Understanding and Modeling the Effect of Steel-fibres on Degradation of High-Performance Concrete subjected to Fatigue

Systematic investigations of the deterioration of high- (HPC) and ultra-high-performance concrete (UHPC) subjected to fatigue loading are currently not available. This holds especially true for steel-fiber modified HPC and UHPC subjected to multi-level and sequence loadings. Furthermore, numerical approaches and models are often calibrated using experimental data from literature, which does not allow a comprehensive calibration due to lacking coordination between experiments and numerical models.

Therefore, this project aims at testing, describing and modelling the ongoing deterioration including damage accumulation of steel-fiber modified HPC and UHPC subjected to fatigue loading, using multiscale approaches and phase-field theory. In addition to single-stage fatigue loads and the fatigue loads include multi-stage collectives. Two high-performance concretes (reference concretes of the SPP2020) are used for the investigations. A high-strength concrete with hooked-end steel fibres with fibre contents of 23 kg/m³ to 115 kg/m³ and an ultra-high-strength concrete with smooth, high-strength short steel fibres and fibre contents of 57 kg/m³ and 115 kg/m³ are used. For the experimental and numerical analysis of the damage evolution, extensive static fibre pull-out tests as well as static and cyclic bending tensile tests and cyclic compression tests were carried out in the first funding period and the degradation was described based on damage indicators. These indicators include crack opening development, residual stiffness development and energy dissipated into damage. The close collaboration between experiment and numerical simulation enabled calibration and validation of the models and material descriptions.

In the second funding period, the experiments will be supplemented by high cyclic multi-stage loading sequences, taking into account sequence effects under flexural and compressive fatigue loading. For the in silico tests, the already existing simulation models are further developed with regard to efficiency, especially in the area of high cyclic loads. For this purpose, cycle jump approaches are developed, calibrated and validated on the basis of specific tests to model and predict degradation at high cycle loadings. Based on the procedure in the planned work program, the interaction of experiments and modelling is evaluated and optimized in the sense of an experimental virtual lab. With the help of this lab, it will be possible in future to numerically evaluate the fatigue behaviour of high-performance concretes in based on a few tests.