Experiments on concrete cubes under non-proportional loading in multiaxial compression

A series of tests of plain concrete cubes was carried out at the cubical cell at the University of Colorado in Boulder. Nine different loading paths (which contained some replications in parts of the loading path) were chosen with the aim of assessing the effects of prior loading to high stress levels on subsequent behaviour and strength. Most of the specimens were loaded under plane strain conditions with a ratio of the two principal stresses of either 1:9 or 1:2. Some contained unloading cycles, some were loaded and unloaded in the third direction at a certain stage of the loading path. Most of the specimens failed at the final equi-biaxial stress path. Conclusions - preliminary due to a small number of samples - are: 1. Severe preloaded concrete exhibits directional behaviour which requires anisotropic formulations for its representation. 2. For piecewise-proportional loading and unloading, the stress-strain relation in each principal direction seems to be unique and independent of the load sequence. 3. The strength of concrete in plane stress seems unaffected by previous load history.