Words by Tony Whitehead
Better understanding of the way that concrete flows and sets — its rheology — will not only affect how we build, but what we build, according to Franz-Josef Ulm at MIT. “There is rapid progress in this field at the moment,” he says. “As we learn how to block the receptors of the cement until the exact time we want it to set, so we can use this expertise to build in different ways. For example, we do not see many curves in buildings these days because of the labour costs involved. But mastering rheology allows greater automation, and this could mean the return of shell structures, curves and cathedral-like arches as designers are freed from square thinking. I believe it will have profound implications for how we shape our built environment.”
At ETH Zurich in Switzerland, researchers are looking at the dynamic casting of complex concrete elements — a digital fabrication process where wet concrete is fed through a movable form, attached to a six-axis robotic arm. This allows self-supporting concrete elements to be extruded like Play-Doh. Unlike traditional slipforming, used to construct the cores of tall buildings, dynamic casting works on a smaller scale and produces more varied shapes.
Postdoctoral researcher Ena Lloret-Fritschi explains that the process uses a type of self-compacting concrete that has been heavily retarded, to which a chemical admixture, or accelerator, is added just before it enters the formwork. “Smart dynamic casting [SDC] relies on close control of the concrete’s state of hydration,” she says. “This is accurately monitored, and the information used to automatically adjust the slipping speed — the rate at which the concrete exits the moving form. In that moment, it is similar to wet clay, just capable of sustaining its own weight and the weight of the material on top. It then hardens almost immediately.”
The process has produced some startling results. Using a robot-controlled, revolving rectangular form, the team produced a twisting column with a corkscrew-like appearance. Columns can also be produced with bends, kinks or variable diameters. The concrete’s strength has been measured at 80-100MPa in compression, and it can also be used in conjunction with steel reinforcement.
So far SDC has been confined to the lab, but its first real-world application is set for later this year, when the team will produce concrete mullions for the NEST building — a project in Zurich designed to test and demonstrate new technology. “By using a form with an adjustable diameter, we can make each mullion thicker in the centre where strength is most needed,“ says Lloret-Fritschi. “In this way, the process supports a more efficient use of materials.”
Because SDC is a continuous process, it is potentially faster than 3D-printing concrete, she adds. “We have created columns at a rate of approximately 1m/hour. The finish is also very smooth, rather than corrugated as with 3D printing. I think initial applications will involve factory-made precast elements, but as the process evolves, there is no reason why it cannot be applied on site.”