Temperature- and moisture-induced damage processes due to cyclic multiaxial compressive fatigue loading

In the present research proposal scientists from materials science (Materials Testing Institute University of Stuttgart) and continuum mechanics (Institute of Applied Mechanics - Chair of Continuum Mechanics) combine forces to contribute to a better understanding of the thermal and moisture induced deterioration processes due to multiaxial cyclic compression of high performance concrete.

As a basic working hypothesis, it is assumed that multi-axial stress states in the concrete structure have a significant influence on processes of macroscopic crack formation. On the other hand, fatigue-related damage processes are significantly influenced by sample conditioning, i.e. by moisture content and temperature. According to the current state of research, fatigue damage occurs on a microscopic level in the cement paste structure. The emphasis of this research proposal is to expand the investigations done in the first funding phase on damage processes at uni-axial stress to multi-axial stress conditions.

Within the working group materials science, a high-pressure triaxial cell is being developed to perform cyclic pressure threshold tests for cylindrical specimens with diameters up to 60 mm and stress states up to 100 MPa. This allows the systematic investigation of temperature- and moisture-induced fatigue damage processes. Following the cyclic multi-axial loads, complex load sequences will also be superimposed later. The characterization of the damage processes is carried out with the aid of BET, Hg porosimetry and XRCT. In addition, imaging methods such as light microscopy and REM are used across scales. It is expected that the knowledge gained will allow a model to be derived to describe damage phenomena on the basis of global damage indicators. With this model it should then be possible for the first time to make qualitative predictions about the development of degradation.

In the working group continuum mechanics twin samples are characterized under static triaxial preloads for a wide frequency range in the tangential space under cyclic loading. In order to test the temperature influence, dry and wet samples in the triaxial cell are loaded with a temperature-controlled fluid via confining medium. In parallel, high-resolution scans of damaged samples are characterized in the open XRCT scanner. Based on these scans, the hydro-mechanical interaction of macroscopic single cracks on the damaged concrete matrix is numerically investigated. Using a highly efficient hybrid FEM, pore resolved simulations will be performed in extension of the numerical analyses undertaken in the first application phase.

The general aim of the project is to better understand the findings of the damage processes gained in the first funding phase and to further generalise derived models for damage initiation and crack development.