A new plant-based bitumen is being incorporated into asphalt mixtures being developed by Aggregate Industries in the hope is that roads will act as carbon sinks. In a sector that is carbon intensive its supporters say that it offers a scalable way to reduce the carbon footprint.

By using advanced recycling technology that has been developed in the waste plastics sector, road builders can cut the amount of virgin fossil fuels used in asphalt. By using biogenic, or plant-based source materials, Shell Bitumen’s latest innovation both reduces the need for virgin fossil fuel inputs, and takes carbon out of the atmosphere, storing it securely in the asphalt used to build roads.

Locking down carbon

It’s not an entirely new development. Instead, it builds on earlier research, finding a way to make it commercially viable. Aggregate Industries are partnering with Shell to test and develop the new product. “We started off, way back where everyone was utilizing recycled materials, and not just looking at reclaimed asphalt but at all sorts of weird and wonderful things from cars, things called frag fluff, which was the material that you’d get from recycled tires,” says Bob Allen, Head of Research and Technical Services, at Aggregate Industries. But this moved on in 2010 when they supported a PhD by Dr Helen Bailey. “Her PhD was looking at replacing bitumen with vegetable oil. And she successfully got a PhD on that. And we did actually get a patent,” says Bob

The problem with Bailey’s innovation was supply of raw materials. It was difficult to get waste vegetable oil. Shell approached the problem from a different angle. For their new product, which has a trial name of ‘Shell Bitumen CarbonSink’, they’ve been working to identify sources of plant-based materials that are reliable, sustainable and perform at least as well as existing products. “You have to look at where that material is being sourced from. You have to have a look at its impact on land use and land use change. You have to look at the environmental credentials of the source,” says Richard Taylor, Global Technology Manager for Shell Bitumen.

He says that there will not be a silver bullet single material that will replace bitumen, there will be lots of different opportunities from the bio world where proportion of that material can be used in lieu of non-renewable products. But crucially these must be available at a scale sensible for bitumen and asphalt production. “It has to be compatible.  We’ve got to be able to blend this into the bitumen, we have to be able to make a product that behaves and performs at least as well as the bitumen that we have today,” says Richard.

Source local

So, the new product will use a variety of sources, which reduces the need for long distance transportation, itself a source of carbon. “If we look at the existing crude oil system, it’s very much an international system. I think that it would make sense for these materials to be sourced locally. The types of things that grow in different regions are very, very different. Put quite simply, you’re not going to get the same biogenic material from all regions of the world,” says Richard noting that the plants and vegetation found in Southeast Asia, compared to Northwest Europe compared to South America, are all very, very different. “I don’t think there’s going to be one global commoditised bio component. It’s very much more likely to be a regional type of thinking, when it comes to the use of biogenic material.”

We could store up to a million tonnes of this renewable biogenic material within the Highways England strategic road network.

Richard Taylor, Global Technology Manager for Shell Bitumen

A new bioeconomy

Reducing carbon is critical at many levels. “If we look forward to a net zero Britain in 2050, people will still travel by roads and actually roads will be a vital part of zero carbon travel. From an industry perspective, the main asphalt producers, and major cement companies, not only have huge challenges with managing CO2 emissions, but also very aggressive decarbonisation strategies,” says Rachael Morgan, Key Account Manager for Shell Bitumen. Rachael says that organisations such as Highways England and other road owners in the UK, are now requesting sustainable products and solutions as part of their tenders. “As key contributors in the supply chain, we have to be at the forefront of sustainable solutions to support the net zero targets for UK roads by 2050 or earlier, but also the global net zero targets for both companies,” she says.

The reduction in carbon by using products like this could play a significant role in helping the UK achieve net zero says Richard. “We could store up to a million tonnes of this renewable biogenic material within the Highways England strategic road network.”

A single UK household uses up to 20 tonnes of carbon a year. So using biogenic asphalt in the English road network could save as much carbon as us used by 50,000 homes in one year. Other aproaches in the UK include improving carbon cycles in nature by reforesting, by protecting peatlands and mangroves, for example. Carbon capture from industrial processes, or through direct air capture is also on the table. “There is a natural world and then there is a manmade world. And in the natural world, it’s about, you know, rebuilding and maintaining those natural carbon cycles, which kind of act as the world’s thermostats, or its then as we move to the technical world, it’s how do we actually intervene in that cycle by preventing gases from industrial processes reaching the atmosphere, or even by taking gases directly from the atmosphere themselves,” says Richard.

“The installation crews are very, very canny and will soon let you know whether they’ve got a problem in laying material.”

Bob Allen, Head of Research and Technical Services at Aggregate Industries

Road trials

At a practical level Shell Bitumen have been responsible for identifying biogenic sources and working out how to transform them into binders for asphalt and Aggregate Industries have been testing the new product in the lab, and in the field to show how it performs both when it is being laid, and when it is being driven on. AI conducted road trials last year. “The trial consisted of the installation of a section of public highway,” says Bob. “We replaced both the surface course and the binder course the layer immediately beneath the surface course and looked essentially at the three main mechanical performance indicators. And those are deformation resistance, fatigue properties, and stiffness.”

As well as these performance characteristics, AI looked at how the material mixes and how it pumps. They even smelled it. “It sounds an odd thing to think about in terms of asphalt. But, you know, the general public and operatives are quite sensitive to changes in in odour,” says Bob.

“The installation crews are very, very canny and will soon let you know whether they’ve got a problem in laying material.”

Fortunately there were no problems. The installation crews were more than happy and found it difficult to discriminate installation of a bio binder compared to a conventional binder. The trials and lab tests also suggested there may be further advantages to the new binder. Neither company is ready yet to make definitive claims about performance but the results so far are very promising. “Testing in the field is essentially looking at the levels of compaction. So we do that by taking cores of the material, and then look at things like the air voids in the mixture. So that’s one way in which we can analyse the laid product and determine whether or not the material is compactible. And when we did that air voids between the standard grade and the bio-binders is very similar”

This is very good news. “My conclusions from all of the performance testing we’ve done, is to conclude that the material performs at least as well as what we would expect from a normal grade binder,” says Bob. “Stiffness and deformation are comparable. There is an indication that the fatigue strength of the material is better. But that’s just from limited data that we’ve generated so far.”

As we were putting this episode together, Shell and AI are preparing for a new pilot installation using the product, to demonstrate how it works in the field. They’ll be using the results of all this testing to explain the carbon reduction achieved by using the product.

Proof of product

Highways England, and other customers, have clear specifications for how asphalt performs and existing labelling can show how much carbon is used to manufacture the binder. But this does not fully show all the impacts of using different binders for example where the carbonaceous material came from. “Has that come from a non-renewable resource? Has that come from the bio world? And if it’s coming from the bio world, how has that been grown? Has there been some land use change? Has there been an impact in the growing of that material? How quickly is that material being re-established and regrown? And how well is that carbon stock being managed?” asks Richard Taylor. The same applies for end of life. “And so if you gave another example, another bituminous material, if you imagine a bituminous backed carpet, tile, or roofing felt that proposition may not hold as well, because the majority of end of life carpets are currently as far as I’m aware, not recycled. And so you haven’t got that loop at the end.”

It seems that the users are not yet ready to fully evaluate the carbon impact of the products they are using. “It isn’t easy for customers to access or assess the full carbon impacts of road building materials. But throughout the project development with Shell and AI for this new product, we’ve had experience in measuring this actually end to end,” says Rachael who says that this will help the industry improve its understanding and help it develop strategies to avoid and reduce emissions where possible.

Some recent standard revisions have begun to consider these wider impacts. The European standard for sustainability of construction works, EN 15084, revision A2, includes this.

As does the British Standard for carbon neutrality, PAS 2060. “But the standard asphalt carbon calculator is a pure sort of carbon dioxide cradle to gate number machine, says Richard. “With a technology which is slightly more complicated like this is how do you articulate that benefit against a good but now quite old system?”

To conclude Richard and Rachael both think that there is scope for more industries to make use of biogenic materials, capturing and storing carbon for any product that can be made circular. Where plant-based materials are used to take carbon out of the bio world, and then kept in the technical world through long term use or recycling.“The essence of it is if we can take carbon that was in the atmosphere in some form, and remove that through an industrial process or through some form of vegetation, if that material can be then incorporated into a circular material, that that’s recycled, and that you can then minimise the degradation and the eventual return and release of that back to the atmosphere as carbon dioxide, then anything that meets that criteria has potential.”

Partner: Shell Bitumen