Here’s an interesting way to reduce the carbon footprint of road construction: replace some of the bitumen in the asphalt with wood.
That isn’t quite as crazy as it sounds. PEAB Asfalt in Sweden is using lignin, a plant-derived polymer that has been extracted from softwood, in place of some of the bitumen. Earlier this year it laid its second test section in Järfälla, near Stockholm in Sweden, following on from a trial in 2020 in Sundsvall, which is further North.
For the local authority in Järfälla, this is another possible way to reduce its impact on climate change, according to Marcus Gry, chairman of the municipality’s technical committee: “Järfälla municipality works in many different ways to be able to reduce emissions. This is an important part of that work while we hope to achieve technical benefits.”
In the laboratory, PEAB has replaced up to 25% of the bitumen with lignin. On the first test section at Sundsvalle it replaced 10% and at Järfälla, 15%.
After the first trial, PEAB reported that the asphalt mix that contained lignin showed no differences in terms of workability and the paving process. Although some observers did notice that it smelled a little different; there was a whiff of wood.
PEAB expects the modified asphalt test sections to behave at least as well as standard asphalt road surfaces. It could even be that performance is superior, although that will only be confirmed by regular monitoring over several years. If that were the case, it would be a double environmental win, since longer-lasting roads reduce maintenance and carbon costs over their lifetime.
PEAB’s trials are part of a larger, €19.8m EU research programme called Rewofuel (Residual soft Wood conversion to high characteristics drop-in bioFUELs). One of Rewofuel’s objectives is to look at how lignin, which is a coproduct of biofuels made from softwood, can be put to good use. The lignin used in the latest trials was manufactured by Stora Enso which produces products based on wood and biomass.
PEAB has been working for years to reduce its carbon emissions as a business. One of the biggest reductions it has achieved has come from switching to biofuels at its stationary plants. The burners used to dry and heat up the aggregate use the most energy-hungry part of the asphalt production process.
As well as the urgent need to tackle climate change, there are other good reasons to find alternatives to bitumen. The hydrocarbon-based oil from which bitumen is produced will one day run out, so it could be that we will need to grow all our binders for roads in the future.
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