1. Chair of Algebra and Number Theory, RWTH Aachen University, 52062 Aachen, Germany
2. Chair of Algebra and Representation Theory, RWTH Aachen University, 52062 Aachen, Germany
3. Institute of Concrete Structures, Technische Universität Dresden, 01069 Dresden, Germany
4. Institue of Structural Concrete, RWTH Aachen University, 52062 Aachen, Germany