Structural Engineer:Mott MacDonald
London’s new super-sewer pushes the boundaries on low-carbon mixes
The mammoth Thames Tideway Tunnel is a 25km, 7.2m-diameter addition to London’s Victorian sewage network to make it fit for the 21st century. Due to complete in 2025, it will intercept and divert wastewater and stormwater to prevent the release of untreated sewage into the River Thames.
This is an essential upgrade to the UK’s infrastructure, which will bring widespread environmental benefits. But client Tideway also wanted to deliver it with the lowest possible contribution to greenhouse gas emissions. It has challenged the project teams to achieve an 8% reduction in embodied carbon (against a 2016 baseline) and set a stretch target of 10%. Since concrete is the principal material, focusing on the carbon impact of the concrete itself will be integral to meeting that goal.
The team on the 10km eastern tunnel sections from Greenwich to Bermondsey and from Bermondsey to Stratford (Tideway East), carried out an exhaustive analysis of the construction process to identify opportunities to reduce the amount of materials and to substitute lower-carbon alternatives.
For each of the mixes used across the five sites, they estimated the carbon footprint by taking into account the volume poured, the composition of each mix and the characteristics of the components. For each component, they also looked at the distance from the quarry or factory to the batching plant and the onward journey to the construction site, as well as the transport method and type of mixer truck used.
From the start of the project in 2017 until August 2021, approximately 63% of the concrete poured for the permanent works structures used low-carbon mixes – 87,000m3 out of a total 137,000m3. When the precast segments are excluded (which required a very high early strength gain for casting twice a day), low-carbon mixes were used for more than 98% of the permanent structures.
These mixes were specified in the initial action plan, but as the project progressed, the team continued to push the boundaries to further optimise their carbon performance. In the low-heat mix, the proportion of GGBS was increased from 67% to 73%, reducing embodied carbon by 26kgCO2e/m3. Overall, 5000m3 of this concrete had been poured up to the end of August 2021, saving 125 tonnes CO2e. In the abrasion-resistant mix, the proportion of GBBS was increased from 40% to 50%, the maximum permitted for this kind of concrete. During the same period, 450m3 was poured, representing a carbon reduction of 20 tonnes CO2e.
Joint venture partners Costain, Vinci Construction Grands Projets and Bachy Soletanche extended the action plan to design low-carbon mixes for the temporary works structures too. Concrete supplier Hanson designed a range of three mixes that included 80%, 85% and 90% GGBS, to replace C20/25 to C25/30 concrete mixes, which are now approved on all Tideway East sites. These enable a reduction of at least 70kgCO2e/m3 compared to mixes currently used for similar applications.
The project team also developed an ultra-low carbon mix for reducing the carbon associated with C30/37 to C40/50 mixes, to meet a higher strength requirement of 35-40MPa. In testing across various sites, it has reached 50MPa at 28 days and more than 57MPa at 56 days and represents a saving of at least 100kgCO2e/m3compared to conventional mixes. Pouring 280m3 for the batching plant foundations saved approximately 20 tonnes CO2e.
The increased use of GGBS in the mixes has a double benefit for carbon reduction. To ensure that the reinforced concrete chambers are watertight, the level of cracking is not permitted to exceed self-healing widths. Cracks are generated by early-age thermal effects, as the concrete shrinks while still relatively weak and also from the tension that occurs when the chambers are full. During the design phase, CIRIA released updated guidance on concrete cracking, based on improved estimates of the heat generation from mixes containing GGBS, which allowed further refinements.
This predicted considerably lower temperatures and permitted greater widths from early-age thermal cracking, as the self-healing effect occurs in wider cracks, so long as they are stable. At the chambers in Greenwich, for example, these two effects combined to enable Mott MacDonald to remove 40% of the designed reinforcement, representing a carbon reduction of around 100 tonnes C02e. This approach was used across the shafts and chambers at all the Tideway East sites.