Engineers in Carnegie Mellon University’s Computational Engineering and Robotics Laboratory (CERLAB) have developed a simulation tool that predicts how sprayed concrete behaves and solidifies, even around rebar. The breakthrough could transform how buildings are constructed, cutting material waste and enabling stronger, more complex structures for tomorrow’s cities.
Concrete 3D printing reduces both time and cost by eliminating traditional formwork – the temporary mould for casting. Yet most of today’s systems rely on extrusion-based methods, which deposit material very close to a nozzle layer by layer. This makes it impossible to print around reinforcement bars (rebars) without risk of collision, limiting both design flexibility and structural integrity of builds.
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Kenji Shimada, a professor of mechanical engineering, and researchers in his laboratoryare breaking through that limitation with their new simulation tool. ‘Spray-based concrete 3D printing is a new process with complicated physical phenomena,’ he said. ‘In this method, a modified shotcrete mixture is sprayed from a nozzle to build up on a surface, even around rebar.’
The ability to print freely around reinforcement is especially important in places such as Japan and California, where earthquakes are an imminent threat and structural strength is critical.
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‘To make this technology viable, we must be able to predict exactly how the concrete will spray and dry into the final shape,’ Shimada explained. ‘That’s why we developed a simulator for concrete spray 3D printing.’
The new simulator can model the viscoelastic behaviours of shotcrete mixtures, including drip, particle rebound, spread and solidification time. This way, contractors can assess multiple printing paths based on a CAD design with the simulator to evaluate whether spray 3D printing is a feasible fabrication technique for their structure.
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The team travelled to Tokyo, where Shimizu Corporation already operates spray 3D printing robots, to validate their model. In the first test, the team focused on the simulator’s ability to predict shape based on the speed of the nozzle’s movement. With 90.75 per cent accuracy, the simulator could predict the height of the sprayed concrete. The second test showed that the simulator could predict printing over rebar with 92.3 per cent and 97.9 per cent accuracy for width and thickness, respectively.
According to Soji Yamakawa, a research scientist in Shimada’s lab, a simulation of this kind would typically take hours, if not days, to run. ‘By making wild assumptions, we were able to successfully simplify a super-complex physics simulation into a combination of efficient algorithms and data structures and still achieved highly realistic output,’ he said.
Future work will aim to increase accuracy by identifying environmental parameters such as humidity, optimise performance and add plastering simulation to create smoother finished products.
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‘There are still so many applications and technologies that we can develop with robotics,’ said Kyshalee Vazquez-Santiago, a mechanical engineering PhD candidate leading the Mobile Manipulators research group within CERLAB. ‘Even in concrete 3D printing, we are working with an entirely new type of application and approach that has so many advantages but leaves so much room for further development.’
The research has been published in IEEE Robotics and Automation Letters.