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Reused concrete arch has no bigger carbon footprint than glulam

Words:
Stephen Cousins

Swiss researchers reused concrete blocks to create ‘ReCrete’ bridge whose embodied CO2 compares to that of the glulam equivalent

Researchers at the Structural Exploration Lab, part of EPFL’s School of Architecture, Civil and Environmental Engineering
Researchers at the Structural Exploration Lab, part of EPFL’s School of Architecture, Civil and Environmental Engineering Credit: EPFL

In a scheme to demonstrate how reclaimed materials could form the basis of entire buildings, a reused concrete footbridge has been developed with an embodied carbon count equivalent to that of a glulam bridge built to the same dimensions.

The 10m-long ‘ReCrete’ bridge, developed by researchers at science and technology institute EPFL in Switzerland, incorporates reinforced-concrete blocks taken from the walls of a local building that was being demolished.

The 25 blocks were cut into individual pieces on site, transported to the Smart Living Lab at the EPFL campus in Fribourg and assembled into a post-tensioned arch.

A comparative life-cycle assessment of the bridge, covering materials manufacture, disassembly and reassembly processes and transport, revealed approximately the same greenhouse gas emissions as a glulam timber arch built to the same span and to withstand the same loads.

The structure has about a third the CO2 emissions of an equivalent conventional reinforced concrete arch, whether cast-in-place as a monolithic structure or made from prefabricated concrete blocks.

Even greater carbon savings are possible though refinement of the methodology, said Corentin Fivet, head of the Structural Exploration Lab (SXL) within the Smart Living Lab: ‘A third of embodied carbon in our bridge related to production results from the transport of the blocks from the demolition site to the assembly site, roughly 70km, and a third comes from a bridge centring formwork used to position the blocks, which is used only once.’ This hints that a further decrease in carbon is achievable, he explained.

The prototype aimed to demonstrate, quickly and efficiently, how reusing concrete elements can be safe and relevant. Researchers gave themselves just two months to find a source building in the region and a demolition company willing to extract the blocks.

The blocks were cut by the contractor using a diamond-tipped saw blade attached to rails on the wall and holes drilled for the prestressing cables.

  • The ReCrete bridge spans 10 metres and exploits existing concrete sections, rather than the conventional approach of pouring fresh concrete
    The ReCrete bridge spans 10 metres and exploits existing concrete sections, rather than the conventional approach of pouring fresh concrete Credit: EPFL
  • Arches are considered an ideal structure for repurposing concrete blocks, because the material is only subject to compression forces
    Arches are considered an ideal structure for repurposing concrete blocks, because the material is only subject to compression forces Credit: EPFL
  • A diamond-tipped saw slices through the 20cm-thick wall of an existing 10-year old building to create blocks for the bridge
    A diamond-tipped saw slices through the 20cm-thick wall of an existing 10-year old building to create blocks for the bridge Credit: EPFL
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The technique uncovered some challenges around tolerances, said Fivet: ‘The main difference to a conventional approach is the blocks have quite a high variability. Although we target the same width, height and thickness, sawing meant the difference could be up to 2cm.’ More accurate sawing is possible, but it incurs a greater cost.

Mortar joints between the blocks always vary in width to make up for the dimensional differences.

Builders are often reluctant to reuse concrete due to the perceived additional risk, partly because the structural properties of existing concrete sections vary and can be hard to predict. To overcome this, SXL used an in-house developed software tool to automate the process of selecting reclaimed elements from a given stock of reused components.

Its exploration of other available testing and tools further supported the case for reuse, said Fivet: ‘The tools needed to assess the quality of cast concrete – including the strength, type of rebar, composition etc – already exist, so we can gain the same confidence in reused material as in new concrete. Additional costs related to testing are balanced out by the savings related to not pouring new concrete.’

Greater reliance on reclaimed concrete could avert the need to produce energy intensive concrete and help eliminate waste, but according to Fivet, it is not a silver bullet solution to decarbonising the concrete industry.

‘It should be used alongside other strategies, such as using recycled concrete and designing leaner structures to help decrease greenhouse gas emissions,’ he said.

The cement sector alone generates around 7% of global CO2 emissions, according to the International Energy Agency.

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