Analysis On Failure Mode,Mechanism And Mechanical Performance Of Flexural-Shear Behavior And Force Performance Of Inferior And Ordinary RC Beams Strengthened With Various FRP

After long-term use of the building structure,due to damage,material aging,the use of functional changes,not applicable to the new design load and other reasons,it is easy to exist a variety of security risks.It can be used to increase the cross-section,outsourcing of steel,prestressed in vitro,fiber composite reinforcement and other methods to strengthen the structure.Among that the fiber reinforced composite material with light high strength,convenient construction,good corrosion resistance and other advantages,In recent years it has been a wide range of research and application,Analysis on failure mode,mechanism and mechanical performance of flexural-shear behavior and force performance of inferior and ordinary RC beams strengthened with various FRP has been expanded.The main contents and achievements of the thesis are described below.Firstly,Bearing capacity evaluation and subsequent treatment of inferior RC components are required in practical cases.For this purpose,experiments on seven inferior RC beams strengthened by FRP sheets and an unstrengthened beam have been carried out in the laboratory.The mechanical behaviors,failure mechanism and ultimate load capacities of these beams were investigated in detail.The experimental results demonstrate that the unstrengthened beam presented a failure mode like a normal beam.However,the failure modes of seven strengthened beams were quite different from the unstrengthened beam.Their mechanical model changed from a comb-type arch to a tied arch with the increase of the external loads and the damage severity.Meanwhile,It is also found that although the strength and elastic modulus of concrete are not qualified,the bending stiffness,as well as the crack,yield and ultimate loads,of the inferior beams can be effectively improved.Moreover,whether the beams were damaged or not before strengthening gave little influence on the ultimate loads.Lastly,the tied-arch model was adopted for deducing the ultimate load capacity formula of inferior RC beams.It was observed that the predictions by the formula well fit the experimental results.Therefore,the proposed formula can be used as a reference for reinforcement designs of inferior RC beams.Secondly,Aiming at the key problems of FRP strengthened RC beams in practical engineering application,such as various failure modes and influence of secondary stress,nine RC beams with different types of FRP reinforcement and one non-reinforced RC beam were used to study the bending test respectively,the reinforcement effect of RC beams strengthened with AFRP,BFRP and CFRP was analyzed and compared,The effects of FRP types,FRP reinforcement,U-type anchoring measures and pre-cracking on the reinforcement effect were discussed.The experimental results show that the three kinds of FRP can significantly improve the cracking load,yield load and flexural capacity of RC beams,and the reinforcement effect of AFRP and CFRP is the best,which is better than that of BFRP.But not in linear proportion.There is no U-shaped anchor reinforcement at the beam end,and the FRP of the reinforced beam bottom is still broken.The pre-split degree has no effect on the flexural capacity of the test beam,but increases the yield load and thus reduces The ductility of the reinforced beam can be enhanced;AFRP and BFRP can both increase the ductility of the beam without pre-cracking,and the ductility of the CFRP is unfavorable.In general,BFRP has the highest cost performance,followed by AFRP,and the lowest CFRP.and the content that the cracking load,yield load,ultimate load,ductility,economy,load-displacement curve,load-strain curve,bending performance and failure mode of the test beam are analyzed.The constitutive relations of different materials and the consideration of secondary loading effects are the primary factors that affect analysis accuracy.By utilizing the Hognestad constitutive model close to actual stress-strain situations and considering the nonlinear changes in the compressive zone of concrete,this paper first deducts the coefficients related to the equivalent stress of concrete under compression,as well as to the depth of compressive zone of concrete.Then the formulas of ultimate bending moments of RC beams are established with respect to three different failure modes of FRP tensile failure,reinforcement yield before or after concrete crushing.Meanwhile,the maximum and minimum FRP amounts for beam reinforcement are given to define the critical conditions between different failure modes.The secondary loading effects with lag strains are also considered in the context.Lastly,the proposed formulas are validated using some tested RC beams.The comparison results demonstrate satisfactory agreement of ultimate loads predicted by the proposed formulas,indicating the application feasibility of these formulas for the FRP reinforcement designs and the ultimate load analysis of RC beams.Lastly,Aiming at the selection of optimal reinforcement plan of FRP fabric based on shear-resistant RC beams,using orthogonal test method,select the orthogonal table that L9(34),considering FRP type,FRP strip clearance and anchoring method(layer number)three factors,the three levels,plus a non-reinforced RC beam,a total of 10 test pieces were fabricated for shear test.The shear reinforcement effect of RC beam is analyzed and compared,and the optimal shear reinforcement scheme is obtained.The results show that the three kinds of FRP can significantly improve the cracking load and shear capacity of RC beams.The improvement degree of AFRP and CFRP is basically the same,followed by BFRP.The strain distribution of each FRP strip is not uniform and the average strain is far less than that of FRP Ultimate strain;AFRP strip spacing of 120mm,oblique side of the band + bead method to strengthen the reinforcement effect is the best.Finally,the calculation method of the shear strength of three kinds of FRP reinforcement is improved.The improved calculation method is wider and the calculation result is closer to the test result.