Damage Processes in Ultra-High Performance Fiber-Reinforced Concrete Under Cyclic Tensile Loading

Application of ultra-high performance concrete (UHPC) offers great advantages for the construction of mass-optimized structural members with high load-bearing capacity and outstanding durability. To attain a resource-efficient design of such structural members, the brittle failure of UHPC should be avoided by adding considerable amounts of high-strength steel microfibers to the mixture.

While models exist for the evaluation of load-bearing behaviour of fibres in structural members made of ultra-high performance fibre reinforced concrete (UHPFRC) subjected bending, shear and torsion, the loadbearing behaviour of fibres under cyclic loads is not evaluated adequately. Investigations conducted on UHPFRC tensile specimens under cyclic tensile loading within the first funding period of the SPP2020 indicate that even after a high number load cycles with constant amplitude (single-stage tests), the fibres show still a considerable fatigue resistance.

However, the cycle degradation of UHPFRC under complex loading scenarios (multi-stage tests) has not been sufficiently investigated and understood yet. Therefore, the present research project aims to conduct fundamental experimental and numerical investigations on the damage mechanism and degradation process of UHPFRC under complex cyclic tensile loads, which are based on the investigations and findings of the first funding period.

On the one hand, continuous and systematic multi-stage cyclic tests are planned for the evaluation of sequence effects and damage accumulation on specimens with various geometries, including two demonstrators. The damage will be measured and documented using novel mechanical and optical measurement techniques like digital microscopy (DM), scanning electron microscopy (SEM), computer tomography (CT) and photogrammetry (GOM), as well as acoustic emission analysis (SEA). On the other hand, an efficient homogenization strategy will be developed and implemented based on the mesoscale bond model from the first funding period in order to describe the degradation behaviour of the composite material UHPFRC under cyclic tensile loads in a continuum-mechanical model.

The experimental results of the first and second funding period will be implemented for the validation of the numerical model. The aim is developing a model to predict the macroscopic response of structural members under monotonic and cyclic loading. The superior goal of the investigations is the combination of experimentally and numerically obtained results and models in an experimental-virtual-lab to enable a degradation prognosis of UHPFRC under cyclic tensile loading. To enable a prognosis for modern UHPFRC structural members subjected to tension and bending, the demonstrators will be simulated in the experimental-virtual-lab to validate its prognosis accuracy.