How to bring steel into a green circular economy

The urban mining of buildings is a true celebration of circular economy and has the potential to go a long way to reduce the embodied carbon of the built environment. While at a smaller scale, raised access floors and ductwork being upcycled and reused are increasingly common, does it make sense for this approach to be applied to the structure of a building? Of course, the answer is Yes.

A standard office building, which is founded on piles and has a superstructure of a steel frame with composite slab, is likely to create around 1200 tonnes of CO₂e during the construction of the superstructure alone. The original embodied carbon of this structure has been spent and has done its damage. If  it cannot be refurbished we could mine this building to place the constituent materials back into the ­supply chain.

Steel is a woeful performer in terms of embodied carbon, but it is a great circular economy material, being recyclable without harm to its structural performance. This may come as a shock to some though – recycling virgin steel  produces around just 50% of the original CO₂e.

If we take this principle further things start to get really interesting. If we recycle steel elements within a building and replace it with a new timber frame building, we could potentially sequester enough carbon from the atmosphere to start creating an embodied carbon deficit.

A steel frame with composite slab is between 30-40% heavier than an equivalent timber frame. By leaving the original foundations in the ground and reusing them for a new timber building, it could be 30-40% taller. If we replaced an existing six storey steel frame building with an eight storey timber frame one, the embodied carbon in the frame would be: 540 tonnes CO₂e to recycle the steel and 450 tonnes CO₂e to build new timber building  – but sequestered carbon in the timber amounts to -1500t CO₂e.

So working through the numbers you can remove 500t CO₂e from the atmosphere and provide a bigger building, while placing materials back into the supply chain for use elsewhere. And other types of urban mining could be more appealing to those less comfortable with use of timber in buildings.

There are a few projects under way in London where the superstructure is all or part formed using reclaimed structural steel from other sites owned by the client. This, as a concept, is a very pure version of the circular economy and urban mining.

In this form the embodied carbon is kept to a minimum and the additional embodied carbon would be associated with storing, cleaning, re-fabricating and painting the steelwork.

If the steelwork is reused rather than recycled, that embodied carbon figure potentially falls from 540t CO₂e  to 150t CO₂e – presenting us with a staggering potential net 900 tonnes of CO₂e benefit.

Scaling up urban mining for the circular economy to make it feasible across the industry needs to be part of the design process at the start of the project, and needs the support of manufactures. There are three critical moves.

First, when a building is demolished all its components need to be scheduled and stored for re-use. This requires space and time.

Secondly, assurance and warranties would be required to ensure the buildings can be designed appropriately and insured so that all or some of these components can be tested and their properties verified/certificated. And finally, a database listing where the components for reuse can be found and bought would be needed.

It requires some joined up thinking, a little investment to get things off the ground and some willing clients to apply these principles to their schemes. 

All of this is very straightforward, but requires some joined up thinking, a little investment to get things off the ground and some willing clients to apply these principles to their schemes. One such client is the massive Grosvenor Group, which has been actively exploring the circular economy in the UK, and there is a hopeful model in RotorDC, a Brussels-based group that deconstructs, processes and trades salvaged construction materials. Perhaps it is a precursor to a giant construction-based eBay for the built environment.

Tom Webster is a director at  Webb Yates Engineers 

Read more of Webb Yates on reducing embodied carbon in structures including on house extensions, towers and design approach. And on using stone and blocks.