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As the most widely used construction material in the world, concrete has a significant impact on the environment. Producing it consumes large amounts of raw materials and is responsible for around 10 % of all human-sourced emissions of greenhouse gases, according to the team behind the ENCORE project. Concrete from demolished buildings and structures also generates a large amount of waste each year.

Standard concrete used for houses, office buildings and other structures is a mix of water, cement, and aggregates, such as sand, gravel, or crushed stone. Industrial steel fibres are sometimes added in a class of materials generally referred to as 'fibre-reinforced concrete'.

Could recycled substitutes be used to replace some of these ingredients to make concrete production more sustainable' Yes, say researchers for the EU-funded project ENCORE, which demonstrated that sustainable substitutes could be used in the place of aggregates and fibres.

The project has produced three model guidelines on the technical standards for using recycled concrete, steel from waste tyres and natural fibres obtained from plants, such as sisal.

These model guidelines are recommendations or 'instructions' for the construction industry on how to produce, process and mix concrete with recycled ingredients, says project coordinator Enzo Martinelli of the University of Salerno, Italy.

"Before ENCORE, the level of knowledge on the mechanical performance of concrete made from waste was mainly empirical and consisted of various proposals merely based on experimental observations," Martinelli says. "Our project has formulated a fundamental approach for predicting the relevant mechanical properties of the materials under consideration."

The project's research and the three guidelines have been disseminated to European and international industry organisations. The project conceived the guidelines as a possible annex to the International Federation for Concrete's 'Model Code for Concrete Structures' for field applications, Martinelli says.

"They are ready to be considered for adoption in future revisions of that document," he adds. "Inclusion would be a key step towards a more rational and wider use of recycled constituents in structural concrete."

Durability and strength testing

During ENCORE, project partners tested different combinations of recycled ingredients. These included waste concrete, steel recovered from old tyres, fly ash left over from coal combustion, and natural fibres from sisal, a type of agave plant.

The products were tested for durability, strength and other important mechanical properties needed to ensure their performance matched that of standard concrete.

Waste concrete was crushed to obtain recycled aggregates, which were then used to partially or totally replace those normally incorporated in regular concrete.

Recycled steel fibres from waste tyres were used to replace industrial fibres used for fibre-reinforced concrete. Natural fibres obtained through sustainable processes from sisal were used to reinforce mortars and as activators of the material's self-healing capacity.

ENCORE developed production protocols for concrete produced using these recycled and natural ingredients, as well as design rules for the final concrete products.

ENCORE also sought to stimulate research and cooperation among the partner universities in Italy, Portugal, Brazil and Argentina through staff exchanges. Some 88 exchanges occurred during the project.

"The research could have a huge potential impact on both the building industry and the management of waste in urban areas and industrial districts," Martinelli says. "Demand for sustainable buildings, renewable materials and reduced CO2 emissions indicates an emerging need for optimising the disposal procedures of wastes derived from both the building technology and other industrial processes."

With the project's end in December 2014, Martinelli became coordinator of SUPERCONCRETE, a new EU-funded project addressing the subject of improving sustainability in concrete production. The project aims to further develop ENCORE's research and disseminate the information to engineers, concrete technologists and decision makers.

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