Next time you go out for a walk, spare a thought for the two humble materials that are more than likely under your feet: sand and aggregate. Water repellent, weather resistant and malleable to human design, they have given strength and resilience to the Earth’s roads for generations. They are also the primary materials used in more than 810,000 kilometres of Australia’s paved roads. Beyond roads, these two humble materials underpin almost every aspect of our expanding built environment. Each Australian requires seven tonnes per annum of quarry materials to support the building of roads, houses and infrastructure to service their needs. Sydney alone uses 20 million tonnes of fine and course aggregate every year. Not surprisingly, then, sources for Sydney’s aggregate are running low. As the city expands, pushing quarries far from the city, transport costs rise, making aggregates more expensive. This fact, combined with the increasing need for roads to multi-task in response to environmental, technical and urban conditions, has driven an army of designers, scientists and businesses to rethink not only the materials we use to make our roads, but also what it is that our roads even do.

This need to adapt and change is not new. The material history of our roads has been shaped for millennia by multiple factors: the nature of trade and capital, the movement of labour, the vehicles that traversed them, and the materials available locally for their construction. As wheels evolved, so did the street. In the late 19th century, tarred wooden blocks lined the streets of Melbourne’s Collins Street and Sydney’s Darling Harbour. This innovation followed on the heels of wood block streets in America, where the superintendent of Boston, Samuel Nicolson, gave his name to the Nicolson pavement. Now, for the first time in Australia, a practical, resilient surface could support the rigid wheels of carriage transport. According to contemporary accounts, these wood block streets produced a sound of “hideous discord” compared to earlier unsealed muddy roads. But they were quieter and cheaper than their stone alternative. Wood block streets lasted well into the 1930s, when they were finally replaced with asphalt. Small gangs of ‘block boys’ sprung up across Australian cities, seizing upon discarded tarred blocks like savaging seagulls and returning home with fuel for the long, cold, depression-era winter. While asphalt had been used sporadically at different times over history, it didn’t really take off on a large scale until the huge underground Swiss asphalt mines of Val-de-Travers commenced commercial production. Asphalt, then a new, cutting-edge material, was exported to all corners of the globe, with the Neuchatel Asphalte company providing the asphalt that would line the streets of London, Berlin and Rio de Janeiro, as well as Melbourne and Sydney. For the inhabitants of Sydney, the benefits of traversing these asphalt roads on their brand-spanking new rubber tyres were “cleanliness, noiselessness and ease of traction”. For over a century, driver expectations changed little, even while the recipes for asphalt and concrete evolved.

'Block boys' at St Peters, retrieving discarded wooden blocks. Image: Sam Hood. 'Block boys' at St Peters, retrieving discarded wooden blocks. Image: Sam Hood, courtesy State Library of NSW. 'Block boys' at St Peters, retrieving discarded wooden blocks. Image: Sam Hood.

For most of the past 100 years, the modern road engineer has been interested primarily in two functions: to provide a visual reference for the travelled path and to ensure a vehicle’s mechanical integrity along that path. However, just as the vehicles of the future are evolving, so are the materials, construction methods and uses of roads. Just as the phone has transformed from a tool for making calls to one that touches almost every corner of our lives, so too the humble road surface, once a tool for conveyance from A to B, is being transformed into a multi-functional, versatile everything-tool, that touches almost every corner of the life of the city. For some involved in the design and construction of urban infrastructure, this evolution is not happening fast enough. “It’s really weird that when we talk about mobility, and when we talk about innovation, everyone is always focussed on the car,” says landscape architect Daan Roosegarde of Studio Roosegaarde. “Billions of dollars in research and development, no problem. But somehow the roads, the infrastructure, are being pushed aside. It should be cheap, maintenance free. Almost nobody cares. I think that’s wrong. I think roads are an interface of innovation, of expression, of information, of safety.”

Gates of light by Daan Roosegaarde, using new luminescent materials to reduce light pollution from roads Gates of light by Daan Roosegaarde, using new luminescent materials to reduce light pollution from roads Gates of light by Daan Roosegaarde, using new luminescent materials to reduce light pollution from roads

An example of this philosophy is his recently completed project, Gates of Light. The project playfully exploits new materials to solve an old problem: how to illuminate roads that are marooned from electrical infrastructure. To illuminate 60 enormous dyke floodgates at Afsluitdijk, an hour’s drive north of Amsterdam, Roosegaarde’s studio has developed a surface containing micropearls that reflect light from vehicle headlights. These lines of ephemeral light not only provide an interactive, poetic, theatrical and safe approach to this historical monument, a celebration of the heroic age of engineering, but they do so while minimising light pollution. This removes the negative impact lighting might have on the natural ecology of the fragile wetland environment the road traverses. A design response such as this represents only the beginning of a new way of reinterpreting the conventional road as a place of innovation and creative expression. A smarter road to lower emissions Engineers and scientists are also rethinking our vehicular arteries and concrete pavements. In a bid to reduce carbon emissions and the environmental footprint of the street, scientists are working towards a deeper understanding of its material components at a micro- and nano-scale. The reduction of carbon emissions is a key goal for many material manufacturers. The cement industry is responsible for about eight percent of global CO2 emissions, according to some estimates. This is hardly surprising, given that concrete is the second most widely used material on earth, after water. With these kinds of numbers, industry executives are starting to look in earnest for novel ways to decarbonize. At a forum on “Reinventing the Building” in July 2018, Julie Hirigoyen, the Chief Executive of the UK Green Building Council, said that “without adopting a whole lifecycle approach to how we evaluate, measure, report on and design-out the environmental impact … of those materials, we stand very little chance of meeting a two degree world.”

Could recycled tyres be the smart road material of the future? They look the same, but some roads are recycling everything from tyres to printer cartridges. Image: Close the Loop As aggregates get more expensive, the search is on for cheaper, recycled materials for our roads. Image: Brittany A Chase

Powering along: turning roads to electricity generators Plentiful acres of road surface have the potential to become a precious additional piece of real estate, to gather and utilise the sun’s energy. For this reason, a new breed of product designer is considering the street not just as a surface to travel, but as a potential component in a complex energy grid. After China, America is the world’s most energy-hungry country. Not surprisingly, the prospect of replacing thousands of kilometres of asphalt with electricity generating solar panels is attracting significant funding from the US government and venture capital. There is already a proposal underway to introduce solar panels onto the iconic US Route 66, turning it into a smart road. But even small countries like Budapest are getting in on the act, with a new startup creating interlocking solar paving units that have been installed on a shopping mall pavement. The units have also been incorporated into street furniture allowing the public to watch the street, dream and charge their mobile devices. Their latest project saw these trafficable pavers built into a wooden pontoon at a marina in Gotenburg, which allows energy to be harvested and delivered to boats and lighting. While solar energy is a mainstay of smart road research, piezoelectricity is increasingly making its presence felt. Piezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical stress. Engineers are looking to harness the electromechanical energy conversion properties of piezoelectric materials to harvest road vibration, weight, motion and temperature energy with promising results. The Chinese have shown significant interest in this material’s potential and have recently discovered a flexible organic piezoelectric substance that could be tested in new applications, including roads.

Platio is a modular solar paving system made from recycled plastic. A trial section of modular solar paving outside a shopping mall. Solar roadway concept.