If you were asked to name developments that are helping to make road travel more environmentally-friendly, you might think of the increasing popularity of electric vehicles, or regulations such as ultra-low emission zones, and the UK government’s planned ban on new petrol and diesel cars.

But, thanks to a raft of recent innovations, the carbon footprint of the roads themselves is also being reduced. Road surfaces can be made from plant-based aggregates, laid using vehicles that run on vegetable oil, and formulated to give a smoother ride, reducing fuel consumption. And those technologies are all gaining traction amongst end users, says Tarmac’s national technical director Jess Tuck.

“We have this range of technologies and solutions that allow us to get to near zero and even Net Zero road pavement construction, which is fantastic. That’s the art of the possible we have that available to us now. Clients are definitely interested in how they can achieve their commitments to Net Zero, and how we can work with them to help them on that journey.

“In bringing these low carbon materials to the road, it’s not just about the materials. We need to also think about the way that we deliver and construct our road pavements.”

Whereas the traditional ‘project management triangle’ balances cost, scope and time, there is now an additional consideration: carbon. Monitoring and controlling carbon impact is key to meeting both regulations and the approval of customers and investors. But, it has to be done without negatively affecting those other variables. In short, the balancing act has become more complex.

The state of the art

The good news for those involved in constructing roads is that there are a number of technologies already available that reduce carbon footprint. Warm mix asphalt, for example, is well-established. Because it’s laid at lower temperatures than conventional asphalt, it requires less energy to heat and dry aggregates, cutting carbon emissions by between 10-15%.

And there are a whole host of other options too, such as recycled asphalt – as Tim Smith, regional technical manager at Tarmac, explains.

“One of the big advantages of asphalt is that you can recycle it. It needs a lot of process control to do it, attention to detail, and a lot of testing of the old asphalt before we mix it into new asphalt. But that gives you some big benefits, as well as potentially reducing the amount of transport of aggregates around the country.

“Certain parts of the country don’t have great aggregates for asphalt, which means a lot of it gets moved around by train. But if you can locally recycle your roads, take it to your local asphalt plant, recycle it and send it back out again, there’s a carbon saving there, as well as on the material itself.”

Asphalt is produced using bitumen, which as a derivative of crude oil, has a relatively high carbon footprint. This can be reduced with the use of ‘bio-binders’, says Smith: plant-based materials that can be blended in with the bitumen.

“What you’re doing there is carbon sequestering: you’re taking the carbon that was absorbed by the plant while it was growing, and you’re permanently trapping that within the road. Some of these are available off the shelf from the conventional bitumen suppliers, and we’re working on evaluating a whole range of different plant-based materials.”

Plant-based aggregates are also being tested, with Tarmac running a trial of negative carbon aggregates on the M11 motorway. It’s a similar concept to bio-binders, says Smith, as it uses an artificial aggregate made, typically, from a plant source such as waste wood. These waste materials are processed, treated, and turned into a suitable substitute for a conventional construction aggregate.

“Those can then be used in relatively small quantities to sequester carbon,” says Smith.

Greener fuels

Fuels are used at multiple points throughout the road construction process, from heating and drying aggregates to make asphalt, to transporting the asphalt to the job site.

“So, one of the things that we’re looking at is alternative fuels,” says Smith. “A lot of those are likely to be hydrogenated vegetable oil in the short team, which is an alternative to diesel.”

Tarmac is currently looking into longer-term solutions, he adds. “In the more medium term, you’re going to be looking at other plant-derived materials that you can use as burner fuel, and in the long-term, green electricity for your transport, or hydrogen in the burners for the asphalt.

“Hydrogen is something we have looked at, and we’re aware there have been some trials, and that there are some big challenges. One of which is the availability of green hydrogen. But there’s a lot of work going on looking at hydrogen as an alternative fuel.”

As well as using greener fuel when transporting the asphalt, there’s the option of making plant such as excavators more sustainable during the road construction process.

Tarmac is currently around 18 months into a five-year contract with National Highways to repair, maintain and renew roads across England as part of the body’s Pavement Delivery Framework. One project within that contract took place in the north-east of the country, where, working in partnership with Volvo, the company delivered pilot schemes for Stockton and Hartlepool councils using electric yellow plant.

“The diggers, the excavators, the loading shovels, working in and around the asphalt plant, were all run on electric,” says Tuck. “We bought in green electricity for the project, and for the production plant – so our asphalt plant ran on low carbon fuels.”

The mix design also used the latest asphalt technology mentioned above, such as biogenic components, and a high recycled content. The carbon-cutting measures didn’t stop there, adds Tuck:

“We used electric delivery vehicles, and then on site, we ran a low carbon accelerator programme and worked with Renault to develop the world’s first electric bond coat sprayer, which was also deployed on the scheme, along with electric rollers. The asphalt plant was modified to run on low carbon fuels, so we really put the latest technology into the scheme and achieved an 83% carbon reduction, which was fantastic.

“Our next scheme, we’re going to take that even further, and where we’ll be utilising long-life surfacing, which has 20% extended life on surface course, and bringing a new biogenic component to that. So this is about enhancing durability, delivering long life and lowering the carbon as we do it.”

Longer life

By enhancing durability, road surfaces need repairing or replacing less frequently, which reduces the lifetime carbon footprint of the roads, as Smith explains.

“We’re looking at more durable materials. Some of the bitumen suppliers have developed some really clever technology that makes bitumen last longer and in turn, the road surface last longer. And that’s something that we’re already using on the M25, for example. By extension, that’s going to give you better sustainability, because you’re not going to have to go out and replace the material quite as often.”

Another characteristic of the road surface that can improve its long-term environmental credentials is rolling resistance.

To the naked eye, road surfaces look fairly flat and smooth. But by making those surfaces even smoother, resistance is reduced, resulting in the vehicles travelling on those roads requiring less fuel to get from A to B. 

“There are some papers out there that suggest that if you were to tweak the characteristics of the road surface, which is something that we can do – and do all the time, usually aimed at maintaining skid resistance – you can also affect the ride quality,” says Smith. “So if you think about something like a heavy goods vehicle, if it’s travelling along the road, if it’s bumping up and down, that’s a lot of energy being used going up and down rather than being going forward.”

It might sound like a marginal gain, but in fact, studies show that it can have a sizeable effect on carbon emissions. The papers mentioned by Smith suggest that this can potentially reduce the road user’s carbon footprint by up to 7%.

“And if you start to do the calculations on that, you rapidly see that user carbon is orders of magnitude higher than construction carbon,” says Smith. “So if you can get that right, and get the right surface characteristics, you can get the right ride quality, and reduce the bumpiness. Obviously, you need to do that and keep it safe – it’s still got to have good skid resistance. But you can produce quite a big saving in theory.”

A new consideration

Measuring and minimising the carbon footprint is going to be a key consideration for many construction projects going forward. 

“One of the things that we historically have always looked at is cost,” says Smith. “Now we look at what’s the lowest possible carbon.

“So we’ll look at each one of the materials we could use, and get an exact carbon footprint. We can calculate that, and look at: what’s the lowest cost, and what’s the lowest carbon per square metre.

“And then that might even go further into durability, and look at whole life cost and whole life carbon. It’s one of the really important things with low carbon solutions, to make sure that it’s got good durability. It’s no use introducing something that’s wonderful low carbon solution and it falls apart in six months. That’s kind of self defeating.”

The process of developing new and commercially-viable technologies for reducing the carbon footprint of roads is not a simple one, and there are high demands placed on these solutions. But with increasing focus on sustainability, it’s undoubtedly where today’s roads are taking us.