Time dependent strain development of early age concrete under step-by-step load history

• Strain developments of concrete under step-by-step load history were studied.
• The validity of superposition principle in calculating creep strain was discussed.
• Strain development is substantially affected by the loading age and stress amplitude.
• Axial shortening is overestimated with larger stress amplitude and shorter loading history.

With the increasing height of high-rise buildings, adverse effects caused by differential axial shortening become significant. Most of the calculation methods that have been developed to analyze the axial shortening of high-rise buildings are based on a common assumption that creep is linearly proportional to stress and conforms to the superposition principle. However, this assumption is rarely verified when it is used to describe the time-dependent deformation development of early age concrete in construction process. This paper presents an experimental study to examine the validity of this assumption in this use. A step-by-step load was employed to approximately simulate the load history of axial components experienced in the construction process and the strain developments were monitored. A comparative analysis between test results and numerical simulations shows that, the strain development of early age concrete under a step-by-step load is substantially affected by the loading age and stress amplitude, which verifies that the assumption, on which previous methods are based, may lead to inaccurate prediction of strain development in axially loaded concrete components. The time-dependent strain is overestimated with the larger step-by-step stress amplitude and shorter loading history.