Steinbeis experts at FiberCrete develop fiberglass- reinforced construction concrete
For a number of years, there has been an increasing tendency to erect buildings with curtain wall facades – exterior walls covered by elements mounted on the outside. This building method requires thin and light free-form facade components. In view of minimum concrete strength requirements, classic reinforced concrete is not an option. Concrete that is strengthened without steel, such as short fiber- and textile-reinforced concrete, offers plenty of potential, however. These new types of concrete are typically made with non-corrosive materials. They are not subject to traditional minimum concrete strength requirements, making it possible to construct intricate concrete structures that are extremely lightweight. An alternative way to strengthen facades is to use short fibers, textile matting, or rods of alkali-resistant glass or carbon. FiberCrete, the Steinbeis Innovation Center based in Chemnitz, has been working on the development, testing, and production of new fiber- and textile-reinforced construction materials and compounds, as well as novel production technologies. As part of a network project, the Steinbeis experts have developed a fiber-reinforced construction concrete called BetoLamina®-Cast, along with the technology to produce thin-walled facade components from the same material. Their technology is now being used for the first time at the Kaiser Hof development in Cologne.
Kaiser Wilhelm Street in Cologne has a rich history and it will shortly be home to a new and exclusive office property, the Kaiser Hof, designed by Meyer Schmitz-Morkramer Rhein. One highlight of the new building is its stylish facade. Measuring approx. 5,000 square meters, it will be made from fiber-reinforced construction concrete. The bright outer facade contains vertical elements at different angles, so-called pilaster strips, to cast unexpected shadows that change continuously depending on the angle of light. Construction concrete is subject to particular requirements. It has particularly flat surfaces which not only adhere to the highest standards for exposed materials, they also appear matte to the eye and offer strong weather resistance.
The facade was developed on behalf of Art-Invest Real Estate Management as part of a close partnership between three companies, who joined forces to form the “Kaiser Hof Cologne Fiber Concrete Facade Company Alliance Working Group” and add a special front to the building. The FiberCrete Steinbeis Innovation Center in Chemnitz was responsible for the concrete formula, also helping with the approval process and technology advice. FIBER-TECH Products from Chemnitz planned and carried out production of the fiber-concrete components, including mold-making and delivery. Medicke Metallic was responsible for the complete planning process for the facade and assembly of the fiberglass concrete parts.
Developing the materials and the technology for the project involved a variety of complex factors, many of which overlapped, from assembling the adapted short fiber concrete to molding the concrete, mounting anchors for attaching the facade to the building, reshaping and reprocessing facade components, and final installation on the building. One of the main priorities during development was mapping the overall sequence for the process, beginning with the mixing of the formula required for the fiberglass concrete to logistical considerations. To develop the material, the experts used a modified white fiberglass concrete called BetoLamina®-Cast, which was also developed at the FiberCrete Steinbeis Innovation Center. Not only does this material offer outstanding mechanical properties, it is also amazingly durable and has excellent visual attributes as an exposed concrete. It is based on a 5-substance system consisting of Portland cement, aggregates, additives, and admixtures developed specially for the project. Because of the complex geometry of the pilaster strips, the freshly mixed concrete has to flow readily, but at the same time remain uniform in terms of material consistency. To achieve this, an admixture was developed specially for the project as a private label product in close collaboration with the building chemicals firm MC-Bauchemie. There were no high-performance or high-speed mixers available at the precast concrete works for mixing the formula, as would normally be the case. As a result, the BetoLamina®- Cast concrete had to be modified for mixing in a simple pugmill mixer. This also ensured that the fresh concrete remained 100% homogeneous.
Currently, there are no certified systems for mounting these kinds of thin-walled concrete components, which are only between 14 and 30 millimeters thick. The project team therefore had to develop a suitable mounting solution itself. The idea was to create a system using inserts, which could be integrated into the mold before casting and thus ensure components are positioned exactly as required. The inserts were designed using bionic principles to allow uniform forces to be applied to specific points on the thin-walled concrete facade components. During development, the experts designed inserts of different sizes and shapes, calculated their properties, and simulated performance using finite element methods. The parts were then 3D-printed (selective laser sintering) and examined using pull-out testing before the calculated results were validated. Particular attention was given to the effect the insert design had on connections between the bearing load of metallic elements and the textile-reinforced concrete, as well as cracking and failure properties. For the pull-out testing, special test specimens with thicker material around the cast inserts were subjected to unidirectional tensile loads at angles of 0°, 45° and 90°.
In total, the facade cladding on the Kaiser Hof building consists of 1,619 three-dimensional concrete elements and 137 different types of plates. In terms of molding technology, the concrete casting tools were designed with the surface requirements of facade elements in mind. The concept behind the molds also had to take the required quantity of facade elements and expected service life into account. Depending on how often individual concrete elements would be needed, there were two material options for making the molds. Facade elements required in larger quantities were produced with fiberglass-reinforced plastic, because this makes it possible to deliver a useful life of 150 molding processes or more. For elements required in smaller quantities, the molds were made from wood for reasons of economy. To finish, store, and safely transport components to the construction site, the concrete elements were mounted on specially developed racks. The parts were then mounted by facade professionals on the construction site. Completion of the development is scheduled for October 2018.
PD Dr.-Ing. habil. Sandra Gelbrich, Henrik L. Funke, Andreas Ehrlich
Steinbeis Innovation Center FiberCrete (Chemnitz)