The process of surfacing a road isn’t complicated. Layers of asphalt, which is composed mostly of bitumen (a byproduct of crude oil distillation), are poured over an aggregate of crushed stone and sand; the asphalt acts as a glue, binding the mixture together to form asphalt concrete.
Maintaining the roads, however, is a costly job. According to the Asphalt Industry Alliance it would cost more than £12bn to restore all road networks in England alone to a reasonable condition.
Simon Hesp, a professor and chemical engineer at Queen’s University in Ontario, believes standard industry asphalt is not sustainable. “The problem with the composition is that it’s poorly controlled … it uses materials with poor performances,” he says. Hesp says the presence of certain oil residues lowers the quality of the concrete and is a key reason why roads are failing and many potholes need to be filled and cracks fixed.
But there’s not just a maintenance cost. Asphalt, dependent as it is on the oil industry, is resource- and energy-intensive, which is why the race is on to develop a greener alternative.
In Sydney an experiment is under way using printer toner waste blended with recycled oil to produce an environmentally friendly asphalt. And in the past few years there have been studies into the development of non-petroleum bioasphalts.
At Washington State University researchers developed asphalt from cooking oil, and last year academics at Wageningen University in the Netherlands found that lignin – a natural substance found in plants and trees – is another suitable replacement for crude oil bitumen. Other investigations have looked into the use of soybean and canola oil (rapeseed oil) and coffee grounds.
The WSU research, led by Haifang Wen and published at the end of 2013, concluded that the introduction of cooking oil can increase bioasphalt’s resistance to cracking . Wenn also claims it’s possible that, if commercialised, such bioasphalts could cost much less per tonne. The price of standard asphalt can fluctuate wildly as it’s dependent on the price of oil.
Hesp isn’t convinced that cooking oil is the way forward. He says, like petroleum, over time it will cause roads to fail because of weak bonds.
Bruno Bujoli, director of research at CNRS (Centre National de la Recherche Scientifique), agrees that the use of cooking oil “chemically modified to reach appropriate mechanical properties” could significantly affect quality. He also sounds a note of caution about food security, saying that asphalt based on vegetable oils could, if scaled up, affect food stocks
Bujoli recently played a key role in developing a bioasphalt from microalgae. It uses a process known as hydrothermal liquefaction, which is used to convert waste biomass, including wood and sewage, into biocrude oil. The chemical composition of the microalgae bioasphalt differs from petroleum-derived asphalt, but initial tests have concluded that it also bears similar viscous properties and can bind aggregates together efficiently, as well as being able to cope with loads such as vehicles.
How it will perform over time is yet to be determined. The findings were published in April.
Bujoli suggests that microalgae – also known for its use in the production of cosmetic and textile dyes – is a greener and more appropriate solution than agricultural oils. The latter, he says, should be kept for food production.
“The benefits of microalgae over other sources include low competition for arable land, high per hectare biomass yields and large harvesting turnovers. There is also the opportunity to recycle wastewater and carbon dioxide as a way of contributing to sustainable development,” he adds.
It’s a neat idea, with an admirable green mission behind it, but how much of an impact can it really have? Technology such as this is still in its infancy, suggests Heather Dylla, director of sustainable engineering at the National Asphalt Pavement Association, a US trade organisation for the paving industry.
“A lot of interesting work is being done in this area, looking at everything from algae, to swine waste, to byproducts from paper making. It’s worth exploring these alternatives, but we need to be sure they provide equivalent or improved engineering properties. We need to understand how they affect the recyclability of asphalt pavement mixtures,” she says.
She points to the “unique” advantage of asphalt when it comes to recycling. “Not only are the aggregates, which make up about 95% of [asphalt concrete], put back to use, but the bitumen can also be reactivated and used again as the glue that holds a pavement together.”
Microalgae could yet put the paving industry on the road to a greener future. For now though, there are plenty of challenges – from price to scalability – for Bujoli and his team to address if the bioasphalt is to be commercialised.
“This is our research focus for the near future. Our current laboratory equipment works in a batch mode,” explains Bujoli. “Scaling up the process will require the design of a large-volume reactor that can operate under continuous flow conditions.”
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