Recycling concrete at scale to create London’s ‘super sewer’

Utility Week Innovate pores over the large-scale recycled concrete innovation at the Thames Tideway Tunnel.

According to the Paris agreement, carbon emissions from cement – which accounts for approximately 8% of global CO₂ emissions, is the second most used material on earth and the most widely used man-made material – need to fall by at least 16% by 2030 for the world to reach its target of keeping global warming within the limit of 1.5^oC.

In 2021, 40 members of the Global Cement and Concrete Association (GCCA) affirmed their commitment to net zero concrete by 2050 and agreed to an ambitious intermediate goal of preventing five billion tonnes of CO₂ emissions by 2030 – bringing together companies from the Americas, Africa, Asia – including India and China – and Europe in the process.

The UK cement industry has cut its emissions by 53% since 1990, with utilities attempting to build on this at large scale infrastructure projects such as the Thames Tideway Tunnel  – the £4.2 billion project to create a 7.2m diameter tunnel winding for 25km at depths of up to 70m beneath the capital.

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Cutting volume of new concrete

Dan Webster, environmental manager on the western section of the Tideway project, tells Utility Week Innovate that the team at Tideway’s Carnwath Road site in Fulham has integrated the production of high strength interlocking concrete legato blocks from surplus building material into business as usual.

“This surplus concrete comes from the concrete lines and transportation bullets used to transport the concrete to the shutters from the batching plant on the surface,” he explains. “The blocks have also been used to construct some of the aggregate storage bins at the site. The remaining blocks produced from the waste have been used on other projects in the UK including Silvertown [road tunnel project] and HS2 [High Speed 2].”

Webster adds that once the secondary lining of Tideway’s tunnels is finished, and the batching plant is removed from site, the project will be left with approximately 350 blocks of the recycled concrete.

“The team has managed to get the permanent works redesigned so that the blocks can be placed within the benching of the shaft at Carnwath Road, reducing the volume of new concrete required for the works by some 350m³,” he says. “This also helps to reduce the thermal effects of the large concrete pour allowing the pour to be completed in larger depths without the risk of thermal cracking.”

The team at Acton Storm Tanks on the western section of the Tideway project has also managed to integrate 180 blocks into their shaft design, according to Webster. “These blocks will be produced from the waste concrete at the Carnwath Road site and transported to the Acton Storm Tanks site. This prevented the further production of a further 180m³ of concrete.”

The project has also managed to reduce cement volumes by minimising, or where possible fully substituting, traditional Portland cement for ground granulated blast-furnace slag and pulverised fuel ash – both by-products from steel manufacturing and the coal industry.

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