Minimass

3D printing and post-tensioning combine to create a reusable beam with half as much material

Structural engineer Andy Coward has combined 3d printing, post-tensioning and standard concrete to develop a reusable beam with half as much material.

A structural engineer with an interest in innovation, Andy Coward became used to sitting in meetings with architects and contractors who wanted to maximise the potential of 3D printing (3DP) with concrete. “We kept coming back to the difficulties of reinforcing it,” he says. “Despite the exciting possibilities of being able to create complex, material-efficient shapes relatively simply, they could not be made strong enough to function as structural elements.

We tended to end up with igloos and street furniture – which is fine, but obviously limited.” Coward thought the problem was being approached in the wrong way: “I felt we were missing something: essentially that 3DP should be seen as just part of the solution. I began thinking about what geometries 3DP could open up and what other technologies it could be used with.” Determined to test his ideas, he founded his own company, minimass, which has just completed an 11m-long demonstration footbridge in Norfolk. Its slightly curving concrete support beams are recognisably 3D-printed but, as Coward explains, that is only half the story.

“First, we print the sides of the beams in outline, effectively creating 50mm-thick concrete formwork. Then we drop reinforcement inside the forms and fill with standard pumped concrete. Because the beams are printed to exactly the size and shape required, they don’t have to be straight, or square in section. That immediately allows us to use less concrete and reduce embodied carbon.”

To further minimise the material involved – Coward estimates the Norfolk demonstrator uses 50% less concrete than a standard solution – the elements are also post-tensioned using an innovative external cable system “Unusually, we have the cable slung some distance beneath the centre line of the element, and held in the correct position by the 3DP geometry of the beams.

We post-tension it using standard PT equipment, but only partially.” When loading is applied to the bridge, the forces at play automatically tension the cable further, to around 25% of its capacity, and this increases  compression of the concrete element. “It works like a bowstring truss – increasing both tension and compression under loading. You see the technique used sometimes in wider structures but not on single elements like this. The result is a highly efficient, low-mass, structural beam.”

The exterior tensioning cables help to make minimass beams reusable, he adds. “Most PT systems have the cables encased in concrete, but our cables can be de-tensioned and removed fairly easily. That’s helpful if you want a temporary, reusable structure, or even if you want to disassemble a more permanent structure and reuse the beams elsewhere.”

To build the prototype bridge, Coward and co-founder Sarah Blake teamed up with Irish 3DP specialists, Harcourt Technologies. It was assembled on site at Constructionarium in Norfolk, a training facility that also provides a site for experimental technologies. The beams use standard, code-compliant concrete containing 6mm aggregate: “This is important as some printing technologies use low or no-aggregate mixes and this increases the cement and therefore the carbon content,” he says. “Our concrete is made 3D-printable by injecting an additive just before it gets to the nozzle. Elements can either be made at a central location or, if the contractor has room, printed on site to minimise transport costs.”

Coward believes his design to be the first ever code- compliant structural concrete element to use 3DP. “Our beams do look different,” he concedes. “But architects we have spoken to really like having the cable visible. It shows the building’s occupants how the structure is functioning.”

But could minimass beams ever compete commercially with traditionally produced in-situ or precast elements? “3D printing the permanent forms this way can actually be very cost-effective, especially if you want a range of shapes and sizes. It uses less concrete, less steel and less labour than traditional formwork approaches. We think a three-person team can produce around seven beams a day.”

Overseeing the construction of the prototype bridge has involved a steep learning curve: “Sarah and I have been very hands-on. As well as designing the bridge, we had to source materials and organise deliveries.” He has clearly found the challenge refreshing, however: “Previously I have worked with Foster + Partners and Danish group BIG. Both practices encouraged creative thinking, but at the same time the reality was it was often not possible to go for the most sustainable solution.”

He sees minimass primarily as a product supplier: “I always felt that I could make more of a difference to carbon in construction by developing products than by shaving a few kilos of material here and there on a project-by-project basis.” Coward and Blake are in discussions to provide structural elements for its first commercial project in Ireland: “That would be for a permanent structure,” says Coward. “But I think we may also be able to find opportunities, especially initially, to supply structures for temporary works on larger projects.”

Interview by Tony Whitehead

^{Published in CQ Autumn 2024}