Recycled concrete has a significant role to play in creating a sustainable and ecologically responsible built environment, says Daniel van der Merwe, architect at the Cement & Concrete Institute (C&CI).
“Well-designed buildings are increasingly being reused after their original use has ended. The reuse of a derelict railway line in New York as urban park called The Highline, has regenerated the Meatpackers District in New York. In London, The Tate Modern – a converted power station – is one of the most visited art galleries in the UK; and, in Johannesburg, the conversions of derelict buildings in the CBD and surrounds have created vibrant cultural and economically active nodes. In Cape Town, the reuse of the Old Biscuit Mill has been a catalyst which has regenerated the Woodstock area,” Van der Merwe states.
He says when considering buildings that has reached the end of its first life, the order of decision making should be: reuse of the building, reuse of the components and then – only as last resort – demolition with the recycling of materials.
“The large volumes of potentially reusable components and ‘building waste’ which are land-filled only to be replaced with similar components, have become unacceptable. Conservation and performance-based legislation, and the cost of demolition and new construction, are driving designing for deconstruction, reuse and recycling as key facets.
“Recycled concrete aggregate made from crushed concrete has proved viable and makes economic and ecological sense. As a coarse aggregate, it is used in the same way as a natural aggregate and is particularly appropriate for use as bedding material, concrete roads and pavements, floors, reinforced concrete, precast and masonry elements, and foundations.”
Van der Merwe says a new concrete mix that combines recycled materials with a tiny amount of Portland cement was recently awarded the US award for Innovation.
“The mix uses only 2% cement with the rest of the mix consisting of fly ash, slag, crushed recycled concrete, recycled water and admixtures. Developed by Cemstone Products, the mix actually performs better than concrete made with virgin materials, and can be used as a high performance concrete for the construction of bridges and structural building elements.
“Key innovative procedures and insights have shown that there will always be unhydrated cement available in recycled concrete aggregate to form cementation pastes for a new mix. By using the fine fraction of the recycled material and reclaimed water, nucleation sites for cement crystal are created that are already hydrated and therefore supports additional hydration.
“Furthermore, the recycled aggregates having some cementitious material, encourage crystal growth and a better micro-structure, making a reduced total cementitious content possible. Recycled aggregate has made it possible to use more supplementary cementitious materials, producing a more dense concrete which requires little or no waterproofing.
“A recycled concrete aggregate mix can use up to 95% recycled content with only 2% added Portland cement. Tests have shown that using the recycled materials leads to a 80% CO2 reduction when compared to new concrete mixes, with a 75% embodied energy reduction.
“Transport costs for aggregate continue to increase, and finding good sources of virgin material close to urban areas are increasingly difficult. Recycled aggregate is closer and more readily available. It is stronger, has a longer life-cycle,and saves money in replacement costs,” van der Merwe adds.