In the world of formwork, employees already rely on modern technology in many areas of their work. Formwork is planned digitally and formwork production is largely automated. This progress is now being extended to the composition of the formwork itself. Depending on the requirements, complex custom formwork can be manufactured much more efficiently using new technologies. Individual elements that are likely to be used only once for a very specific component are normally made of wood or plastic – they are milled, smoothed, sealed and then put together. This method also has the best cost-benefit ratio for the majority of formwork objects. However, there are also situations where other manufacturing processes would be a better choice: in collaboration with voxeljet AG, formwork specialist Doka has undertaken a pilot project to produce a 3D-printed formwork shell for an exposed concrete staircase with complex geometry. The result? Significant time savings and perfect concrete surfaces.
High standards for the staircase shape and surface
A half-turn staircase with intermediate landing, mounted to a load-bearing wall, was required in the shell of the headquarters of Sächsische Aufbaubank (SAB) in Leipzig, which were constructed by the Rohbau SAB Leipzig consortium – consisting of Ed. Züblin AG, Saxony division and GP Papenburg Hochbau GmbH, eastern branch. Like the rest of the new building, the staircase, which links the underground garage with the foyer, was architecturally demanding. It is 40% – up to the intermediate landing – sited against a wall, while the rest is self-supporting. The inner surface was also designed as exposed concrete. The necessary after-treatment was performed using sandblasting to create a surface largely free of holes and consistent in colour. The geometric and optical requirements meant that high standards had to be met by the specially designed formwork system. The contracting consortium called in Doka to take on this challenge.
Analysing the formwork design
Doka’s first step was to categorise the different surfaces based on their complexity: curved, single-axis surfaces in cylindrical or conical forms could be formed efficiently using conventional methods. However, one of the special features of the staircase was the curved, three-axis surface of the rounded overhang extending up to the side of the wall. Producing the necessary custom formwork using conventional methods would have been incredibly time-consuming, to say nothing of the challenges of installing it on site.
Digital model printed in 3D
Doka therefore opted for an innovative solution: in cooperation with voxeljet AG, a provider of 3D printing systems for industrial use, it used an additive manufacturing process to produce a special shell for the custom formwork. The starting point was a CAD model: Doka first created a highly detailed virtual design of the seven formwork shell parts. Based on these specifications, voxeljet used a powder binder jetting process to produce the formwork shell parts with a wall thickness of just 21 mm. This process involves applying a particle layer, in this case sand, in several extremely thin layers using resin as an adhesive in the appropriate places to form the desired component. The process is repeated several times until the entire component has been made. No supporting structure is needed: unbound material is removed after the process is complete. The 3D-printed components are produced without tension and only need to be infiltrated with epoxy resin and then sanded and lacquered.
90% reduction in assembly time
The 3D curved elements were designed to easily combine with Doka’s standard formwork (Xlife panels): holes and connectors were integrated into the elements. This was the basis for a very simple assembly plan using a plug-in system. As a result, the on-site assembly work was completed quickly and efficiently. The high level of prefabrication of the individual parts meant that the fitters took only a tenth of the time they would usually need. Assembling the formwork sets was also incredibly easy thanks to the high precision with which the components had been made based on the digital data.
The corners of the elements were a perfect fit and could be used as measuring corners. Digital data processing was used for all of the 3D-printed geometries. This resulted in high-precision manufacturing, which was also reflected in the results: the concrete elements were exactly as planned. Quality assurance posed no problem thanks to the closed, digital data chain.
Stable form to withstand wind and weather
The excellent weather- and scratch resistance of the finished epoxy and sand mix also contributed to the outstanding results. Before the concrete was poured, the finished formwork was stored on the construction site for a month, from March to April. If conventional custom formwork had been used, frost, damp conditions and warm weather could easily have led to cracks and surface defects. The new process ensured that no significant corrections or repairs were needed. Following the success of the concrete staircase, it is clear that there are many more potential applications for 3D-printed formwork shells.