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Zurich and the art of concrete recycling

mixed mineral aggregate

Is it possible to build houses with recycled con­crete and what impact does this have on costs, environment, safety and aesthetics? Concrete is essential in modern construction. It is a highly flexible, resistant and stable material. But there are downsides: gravel, one of the main compo­nents of concrete, is a finite resource; cement is energy-intensive in production, and mining and transport degrade the environment.

At the same time, construction and demoli­tion (C&D) waste is one of the largest waste streams in the EU. According to the European Commission, 25-30% of all waste generated in the EU consists of C&D materials such as concrete, bricks, gypsum, wood, glass, metals and others – many of which can be recycled.

Instead of landfilling, cities today are looking at ways to use these resources more efficiently and in an environmentally friendly way. In the sense of ‘urban mining’, one option is to recycle the mineral components of C&D waste to make new concrete. This allows the construction sec­tor to preserve natural resources, avoid landfill and protect precious landscapes and habitats.

Reusing old materials in new buildings increases their resource efficiency and makes the construction sector less dependent on scarce resources such as sand and gravel. It can also improve the energy balance of new build­ings, especially if materials are recycled and reused locally. It is a step towards closed loop resource use. But many cities are not aware of the opportunities that recycling construction materials bears.

The Zurich art museum, the Kunsthaus, is currently undergoing a major extension. Designed by architect David Chipperfield, the new complex is made almost entirely of con­crete with recycled aggregates and CEM III/B cement (where conventional clinker is replaced by slag sand), including the indoor space shaped by widely exposed concrete.

This is just one example of the city’s public buildings that include a high proportion of recycled concrete. Zurich strives to use only recycled concrete in any new construction: this includes an ice-skating rink combined with a public outdoor swimming pool, several schools and new housing areas.

The city has more than 15 years of experience in building with recycled concrete, and it is pushing the boundaries of what was thought possible. Newer buildings achieve rates of up to 50% recycled concrete in the final product. Other countries, such as Finland, maintain a maximum of 10% of added non-concrete mineral materials for producing concrete and use it in road construction, so limiting the amounts of recycled mineral aggregates that can be added to concrete.

Recycled materials are also used in Zurich’s road construction and maintenance: in the subbase (foundation layer) up to 30% of recy­cled concrete is used, and for the road base, up to 60% of reclaimed asphalt is permitted and used, or even up to 80% on pavements and places with low traffic. Tests with a new mixture of reclaimed asphalt look promising, and the previous difference of quality may be elimi­nated. The amount of reclaimed asphalt may be increased in the future.

A precondition to the successful reuse of C&D waste is the sorting of construction materials at the demolition phase. In Switzerland and in some other EU countries, this is already mandatory.

“It takes years of experience to reach these levels of recycled components in the final product,” says Michael Strauss, responsible for quality control of materials at civil engineering and recycling company Eberhard Bau. “We refined our recycling process to the point that recycled mineral aggregates can be used with­out any impacts on the quality of the construc­tion material.”

The business is a pioneer in providing recy­cled aggregates for the construction sector. The sorting and recycling process results in gravel that can replace up to 50% of natural sand and gravel in concrete production.

Most C&D waste does not consist of pure concrete, of course, and also contains bricks and other mineral components. This mixed material lowers the quality of the final product. Recyclers such as Eberhard are constantly look­ing into crushing and sorting technologies to meet the growing demand for recycled concrete while maintaining quality.

Sonja Gehrig, project manager of sustainable public procurement and co-operation, environ­mental and health protection service at the City of Zurich, says: “One major Swiss certification system for ecological buildings, Minergie-ECO, now requires the use of recycled concrete. This has caused a huge spike in demand to which the market has responded by producing more and more recycled construction materials.

“This showed us that supply will follow demand, and policy-makers can push the mar­ket towards higher recycling rates.”

Despite these successes, there is scepticism. Designers, architects and builders question the aesthetics and safety of the new material, so Zurich has found a clever way to address this. In addition to scientific testing, the city has built a mock-up wall consisting of different types of concrete, placing regular next to recy­cled. The wall physically shows that recycled concrete works well and looks good, and it has helped to convince many.

“Mining urban waste materials for new con­struction does make sense, but only if these are locally available,” says architect Philipp Noger of Zurich’s Office of Building Construction.

“Recycling construction materials is particularly useful in dense urban areas, where a con­stant stream of C&D waste occurs. Using recycled concrete within near distances is often cheaper than new concrete. We also need to design for reuse and recycling, to reduce the overall amount of mixed materials in demolition.”

The wheel is moving to use C&D waste more efficiently. A research project by the Fraunhofer Institute is looking into new processes that upcycle the smallest mineral particles, which are unsuitable for use in traditional recycling. The researchers have developed new construc­tion materials, such as lightweight, aerated concrete bricks, which can be used for family homes and have good insulation qualities.

Recycling C&D waste for the production of new concrete addresses resource scarcity but does not solve the issue of carbon dioxide (CO2) emissions from concrete production. Produc­ing clinker, the key element of concrete, requires a process during which CO2 is released. As a result, 1.5 billion tonnes is released into the atmosphere each year – as much as Europe’s 300 million cars.

But using slag ashes – a leftover product from iron smelting – can cut the CO2 emissions by 25% compared with concrete produced with regular clinker. Concrete, containing recycled material or not, remains a CO2-intense prod­uct. For a low to zero-carbon future, the con­struction sector will also need to look into other materials.

Paula Land is an officer for sustainable economy and procurement at Local Governments for Sus­tainability (ICLEI), the global network of cities, towns and regions committed to sustainability

A forum for learning on waste and sustainability

To showcase its experience and inspire European public authorities in the field, Zurich’s Environmental and Health Protection Service hosted a three-day visit for professionals as part of the UrbanWINS CityMatch project run by ICLEI.

Co-operating with the Procura+ European sustainable procurement network, CityMatch is an exchange programme that provides a forum for learning and exchange on waste management and sustainable procurement.

UrbanWINS is an EU-funded project that supports eco-innovation in waste prevention and management with the involvement of citizens, and pilots actions in eight European cities.

For this second edition of CityMatch, experts from the Metropolitan City of Rome Capital and the Helsinki Region Environmental Services Authority (HSY) travelled to Zurich to learn about circular procurement and the reuse of concrete used for buildings (recycled concrete) and road construction (concrete and recycled asphalt).

Roni Järvensivu, environmental quality and safety senior specialist of HSY, says: “This CityMatch has been an eye opener for us on how to use recycled concrete in buildings.” And Margherita Carè, architect for Metropolitan City of Rome Capital, adds that their plan was to “go home and see how we can change our standards for using recycled concrete, taking the Swiss examples as a baseline”.

UrbanWINS is a three-year project that started in June 2016. Eight pilot cities have applied participatory approaches to find out what they consume and dispose of using the urban metabolism approach [a model to facilitate the description and analysis of the flows of the materials and energy within cities] to stimulate innovation in waste prevention and management.

  • Registration to the third CityMatch, with a focus on contract management and procurement for public transport and cleaning, taking place in Malmö, Sweden, on 6-8 March 2019, is now open.
  • On 4 April 2019, the UrbanWINS team will host its final conference in GreenBizz, Brussels, to present its main outcomes and results to a wide range of stakeholders who can make use of them. Among these outcomes, the UrbanWINS toolkit will support decision-makers to improve their waste prevention and management policies.


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