Study On Long-term Behavior Of RC Beams Strengthened With Externally Bonded CFRP

The application of fiber reinforced composites as external reinforcement materials in concrete structure reinforcement is more and more extensive.In general,the reinforcement component consists of three materials:the original concrete,reinforced FRP materials and interface bonding materials.Due to the different material properties and mechanical properties of the constituent materials,the long-term performance under sustained load is also different,resulting the variation of mechanical properties in reinforced structures.Whether the reinforcement design based on the short-term mechanical properties of the material can ensure the effectiveness during the subsequent service is still a problem that needs further study.In this paper,the viscoelastic characteristics of CFRP-concrete interface under sustained load are studied based on the double shear test,and the viscoelastic constitutive model is established.Then the analysis model for long-term mechanical performance of CFRP strengthened beam is established based on the theoretical analysis,and the validity of the model is verified by comparing with the experimental results.The main contents of this paper are as follows:1.Firstly,the creep of concrete,interface layer and externally bonded FRP materials as well as the analysis theory of the reinforced components in related research overseas and domestic were summarized and analyzed,meanwhile,the problem in existing research is pointed out,furthermore,the objects and contents of this paper were established.2.A long-term double shear test method including pouring template,positioning fixture and loading device was designed based on the summary of existing double shear test device and method,this method can guarantee the pure shear stress state of the interface during loading and keep the stability of the load during the whole period of experiment.Based on this method,24 sets of double shear tests with different load levels and different loading times were developed.The results showed that:after subjected to sustained loading,the interface slipso correspond to the peak stress ?nax in the process of debonding decreases with the holding time,and has no significant correlation with the time of loading.with the sustained load level is lower than 0.5Pmax,the peak stress of the interface does not changed obviously,but when the load level increased to 0.5Pmax and above,the interface will be damaged with the increase of the sustained loading time,so that the peak stress at the interface decreases,and the bond-slip relationship of the interface in this region is closer to bilinear,beyound this region,the bond-slip relationship has not changed.Finally,the bond slip constitutive model of the interf'ace under sustained load was established based on the experimental results.3.According to the viscoelastic theory,the modified generalized Maxwell model which contains five components was used to establish the viscoelastic constitutive model of the CFRP-concrete interface,and the physical meaning and expression of each parameter in Maxwell model were given subsequently.Based on one sets of the experimental data,the parameters in the constitutive model are fitted,then the model is used to analyze the remaining specimens.The comparison between theoretical results and experimental results shows that the model can simulate the viscoelastic characteristics of the interface very well.Meanwhile,the effects of concrete creep and interfacial creep on the long-term mechanical properties of CFRP-concrete interface were studied respectively,the results showed that the effect of concrete creep on the long-term mechanical properties of the interface is small in the high shear stress region,but it's a major factor which influences the long-term mechanical properties of the interface in the low stress region away from the loading end.4.The mechanical tests of three reinforced beams and one ordinary reinforced concrete beams under sustained load were carried out using the long-term loading device which is designed by the author self.The results showed that the externally bonded CFRP has a certain inhibitory effect on the creep deformation of the beam,which can improve the long-term rigidity of the beam,the compression strain of concrete,the tension and compression strain in steel bars,and the CFRP strain increases with time,and the growth rate is grate in the first 30 days of loading but tends to be gentle in the later period.5.An incremental equilibrium equation of the strengthened beam is conducted via the variational forms of virtual work principle,in order to consider the viscoelasticity of each constituent material,the differential constitutive relation of concrete,adhesive layer and external CFRP was established based on the generalized Maxwell model.The ACI Committee-209 code was used for simulating the creep character of concrete and the creep of interface layer is analyzed using the analysis in Chapter 3.The deformability of the adhesive layer in shear and through its thickness were also accounts for,then the analytical model of reinforced beam under sustained load is established.On the basis of this model,a smeared cracking model and a constitutive model of concrete based on tension-softening is adopted for the modification and enhancement of this model to consider the effect of concrete cracking on the long-term mechanical properties of strengthened beam.The analysis results showed that the higher interface shear stress and normal stress in the local area of CFRP end are the main factors that lead to the edge debonding failure of the strengthened beam.By considering the effect of concrete cracking,the analysis results of the enhanced model of the cracking area in mid-span are closer to the experimental results,and there is no significant difference between the two models for the edge region far away from the cracked area.