New insights into creep and creep recovery of hardened cement paste at micro scale

Indentation technique has been a popular way to measure the micro creep of cementitious materials. However, several aspects concerning the measured indentation creep (e.g., recoverable or irrecoverable, the underlying mechanisms) are still unclear. This study investigated the micro creep and creep recovery of mature cement paste by indentation technique. Continuous stiffness measurement tests were first carried out to determine the indentation force required to measure the homogeneous properties of cement paste. Then the micro creep and creep recovery of cement paste were measured via microindentation tests that contain a holding stage at the maximum indentation force and a reholding stage incorporated into the unloading stage. It is found that both the recoverable creep and irrecoverable creep can be measured by the microindentation tests. The micro recoverable creep of cement paste that mainly evolves over the loading stage is about 30-40% of the total creep, and the logarithm of the micro creep recovery rate decreases bilinearly with respect to time, which are consistent with the observed creep recovery at macro scale. The long-term creep rate of cement paste appears independent of loading duration, holding duration, indentation force amplitude, and unloading duration of the microindentation tests, suggesting it is an intrinsic material parameter. The identification of recoverable and irrecoverable creep at micro scale can contribute to a better understanding of the creep mechanisms of cementitious materials. (C) 2020 Elsevier Ltd. All rights reserved.