Triaxial Experimental Research And Numerical Simulation Of Concrete Under Mixed Load Paths

Many large concrete structures are under complex stress state, such as concrete filled steel tube structure, high-rise buildings, hydroelectric dams, tunnel and underground projects,and unclear reactor pressure vessel. Multiaxial mechanical properties of concrete are different from uniaxial mechanical properties. There is an urgent need to perform multiaixal experiments to investigate the behaviors of concrete.Multiaxial experiments were carried out to investigate the mechanical characteristics of concrete, involving loading paths consisting of a stress path, a strain path and a mixed path of both stress and strain. In previous literatures there are many experiments under stress loading path. Due to limitations of research thought and test apparatus, triaxial experiments under strain loading path and mixed loading path are less.The paper investigates the behavior of concrete specimens (cubes, d=100mm)subjected to a mixed path of both stress and strain loading. The specimens were statically loaded into a true triaxial apparatus. During the first phase, the stresses in all three directions were simultaneously increased to a specified value of p. Then, the strain in Y-axis was increased monotonically while maintaining a constant minimum principle stress in Z-axis as well as a constant strain rate ratio between the X-axis and the Y-axis.Three different values of p were chosen, which were 10, 15 and 20MPa. In addition,three or four strain rate ratios were chosen, which were 0.25, 0.5, 0.75 and 1.0, for each value ofp. The following works are mainly conducted:(1) Based on the experimental datas of strength and volume, the characteristics of them were analyzed. The experimental results shows that compressive strengths of concrete increase with the minimum principal stress increasing, and decrease linearly with the increase of strain rate ratio, which are higher than uniaxial compressive strength, and the maximum value is 3.4 times to uniaxial compressive strength; X-axial stress, mean normal stress and mean shear stress were increased with the minimum principal stress and strain rate ratio increasing. By contrast, Y axial stress was decreased with the strain rate ratio increasing; hydrostatic pressure did not affect the initial shear modulus when the concrete specimen experienced only monotonic increase of hydrostatic pressure, and a coinciding part was found in curves of shear stress versus shear strain for different experimental groups; peak value of volumetric strain increases with the minimum principal stress as well as strain rate ratio increasing. There is dilatancy for concrete, volume decreases firstly, and then increases; peak strength and peak volumetric strain appear at the same time when the minimum princpal stress was 10MPa. The greater the minimum principal stress, the longer the time interval of their appearance, and the peak volumetric strain is later.(2) According to analysis of experimental data of three axial strains and strain curves, the following conclusions were obtained: in the latter part of loading process,relationship between z-axial strain and y-axial strain approximates to linear, as well as the relationship between z-axial strain and x-axial strain, but the corresponding slopes of fitting lines are different with each other under different load paths. Comparing differential equations with fitting equations of curves, when the minimum principal stress is constant, the same numerical differentiation of z-axial strain versus x-axial strain and z-axial strain versus x-axial strain are calculated for different strain rate ratios,which is related to failure modes and inclination of shear bands.(3) Failure modes and mechanism of two failures in one specimen, as well as characteristics of two peak point were mainly investigated, strength criterion is obtained.In the total loading process, the first and second peak stresses occur in the maximum principal strain axis (Y-axis) and the intermediate principal strain axis (X-axis),respectively.The maximum principal stress and the intermediate principal stress do not always exist in a fixed axis, which is related to strain rate ratios. Some damage formed due to previous load-histories including the peak stress of Y-axis results in a decreased carrying capacity in X-axis. Therefore, peak stress of the intermediate principal strain axis (X-axis) is lower than that of the maximum principal strain axis (Y-axis). Moreover,Y-axial strain corresponding to the first peak stress and X-axial strain corresponding to the second peak stress are essentially equal. There is a limit state exists in concrete, if the strain difference between y and z axes or x and z axes is large enough,carrying capacity values of Y axis and X axis are the same, which is related to minimum principal stress and independent of strain rate ratios.(4) Triaxial tests of concrete were simulated by means of large-scale commercial software of ANSYS. According to results of numerical simuliation, it can be seen that:modeling approach, failure criterion, yield criterion and constitutive relations adopted in ansys imitate triaxial tests well. So we can expand the anlysis and study of the other triaxial tests by changing the parameters in our procedure as presented herein.