Effect of temperature gradient on transient thermal creep of heated and stressed concrete in transient state tests

The stress-strain constitutive relation of concrete subjected to thermomechanical loads is extremely complicated since its deformation relies on the sequences in which the heat and loads are applied. Existing experiments demonstrated that the development of quasi-instantaneous transient thermal creep, generally referred to as TTC, was dependent on the virgin heating of a mechanically pre-loaded concrete specimen. However, the mechanism behind it is still not fully understood. In this paper, a numerical method is developed to analyze the stress fluctuation and corresponding plastic strain caused by the combination of pre-load and temperature gradient. The results show that the TTC calculated using existing semi-implicit methods is reasonable only at early stage but could be wrong at high temperatures. The sharp increase of TTC beyond around 500 degrees C observed in many tests could be attributed to the implicit inclusion of the extra plastic strain resulted from temperature gradient. By excluding the extra plastic strain generated at different temperature levels, a more accurate formula for calculating TTC is proposed. (C) 2019 Elsevier Ltd. All rights reserved.