Construction with 100% recycled concrete

Celebrated on 12 February 2024, the topping-out ceremony for a new three-storey school building in Hamburg-Eilbek also marked the public premiere of the ‘Hamburger Mische’ or ‘Hamburg Mix’, a concrete with 100% recycled aggregate. This is the result of several years of research and development work by OTTO WULFF with a team of partners from Hamburg University of Science and Technology, the Hamburg Senate Chancellery and the Construction Competence Centre, OTTO DÖRNER Kies und Deponien, EGGERS Tiefbau, and e-hoch-3 eco impact experts. The aim is to promote the use of recycled concrete (hereafter: R-concrete) in resource-efficient construction. The project is part of the EU programme ‘Circular Construction in Regenerative Cities’ (CIRCuIT), which is examining circular economy approaches for the construction industry.

‘Hamburg Mix’ – the optimised formulation for recycled concrete

When producing R-concrete, the main challenge is to achieve good workability and long-lasting consistency (slump retention) in the concrete for transport. Due to the fine grain content, it requires significantly more water than normal concrete. OTTO WULFF therefore brought Hamburg-based ready-mix concrete manufacturer Lebbin Beton, technical inspectorate ABN Gesellschaft für Baustoffüberwachung Nord mbH and MC-Bauchemie on board to solve this problem. Initially, various formulations with recycled aggregates were tested. A recipe with the concrete plasticiser Centrament N9 and MC-PowerFlow evo 530, a flow promoter from MC’s latest generation of superplasticisers, proved to be the best solution. The ‘Hamburg Mix’ was successfully used for the first time in the ‘model build’ on the OTTO WULFF company premises, and has now been successfully applied in the construction of a new school building in Hamburg. The R-concrete is used for floor slabs and walls and can be applied and compacted on the construction site just as easily as conventional concrete. As the ‘Hamburg Mix’ consists of 100% recycled material and is not standardised, approval was required from the Hamburg building inspectorate on a one-off basis – an effort that was well worthwhile, given that both the construction outcome and the ecological balance cannot fail to impress.

Benefits of recycled concrete

The use of recycled aggregates in concrete production has many advantages. The consumption of primary resources such as natural stone and gravel can be reduced, as can the amount of waste material that has to be disposed of in landfill sites. Gravel and sand are also becoming increasingly scarce. Against this backdrop, transporting these bulk materials from other countries makes little sense, especially when Germany already has sufficient quantities of used materials that are simply dumped. The projects implemented by OTTO WULFF impressively demonstrate the possibilities and advantages of building with 100% R-concrete. Thanks to close cooperation between the project partners and MC-Bauchemie, a pioneering example for circular economy in the construction industry and for sustainable construction has been successfully achieved.