Experimental investigations of concrete at a very high number of load cycles as basis for modeling of concrete fatigue taking into account viscous and damage-induced strains

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The service life of civil engineering structures is dominated apart from static actions more and more by cyclic actions which can cause fatigue failure. Concrete is the most used building material in civil and building engineering and therefore of considerable importance. Well-founded knowledge concerning the number of load cycles endurable by concrete under cyclic compression loading is currently limited to the so called Low Cycle-Fatigue (LCF) range up to N = 10^5 load cycles as well as the so called High-Cycle-Fatigue (HCF) range up to N = 10^7 load cycles. Fatigue scenarios with a number of load cycles N > 10^7 are classified as belonging to the Very-High-Cycle-Fatigue (VHCF) range, for this fatigue range currently no significant research work exists.The research project includes a detailed and fundamental examination of fatigue behaviour for very high numbers of load cycles under consideration of an additive strain model approach. Elastic, viscous, damage-induced and thermal strain components are quantified on the basis of cyclic load tests in the VHCF range in combination with specifically coordinated creep and shrinkage tests as well as temperature measurements. The strain components are considered taking into account interaction effects, as the recorded specimen heating in the VHCF tests are explicitly taken into account as the external ambient temperature in the HCF and creep and shrinkage tests. By using a high-frequency testing technique, fatigue tests are obtained in a temporally condensed manner so that load cycle numbers N > 10^7 can be realised. A detailed investigation of the fatigue behaviour of concrete is carried out at three different stress levels with test frequencies from f = 1 Hz to f = 150 Hz with a significant number of test results. To connect the test results to fatigue processes in concrete subjected to low-frequency stress, specific investigations in the HCF range are supplemented. Possible influences of scaling effects are recorded by the parallel investigation of two different specimen sizes. For the first time extensive tests make it possible to obtain basic knowledge in the VHCF range of concrete and to realise the connection between test results from time-related, high-frequency and low-frequency fatigue tests. This is the basis for understanding the fatigue behaviour of concrete on the basis of the respective strain components in the VHCF range, for anchoring it in a modelling based on test engineering and for using it for design in the future.