Research project aims to make reusing wood easier
The construction industry needs to reuse many more materials, but how can you determine how much the old materials can withstand, and who will provide the guarantee? In collaboration with several private partners, researchers at the University of Southern Denmark are working on finding a solution.
Close to 0 percent.
According to the Danish Ministry of Environment, this is the percentage of building materials currently being reused in Denmark. If the construction industry is to move in a more environmentally friendly direction, this needs to be increased significantly. However, as it stands today, there are numerous challenges that hinder the large-scale use of recycled materials.
One of these challenges is the lack of documentation.
This is explained by Henrik Brøner Jørgensen, associate professor at the University of Southern Denmark (SDU) and head of SDU Structures.
- When building with new materials, you know how strong they are. There are standards that guarantee, with a certain probability, that there will be no failures under a specific load. Engineers can then calculate and design the construction based on this, explains Henrik Brøner Jørgensen.
- We don’t have the same documentation for recycled materials today.
This means that, to be on the safe side, buildings with recycled materials are often over-compensated. And even more importantly, it means that there is generally no one who can guarantee that the materials will hold up.
1:1 reuse of truss beams
Researchers at SDU have therefore teamed up with several private companies to try to gather data on recycled materials so that they can potentially be included under the same certifications and standards that exist for new materials.
In one specific project, the focus is on old trussed rafters from the Åhaven public housing area just outside Odense, where about 350 row houses are being demolished due to moisture problems.
- Due to the project's size, there are a large number of trussed rafters, which means there is a large so-called population to test, allowing us to gather some statistics on the strength of the wood, says Michael Rex Skov, an architect and one of the project's partners.
- Today, recycled materials are primarily used in what is called secondary construction. This can include sheds, temporary doors, and fences on construction sites. But what is interesting about this project is to find out if we can reuse the rafters 1:1. It is also exciting because it means that, as an architect, you have to start from the premise given by what is available.
Good business
At Stark, another project partner, they also believe that the lack of knowledge about the strength of recycled materials is one of the major challenges.
In many cases, you are not allowed to use recycled materials at all, for example, in public housing, because they are automatically classified as 'risky,' explains Andreas Juhl, a sales advisor and sustainability ambassador at Stark in Odense.
- If you really want to push this forward, we need new legislation. And that is one of the reasons we are testing these trusses, so we can verify that the materials can withstand what is expected, says Andreas Juhl.
If the strength of recycled materials can be mapped and made legally usable in more places, Andreas Juhl is convinced that it will not only be environmentally sustainable but also economically viable.
- At Stark, we got involved in this because we believe it is the right thing to do, because we want to lead the way, but also because it can be good business.
Destructive tests
The tests on the wooden rafters are being conducted in the laboratory at SDU in Odense, where the researchers first perform a visual strength assessment, including counting and measuring knots, to get an expected strength. Then they perform a destructive test to measure the bending strength, i.e., the actual strength of the wood, which they can then compare to the expected strength.
When the dataset is sufficiently large, visual assessment alone will suffice, and it will be assumed that it is accurate. It can also be used in other future projects and for other wooden trusses.
- If you want to reuse rafters from a single-family house, you might not have 90 rafters to test in a lab. Here, you could rely on a few tests and use the statistical basis from our large dataset to translate into load-bearing capacity, says Henrik Brøner Jørgensen.
Although all the tests have not yet been completed, preliminary results indicate that the old wooden rafters from Åhaven are sufficiently strong.
-They fall within a strength class called C18 or C24, which is a strength class you would expect from this type of wooden truss. We expect it to have roughly the same strength as when it was first installed, and it certainly has sufficient strength to be used in practice.
About the project
Researchers from SDU Structures have partnered with Rex Skov Arkitekter, Stark, LH Hockerup, Enemærke & Petersen, and Brav Consulting Engineers to investigate how old wooden trusses can be reused.
The project is funded by We Build Denmark and the Danish Business Promotion Board.
The wooden trusses come from the row houses in Åhaven, just outside Odense, which are being demolished due to moisture problems. A wide range of other materials, including bricks and laminated timber beams, are also being reused from the construction.
SDU is responsible for testing the trusses and building a statistical basis for the variation in wood strength. The goal is that in the future, old wooden trusses can be sorted based solely on a visual strength assessment.
This would solve a major problem in the reuse of building materials, namely the lack of documentation and therefore also the lack of guarantees.