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Lower carbon roads?

There are lots of stories about low carbon and even net zero carbon roads at the moment. This is good news, because it means that road authorities are willing to look at new ways of doing things to cut carbon emissions. But what does it all mean?

The UK asphalt industry was asking this question back in 2009, when a collaborative project, which also included the local authority Association of Directors of Environment, Economy, Planning and Transport (ADEPT) and the Transport Research Laboratory (TRL), came up with a protocol for calculating the carbon footprint of asphalt. Called asPECT, the Asphalt Embodied Carbon Tool, it has been updated since then and is still very much in use.

The protocol sets out ten carbon-producing steps over the lifetime of asphalt. Steps 1 to 8, from raw material acquisition to installation and scheme specific works, take us up to the point where the asphalt is laid and about to go into use. Often comparisons between materials and pavement designs are made up to this point only. But a more meaningful exercise is to go all the way to step 10 to include maintenance and end-of-life emissions.

It’s impossible to say how much embodied carbon is in a tonne of aggregate. It depends on the sources of the various materials, distances travelled and sources of energy used in the production process. For a standard hot mix as delivered, it could be in the order of 50 – 70 kg carbon per tonne.

The two biggest components of the embodied carbon of as-delivered asphalt are the embodied carbon of the materials that go into it – largely aggregate and binders (bitumen) – and the energy required to dry the aggregates and heat the mixture. They both account for between 40% and 45%, with the remainder due to mixing and transport.

Anything that can reduce those two big chunks of embodied carbon will lower the carbon footprint of asphalt up to operation. So, using warm mix technologies, where the heat required at the asphalt plant is lower, or using a proportion of recycled asphalt planings (RAP) to reduce the need for virgin aggregate and bitumen – or both – leads to carbon savings.

Broadly speaking, warm mix can cut emissions by around 15%, depending on the plant and additives used. Adding RAP lowers emissions by around 4% for every 10% of RAP added. Up to 20% RAP is relatively easy to achieve; higher proportions of RAP need more modifications to the mixing plant and eventually specialist plants. ‘Net zero’ roads might combine warm mix and RAP, with any carbon produced being offset – probably by planting trees somewhere.

But activities associated with that road will produce carbon over its lifetime. When comparing whole-life carbon of roads, carbon emissions per year of life are calculated; it makes sense that a road which has to be resurfaced after seven years is responsible for far more emissions than one that lasts for 14. Lifetime emissions include those due to interventions such as surface dressing or slurry sealing and repairs.

And then there’s the end-of-life emissions. Whole-life carbon comparisons also have to take into account what happens to the planings. If they’re heading to landfill – which many do – more carbon has to be added into the equation. For instance, the emissions per tonne for landfill at the time the latest asPECT guide was written in 2020 was 1.264 kg per tonne of planings.

With all these elements of asphalt’s carbon footprint to consider, it is important to make decisions about the materials and technology used on a whole-life basis, rather than simply comparing the carbon footprints of different types of material. Surface treatments lower the carbon per year because they can significantly extend a pavement’s lifetime. Long-lasting pothole repairs have the same effect.

It’s pointless to use a ‘low carbon’ pothole repair material that needs to be replaced within 12 months. It makes more sense to go for long-lasting repair methods such as Thermal Road Repairs. Because our system heats up the material in and around a pothole and mixes it with new materials, the old and new asphalt become one so that there are no joints or weak points where the fix can fail again. This not only cuts the carbon emissions during the repair process, it decreases whole life carbon because there will be no repeat repair and the whole road will last longer.

Put simply, ‘low carbon’ materials are good – but only if they lead to a long life too.

Thermal Road Repairs is a green technology company which supplies systems to improve the quality, cost and time efficiency of road repairs and paving – at a far lower environmental cost than traditional methods. We invest significantly in R&D, to create new technologies and to continuously improve our existing ones.

High output. Low emission. Permanent solution.


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