Fatigue damage versus creep deformation – differentiation using the development of stiffness

Concrete structures, especially bridges, can exhibit deformations due to creep, induced by cyclic loading at serviceability limit state. These deformations can exceed the deformation when static creep is considered. For loading levels corresponding to serviceability stress-levels, structural damage in terms of micro-cracking is not supposed to occur. In contrast, in fatigue testing viscous deformations (without structural damage) are not supposed to happen. Therefore, measured deformations are usually regarded as fully damage-induced. A collaborative research project, supported by the German Research Foundation (DFG), was set up to experimentally determine the influence of superposition of viscous creep deformations in static creep and cyclic loading. Special attention was paid to the transition from linear to non-linear creep due cyclic/fatigue loading. Linear creep is regarded as a purely viscous deformation, non-linear creep as a superposition of viscous deformation and damage-induced deformation. Finally, deformations in fatigue loading are regarded as predominantly induced by structural damage respectively micro cracking. The overall aim was to test the hypothesis that development of stiffness during cyclic loading can be used to differentiate viscous deformations from deformations induced by micro-cracking. As long as only viscous deformations occur, stiffness is supposed to stay constant. Damage in terms of micro-cracking is supposed to be characterized by a gradually decreasing stiffness – as usually measured in fatigue loading. The experimental program covers different load-levels, loading-amplitudes, loading- frequencies, as well as different moisture contents of normal strength concrete. Conventional static creep-tests as well as cyclic loading tests were performed for direct comparison. Different loading- frequencies in cyclic loading enable both, an evaluation of damage-induced and viscous deformations, in terms of duration of loading as well as number of cycles. It is shown that loading at serviceability stress-levels up to about 45 % of the compressive strength show a stabilization or even increase of stiffness. However, stress-levels in non-linear creep and fatigue with load-levels exceeding 45 % of the compressive strength show a continuous degradation of stiffness. Furthermore, the loading frequency affects the development of stiffness. Finally, it can be concluded, that the development of stiffness is suitable to differentiate viscous deformations from deformations induced by structural damage/micro-cracking.