A circular economy (CE) is a radical departure from the centuries-old economic model of taking materials from the ground, making products from them and eventually throwing them away as waste. A CE does not have this linear process.  

A circular approach designs out waste in the first instance while circulating products and materials, ideally before degradation has reduced their potential uses.  This is underpinned by a transition to renewable energy and materials.

There are dozens of suggested definitions for a CE. The influential Ellen MacArthur Foundation (EMF) has attempted to encapsulate it in a single sentence: “The circular economy is a systems solution framework that tackles global challenges like climate change, biodiversity loss, waste and pollution.”

The EMF's butterfly diagram explores the many elements of a CE and their interrelationships: 

Eva Hinkers, chair of Arup’s Europe region, says infrastructure is crucial to the fight against climate change. “The built environment sector is a major consumer of natural resources and is currently responsible for almost 50% of the consumption of raw materials, and around 40% of global CO₂ emissions,” she says. 

According to the UN’s International Resource Panel (IRP), between 1970 and 2017 annual global extraction of materials grew from about 27 billion tonnes to 92 billion tonnes, a yearly growth of 2.6%. Metals extraction increased 3.5 times over this period and that of non-metallic minerals (sand, gravel and clay) rose 4.9 times, the highest growth of all resource groups.

The UK alone uses aggregate, cement, brick, plasterboard, stone and glass at a rate of more than half a million tonnes per day.

The EMF estimates that a CE could generate a 38% cut in global CO₂ emissions from building materials by 2050 through reducing demand for steel, aluminium, cement and plastic. It could also make the sector more resilient to supply-chain disruptions and price volatility of raw materials. So, for civil engineers, how infrastructure is designed, built, constructed and used will matter hugely if climate targets are to be met. 

The EMF’s 2019 report, Completing the Picture, sets out circular scenarios from which it is possible to see how civil engineers could play a leading role in partnership with commissioners, designers, the supply chain and other stakeholders:

A circular economy is broader than net zero, which has wider public awareness. Many see energy efficiency and the transition from fossil fuels to renewables as key to delivering net-zero emissions, but these areas account for only 55% of greenhouse gas emissions. The other 45% is embodied in everyday products, including infrastructure, which CE advocates say can only be tackled by circularity. 

“We have to recognise that in building infrastructure with a degree of resilience or adaptability, the best design might not be the lowest-carbon option,” says Leon Black, professor of infrastructure materials at the University of Leeds and co-director of the UKRI (UK Research and Innovation) Interdisciplinary Circular Economy Centre for Mineral-based Construction Materials. 

“For example, a concrete beam when it is recovered or recycled might need more cement for that longer life – adding to the overall carbon. And recycled materials in concrete can diminish performance. Civil engineering is a balance between CE and net zero and we must be careful they are not interchanged. Both are valid aims, but they are in a complex relationship.”

Julie Bonfait, a CE and sustainability specialist with environmental consultancy Ricardo, believes that CE targets concerning waste generated and treated, recycled content and biological material used can drive carbon emissions reduction and support net-zero ambitions. 

“This is being endorsed by the standard for managing infrastructure carbon, PAS 2080, which presents a hierarchy of options for carbon reduction from building nothing to building less, followed by building smart and, finally, by building efficiently,” she says.  “Think about the longer term. Don’t measure only on the financial cost – you need a range of KPIs that includes social and environmental aspects as an agreed set of metrics.”

Measuring circularity in an economy is a relatively new concept and is less established than, for example, carbon footprint protocols. Metrics must cover variables that are hard to measure, including product comparability, usage intensity, durability, and the number of users that benefit from invested resources, be they products or services. 

The most significant metric initiatives include  Circulytics, developed by the EMF, and Circular Transition Indicators, created by the World Business Council for Sustainable Development.  

Meanwhile, the Netherlands-based Madaster Circularity Indicator has been designed for the construction industry.

Among those offering toolkits to help explain the range of available CE strategies from the design process onwards is Arup. “The Circular Buildings Toolkit helps to future-proof real estate, helping investors, asset owners and developers to prevent their assets from becoming stranded while simultaneously generating future value, allowing the sector to deliver on its climate-neutrality ambitions,” Eva Hinkers says. “By recasting buildings from materials ‘bingers’ [consumers] into materials banks, the Circular Buildings Toolkit can help the real estate sector to generate further value.”

Julie Bonfait adds: “When a bridge is decommissioned, how easy it is to take it apart? It’s a mixture of concrete, plastic and stone, so separation is key. CE principles will have an impact on net-zero aspirations, but carbon savings will come from the re-use of materials being recovered.”

Infrastructure projects have a long and typically disjointed supply chain – making common thinking over a CE approach challenging. It would take resistance in only part of the chain to undermine any circular approach.

Leon Black cites the example of a proposed tunnel in Spain where the design was focused on the required performance of the concrete rather than its typical composition. The project’s insurers rejected that approach as they were more comfortable with traditional, composition-based concrete.  “Understanding the barriers means they can be overcome when necessary and practicable,” Black says.

As already mentioned, much of the current sustainability focus is on recycling and cutting the use of fossil fuels. Increasingly, though, there is a focus on ‘sufficiency’ – where consumption avoidance is prioritised over strategies such as recycling. Conscious consumption choices for sustainability are encouraged so that consumers make do with less to reduce overall resource use, avoiding unnecessary purchases, repairing and maintaining existing products, and buying second-hand, refurbished or remanufactured items where possible.

This view is supported by a 2022 paper from three European academics, The Sufficiency-Based Circular Economy — An Analysis of 150 Companies. “[The] circular economy potentially offers a way to decouple environmental impacts from economic growth; in other words, to generate more profit while reducing environmental impact… Yet, because of this fit with ‘business as usual’, there is a risk that the circular economy, when interpreted narrowly, perpetuates the current state of resource use and climate impact, or even worsens it.”

According to Bonfait, having the mindset to embark on a circular path is the big challenge. “Most people think short term so tend not to think of the decommissioning of infrastructure, which could be 50 or more years away," she says. 

"This is where the true cost comes in because what’s seen as a ‘cheap’ building does not include the overall lifecycle of that building. Negative impacts have a cost to be put to the owner, builder or manufacturers. We are used to extended producer responsibility for packaging and some other products, but it needs to be developed more widely. After all, the CE concept is pretty new.”

The concept has driven the philosophy of German architectural firm Werk.um, which wrote an open letter to the German government arguing that if the average residential space per person came down from 47 sqm to 46 sqm, this would free up 80 million sqm – more than the national house-building target.

It is putting the theory into practice by renovating a 1955 block of residential flats in Russelsheim, near Frankfurt, for a property company. Partner Arne Steffen says that by rethinking how a housing unit is occupied, little-used areas such as spare bedrooms can be relocated and shared between residents on each floor, allowing the renovation to make better use of the remaining area per person.

The new layout needs little additional building materials, but significant communication and organisational input is required. The cost of the renovation will be about 15% more than a conventional refurbishment, but more tenants can be housed, boosting the owners’ income in the longer term.

Road tunnels typically require high levels of lighting, especially during the day, as drivers adjust to the transition from driving in daylight to tunnel darkness. 

In Spain, the cost of illumination can exceed €1m a year per tunnel and requires large quantities of raw materials (lamps, wiring and aluminium), which in turn need maintenance throughout their life. Engineers have been working on the idea of introducing more natural light in and around tunnels portals, where there is typically high energy consumption due to the increased lighting need. One of the most promising techniques is the use of pre-tunnel pergolas with cut-out sections to allow daylight through, which can yield energy savings of around 40% in the high consumption zone.

These pergolas can increase their circular economy credentials by reducing reliance on typically carbon-heavy concrete for their construction. As they are situated at the tunnel portals these structures do not have to support heavy loads so engineers have trialled building them using cement originating from the sewage sludge treatment process. In addition, the pergola's modular design can facilitate deconstruction, transport and re-use at the end of their life.