A Study On The Constitutive Model Of Concrete Using ATENA And The Compression Mechanism Of Constrained Concrete

This thesis studies the concrete constitutive model suitable for describing the compressive performance under multiaxial constraints.The constitutive model mainly includes a three-parameter failure surface function in the form of a quadratic function and a plastic potential function in the form of a polynomial.Since the tests of high-strength stirrup reinforced concrete are carried out less frequently,the parameters of the constitutive model are calibrated using test data of the FRP-reinforced concrete that is also passively constrained.The main parameters of the failure surface function are studied later in detail,including the average uniaxial compressive strength(fc),the average uniaxial tensile strength(ft),the out-of-roundness eccentricity parameter(e),the hardening function k(?)and the softening function c(?)in the failure surface.Based on the calibrated concrete constitutive model,modeling and starting analysis of a point stress state of high-strength concrete combined with high-strength stirrups using ATENA-GID finite element software.It is found that:(1)For the parameters in the failure surface function,only fc is a free parameter,and the remaining parameters can be expressed by the concrete uniaxial compressive strength fc.The existing experimental data are used to perform a regression analysis on the above parameters,and thus an expression for each parameter with respect to fc,which reduces the number of parameters in the loading surface function,and simplifies the form of the loading surface function.(2)For the plastic potential function,the original is of a more complex polynomial form with multiple parameters.Combined with the special force form of triaxial constrained concrete,the form is simplified,and finally plastic potential function in form of two parameters A and B are obtained.Therefore,combined with the FRP confined concrete test data of different concrete strength levels,the parameter values in the simplified form are fitted,analyzed,and finally the parameter values under different concrete strength levels are obtained.(3)The stress-strain curve of the constrained model is studied,focusing on the analysis of the influence of the stirrup strength,stirrup spacing,stirrup diameter,volume stirrup ratio,effective binding force on the compressive performance of the constrained concrete.The results are as follows: high strength stirrup restraint of high-strength concrete can effectively prevent the abrupt decrease of the stress-strain curve of concrete;the restraining force increases as the spacing of spiral stirrups decreases,the diameter increases,the yield strength increases,and the volumetric ratios increases,the effectiveness of stirrups on concrete,and the peak strength of the restrained concrete gradually increases,and the falling section of the stress-strain curve is gentler,then improving the deformation performance of the concrete;with the same value of effective binding,specimens with higher volumetric ratios of ordinary tie yield strength have better performance than those with lower volumetric ratios of high-grade tie yield strength;among the above influencing factors,the effect of volumetric ratios on the strength and deformation performance of the constrained concrete is significantly greater than other factors.