Cutting carbon at Gallows Corner flyover refurbishment

Transport for London commissioned Costain as its main contractor to renew and replace every element of the complex structure except its foundations. Once complete, all speed and weight restrictions would be removed from the strengthened flyover, in turn improving journey times, reliability and safety for road users. New pedestrian and cycle crossings would also be included to make walking and cycling safer.

Part-funded by the Department for Transport and the Mayor of London, the project involved close partnership working between commissioners, contractors and suppliers. To support its successful delivery, Poundfield was contracted to produce reduced carbon precast concrete decks which would help ensure the strength and integrity of the new structure for decades to come.

The challenge

Costain was tasked with designing and delivering the project with a reduced carbon footprint to align with Transport for London’s environment and climate ambitions. In addition, keeping the busy route open for as long as possible to minimise disruption for Londoners was also a key consideration.

As a result, Costain commissioned its suppliers – including Poundfield – to fabricate as much of the new structure off-site as possible before transporting it to Romford for assembly. In addition, suppliers were also challenged with reducing the use of carbon-intensive materials during manufacture to minimise the project’s environmental impact.

How we helped

During the design phase, Costain used Transport for London’s carbon assessment tool, CAT, to identify the precast decks we were to supply as a potential carbon hotspot. To address this, we worked closely together to find a greener solution which would not impact on quality or durability.

As a result, the slabs were cast using a low-carbon mix in which 50% of the cement was replaced with ground granulated blast-furnace slag (GGBS), a sustainable by-product of blast furnaces used to make iron.

We manufactured more than 90 decks which were 8.4m long by 3.5m wide and weighed on average 25 tonnes. We also assisted with the development of a levelling screw specifically for use at Gallows Corner, to allow precision adjustment of the precast elements during construction. The steel moulds used to create the slabs were designed to be reused, which meant they could be utilised to pour 65 standardised panels and 26 bespoke, in turn reducing material waste at source and improving quality control. Pouring the concrete off-site in our factory in Suffolk enabled us to closely control the production process, which meant that none of the standard decks needing reworking, in turn boosting efficiency and providing value for money. During production, high levels of quality control were maintained by fortnightly factory inspections with Transport for London, while installation methods were also trialled to ensure the project was right first time.

Using precast products ensured the slabs could be installed quickly, in turn helping to minimise disruption for drivers and those living around Gallows Corner.

The impact

By working closely together with Costain from an early stage, we were able to deliver a 37% (155 tCO2e) reduction in embodied carbon for the flyover deck compared with the original design. This is the equivalent to 7,750 mature trees absorbing carbon for one year or driving a car approximately 775,000 miles, which is more than 30 times around the circumference of the world. Costain has since been shortlisted for a NCE (New Civil Engineer) Bridges Award in the “innovation in carbon reduction” category for this work.

In addition, the steel reinforcement used in the precast slabs was produced using an electric arc furnace, which saved an additional 406 kgCO2e/t (56% reduction).

Conclusion

By working collaboratively and starting conversations early, Costain, Transport for London and Poundfield were able to successfully reduce the embodied carbon of this project. At the same time, buildability, safety and programme certainty were all improved, allowing the new-look Gallows Corner flyover to continue reliably serving Londoners for many decades to come.