Creep at early ages of ultrahigh-strength concrete: experiment and modelling

Ultrahigh-strength concrete (UHSC) is a promising material for engineering applications, owing to the rapid increase of compressive strength at early ages. In this study, the creep behaviour of UHSC with an ultralow water/binder ratio (= 0·16) under loading at early ages was investigated. The influence factors of loading age (1, 2, 3 and 7 d), normal coarse aggregate and steel fibre content (0, 1 and 2%) were taken into consideration. Experiment results showed that the addition of coarse aggregates and ultrafine steel fibres were obviously effective in decreasing the creep deformation of UHSC but that the specific creep of UHSC with 2% steel fibres was higher than that of UHSC with 1% steel fibres. Specimens loaded at earlier ages had higher creep. At a microscopic level, it could be seen that the creep of UHSC was influenced by hydration products and concrete internal defects. From a macroscopic perspective, it could be seen that this was the result of a combination of strength and elastic modulus development factors. By introducing a coordination factor for the development of strength and elastic modulus, a modified creep model was proposed to predict the creep of UHSC.