(P)RECAST

In October 2023, 42 hollowcore slabs were removed from a condemned housing block on the Gellerup estate, just outside Aarhus in Denmark.

It’s part of a major programme of renewal – like the UK, much of Denmark’s social housing was built rapidly in the mid 20th century, and many of those buildings are now being demolished. What’s different about this particular redevelopment is that 30 of those slabs will have a second life, in new terraced homes built on the same estate.

It’s all part of a research initiative known as (P)RECAST, which was set up to explore the process of reusing structural elements, from extraction to storage to reinstallation. “We managed to secure an agreement with a precast supplier working on the new buildings at Gellerup,” says Bjørn-Tore Johannesen, sustainability engineer at GXN, which has been involved in the project from the start. “So it was a perfect circle.”

There are 13 organisations involved in (P)RECAST, and if that sounds like a lot, it’s because the challenges are logistical as much as technical. The team includes demolition companies, manufacturers, engineers, sustainability consultants and developers, among others, each taking a deep dive into methodologies and commercial strategies for their part of the value chain.

Danish public housing is an ideal test bed. The vast majority was built within a short time window from 1960 to 1980, assembled from standardised designs using precast concrete slabs and walls. With so many buildings coming to the end of their service life at the same time, a solution for reusing their components could be a game- changer for future housing renewal projects.

The first step was to identify elements for removal. GXN, a design-driven research studio founded by architect 3XN, developed a screening tool, based on the existing process of environmental screening prior to demolition. “We started with quite a simple Excel spreadsheet, then we added as much data as we could about the condition and dimensions of the element, giving us a traffic light system for whether or not to pursue it,” says Johannesen.

The screening tool provides a basis for data flow throughout the project. Architects can connect it to 3D-modelling software, and structural engineers can easily see the known technical capabilities of each component. But first, it enables demolition contractors to plan the deconstruction strategy.

The slabs were 180mm thick, 1.2m wide and 4.2m long – as with most hollowcore slabs used in Denmark in the 1960s, they were made with mesh reinforcement, which limits the structural span. The team carried out a series of test extractions. They investigated methods of removing the slabs using machinery that was already on site, such as diggers and trucks, but concluded that the most viable approach was to erect internal and external support systems, before cutting the joints and lifting the slabs out by crane. “It’s an expensive process, and not budgeted for in conventional demolition,” says Johannesen, “but on a bigger scale you could minimise the costs.”

Initially the extracted elements, each of which weighed about 2 tonnes, were taken to a storage facility 20km away. Testing was then carried out on sample slabs at the Danish Technological Institute in Taastrup. They were assessed for shear and flexural load-bearing capacity, as well as deflection under short-term and long-term loads.
The testing process revealed that the loadbearing capacity was greater than the declared capacity from 60 years ago. Even so, the team needed to find a “sweet spot” in the new buildings where the structural demands on the slab wouldn’t be too high.

The slabs were now 4m long – they had lost a few centimetres during the extraction process and had then been precisely cut in preparation for reinstatement. This was sufficient for their use on the upper floor of the new terraced housing, with one slab used per building alongside new hollowcore slabs. “In technical terms, they could be used in more structural locations,” says Johannesen. “But that would require a standard way of documenting that they’re fit for purpose, which we’re currently lacking.”

The reinstallation process was relatively simple. The thickness and width of the slabs were unchanged: there had been no screed topping in the original structure, and the team had been able to cut the longitudinal joints and remove the mortar cleanly. They just needed new lifting anchors and plastic sleeves to receive the load- transferring connections. “The only difference from any other installation is that the elements weren’t produced a month ago. They were produced 60 years ago.” The undersides of the slab will be plastered and painted.

The next step is to make the case for new regulations, to enable element reuse on a larger scale. Meanwhile, GXN is extending its research to the commercial sector, and in-situ concrete. It has been working with developer British Land on the extraction of a portion of a floor plate from a 1970s London tower, with the aim of studying its potential for reuse as precast planks in another application. As GXN says, it’s not just about recycling a material, it’s about preserving specific performance qualities. If Gellerup is any indication, 50-year-old concrete still has plenty of performance left to give. Words by Nick Jones

Interview by Tony Whitehead

^{Published in CQ Winter/Spring 2025}