Holes in the conrete

At first glance, the cylinders on Prof. Holger Steeb’s desk, each of which is about the same thickness as a thumb, resemble knick-knacks, holiday souvenirs perhaps. However, as the holder of the Chair of Continuum Mechanics at the University of Stuttgart’s Institute of Applied Mechanics (Civil Engineering) (MIB) explains, they are, in fact, important objects for his research, being sandstone drill core rock samples, lithographic limestone and high-performance concrete. They appear to be extremely solid and impermeable, but, as Steeb makes clear: “Each of these materials is porous and permeable to fluids. The only question is over what period of time?” And that brings him to the core focus of his research.

Since his appointment in Stuttgart in October 2015, Steeb has been building up the Porous Media Lab, which rather than being a classical laboratory to which samples are sent for analysis, functions as a partner for numerous research institutions in Germany and abroad. The laboratory is a central component of the University of Stuttgart’s SimTech Cluster of Excellence and the Collaborative Research Center “Interface-Driven Multi-Field Processes in Porous Media – Flow, Transport and Deformation” (SFB 1313). As Steeb explains: “we are working on creating links between experiments and models”.

One typical question of such fundamental research is what physical processes take place when rock is penetrated by fluids. Steeb’s group carries out experiments, but the knowledge gained also flows into simulations, which, for example, calculate the flow behavior of fluids in rock. This research is applied in connection with drilling operations for the exploitation of geothermal energy (the Mine water Project), in the development of intelligent road surfaces or water- and gas-proof construction materials. For example, asphalt and concrete are prone to cracking under the influence of water and other mechanical loads during their lifetime. To prevent this, researchers want to discover which processes take place at the so-called pore scale, which in the case of rocks, are areas with a typical length of 10 to 50 millionths of a meter.

Steeb summarizes the scientific challenge as follows: “It’s not enough just to look inside porous media.” It is vital to understand what happens inside the pores under mechanical influences. Researchers in the fields of medicine and the geosciences are using ultrasound or seismic experiments to generate data of increasingly high resolution. But more detailed knowledge about porous media is required to interpret this with more precision and this is what the Porous Media Lab wants to provide.

Steeb and his 15 colleagues are using X-ray tomography and other imaging techniques to look into the structure of pores and generate computational models of the porous materials as a basis for simulations. “Making it visible is a prerequisite for gaining any understanding of a material whatsoever”, explains Steeb. This understanding leads the researchers to multi-scale mathematical models and eventually to a wide range of simulation applications.

Text: Jens Eber