Research On Shear Behavior Of RC T-section Beams Strengthened With NSM FRP Bars Considering Effect Of Secondary Loading

In recent years,due to the excellent mechanical properties and mature production process of fiber reinforced plastic(FRP),it has become the new material used in the field of civil engineering structural reinforcement.Compared with externally bonded FRP reinforcement,the advantages of near surface mounted FRP reinforcement are construction convenient and strong corrosion resistance.NSM FRP reinforcement has become the focus of scholars and practical engineers at domestic and foreign.At the present stage,the study of the reinforced concrete structures with NSM FRP reinforcement are mainly focused on flexural and shear behavior.A large of number of research has been done on RC beams strengthened by direct NSM FRP reinforcement,but the impact of secondary loading was not considered.In this paper,the shear behavior of T-beams strengthened with NSM FRP reinforcement under the influence of secondary loading is studied.It is expected to provide reference value for the development and application of NSM FRP reinforcement technology.In this paper,nine T-beams were used to study the shear behavior of RC beams reinforced with NSM FRP reinforcement based on the parameters of preload,unloading degree,the type of FRP bars and the anchorage lengths.The failure forms and stress process of the test beams,and the development of inclined crack in the experimental beams during the loading process were discussed.The load-displacement relationship curves and load-strain relationship curves of concretes,stirrups,longitudinal steels and FRP bars were compared and analyzed under the different parameters.Test results show that compared to the contrast beams,the shear strength of the strengthened beam is significantly enhanced when the inclined crack appears,the diagonal crack propagated and the beam failed.All the test beams eventually appeared shear failure,but the failure characteristics of specimens were not same.With the increase of initial load,the failure characteristics were changed from the concrete crush of compression zone into the splitting of concrete cover,and the ultimate bearing capacity tends to decrease;With the increase of unloading degree,the ultimate bearing capacity and span displacement obviously increase,but the width of inclined crack decreases.It can be seen that the unloading reinforcement of secondary loading test beams has contribute to restriction on inclined crack propagation;The ultimate bearing capacity of the test beam strengthened with NSM CFRP reinforcement is greater than that of the test beam strengthened with NSM BFRP reinforcement,reflecting the high tensile strength of CFRP tendons.Compared with the strengthened beam without anchorage,the ultimate bearing capacity and deformation capacity of the strengthened beam with FRP end-anchored treatment are both increased and the failure characteristics of the specimen were transformed from crushing of the flange concrete in the compression zone into crushing of concrete at the supports.In the process of the test,the stirrups and FRP bars in the shear span were subjected to two stress redistributions respectively during the cracking of the diagonal crack and yielding of the stirrups.The experimental beams were simulated by the finite element program ABAQUS,and the simulated results were compared with the experimental data,which has verified the rationality of finite element simulation.And then the parameters of FRP embedded angle,spacing,anchoring form and shear span ratio were deeply analyzed in the finite element program ABAQUS.Through the finite element analysis of RC beam reinforced with FRP,the effect of each parameter on the shear capacity of the RC beams can be researched.The simulation results show that the embedded angle,spacing,anchoring form and shear span ratio are all important factors affecting the bearing capacity of the reinforced beams.The bearing capacity of the beams increases with the decrease of the embedded angle,spacing and shear span ratio.At the same time,the embedded angle and shear span ratio are the important factors to change the failure form of the specimens.The bearing capacity and ductility of the beams can be improved by anchoring reinforcement.The anchoring form of U-shaped FRP bars provides the best reinforcement for specimens.Based on the experimental study and finite element analysis,the shear strength of RC beams strengthened with FRP under secondary load was calculated based on the truss-arch model and the shear strength of RC beams strengthened with embedded FRP bars.The results show that the theoretical values agree well with the experimental values.