Influence of load-induced temperature fields on the fatigue behaviour of UHPC subjected to high frequency compression loading

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Newest experiments regarding the fatigue behaviour of fine-grained (ultra-)high-strength concrete (UHPC) under cyclic loading show that induced temperature fields due to high frequency compression loading have a non-negligible influence on the damage process and the fatigue strength of UHPC. This is because UHPC has a significantly denser material composition compared to normal concrete, which causes greater internal friction with corresponding heating in the concrete microstructure under cyclic loading. Beside the concrete composition, the load frequency and regime (minimal and maximal stress) and specimen's geometry are recognised so far as decisive factors for the resultant heat. Systematic experimental and numerical studies on the impact of load-induced temperature fields on the degradation of the concrete material by fatigue loading, particularly for UHPC, are still pending.Today, the research on more accurate material models for concrete is not finished due to the complex mechanical behaviour of cohesive friction material. There are no material models that can represent load-induced temperature stresses and take account of their impact on the damage process and the fatigue strength. For realistic consideration of various mechanical phenomena, the Microplane concept of Bazant, which is based on a thermodynamic approach, has the greatest potential. Therefore, this concept is used as a basis for the development of a new material model to describe the fatigue behaviour of UHPC subjected to high frequency compression loading.The central objective of this research project is to experimentally identify the effects of load-induced temperature fields on the fatigue behaviour of UHPC under high frequency loading and to model these influences by using a new material model based on thermodynamics. Firstly, experimental tests on concrete specimens under static load are conducted in order to generate the basic parameters, like characteristic strains and strengths including multiaxial behaviour and thermal conductivity, for simulation of the basic structural behaviour of UHPC. During numerous cyclic tests, the effects of cyclic loading on strains, cracks, concrete microstructure and resulting temperatures in concrete samples are recorded. Here, concrete composition and age, sample size, load frequency and regime as well as the testing machine are varied. Through analysing the development of damage on the basis of test results, corresponding model parameters are derived and implemented in a new material model to describe the fatigue behaviour of UHPC under high frequency loading. The experimental studies will be complemented by numerical simulations with extensive parametric studies. Research results are the determination of the importance of different factors on the fatigue strength of UHPC under high frequency loading, a newly developed material model to measure these factors and an associated material module, which is implemented in standard software.