How can circular economy principles reduce material demand in the built environment? This article looks at how the concrete lifecycle can close, slow and narrow material loops, and what clients, designers, contractors, producers and policy makers can do about it.
By Dr Andrew Minson and Nicolas Antoniou, Global Cement and Concrete Association
Photo: Pexels
1. Introduction
The linear model of extraction, use and disposal has given way to recognition of the need for circularity. In a circular model, material loops are closed, slowed and narrowed as much as possible.
- Closed: waste is diverted from disposal and turned into secondary raw materials.
- Slowed: projects, products and their materials stay in the economy for longer. For example, the lifetime of projects and elements is maximised and waste is minimised.
- Narrowed: the same economic value is generated from less material. For example, through better design and improved manufacturing and construction methods that minimise waste.
This article covers circularity across the concrete lifecycle and how to maximise it. It focuses on the aspects that built environment stakeholders directly influence. Cement manufacture adds further circularity by treating non-recyclable and non-reusable waste to recover minerals and energy, but that is outside the scope of this article.
Even a perfect circular model can eliminate waste but not the extraction or sourcing of new materials. Society needs more homes, buildings and infrastructure than the material that can be reused from the existing built environment. When new materials are needed, they should be reusable in a circular way, come from plentiful stocks (as is the case with limestone and the other minerals used to make cement and concrete), and be responsibly sourced.
2. Closing material loops
Concrete is an asset that should stay in use for as long as possible. At the end of its life, its constituents should become feedstock for new products. This closes the material loop.
At end of life, recycled concrete aggregate can be used in new concrete or in unbound applications such as road construction and earthworks. This has been possible in practice, and through standards, for decades and is widely implemented.
Recent research and changes to some cement standards now allow more value to be taken from recycled concrete. Once separated from the coarser aggregates, recycled concrete fines can be used as supplementary cementitious materials to replace some of the clinker in new cements.
Evidence from countries such as Japan, the Netherlands and the UK shows that concrete recycling rates can approach 100% when the right regulation is in place. That means reducing landfill and enabling collection and processing into quality-controlled, cost-competitive products. These countries show both what is possible and how to achieve it. Wider adoption of these measures will help the construction industry, and the demolition industry in particular, recycle more concrete at end of life.
Concrete is often reinforced with steel bar or strand. This reinforcement can be added to recovered metal for electric arc furnaces, which use scrap steel as feedstock.
3. Slowing material loops
Keeping projects, products and their materials in use for longer reduces demand for new materials. This slows the material loop.
Retaining or reusing a whole project is also called retrofit or refurbishment. Deep retrofit is when only the structure is kept. Concrete structures suit reuse because the material is resilient, durable and robust. Concrete buildings also usually have structural redundancy, which makes them easier to adapt for a different use.
When a concrete frame is reused, its floors and walls can provide thermal mass. This is an important extra benefit when assessing whether a frame can be reused. Using that thermal mass reduces the need for mechanical heating and cooling, which cuts operating costs, carbon and material impacts. It is a good example of "reduce by design".
Buildings designed today can be designed as "long life, loose fit". This flexible approach makes future reuse more likely. The structural frame stays in valuable use while internal spaces and facades change as needs, and even fashions, change. In schools, for example, columns and walls can be placed along facades and corridors rather than between classrooms. If class sizes change, the non-structural separating walls can be moved, so three classrooms for 30 pupils could become two classrooms for 45. If the school changes use altogether, the walls can again be moved easily. These design features work with any structural material, but a long life is only achieved with materials that are inherently robust and resilient, like concrete.
Concrete elements can be reused because of the material's durability, strength, fire resistance, range of types and forms, and flexibility in design and use. Elements can be designed specifically to be taken apart and reused in future projects. Examples include many factory-made concrete systems based on blocks, slabs, panels or full modules, where components or even whole rooms are joined on site.
There are examples of temporary structures, but the built environment is mostly long-term assets: homes, schools, hospitals, commercial buildings and infrastructure. These are more economical to repurpose as whole projects than element by element.
A growing circular economy lever is the client's upfront choice of a design life beyond 50 years, which keeps material in the loop for longer. A building's first life is typically designed for 50 years. Partitions, ceilings, heating, cooling and facades typically have design lives of 10, 20 and 25 years. Concrete structures can easily be designed for a longer life, and standards already exist for material and element design to give the durability needed. The extra material and embodied carbon are small compared with the extra service life gained.
4. Narrowing material loops
4.1 Reduce material demand through the design of projects and concrete products
Whether designing concrete products or whole projects, engineers can use concrete's versatility to minimise material demand. For example:
- Form: concrete can be cast in many shapes and forms, even with voids.
- Casting: concrete can be cast in a factory or on site.
- Reinforcement: concrete can be reinforced and prestressed.
- Strength: concrete has a wide range of possible strengths.
This gives engineers many options to optimise designs and minimise material while still meeting functional requirements and codes.
Designers can also use concrete's inherent properties to minimise material demand:
- Resilience to fire, moisture and pests: this reduces maintenance and the need for treatments other materials require, such as fire retardants and preservatives.
- Acoustic performance: concrete's density reduces noise between spaces, avoiding or reducing extra acoustic finishes. This saves resources at initial build and in later refurbishments.
- Thermal mass: this can reduce the need for heating, ventilation and air conditioning systems, saving resources at initial build and the energy those systems use over time. Thermal mass can also be a key reason to reuse whole concrete frames at the end of their first life, as it enables a low-energy second life.
Concrete's range of finishes lets architects reduce the need for other materials like plasterboard and ceiling tiles:
- Concrete takes the shape and relief of whatever surface it is cast against: patterns, textures and even photographic images.
- Concrete can be coloured with pigments.
- Concrete surfaces can have exposed aggregates for different looks.
4.2 Reduce material demand through concrete mix design
Concrete is a mix of cement, aggregates, chemical admixtures and water. Cement is itself a blend, based on Portland cement made by grinding Portland clinker. Because concrete is a mix and cement is a blend, manufacturers can optimise the recipe to deliver the required performance while minimising material demand and carbon emissions. Usually both are achieved by reducing the clinker content of the cement and the cement content of the concrete.
The cement industry's use of GGBS and fly ash, by-products of iron production and coal-fired power stations, is a good example of circularity. Both can be used as supplementary cementitious materials (SCMs), reducing the need for virgin material in clinker production. Project specifications should allow SCMs in concrete mix design.
4.3 Reduce material demand in construction
Reducing waste during construction matters. Concrete's versatility means many construction methods can be chosen to suit each site, from factory-made masonry laid on site by hand or robot, to ready-mixed concrete poured in place into a bespoke mould to exact geometry. This versatility, with the right incentives for contractors, can minimise site waste and avoid extra materials.
5. Conclusion
Clients, designers, contractors and material producers can all minimise material demand by closing, slowing and narrowing material loops. Policy makers have a critical role in making this mainstream.
- Client-focused policies, including tax regimes and incentives, should promote circularity, such as extending the life of built assets and reusing structures in preference to demolition and new build.
- Designer-focused policies should encourage design for circularity through whole-project, whole-life assessment. This includes recognising longevity, durability, repurposing, a second life for durable materials and, where appropriate, design for disassembly.
- Contractor-focused policies should penalise site waste, encouraging reuse of materials on site and less over-ordering and wastage.
- Material producer-focused policies should improve access to waste flows through measures that reduce or eliminate landfill.
