Interdisciplinary building materials
research using multiscale approaches

Prof. Dr. rer. nat. Bernhard Middendorf took over the “Department of Construction Materials and Building Chemistry” within the Faculty of Civil and Environmental Engineering at the University Kassel from Prof. Dr.‑Ing. habil. Michael Schmidt on 01.10.2012.

High-performance concretes with multifunctional properties had been developed and successfully used for the fabrication of individual components and the building of complete structures in the department during the preceding years. These materials make it possible to produce these in a significantly more slender, more durable and more environmentally friendly form, but at more rational cost than comparable items made of conventional concrete, particularly when their entire life-cycle is taken into account. The department, which is an integrated part of the IKI Institute of Structural Engineering, pursues a holistic approach in the optimisation of such high-performance materials. Within its work team, engineers, natural scientists and materials scientists jointly research into innovative inorganic mineral materials up to the stage of their practical testing in structures. ZKG INTERNATIONAL spoke to Prof. Middendorf on the current and future orientation of the department.


ZKG: Professor Middendorf, what were your reasons for applying for the professorship at Kassel?
Prof. Middendorf: The University of Kassel has developed extremely positively during the past ten years, in the field of basic and applied research; interdisciplinary networking and cooperation between the faculties are exemplary. In addition, the university also enjoys optimum national and international links and networks, and it has an interdisciplinary workgroup for the co-development of important future technologies and the promotion of efficient technology transfer, in the form of the “Center for Interdisciplinary Nanostructure Science and Technology” (CINSaT), a focus for nanostructure-science activities enjoying broad support from the faculties of Mathematics and Natural Sciences, Electrical Engineering and Computer Science, Mechanical Engineering, Civil and Environmental Engineering, and Philosophy. An official civil-engineering materials testing institute (AMPA) which, on the one hand, rationally augments the range of activities and, on the other hand, maintains important contacts and interchange with the construction industry, is also attached to the Faculty of Civil and Environmental Engineering.

Thanks to my predecessor, Prof. Schmidt, and his former senior lecturer – and current professor at the Tech-nical University (TU) of Berlin – Prof. Dietmar Stephan, the department has also gained an excellent reputation in the building materials sciences and has established extensive experimental facilities which will provide my team and myself with numerous opportunities for extremely exciting research. In addition to the facilities for the microstructural analysis of building materials, I am also able to make use of the faculty’s mechanical testing equipment (at AMPA and the IKI), which further extends our capabilities. It is not only the experimental facilities which are scientifically stimulating, however – my department is remarkable, in particular, for its highly qualified and motivated interdisciplinary staff, a team which is also notable for its high level of independent initiative. If you take these potentials as a whole, you will very quickly realise that my new department most certainly provides a firm basis for the future pursuit and co-development of building materials research at the very highest level.


ZKG: What are the current focuses of research at Kassel?
Prof. Middendorf: Prof. Schmidt and Prof. Fehling (Department of Structural Concrete) concentrated on the setting-up of the field of research into ultra-high-performance concretes (UHPC). The German Research Foundation Priority Programme, “Sustainable building using ultra-high-performance concrete” (SPP 1182), inter alia, was initiated and coordinated at the University of Kassel. Other publicly supported projects for the practical use of UHPC are currently also underway. In this context, we should, perhaps, also mention the potentials for the use of UHPC as a whitetopping material, in low-noise road surfaces, in paving stones and in wind farm structures.


ZKG: To what extent are you continuing your predecessor’s initiatives, and where do you intend to set new emphases?
Prof. Middendorf: The UHPC building material I’ve mentioned is closely associated with the “Department of Construction Materials and Building Chemistry”, because there has been intensive – and successful – research into UHPC technology for more than ten years at the University of Kassel. It is now intended to further intensify the introduction of UHPC into the building industry, i.e., the use of this material in construction projects in which its specific properties and benefits can be utilised. My team and I are in an advantageous situation thanks to Prof. Schmidt who, in his final years of active work, successfully acquired research projects on the subject of UHPC, some of which will run to the end of 2014. Work for these projects will continue and will doubtless generate new questions which will need to be answered, in other research projects, in the future.

Unlike my predecessor, I come from a natural sciences background, so I also have a more fundamental research orientation. One ever-recurring element in my research is “microstructure”, which influences the essential properties of building materials – you only need to think of mechanical properties, and the durability aspect.

The two main emphases of my research activities, “Development and application of building materials with systematic properties” and the “Interaction of additives and admixtures with inorganic mineral binders” are also closely linked to the quantitative description of microstructure. And, in addition to the development, production, optimisation, testing and application of UHPC which we just mentioned, thanks to my scientific work at the Technical University of Dortmund, I am also able to build on my many years of experience in the field of high-strength air-curing foamed concretes. Since no substantiated constitutive laws for foamed concretes have as yet been scientifically elaborated, I will, in the future, be working, jointly with Prof. Dr.-Ing. Just, of the EBZ European Training Centre for Property and Housing Management in Bochum, and with other specialist colleagues, on projects for the optimisation of minerally bound foams and on the multiscale modelling of the behaviour of porous building materials.

In the future, I’ll be using microscopic and calorimetric methods to investigate the influence of various organic and inorganic admixtures as inhibitors on crystallisation processes, and on those of gypsum crystals, in particular, in greater depth. We’ll use this nanoscopic approach to research setting process reactions, and their influence on the size, character and habit of the resultant gypsum crystals, and thus on the porosity of the structure.

Structure also influences significant characteristics variables of calcium silicate bricks, and not only brick strength, but also, and in particular, bulk density, which is extremely relevant in structural noise suppression. We are currently studying the modification of C-S-H phase formation when seed crystals are added, in the context of a research project. In another project, my team and I are researching into the influence of humic substances on structure formation in calcium silicate bricks.

In the past, the focus has been almost exclusively on strength, stability and durability; nowadays, one has also to take the low-energy production, compatibility, environmental impact and recyclability of building materials into account. Due to increasing social awareness, sustainability, in terms of resource-conserving building materials and methods, is increasingly becoming an important element in all construction projects and in my department, we are currently intensively working on developments in the field of self-compacting filling materials and other projects. So our aim must be to reduce the primary energy input, environmentally relevant emissions, and the production costs of binders, without sacrificing quality and versatility of application.

Another important part of my future activities will be the durability and preservation of buildings. Buildings and other structures are high-value assets that require correct and careful maintenance and repair. Even nowadays, they continue to be “repaired” using incompatible building materials, which can result in high consequential costs, on the one hand, and loss of building fabric, on the other hand, something particularly tragic in the case of culturally and historically important buildings. There is, therefore, a great need for thoroughly scientifically founded, practically orientated support for the technical and economic optimisation of building repair products based on mineral building materials. We are currently working on a number of projects for the characterisation of masonry decay processes in historic structures, on salt crystallisation and on the development and testing of building materials for repair purposes. We are also active in drafting repair strategies for historic properties, and are, in addition, closely linked to the Institute for Stone Conservation (IFS), in Mainz.

ZKG: How will this be methodologically supported, and what preconditions need to be created to permit successful research in these fields?
Prof. Middendorf: In my new department, we can perform a large range of tests and analyses for the characterisation of the pore volume and the building material matrix even now. I have extensive equipment at my disposal, but I do also intend to obtain a high-resolution computer tomograph (µ-CT) for three-dimensional mapping of microstructures. This instrument will be used cooperatively and on a supra-disciplinary basis with other workgroups, which will, in addition, also serve to promote interdiscip-linary interchange and thus the creativity of the users. The results obtained will be incorporated into the development of material models, which can then also be studied using computer simulations and compared against experimental results. The prime aim here at all times will be the use of results for the development of even more efficient building materials with even better performance.


ZKG: And: your vision of the building materials of the future? Where are we going?
Prof. Middendorf: In future, energy and labour will account, to a significantly greater extent than even now, for a major portion of building costs. The building materials of the future must, therefore, be produced with less energy input, and these optimised building materials must also be used, wherever possible, around the world. Still greater internationalisation of research will be necessary to achieve this.

We must, also, make even greater efforts to find potentials for using building shells for energy recovery.

It will also be necessary to study materials recycling still more intensively in the future. In my opinion, far too little attention is focused on building materials recycling even now, despite all the analyses of the energy efficiency of structures. Many building materials, and multicomposite building materials, in particular, can only be landfill dumped when their useful life has ended, because they cannot be separated into individual materials. Mono-material systems harmonised with each other could, on the contrary, offer the same performance, but combined with significantly better recyclability. It is, for example, conceivable that building shells could be constructed of thin load-bearing UHPC “skins” which would then be enclosed in the TU Dortmund’s cement-bound foamed concrete, ensuring extremely efficient thermal insulation. The visible surface of the component could consist of a sealing and decorative cement rendering. ­After demolition, the result would be a recyclable building mono-material that can be kept in the materials cycle without complication.

Roof structures for single-family and multi-family residential buildings would be conceivable using such multifunctional concretes, for example. One benefit would be the fact that the geometrical and design versatility of concrete as a building material would make it possible to integrate photovoltaics systems directly into the concrete roofing elements. It goes without saying that this form of modular building would also need to permit aesthetic variations to gain social acceptance.

Finally: The primary aim of all building material development in future will be the conservation of energy and resources – this is a clear and unmistakable trend.


ZKG: Thanks for the interview

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