Creep of early age concrete under variable stress

This contribution aims at describing the creep behaviour of early age concrete with particular focus on the hardening process and the stress history. The influence of the hardening process on basic creep behaviour is investigated through tensile creep tests at different loading ages under constant stress. Influences of preloading are analysed through creep tests with several different stress histories. Creep tests with a stepped stress change are performed to check the applicability of superposition principle. Tests with slow and continuous stress application are targeted at describing the influence of loading velocity. The modelling of the viscoelastic behaviour is done using a rheological approach in terms of a generalized Maxwell model with aging spring stiffnesses and dashpot viscosities. The results show a good correlation for all investigated stress histories. This implies that the viscoelastic behaviour of early age concrete in tension can be described by theory of linear viscoelasticity.