permanent formwork,the relevant theoretical calculation formulas of new composite beams are studied.The main research contents and results are as follows:(1)Through the tensile property test of GFRP bars,the changes of the ultimatetensile strength,elastic modulus,ultimate elongation and failure morphology of GFRP tendons with different diameters are studied.The uniaxial tensile stress-strain curve of GFRP bars is obtained.Through the UHPC material test,the influence of steel fiber content on its basic mechanical properties was studied.The best steel fiber content is 3%,and the stress-strain curve of axial tension and compression is tested.(2)The four-point bending test research shows that the new composite beam satisfies the assumption of flat section;Prefabricated UHPC permanent template effectively inhibited the development of cracks in GFRP bars,changed the distribution of cracks,and significantly reduced the number of cracks.With the increase of reinforced beams(0.63%,0.95%,1.43%),the ultimate load and stiffness of the test beam were significantly improved.The ultimate load was increased by51.4% and 77.4%,and the ultimate deflection was decreased by 4.9% and 5.7%,respectively.There are three modes of destruction in the beam.With the improvement of GFRP bars to replace steel reinforcement beams(0,1/3,1),compared with pure steel beams,the ultimate load is increased by 26% and 8%,Compared with pure GFRP reinforced beams,the stiffness and deformation performance of mixed reinforced beams are significantly improved,and there is no peeling failure of UHPC formwork;The mixed reinforced beams give full play to the high strength of GFRP bars and the high elastic modulus of the steel bars,increase the ductility of the test beams,and have good engineering practicality.(3)The results of parametric analysis of the new composite beam using numerical simulation software show that: The results show that,compared with pure GFRP bars beams,the bending performance of bottom formwork,side formwork and full formwork hybrid beams is improved,the cracking load is increased by 49.1%,55.7% and 67.2% respectively;the ultimate load is increased by about 20%;the ultimate deflection was reduced by 10.1%,19.9% and 21.9% respectively.With the increase of the reinforcement ratio(0.63%,0.95%,1.43%)and the strength of concrete(C30,C40,C50),the cracking load of the beam is basically unchanged,The ultimate load increased by 24.6%,46.9%;4.4%,10.2%;the ultimate deflection decreased by 7.4%,20.7%;7.4%,14.6%,respectively.With the increase of the thickness of the prefabricated UHPC permanent bottom formwork(20mm,35 mm,50mm),the cracking load of the new composite beam has obvious improvement effect,and the cracking load,ultimate load are increased by 40.6%,87.9%;9.3%,18.4%.The limit deflection was reduced by 6.1% and 12.1% respectively.(4)In this paper,based on the current specifications,considering the role of UHPC permanent formwork,a new formula for calculating the cracking load and ultimate load of composite beams is proposed.The results of the test data verify that the agreement is good.The test value of the ultimate deflection of the new composite beam is less than the ultimate deflection obtained by the effective section moment of inertia method,which may be due to the better restraining effect of the prefabricated UHPC permanent formwork on the core concrete.The extremely high tensile strength of UHPC and the bridging effect of steel fiber are both can play a role in inhibiting crack development and reducing deflection.