Tom Wood / sustainability and climate change associate / WSP / UK
Increasingly we have approaches that can offer highly recyclable construction. What holds us back is the trade-offs.
First, we need to be clear on the distinction between recycling and downcycling. In a circular economy, recycling means using the materials again at a similar value level. In the UK, we recycle a high level of the waste from construction and demolition, but most of that is actually downcycled. For example, concrete is crushed and becomes, at best, an aggregate replacement, offsetting the need for virgin aggregate. At worst, it’s just used to fill holes for landscaping. That’s certainly better than landfill, but a lot of the value is lost.
Then there’s the difference between recycling and reuse. You could deconstruct the building, take out a steel beam, melt it down and create something new out of the same steel. That type of recycling maintains its value but takes quite significant amounts of energy. Reuse is higher up the hierarchy and more desirable in circular economy thinking. You could reuse the beam for the same purpose in another building, but a strict reuse without altering the material or the building element is harder. The beam may have deteriorated, and the size, form and specification of the original elements may not be right.
Standardising building elements would be one way to maximize reusability. They also need to be extremely durable so they last, not only for their first lifetime, but also cope with being deconstructed and moved to a different site for their second use. If you put all your effort into making a building where everything is reusable, you might have to over-engineer it or use such strange elements that you end up with a poorly performing building.
”If you put all your effort into making a building where everything is reusable, you might have to over-engineer it or use such strange elements that you end up with a poorly performing building”
I can foresee some big challenges around the financing and economics too — buildings are generally so long-lived that the people who build them are not the ones who will reap the benefits from recycling and reusing the materials.
One way of keeping materials at a high level is to design things to last as long as possible. But sometimes longevity conflicts with recyclability. Traditional masonry approaches are highly durable, but very poor in terms of recyclability or even reuse, as it’s essentially a composite material. The bricks or blocks are stuck together with mortar and you have to chip it all off before you can reuse them. That takes a lot of work, a lot of energy and they are often damaged in the process. There are mortarless approaches — aggregate construction blocks that slot together — but it’s a challenge to ensure the air and watertightness of the joints.
That’s another potential trade-off: between recyclability and operational performance. If you want to achieve an absolutely exemplary, low-energy building, you generally ensure that it’s highly insulated, very airtight and has lots of thermal mass. It’s difficult to do that with construction methods that also make life easier for recycling. A shack made of wood panels and metal sheets is highly recyclable because the materials are not bonded together. But the flipside is that it’s not going to last very long and there are probably lots of gaps where wind and rain can blow in.
Precast concrete or highly insulated modular components could potentially provide thermal mass and be able to be deconstructed as well. Materials like polished concrete or stones can be very expensive and use a lot of energy to manufacture. But they often present a good opportunity for reuse because if the design is done right, you can just take them off and use them again.