Research on tri-axial compressive test of concrete under quasi plain-strain condition

Using proportional loading method which is similar to that under plane-strain condition, four true tri-axial loading tests were performed on concrete cubes with dimension of 100×100×100mm. The specific method went as follows: in the first stage, under the condition of keeping the three axial main stresses equivalent, compressive stress was increased to 10Mpa, which was maintained for a period of time. In the second stage, strain in y axis was increased monotonously while main strain in z axis as well as stress rate ratio of y and x axis remained constant respectively. Selected stress rate ratios contained four categories: 1:0.05, 1:0.1, 1:-0.05 and 1:-0.1. The test indicates that, stress relationship between y and z axis appears to be curves. For different stress rate ratios, the transcending branches of these curves coincide with each other but differ near their peak points. Here presents the strain relationship between y and x axis: during the second phase of loading, strain in x axis decreases monotonously with the increase of strain in y axis. When strain in x axis is more than -0.02, the relationship develops in curvature; when it is less than -0.02, the relationship tends to be linear, for which, the straight parts parallel to each other in different stress rate ratios. Moreover, it is found that it is conditional whether the concrete volume expansion occurs before the peak point. Compared with previous researches, in this paper, the test implements the accurate control of strain and stress for the transcending also descending part.