Researchers at the department of Structural Engineering, Ghent University, have made a bridge at lab scale with the smallest possible amount of concrete. This project was 3D printed in collaboration with Vertico, a Netherlands-based 3D concrete printing firm that made a mobile 3D printer. The shape of the bridge was optimized and combined with an optimal trajectory of the steel cable at the bottom of the bridge to minimize stress peaks. The team reduced the materials used in the construction of the bridge by 60 percent through the concrete 3D printing process. For more information see the IDTechEx report on 3D Printing Materials 2019-2029: Technology and Market Analysis.
Concrete bridges have been printed before. There was the bicycle bridge in Gemert (The Netherlands) in 2017. There, the prismatic concrete elements were printed separately, tilted after hardening and finally pulled together by means of steel cables. This technique is common in traditional bridge construction but then the prismatic elements are typically casted in formwork instead of printed. This laborious first step is no longer necessary.
One of the major advantages of 3D printing is the freedom to create all kinds of smooth shapes. This is esthetically refreshing, but it also allows engineers to place concrete solely where it is needed the most. In this way, you reduce the material consumption and the construction time which both provide an ecological and economical advantage. The researchers started with a 3D model of the bridge and had algorithms search for the shape of the bridge that required the smallest amount of concrete. This appears not to be the well-known rectangular beam shape but a rather organic structure instead. Eventually, the topologically optimized model was sent to the concrete printer.
During printing of the bridge, the researchers provided a cavity in the bottom of the bridge to pull a cable through afterwards. After hardening of the bridge, this cable was gently tensioned to make sure that the weight of the bridge and future loads wouldn't cause additional tensional stress in the concrete, which it can't sustain very well. This technique also reduces the theoretically required amount of concrete to provide sufficient strength. Thanks to 3D printing, this material saving can now be actually employed.
The printing of a concrete bridge at lab scale fits the research ambition of the department of Structural Engineering because it allows to study different sides of the printing technology. The department is fully aware that this technique has real potential to break through and therefore deploys several researchers to cope with the many challenges specific to this technology. One of them is trying to combine the appropriate fluid behavior with the required mechanical properties of the hardened element.
"This bridge showcases the range of possibilities that 3D concrete printing offers," explained Volker Ruitinga, founder of Vertico. 3D concrete printing can provide the construction industry with automation and innovation and the 3D printed footbridge demonstrates the possibilities of the technology on a relevant and significant scale.
Source and top image: University of Ghent
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