Hysteresis Performance And Restoring Force Model Of Reinforced ECC Columns

ECC material has the advantages of superior crack control ability,high ductility,wear resistance and good damage tolerance,and it can also improve the defects of concrete easy to peel off when it is damaged.In recent years,in order to protect the ecological environment,the state has issued relevant documents,so that the output of concrete raw materials in China has declined sharply.Therefore,it is very important to replace concrete with new ECC materials.Up to now,a lot of researches have been carried out on the properties of ECC materials and the mechanical properties of ECC members in compression,bending,shear,pure torsion and compression bending at home and abroad.Meanwhile,excellent results have been achieved,which make up the blank of research in ECC related fields in China.However,there are few researches on ECC columns under the combined action of compression,bending,shear and torsion,and the relevant stress mechanism and failure mode have not been revealed.To a certain extent,it limits the development prospect of the application of reinforced ECC components or structures in design and engineering.Based on the mix ratio test,the research group designed and poured 11 ECC columns and 1 concrete column with the same parameters.The stress mechanism and failure characteristics of ECC composite columns are studied by monotonic and low cycle cyclic loading tests.Based on the low cycle cyclic loading test,this paper studies the influence of shear span ratio,stirrup spacing,fiber volume ratio and matrix material on the seismic performance of composite columns.In this paper,the test column consists of 5 reinforced ECC columns and 1 ordinary concrete column,and the rest parameters are studied by members of the same group.Based on the experimental results of ECC columns,it is found that the hysteresis curves of torque T-twist rate θ of the 5 specimens are all in inverse s shape,and the pinch phenomenon is obvious.At the initial stage of loading,the hysteretic curve shows a linear rise,the loading and unloading stiffness are basically the same,and the residual deformation after unloading is small;after cracking,the hysteretic curve of reinforced ECC column does not show obvious inflection point,and each specimen shows obvious inflection point at the next level of displacement angle amplitude after cracking;when the same level of displacement angle amplitude after yielding,the research finds that the same level of displacement angle amplitude after three cycles Under cyclic loading with one displacement amplitude,the last circle is obviously smaller than the torque bearing capacity of the first circle,and its unloading stiffness has little difference.The skeleton curve of all specimens has the obvious "three fold line" feature,and its ECC column skeleton curve can be divided into three stages: elastic stage,strengthening stage and strength degradation stage.According to the above skeleton features,this paper establishes the calculation skeleton curve by solving the eigenvalue through the method of combining experiment and theory.Based on the regression analysis of test data,the influence of cyclic loading on the performance degradation of ECC composite columns is determined,and the main factors are considered,and the formulas of degradation rate of unloading stiffness and strength are proposed,and the hysteretic rules are determined.Based on ABAQUS,the finite element analysis model of ECC composite torsional column is established,and the simulation analysis of ECC column is carried out.The simulation results of hysteretic curve of ECC column are consistent with the test results and the stress nephogram with the failure characteristics of each stage of the test.It is feasible to use ABAQUS to simulate the seismic behavior of ECC composite columns.Finally,the influences of axial compression ratio,shear span ratio,torsion bending ratio,stirrup spacing,steel cap thickness and loading beam height on the seismic performance of ECC composite columns are studied.The analysis of the design parameters shows that the parameters axial compression ratio,shear span ratio,torsion bending ratio and steel cap thickness have great influence on the cracking torque,peak torsion rate,peak torque and the degradation of stiffness and strength of ECC composite columns.