Fiber-reinforced concrete architectural panels can be used in a variety of formats in lightweight construction to provide thin walls and answer the most demanding design requirements in terms of finish and surface quality. Indeed such prefabricated parts are practically predestined to add an aesthetic touch to facades. Detailed research has been underway for years, looking into different ways to combine new fiber-based reinforcement materials with conventional concrete construction materials and thus create fiber-reinforced concrete (FRC) and textile-reinforced concrete (TRC). As members of a project consortium, the Chemnitz University of Technology, Hentschke Bau GmbH, Fiber-Tech Products GmbH and the Saxony textile research institute Stfi have recently developed a new textile-reinforced facade system. Reference construction projects will be central to transferring their developments into the world of business. The Chemnitz-based Steinbeis Innovation Center FiberCrete (FC) is providing the companies with scientific and application support.
The Steinbeis experts were called in to work on the renovation of a high-rise building in Frankfurt called the Poseidon House. The building needed a more energy-efficient facade. To redesign the appearance of the 3D facade, the experts replaced aluminum panels with a highstrength, fiber-reinforced architectural concrete with a high-quality appearance. The white facade measures around 13,800 square meters and has around 11,500 surface-mounted panels. The renovation project was planned by the architects Schneider and Schumacher from Frankfurt in collaboration with Josef Gartner GmbH. Arge Hentsche Bau GmbH and Fiber-Tech Products GmbH were commissioned with the production of the facade panels. The Steinbeis Innovation Center Fiber- Crete looked after scientific aspects including formal applications for planning permission.
The facade panels used for the renovation project are made from fine-grain concrete, which was modified to include alkaline-resistant short glass fibers. This was to reduce sediment forming in the fresh concrete and shrinkage cracks in the hardened concrete. The mixing technique used to produce the modified concrete had a key influence on the homogeneity of the fresh concrete and thus also an impact on the properties of the hardened concrete. The morphological properties of glass fiber can complicate the production of homogenous fresh concrete. To avoid so-called spike irregularities in the mixing process, a high-intensity mixer was used to significantly reduce the cutting force applied to the fresh concrete during mixing. To make it easier to mold each section, the architectural concrete was also made more runny. The visible surfaces of the facade panels, each measuring up to 5 meters in length, are pore-free and contain no color blemishes or marbling. The concrete panels can withstand pressures of over 100 MPa and have a 3-point flexural strength of around 20 MPa. Durability was also confirmed and validated.
Production of the formwork panels was designed around the number of sections needed to make the facade and how long the molds would be needed for. There were 240 different types of sections needed to make the concrete panels in the facade. The molds for facade panels required in higher volumes were produced with glass-fiber reinforced plastic (GRP). An important factor with the design of these molds was the texture of the formwork face, since this had a pivotal influence on the properties of the surface of the architectural concrete. The experts selected a non-absorbent face based on a kind of gelcoat used in GRP molds.
Because of the special shape of the facade panels and their thickness (only between 13 and 25 millimeters), it was not possible to find a conventional mounting system with appropriate building approval. Instead, new mounts had to be developed. Mounting points were positioned on each facade panel using anchors with internal threads (type M8). These internally threaded anchoring points have a sleeve with a continuous thread, a pressured washer, and a sealing cap to prevent runny concrete entering into the insert during molding. To design the high volume of mounts needed for the facade panels, pull-off testing was carried out on test panels of different sizes in the laboratory. Depending on the length of each panel, four or six mounts were needed. The upper mounts bear the main weight of each panel and also have to withstand the influence of wind. The middle and lower mounts only help counteract wind suction.
The fiber-reinforced concrete developed for the project did not entirely meet DIN standards. The method for mounting the finished concrete panels – with embedded anchoring devices featuring an internal thread – also did not fall under DIN standards. As a result, project partners had to apply for usability permits under building regulations, providing not just individual approval for the fine-grain concrete but also for the mounting system. This was made possible by the close collaboration between Hentschke Bau GmbH, Fiber-Tech Products GmbH and FiberCrete, the Steinbeis Innovation Center in Chemnitz.
This example of research with a direct relevance to business application demonstrated that the fiber-reinforced architectural concrete that was developed combines features of strength, surface quality, durability and recyclability. The increasing use of innovative, fiber-reinforced, inorganic, non-metallic, high-performance materials makes it possible to create thin-walled, single- and double-curve free-formed surfaces for use in construction – with particularly practical benefits to light construction. This paves the way for architects and planners to explore new potential designs, especially on “organically” formed buildings. The light construction materials used in such areas will become increasingly important in the future, not just for design reasons, but also because of the potential to save resources.
Dr.-Ing. Sandra Gelbrich, Henrik Funke, Andreas Ehrlich
Steinbeis Innovation Center FiberCrete (FC) (Chemnitz)
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