Steinbeis experts help develop additive manufacturing technology for complex precast concrete elements
Producing precast concrete elements often involves the use of multiple-component formwork. This is due to the geometry of elements, and it can often lead to problems during demolding and thus damage to components. It is also important to consider environmental and economic factors. It is against this background that the Steinbeis Innovation Center FiberCrete is working with WASA Compound to develop a production technology for the sustainable manufacture of flexible and recyclable formwork elements. The joint project was initiated as part of a research project funded by the German Federal Ministry for Economic Affairs and Climate Action under the Central Innovation Program for SMEs (ZIM).
Creating hollow cavities requires formwork cores that structure the internal shapes of precast concrete elements and are as easy as possible to demold. Due to the shrinkage behavior of concrete during setting, it has a tendency to contract and set on the inside of formworks. This can lead to problems during demolding and damage not only to components but also to the mold itself. A further drawback with the multiple-part nature of formwork, which is dictated by the geometry of parts, is the need to integrate draft angles. In addition, conventional designs – based on methods involving the use of mold materials like wood, glass fiber-reinforced plastic (GFRP), and other rigid materials – do not offer the possibility to include undercuts in precast concrete elements. As a result, it is vital to reconsider conventional designs – such as carving ecologically questionable polystyrene – and find replacements for new and expensive adaptive formwork concepts.
Thanks to a molding system developed by the Steinbeis experts and their project partners using a recyclable material, it should now be possible to produce precast concrete parts in complex shapes based on methods that not only save resources, but also minimize costs. For this project, which runs from October 2023 to April 2025, the expert team is developing processing technology that will allow flexible and recyclable formwork elements to be produced sustainably. The aim is to create molding models using extrusion and the machining of low-clinker cement, initially in a fresh state before being hardened. Once milled into its final shape, these models will make it possible to mold a variety of casting resins and still make efficient use of resources. “The model can then be recycled and returned to the material cycle without negative impact on the rheological or mechanical parameters of the concrete. The key to success when it comes to the efficient and sustainable production of precast concrete elements is the resulting material cycle,” explains Steinbeis Entrepreneur Sandra Gelbrich. Combining this method with conventional formwork elements makes it possible to produce complex concrete elements that are easy to demold, without draft angles or localized undercuts.
Material development
For the research project, a low-clinker fine concrete was developed based on CEM III/A cement, mineral powder with a grain size ranging from 0 to 200 μm, additives, and water with the aim of creating a material that is as inexpensive as possible and can be produced using additive manufacturing techniques and machined. The new concrete was developed through continuous experimentation into a five-constituent solution, taking into account parameters such as extrudability, required extrusion pressures, post-extrusion optical properties both before and after curing, adhesion of the individual additive layers, stability during the additive process, shrinkage, and cracking. The concrete was then characterized in both mechanical and rheological terms by using, among others, four-point flexural testing, compressive strength testing, and shrinkage tests.
Technological development
The near-net shape model is produced additively by extruding concrete with a KUKA robot. A second KUKA robot is used for post-processing using milling or brushing tools while the concrete is still fresh. Employing the two robots in coordination, the milling program can be matched directly to the extruded form. The milling and brushing processes were characterized in detail, including potential processing windows. Ideally, milling should be carried out within two and a half to six hours, and brushing should be completed one or two hours after extrusion of the concrete. The surfaces are finished when hardened using a five-axis CNC milling machine, and this should happen no sooner than 24 hours after extrusion of the concrete. “The resulting smooth formwork surfaces are ideal for molding with casting resin. The model can then be recycled with a turbo rotor and reused in the extrusion material as a substitute for mineral powder,” concludes Steinbeis expert Marvin Abstoss.
Putting the technology through its paces
To apply the technology, a concrete planter was designed with no draft angles on the inside. Adding an undercut makes it possible to include an LED strip to provide indirect illumination. The aim is for the component to combine new technology with conventional formwork elements. The outer elements can be produced along conventional lines using GFRP, while the core is made using the near-net shape extrusion technology that was developed for the research project with subsequent milling when fresh and hardened. For manufacturing reasons, the model for molding the core with a casting resin was divided into four sections.
Further research will involve extruding the model, milling while fresh, and milling the part into its final shape in a hardened state. The model will then be molded with a casting resin. A wooden core will be fixed inside the molded element and can be removed from the casting resin during the first stage of demolding.
The thin-walled, flexible formwork element can be demolded without complication so that no draft angles are required and the undercut can be produced. Basic preliminary testing has already been conducted and successful production of a simplified concrete planter without draft angles has been completed.
Contact
Marvin Abstoß (author)
Assistant
Steinbeis Innovation Center FiberCrete (FC) (Chemnitz)
www.fibercrete.de
Prof. Dr.-Ing. habil. Sandra Gelbrich (author)
Steinbeis Entrepreneur
Steinbeis Innovation Center FiberCrete (FC) (Chemnitz)
www.fibercrete.de