Breakthrough for recycled construction timber

Published: 23.06.23

The burn rate of recycled wood – regardless of its age – remains within the Eurocode 5 limit values and thus does not differ from new CE-marked wood in terms of fire safety. The Spireprojekt Upcycling Orangeri 2.0, which was recently completed, has proved this. The next step is to find non-destructive methods for evaluating the wood.

Properties with wooden load-bearing structures can be found all around Denmark. Some of them have remained standing for hundreds of years. When they one day have to make way for new construction, for example, the construction timber is usually downcycled after demolition as material in wooden panels or – in exceptional cases – reused in load-bearing structures in smaller buildings, such as the new orangery in the Plus Bolig housing association in Aalborg, which served as the focal point of Spireprojekt Upcycling Orangeri 2.0.

"The potential for more sustainable construction is enormous if the construction timber can be reused in larger buildings and thus on a large scale," says Claus Langhoff, R&D Project Manager at DBI.

DBI's contribution to the project has included fire testing pieces of the rafters from the roofs of older buildings, which have then been reused in the orangery’s load-bearing structures.

No difference between old and new wood

The fire properties are the current obstacle to reusing construction timber in larger buildings due to requirements for load-bearing capacity in the event of a fire. Lack of knowledge in this area leads fire safety officers to go with the safe choice in the form of new, CE-marked construction wood. The results from DBI's fire testing can help to change this.

"The project has shown that recycled construction timber has a char rate of no higher than 0.65 mm per minute. This means that it is within the Eurocode limit value. In other words, the result is a good indication that, in terms of fire safety, there is no difference between new and recycled wood when the load-bearing capacity is calculated," says Claus Langhoff.

PhD project takes over

Even though the Spireprojekt Upcycling Orangeri 2.0 is complete, DBI continues to fire-test recycled construction timber for the PhD project called 'Fire performance assessment methodology to ensure second-life potential of reused load-bearing timber elements', for which PhD student Aline Uldry is responsible.

"We will carry out many comparative tests to obtain statistical certainty that there are no greater risks involved in using construction timber that has been in a building for 50–100 years than construction timber straight from the factory," says Aline Uldry, continuing:

"It would be a fantastic result if we could document that old and new CE-marked timber has the same fire properties. If some of the old timber differs and burns faster, we need to be able to explain how and why."

Non-destructive tools

In connection with this, Aline Uldry is testing various methods and technologies in parallel for the non-destructive selection and classification of the timber. These include not only well-known methods such as measuring the density of the wood and humidity and visually inspecting for rot and decay but also bring into play analysing the wood with ultrasound and spectrometry (chemical analysis using light).

"The point is that it is impossible to fire-test every single board that is to be recycled. It doesn't make any sense – in terms of time or money. We need to develop non-destructive tools so that the people who sort the construction timber after demolition can quickly and easily determine what it can be reused for," says Uldry.

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Spireprojekt Upcycling Orangery 2.0

The project was financed via WE BUILD DENMARK with funds from the Danish Agency for Higher Education and Science and was carried out in collaboration with Frandsen & Søndergaard A/S, Titan Nedbrydning A/S, Arkitektfirmaet Nord A/S, and Plus Bolig og BUILD. DBI was the project manager.