Deep in the Swiss Alps, a team of designers from ETH Zurich have constructed the first 3D-printed structure to be made from loadbearing elements, using a novel method of integrating reinforcement within the automated process. The 30m-tall Tor Alva is also notable for its ornate, richly textured design – inspired by the region’s baroque architecture and its equally baroque cakes.
The four-storey tower is a cultural venue and viewing platform, nestled beneath the historic Julier Pass in Mulegns, a small village that has dwindled over recent years to just 11 full-time residents. A spiral staircase leads visitors up through the middle of the structure to a vaulted hall on the top floor, with seating for 45 people. By hosting music, theatre and art installations in this spectacular eyrie, the aim is to bring new life to Mulegns.
The structure is based around 32 unique white concrete columns. These free-form elements seem to flow seamlessly from their bases into their capitals, which then merge into the floorplates for each level. In the lower rooms, the columns are heavy and squat, and split into two thick branches; as the tower rises, they become taller and branch into four, making the spaces lighter and airier.
The organic form was designed by architect-programmers Benjamin Dillenburger and Michael Hansmeyer, and is a continuation of their Digital Grotesque installations, a series of unimaginably complex structures generated solely by algorithm from a broad set of parameters. The custom software defines the precise geometry of the structure and converts it into commands that can be read by the printing robots.
The columns were fabricated in 2m-high sections at ETH’s Robotic Fabrication Laboratory in Honggerberg. Each comprises two interconnected, hollow rings of concrete, with a separate outer layer that wraps them into a single form. Both layers have a crenelated inner lining, which creates a series of channels for the vertical rebar to thread through. The voided form uses concrete only where it is needed, which ETH says has reduced the volume of material by half, more than compensating for the higher cement content needed for 3D-printable concrete.
The printing robots generated these elements in 5mm layers, working at a rate of 1m an hour. This required a bespoke concrete mix: soft enough to be piped and to bond the delicate structures, hardening rapidly to support the subsequent layers. To achieve this, two additives were blended into the concrete via an inlet mixing chamber just before it left the pressurised nozzle. “It’s a bit like a slow shotcrete,” says Robert Flatt, ETH Professor of Physical Chemistry of Building Materials, who developed the process. “As the material comes out of the nozzle, it’s still fluid, like clotted cream. But then it stiffens rapidly, so more like cream being poured into a freezer.”
This approach precludes the need for formwork, reducing waste and allowing an almost limitless array of concrete forms. It also means that surface patterns are no longer dependent on formliners. The ETH team have exploited this by creating a densely textured finish in two layers. The first is the typical horizontal ribbing – liked piped icing – that derives from the extrusion process.
But on top of this the designers have superimposed a spiral that wraps round the columns. This was generated by a code that prompted the printer’s robotic arm to deviate slightly from its circular path at programmed intervals. The cream-like concrete then oozed over the edge of the column before quickly hardening as a droplet. These droplets were staggered incrementally, layer on layer, until the spiral relief emerged.
While one robot applied the concrete in layers, a second placed a ring of reinforcement every 20cm. To conform to the variation in column dimensions, the rings were individually shaped using digital fabrication tools, developed by ETH spin-off Mesh. Because the concrete layers are just 15-20mm thick, the amount of cover was limited, presenting the threat of carbonation-induced corrosion. As a result, non-corrosive stainless steel was specified for the reinforcement.
“The interesting thing here is that you pay a certain price in CO2 for the stainless steel, but the concrete that's printed tends to carbonate a bit faster because the walls are thinner and the reinforcement is non-corrodable,” says Flatt. “So you actually recapture a lot of the CO2 that’s released during cement production.” The actual figures are currently under peer review – the team have just submitted their full lifecycle assessment, together with their research on the quantifiable effects of carbonation in 3D-printed concrete, for publication.
The researchers behind the reinforcement design also established a new testing method that adapts the standard slant shear test to 3D-printed concrete elements, allowing their loadbearing capacity to be reliably calculated for the first time. This is a key requirement to ensure that such buildings can be tested as rigorously as conventional reinforced concrete structures.
The bases and capitals for the columns were also cast at the ETH lab, but using a new type of digitally fabricated mould produced by another ETH spin-off, Saeki. The polymer-based formwork combines the precision of 3D printing with a high level of reusability and recyclability, due to the absence of additional linings or resins. For each floor, a single set of formwork was sufficient to cast up to eight large-scale elements, each weighing over one tonne.
It took five months to fabricate the structural elem¬ents at ETH’s factory. They were taken to a warehouse in Savognin, further down the valley from Mulegns, where the 2m-high sections, capitals and bases were joined together with grout and vertical rebar. They were then delivered to the site by lorry.
The horizontal cast elements incorporate all the holes and fixtures necessary to bolt the columns to the floorplates. This enabled the modules for a single storey to be assembled in just one day. The fixing details were an important aspect of the design: after five years, the idea is for it to be disassembled and re-erected on another site, so it is essential that the floorplates and columns can be easily separated for transportation.
Flatt says that the team are now looking at more industry-oriented applications for their growing arsenal of digital concrete technologies. “The idea is to try and show how to combine digital fabrication with ordinary fabrication in the most cost- and carbon-effective ways. The key thing is to see which parts of a structure it’s really worthwhile to produce tailored parts. You might have an ordinary column and an ordinary slab, but then you might need a mushroom head to the column to avoid punching stresses. Why not just 3D print that top part? We’re trying to bring those two worlds a bit closer together.”