The creep of concrete under the cyclic uniaxial compression

The thesis describes an investigation into the creep deformation of plain concrete subjected to a cyclic compressive loading. Under such loading considerably increased creep occurs, especially in the early stages, over that observed for a static stress equal to the mean of the cyclicstress. The increase is greater the greater the amplitude for a given mean stress, and greater the greater the mean stress for a given amplitude. The creep-stress-time relationship can be expressed in terms of a power function expression for early time intervals and a logarithmic expression for later stages, if a linear creep-stress relationship is assumed. The latter assumption is reasonable for amplitudes of less than 0.4 of ultimate as long as the maximum stress is less than 0.55* The increased creep in this linear range is explained in terms of the repeated stress reversals in the absorbed water layers between gel particles causing a breakdown in the structure of the layers and hence increased mobility of the gel particles. Activation energy calculations indicate that movement of gel particles rather than water movement is responsible for the creep process. At stresses outside the linear range there is a large increase in creep which is shown to be due to load oriented microcracking. Though causing increased creep, a cyclic stress below the fatigue limit has a beneficial effect on strength and modulus, the tendency being for concrete to achieve internal structural stability more rapidly than under a static stress.