They were gently removed from roof constructions when an apartment block was demolished in Gellerup last year. Yesterday some of them were reused in the construction of new terraced houses - just around the corner in Gellerup.

Eight old reused concrete element slabs were lifted into place – one reused element in each terraced house. In the meantime, they have been thoroughly cleaned and tested by the Danish Technological institute, to ensure that the durability of the reused decks is just as good as the new ones that make up the rest of the construction in the newly built houses. All in all, 30 element decks are to be reused in the new houses.

The installation marks the preliminary culmination of three years of work in the research project (P)RECAST, involving participants from across the entire value chain in the construction sector. The project aims to explore the possibilities of directly reusing precast concrete elements from demolished buildings —as an alternative to the typical practice of crushing the elements for use in road construction or as sand and gravel in new concrete.

The work was followed closely yesterday by a group of participants:

"It’s a huge milestone that we’ve managed to reintegrate concrete element decks that are over 50 years old into new construction. While it may seem simple, there is a significant amount of development work behind it," says manager Katja Udbye Christensen from the Danish Technological Institute, who is the project manager for (P)RECAST.

"It was impressive to see the concrete decks used in the current construction," says Head of Section Annette Breedholm Rasmussen from the Department of Civil and Architectural Engineering, Aarhus University, who is also involved in the project.

"It's important because we need to gain experience with integrating whole and reused elements. That way we can expand this practice to the construction sector in general," she adds.

There are plenty of used concrete elements to reuse. A large portion of the Danish building stock consists of concrete elements — particularly in residential blocks from the 1950s and 60s, like those in Gellerup. Some of these blocks are now slated for demolition, but until now, there has not been a tradition or method for reusing the elements, even though they may have a long remaining lifespan. This project aims to pave the way for just that, and the Department of Civil and Architectural Engineering at Aarhus University contribute to method development and testing within this field:

"Our primary role has been to figure out how to integrate the used concrete element slaps into new houses after they have been carefully taken down, and to ensure that they are still of good quality. How do we assemble the different elements? Should they be jointed in the same way as new concrete concrete hollow core slabs - or in a different way?" says Annette Breedholm Rasmussen.

Additionally, the department collaborates with the Danish Technological Institute on testing and quality assurance:

"How do we ensure they have the load-bearing capacity and quality they need - and how do we document that? The elements installed yesterday were tested by the Danish Technological Institute, and we have tested similar ones. The test results show that they can withstand at least as much pressure as indicated on the certificate from when they were installed 50 years ago," she says.

(P)RECAST is supported by the Danish Environmental Protection Agency and involves a number of participants, including the Danish Technological Institute, Brabrand Boligforening, GXN, Aarhus University, the industry association Dansk Beton, COWI, Søndergaard, Tscherning, Aarsleff, Danish Standards, Peikko Denmark, Hi-Con, and AP Ejendomme.

FACTS 
(P)RECAST and reintegration
The (P)RECAST project aims to address some of the challenges that stops us from reusing concrete elements from existing buildings.

The project is supported by Environmental Technology Development and Demonstration Programme ( MUDP) under the Danish Environmental Protection Agency and runs from January 2022 to June 2025. The following companies participate in the project: Danish Technological Institute, GXN, Aarhus University, Aarsleff, Søndergaard, Tscherning, COWI, Brabrand Housing Association, the industry association Dansk Beton, the Danish Standard Foundation, Peikko Denmark, Hi-Con, and AP Real Estate.

The reused concrete element decks were taken down from Gellerupparken in the fall of 2023 by Kingo.

The reintegration was carried out by JCN Bolig, and the decks were supplied through Boligbeton.

Environmental impact of construction

• Cement and concrete production accounts for approximately 8% of global CO2 emissions.
• Construction and civil engineering account for 40% of Denmark's total waste production.