Phoenix Bridge takes 3D printed technology a step nearer commercial viability

What  Lightweight concrete bridge
Where  Lyon, France

Phoenix Bridge in Lyon, a collaboration with Zaha Hadid Architects and ETH Zurich, was unveiled at the end of last year at material solutions manufacturer Holcim’s nearby R&D centre. A 3D printed concrete structure, it is a reinvention of the Striatus Bridge that the group designed for the 2021 Venice Architecture Biennale, but this pushes the technology further and significantly cuts its embodied carbon. This is all part of Holcim’s aim to make 3D printed concrete commercially viable for the construction industry. The ‘concrete ink’ developed for this second iteration contains 40% less embodied CO2 than the original, claiming to have a 25% lower overall carbon footprint than a comparable reinforced concrete structure. 

The bridge relies on compression alone and requires no further steel reinforcement. Now made of 104 rather than 53 individual pieces, it is also far easier to handle, construct and dismantle – especially since ETH’s Block Research Group has optimised a strategy of using standardised formwork, so bespoke elements can be reduced to a minimum.

‘Stone doesn’t want to be a beam, it wants to be an arch,’ says Shajay Bhooshan, head of Computation and Design Group (CODE) at Zaha Hadid Architects. Together with Block Research Group, it’s spent the last decade looking at applying digital design and engineering in masonry construction. ‘A key aspect of masonry is its potential for dry construction, which is why we have relied completely on compression here. This obviates the need for steel reinforcement, which makes it far easier to recycle – part of the circular objectives alongside reuse, reduce and repair.’ 3D printing of the concrete also allows for material economy. ‘The blocks’ cross-section is mostly air; we are only using the material where we need it,’ adds Bhooshan. The policy worked on the block fabrication too, he adds. ‘Being smaller, each block had less curvature and so produced less “raft”, the sacrificial material surrounding each piece as it’s being printed.’ To fabricate the individual blocks, ZHA worked with ETH and Holcim to develop a customised programming code.

But the innovation here is not just having used less, it’s the nature of what was used – not least that Phoenix recycled 10t of material from its predecessor. The concrete ink was specifically tailored too. ‘As the bridge is shallower and lighter we could go from a compressive strength of 90MPa to 50MPa, and the higher the strength, the higher the carbon footprint,’ explains Holcim head of global R&D, Edelio Bermejo. ‘One third of the material in the new ink is recycled and we’ve used 100% recycled clinker to make 100% recycled cement.'

The ink recipe has been resolved, says Bermejo. Maximum aggregate size is determined by the ink nozzle and optimum sand/cement proportions have been established, along with the make-up of the accelerator used for setting. Critical is the viscosity of the ink, which depends on the variables of temperature and water content, he explains. ‘When you lay the ink, the next layer will come in 2-3 minutes, so it must set fast enough for that but not so fast that it sets in the nozzle. There’s a significant time difference to printing at 15˚C or 25˚C. Precision is key, so adjustments must be exact.’

So with all that necessary precision, can it ever be viable commercially? Bermejo thinks so – the firm is already printing 3D schools and housing in Malawi and Kenya with COBOD robots to prove the technology can be implemented anywhere, and is going through ETA certification ito progress a housing project in France. ‘The system and ink have been perfected; now it’s just regulatory compliance demands and insurances that need to catch up with the technology,’ he says. And, it seems, despite these humble beginnings, scale is no object. ‘They built the great aqueducts and rail bridges with bricks,’ notes Bhooshan. ‘This could do that.’