From reducing waste and carbon emissions to saving cost and retaining value, circular economy principles offer huge opportunities in the built environment. But a circular economy must not spin out into a spiral economy, as the quest for low-carbon concrete shows.
When we think about the built environment, almost the first material that comes to mind is concrete. As the most widely used construction material in the world, there is intense focus on the carbon emissions associated with its production, and how to rethink the use and reuse of concrete.
Examples of down-use and downcycling, where lower value uses are made of products and materials, are often provided under the guise of the circular economy. This could be best described as a spiral economy, where the value of materials and elements is reduced until it has reached the lowest level, often as a waste product.
“We must place more emphasis on the quality and value of materials that have a subsequent life. It is important to divert waste from landfill, but this should be seen as the absolute minimum level of achievement that should be realised.”
When recycling results in more carbon-intensive concrete
Circular economy principles are one of the most important decarbonisation levers that we have. Materials and products represent a large investment in cost, energy, and carbon, so reuse offers an immediate saving compared to creating new materials. However, we must place more emphasis on the quality and value of materials that have a subsequent life. It is important to divert waste from landfill, but this should be seen as the absolute minimum level of achievement that should be realised.
The default end-of-life scenario for onsite concrete is crushing and reusing as an aggregate. Although this is clearly a better outcome than sending to landfill, it represents a very low value material compared to the benefits of whole concrete. Although crushed concrete aggregate does have the potential to be used as a ‘recycled’ aggregate in concrete, it can actually drive an increased cement demand and may even result in a more carbon-intensive new concrete.
There are several companies looking at how we can make better use of crushed concrete aggregates, either by a more sophisticated form of recycling, or by using their residual reactivity to sequester carbon before it is reused. As these actions are many years from being scaleable, the crushing of concrete should be recognised as the least valuable form of reuse, just one step above disposal.
Concrete’s most beneficial qualities – robustness, durability, and mass -, do not lend themselves to simple forms of deconstruction. Yet, if we have become so adept at construction in concrete, surely we can get better at deconstruction. Even monolithic structures, where the concrete has been poured in situ, have the potential for element reuse, albeit with many challenges.
Due to the carbon intensity associated with new concrete, it is almost always preferable to reuse concrete in its solid state – even in a lower quality application – than it is to crush it. Reusing concrete is always better if it displaces new concrete, as it also avoids carbon emissions.
Stay circular, not spiral!
Concrete is an incredibly useful material in the built environment, but it is recognised that its impacts are significant and particularly stubborn with respect to carbon. Fortunately, we are literally standing on a huge and largely untapped resource of existing concrete structures, the majority of which are in good working order. The recently published guidance is just one aspect of our support to the industry and our clients in unlocking this resource. At Ramboll, we’re looking forward to using our expertise and ingenuity to find ways to create new lives for our existing buildings and materials and to make sure that the circular economy stays circular not spiral.
Author: Paul Astle