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The project RIBB3D challenges the conventional design of floor slabs and allows to reinforced concrete structures in a material-​efficient manner. By using robotic 3D printing to create customised plastic formwork for an uncommon two-​way ribbed floor slab, the new approach uses 40 percent less concrete than a conventional solid slab, with no compromise on performance.

Reinforced concrete floor slabs typically account for more than half of the carbon footprint of a multi-​storey building. Most often, concrete floor slabs are built as solid slabs, not because of their structural efficiency, but rather because of the inexpensive planar formwork that can be used to cast them. The new RIBB3D solution succeeds in overcoming these limitations and uses digital fabrication processes to efficiently produce non-​standard, optimised geometries.

Design and fabrication informed by flow of forces
RIBB3D combines structural engineering with digital fabrication tools to develop a workflow to design ribbed floor slabs based on the alignment of floor ribs with the internal force flow. First, the researchers calculated the directions of the principal bending moments using finite element analysis (FEA) software. Thereafter, they imported the resulting vectors into a parametric design software, where the rib layout was generated. Based on the parametric model, the researchers performed structural analysis and ran optimisations loops to minimise material consumption

This workflow was then used to design a multi-​bay slab supported by columns on a grid of 8 x 8 m, of which a section measuring 2.7 x 2.7 m was fabricated. An industrial robot with a thermoplastic pellet extruder 3D-​printed the formwork of the full-​scale slab prototype. The structure was reinforced with conventional steel reinforcing bars and cast with standard self-​compacting concrete.

Less material, same performance
The interdisciplinary research team combined the novel formwork concept with established technologies for concrete and reinforcement to ensure compliance with building code regulations. The structural test to failure of the full-​scale section of the slab revealed that the optimised design could prevent brittle punching failures of the column-​slab connection, reaching an 80 percent higher ultimate load than a reference solid flat slab.

The results show that an optimised 2-​way ribbed slab topology reduces the use of concrete by around 40 percent with respect to a conventional flat slab, while maintaining the same standards of structural safety, robustness and comfort, using conventional concrete. Combined with low-​carbon concrete, the carbon footprint of the concrete can thus be reduced by roughly two thirds. This highlights the potential of using 3D printed formwork to produce material-​efficient floor slabs in an automated process, paving the way towards sustainable concrete construction.”

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