SET in the heart of Cambridge, the chapel at King’s College is rightly famous. Built in the Gothic style, and finished in 1515, its ceiling is particularly remarkable. From below it looks like a living web of stone (see picture below). Few know that the delicate masonry is strong enough that it is possible to walk on top of the ceiling’s shallow vault, in the gap beneath the timber roof.

These days such structures have fallen out of fashion. They are too complicated for the methods employed by most modern builders, and the skilled labour required to produce them is scarce and pricey. Now, though, new technologies are beginning to bring this kind of construction back within reach. Powerful computers allow designers to envisage structures that squeeze more out of the compromise between utility, aesthetics and cost. And 3D printing can help turn those complicated, intricate designs into reality.

In a factory that makes precast concrete, 16km south of Doncaster, in northern England, a robotic arm hangs over a wide platform, a dribble of hard pink wax dangling from a nozzle at its tip. The arm is mounted on a steel gantry which lets it move about in three dimensions, covering a volume 30 metres long, 3.5 metres wide and 1.5 metres deep. Called FreeFAB, the system uses specialised wax to print ultra-precise moulds that, in turn, are used to cast concrete panels. Hundreds of these panels are being installed in passenger tunnels as part of Crossrail, Europe’s biggest construction project, which is digging a new east-west railway line across London.

Run by Laing O’Rourke, a construction firm, FreeFAB is the first 3D-printing technology used in a big commercial building project. Show offices and show homes have been printed in places such as Dubai and China, but are, for now, just concepts. The problem, says Bill Baker, an engineer who worked on the Burj Khalifa in Dubai, the world’s tallest building, is that printed concrete is currently produced in layers, which are fused together to make a thicker panel. But the boundaries between the layers introduce weaknesses that make the panels unsuitable for real buildings. “These things can peel apart,” he says.

Breaking the moulds

FreeFAB gets around that problem by printing moulds rather than trying to print structural material directly. Invented by James Gardiner, an Australian architect, it has big advantages over traditional mould-making techniques. One is that it creates far less waste. Ordinary moulds are made from wood and polystyrene, and can only be used to produce a single shape. Once they are finished with, they are scrapped and sent to landfill. FreeFAB’s wax can be melted down and poured back into the tank, ready to be re-extruded into a new form. It took Dr Gardiner three years to find a wax which could be printed, milled and recycled.

The system also makes it cheaper to make even complicated moulds. Production of traditional moulds is highly skilled work. Making a mould for a concrete panel that curves along two different axes, like the ones used in Crossrail, takes about eight days, says Alistair O’Reilly, general manager at GRCUK, the firm in whose factory FreeFAB is installed. FreeFAB can print one in three hours. That speed makes it possible to meet the design demands of more complicated buildings. Subtly curved panels can be used inside houses to deaden sound and keep certain rooms quiet, for instance. Doing that with traditional methods would be too expensive. FreeFAB—or something like it—could make such components much cheaper. And because the concrete itself is not being printed, the panels are just as strong as ones made in the traditional way. FreeFAB’s parts do not peel, and have withstood twice the required force in bomb-proofing tests.

It is early days. The factory in Doncaster has had teething problems—it has proved tricky to print moulds without flaws big enough to be visible in panels cast from them. For now the factory supplies concrete cast from a mix of traditional moulds and 3D-printed ones. But if the technology matures enough, Laing O’Rourke plans to spin it out as a startup focused on this new way of creating buildings.

If that happens, Philippe Block, an architectural engineer at the Swiss Federal Institute of Technology, in Zurich, might be an early customer. Dr Block makes floors that have the flowing, veined look of biological membranes. Just a few centimetres thick, they are modern versions of the chapel ceiling at King’s. Instead of building floors that rely on steel reinforcement to hold them up, Dr Block builds them under compression, so that each bit of the floor holds up the rest in a shallow vault. Each is bespoke, designed by a computer to efficiently deal with the specific loads it must bear. This allows him to build much thinner structures out of materials much weaker than reinforced concrete.

Such floors are useful as well as beautiful. In skyscrapers, for instance, the floors and the structures that support them account for a good deal of the building’s mass. Dr Block calculates that his new, thinner floors would need only about a third as much material as a typical floor slab. At the same time, their thinness allows him to claw back enough vertical space to fit three floors into the space that would be taken by two floors built in the standard way.

Walking on an eggshell

Dr Block has already tested many versions of his ideas, most recently at the Venice Architecture Biennale in 2016 (pictured, above). There, he and a team constructed a 15-metre vaulted “tent” out of 399 blocks of cunningly shaped limestone, each precisely milled to match the pattern of forces necessary to hold the vault up. Called the Armadillo Vault, its dome was half as thick as an eggshell would be at the equivalent size.

The next test is in a real building, specifically a demonstration house called NEST in the Zurich suburbs. Dr Block’s group will make the floors for a new part of the building called HiLo. The main bottleneck in the production of Dr Block’s structures is the creation of each element. It is expensive and slow to mill all the parts from blocks of stone, or to build traditional moulds for each individual component. So Drs Block and Gardiner are planning to work together on HiLo, using FreeFAB to print moulds that will produce segments of the floors. If all goes according to plan, the work should be done by 2018.

That could be just the beginning. Dr Gardiner talks of using ductal concrete, which is reinforced with steel fibres that make it lighter than concrete reinforced with steel rods but just as strong, to build thin bridges that span rivers in a single bound. For now, that is a project for the future. But all the components are in place.