Hysteretic Behavior Of Ultra High Performance Concrete Members Under Combined Action Of Compression And Flexural

In order to clarify the influence of axial compression ratio and steel fiber content on the mechanical properties of Ultra-high Performance Concrete compression-flexural members under monotonic and cyclic reciprocating loads.Based on the national Natural Science Foundation of China "Research on seismic Performance and Design Method of active powder Concrete compression members"(Project Approval Number : 51878262),six UHPC compression-flexural members with different loading modes,axial compression ratio and steel fiber content were tested.Based on the summary of test data,a tensile and compressive hysteretic constitutive model of UHPC considering the influence of steel fibers was proposed.The model was embedded in Open Sees platform to conduct numerical calculation and parameter analysis on the mechanical properties of UHPC bending specimens in this paper.A calculation method of normal section bearing capacity of UHPC members under monotonic and cyclic load is proposed.The main research contents of the thesis are as follows:1.Six UHPC compression-flexural members with different loading modes,different coaxial pressure ratios and different steel fiber content were tested and studied.The results show that:(1)The load-displacement curve of specimens under cyclic and reciprocating loads is basically the same as that under monotonic loads.Under the two loading modes,the initial stiffness of specimens is basically the same.At the yield point,the load of the specimens under cyclic reciprocating load is obviously lower than that under monotonous load,which may be because the steel fiber buckling failure under reciprocating load weakens the bearing capacity of the specimens.With the further increase of displacement,the steel fiber gradually pulled out,and the bearing capacity deviation of specimens under different loading methods gradually decreased.(2)With the increase of axial compression ratio,the yield displacement of specimens increases,the peak load increases,the UHPC reaches the ultimate compressive strain earlier,the ultimate displacement decreases,and the ductility and energy dissipation capacity decrease significantly.When the steel fiber content is 2%,with the increase of axial compression ratio from 0 to 0.2,the cracking load of specimens under mono and cyclic reciprocating load increases 86.1% and 107.3%respectively,the peak load increases 61.9% and 68.7% respectively,and the ductility coefficient decreases 47.4% and 36.4% respectively.The cumulative energy consumption of specimens under reciprocating load decreased by 31.0%.(3)The bridging effect of steel fiber on the matrix limits the development of cracks,improves the phenomenon of concrete crushing and spalling.When the axial compression ratio is 0.2,with the increase of steel fiber content from 0% to 2%,the cracking load of specimens under monotonic and cyclic reciprocating load is basically the same,the peak load is increased by 35.8% and 28.4%,the ductility is increased by50.0% and 47.4%,and the cumulative energy consumption of specimens under reciprocating load is increased by 177.0%.2.Based on the results of monotonic and repeated axial compression and axial tension tests of UHPC in this paper and other literatures,a UHPC tensile and compressive hysteresis constitutive model is established.The results show that:(1)Based on the monotone compression constitutive model,the loading and unloading path of UHPC under compression is proposed,and the repeated compression constitutive model of UHPC considering the influence of steel fiber is established.The calculation results of the model are basically consistent with the repeated compression test data.(2)The formulas for calculating the tensile strain,peak strain and limit strain and the characteristic parameters of steel fiber were established,the tensile loading and unloading paths of UHPC were proposed,and the repeated tensile constitutive models of strain hardening and strain softening UHPC were established.(3)The tension and compression hysteresis rule was introduced to connect the UHPC constitutive model under repeated compression and tension,and a tension and compression hysteresis constitutive model was proposed to simulate the mechanical properties of UHPC members under cyclic reciprocating load.3.Nonlinear analysis of seismic performance of UHPC compression-flexural members shows that:(1)The UHPC tension-compression hysteretic constitutive model constructed in this paper can better simulate the mechanical properties of UHPC compression-flexural members with different coaxial compression ratios and different steel fiber content under monotonic and cyclic reciprocating loads,and can be applied to the nonlinear seismic analysis of UHPC structures.(2)The parameters of axial compression ratio,steel fiber content and longitudinal reinforcement ratio were analyzed by using the verified UHPC material model.The results show that when the designed axial compression ratio is between0.1 and 0.8,the increase of axial compression ratio significantly increases the cracking load and peak load of members,but the increase of peak load gradually slows down and reduces the ductility of members.When the content of steel fiber is0～3%,the increase of the content of steel fiber can significantly improve the peak load and ductility of components,which mainly improves the mechanical properties of components by improving the tensile properties of UHPC materials.When the ratio of longitudinal reinforcement is between 0.9% and 3.7%,the increase of longitudinal reinforcement ratio has little influence on the initial stiffness and cracking point of the specimen.(3)The calculation method of normal section bearing capacity of UHPC members under monotonic and reciprocating loads is proposed.Because the buckling failure of steel fibers under cyclic loading has weakened the bearing capacity of specimens,the calculation method of normal section bearing capacity under mono load is still not safe.By introducing the UHPC tensile strength reduction factor into consideration,a calculation method of normal section bearing capacity of UHPC members under monotonic and reciprocating loads is proposed.The calculated results are in good agreement with the test results.