If there's one thing that defines the modern human experience, walking, driving and cycling on paved roads has to be up for consideration. Paved roads form the arteries of modern economies, making it quick, easy and efficient to transport goods and people, regardless of weather or environmental conditions. Today's paved roads are a drastic improvement on previous iterations of road surfaces, such as cobble stones, gravel or even wood. But can they be improved further?
Of the 64 million or so kilometres of roadways covering the planet, around 25 million are paved, according to the CIA's World Factbook. And the vast majority of those roads are covered in either asphalt (sand, gravel and stone, using bitumen as a binder) or poured concrete. These materials are extremely hard-wearing, can withstand great weight and fluctuations in temperature, and are relatively easy to maintain and repair. Nevertheless, they have their limitations.
Asphalt is particularly prone to potholes and is susceptible to damage from extremes of weather and temperature. And, since bitumen is an oil product, it is not truly sustainable long-term. Both asphalt and concrete road surfaces are also expensive to build and maintain.
In response to these challenges, a number of alternative solutions have been proposed by companies and researchers around the world. What are these alternative road surfaces, and how likely are they to catch on?
MacRebur: adding recycled plastic pellets to the bitumen in asphalt
While the exact make-up varies, the recipe for a standard asphalt road today is about 5 per cent bitumen and 95 per cent sand, gravel and stone. The bitumen provides a semi-malleable binder. What if part of that bitumen could be replaced by plastic?
This is the idea behind MacRebur, a Scottish firm launched in 2016. MacRebur recycles ordinary household plastic waste and grinds this into pellets and flakes, which can then be added to the asphalt mix as a complementary binder. This process requires no modification to asphalt manufacturing plants. MacRebur's product does not replace bitumen entirely but, depending on the surface being built, it reduces the amount required.
Besides reducing the amount of plastic sent to landfill, MacRebur also claims its product is more durable and cheaper to maintain than standard asphalt, which means cost-savings for councils. Despite being relatively young, the company's material has been used successfully in trials around the UK.
Aston University scientists create new 'bio-bitumen' from bin waste
Why stop at just recycled plastic? A team of researchers at Birmingham's Aston University recently released findings of a new 'bio-bitumen' product made from general household waste. The scientists experimented with heating rubbish – including plastic, organic materials, paper and textiles – at around 500°C in the absence of oxygen, in a process called pyrolysis. The result? A gloopy black substance which looks like – and shares many properties of – bitumen.
While the bio-bitumen hasn't yet been used in definitive trials, Birmingham Council and Highways England have expressed an interest in testing the product. To begin with, it would still need to be mixed with bitumen in the same way as MacRebur's pellets – yet the team behind the discovery are confident that their product could eventually replace bitumen altogether.
Solar Roadways: getting more out of paving
Today's roads really only offer one practical function: providing a flat surface to drive or walk on. American start-up Solar Roadways has a different vision – to turn roads into a source of energy.
The company has developed a kind of solar panel that can be walked and driven on (at least by lighter vehicles), and this offers some rather extraordinary possibilities. Most notably, the solar panels would be able to generate electricity. Solar Roadways claims that if its product replaced all paved surfaces in the US, they would be able to generate three times the country's current electricity needs.
To begin with, the product is most likely to be used on driveways, allowing home-owners to generate their own electricity, or on municipal pavements or car parks for street lighting and so on.
Besides the selling point of electricity generation, Solar Roadways' product has other benefits too. The paving is modular, meaning that should one specific panel break, it would require only a single person to come out in a truck, remove it and connect up a new one – which is far easier than filling in potholes.
And there are more futuristic applications, too. The product contains LED lights, which mark out lanes (rather than using paint) and could be 'programmed' to display messages on the surface such as 'slow down' or 'speed camera ahead'. And on snowy days, the surface could also increase in temperature, melting ice and improving safety.
At present, the firm has won rounds of funding from the US Department of Transport and has engaged in trials in certain cities, as well as use in private driveways.
Using lignin to create a bio-asphalt cycle path in the Netherlands
Lignin is a type of organic polymer found in the walls of plant cells which gives them their shape and structure. It is also a by-product of the paper industry, among others. In 2015, the Wageningen Food & Biobased Research Centre in The Netherlands first trialled the use of lignin in conjunction with bitumen in a 100m stretch of road which is regularly used by cars and heavy vehicles. So far, the trial has been a success.
Research is ongoing to completely replace bitumen in the mix, but the centre recently built a cycle path at Wageningen University using three different types of lignin, all of which were sourced as by-products of other industries. The goal is to find the cheapest possible source of lignin, and while it is currently expensive, the team behind the research expect a breakthrough in supply in the coming years.
Dutch city to trial new PlasticRoad
While many innovations in road materials seek to imitate and replace bitumen, one Dutch project has taken a different approach by creating completely pre-fabricated road sections out of recycled plastic.
The aptly-named PlasticRoad claims to use 100 per cent recycled plastic from material that is currently being incinerated – and the team also plan to use plastic collected from the sea. Engineers at the firm have found a way to recycle this plastic into hollow blocks, which provide the double function of a road surface that also allows cables, and even run-off water, to pass below. The blocks are modular and simply need to be laid out in sections – dramatically cutting the time it takes to build roads, as well as making maintenance much easier.
Construction of a pilot 30m cycle path is set to begin in September 2018 in the city of Zwolle, with a second to follow soon after.
Early stages for the alternatives
There is clearly a lot of innovation going on as researchers and businesses explore alternatives to asphalt and concrete – both of which have come to dominate road-paving materials over the last hundred years.
However, so far most of these alternatives are very early in their development, with a lot of work, testing and investment required before they can ever pose a serious threat to either asphalt or concrete. For now, those two materials remain kings of the road.
One for the road
Do these alternative road materials have a future? Andrew Dawson is an associate professor at the University of Nottingham's Faculty of Engineering and conducts research on paving. He notes that "major changes to the way we build roads aren't likely to become widespread", so companies like SolarRoadways may have most success where "the energy they deliver is in demand and can be easily transferred from the road source to the consumer".
On the other hand, he certainly sees applications for those materials that work as an additive to existing asphalt-style recipes. He notes that "whether such materials can ever totally replace conventional bitumen and cement as the binders is anyone's guess", but he does expect that "their ability to modify the behaviour of conventional binders at lower environmental and/or financial cost will ensure that their use becomes normal in years to come".
