Research On The Dynamic Response And Failure Mechanism Of FRP Strengthened RC Member Under Lateral Impact Load

With the development of China's urbanization process,the upsurge of infrastructure construction has been rising,and reinforced concrete(RC)structure is one of the most commonly used structural forms.These RC structures may face a variety of impact problems during their life cycle,such as impact loads from derailment trains,ships that deviate from the course or the bomb explosion,thus losing the bearing capacity and causing serious safety accidents.In recent years,the use of fiber reinforced polymer(FRP)to enhance the impact resistance of existing structural members has become a research hotspot.In this study,the effect of FRP reinforcement on the dynamic response and failure mode of circular RC members(FRPRC members)under lateral impact loads is studied.The main contents and results are shown as follows:(1)The impact test on the RC and the FRPRC members with unequal span(the impact point at the span of 2/9)is carried out,and the typical failure modes and the mechanical properties of the members are obtained.The test results show that,under the impact,the short span of the RC member turn to be shear failure mode dominated by diagonal cracks,and the RC member strengthed with FRP is shown as bending failure with vertical cracks longitudinal bars rupture.Compared with the RC member,the impact peak value of the FRPRC member changes little,but the impact platform value increases,and the duration of the platform segment is shorter.(2)The constitutive models of concrete,steel bar and FRP considering strain rate effect and fracture is introduced,and a finite element model is established.The results of the finite element analysis are compared with experimental results in terms of the failure mode,the impact force time curve and the deflection time history curve,and the correctness of the parameters of the finite element model is verified.(3)Using finite element analysis,the failure mechanism of RC and FRPRC members is compared and analyzed.The analysis includes the distribution of bending moment and shear force along the length of members,the changing pattern of bending moment and shear at the cross section of the impact point.The analysis results show that in the process of impact,the shear of the critical section of the RC member(the cross section of the impact point)descends immediately after reaching the peak,loses most of the bearing capacity,and the cross section can not form the effective bending moment;in the whole member range,the main strain of the steel bar is at the stirrup.The flexural and shear strength of the RC member strengthened by FRP increased,and the bending moment and shear force of the FRPRC member remained at a high level after reaching the peak value,then began to decline until the rupture of longitudinal bar.And the maximum strain of the steel bar changed from stirrup to the longitudinal reinforcement,which resulted in the growth of the rate of the longitudinal reinforcement strain.The above patterns further prove that after strengthening FRP,the failure mode of RC member is changed from shear failure to bending failure.(4)The influence of different parameters on the working mechanism of RC and FRPRC members under impact was studied.The parameters included impact velocity,reinforcement ratio,FRP package range,number of FRP layers and concrete strength.The results show that:(a)The change of the impact velocity and the longitudinal reinforcement ratio will not change the failure mechanism of the two members.The RC members are all shear failure and the FRPRC member is bending failure.However,with the increase of the impact velocity and the longitudinal reinforcement ratio,the impact of the FRPRC member can be effectively improved and the deflection is reduced.(b)For the current study,when the FRP wrapping area is about 0.5 times the total span and the number of wrapping layers is 1,it can already reach the ideal strengthening effect.On this basis,further wrapping area and wrapping layers have little effect on the working mechanism.(c)For RC members,the higher grade of strength grade of concrete is,the smaller the damage of members is,resulting in the smaller deflection,but the effect of this parameter on FRPRC members is not obvious.