Cracked asphalt surfaces could be helped to recover using sunflower oil capsules buried inside roads.

 

Sunflower oil used in cooking could soon become an essential ingredient for constructing asphalt roads with ‘self healing’ properties, following a trial by Highways England, reports Mike Walter.

 

Television show MasterChef provided the inspiration for one engineering professional to develop not an elaborate culinary dish but a novel way for highways to repair themselves, before small surface cracks get larger or potholes start to form. Alvaro Garcia Hernandez, an assistant professor at the Nottingham Transportation Engineering Centre, was sat watching an amateur chef on the show create a dish where sunflower oil was ‘encapsulated’ inside the shell of another item of food.

 

Alvaro wondered if this idea of suspending liquid inside an outer casing could be applied to asphalt, so that the contents of a capsule can be released to provide healing properties to a road matrix.

 

Sunflower oil has been found to reverse the effects of ageing in bitumen used in asphalt by reducing the viscosity of the binder. But a way was needed to encase the oil securely in a capsule so that its release could be triggered only when necessary, as the surfacing deteriorates.

 

When the capsules break the oil mixes with the bitumen that surrounds the aggregate in the road to fill cracks, helping to prevent small defects in a highway from becoming more serious.

 

He joined forces with his Chilean friend Dr Jose Norambuena-Contreras, an assistant professor at the country’s University of Bio-Bio and postdoctoral researcher at the University of Nottingham. The pair have spent recent years perfecting the production of calcium-alginate microcapsules and their distribution through an asphalt surfacing mixture, prior to its application.

 

The University has since formed a collaborative partnership with Highways England to drive forward the self-healing technology. Following the successful completion of laboratory and pilot trials Highways England is set to lay what is claimed to be the world’s first trial of self-healing asphalt containing capsules on its road network, at a location likely to be somewhere in the Midlands.

 

International students from the University of Nottingham have since run accelerated load tests on the samples in its laboratory to simulate traffic loadings, speed up the ageing process of a bituminous surface course and to allow the capsules to activate. Cracked asphalt samples were found to recover to full strength two days after the capsules released their contents.

 

Around five kilograms of capsules were needed per tonne of asphalt; a mix ratio of around 0.5%. Preliminary results are said to demonstrate that the capsules can resist the mixing and compaction processes without significantly reducing the physical and mechanical properties of asphalt and that the capsules increase the self healing properties and durability of road material.

 

“I previously worked to develop self healing roads in the Netherlands and Switzerland that feature metal fibres,”

 

Alvaro says. “An induction heater would pass over the surface to melt the bitumen and fill cracks in the road. But the problem with that is you need a large machine, which is expensive and requires a road to be closed to traffic.”

 

He adds: “We needed a different way of self healing roads without the use of an external aid, so decided to design capsules containing oil that can break by themselves when the mechanical loading on a carriageway has caused it to crack.”

 

One of the most difficult aspects of the technology is designing capsules that are brittle enough to release their contents when needed but are strong enough to withstand the pressures exerted on a road by heavy and repeated traffic, explains Jose Norambuena-Contreras.

 

“The capsules also need to be large enough to contain the right amount of oil to react with the bitumen, but not too large that they may damage the asphalt.”

 

Several years of producing different strengths of capsule that can withstand a variety of environmental  conditions has led the team at Nottingham to arrive at a product ready to be tested on a live site.

 

Highways England’s senior pavements advisor and project sponsor Robin Hudson-Griffiths MCIHT says of the trial: “If we can reduce the number of maintenance interventions to repair worn surfaces by using innovative techniques like this, then it is better for motorists and better for us. It might also extend the life of a road by two or three years, reducing costs and delays associated with resurfacing.

 

“There is great interest in this trial from our pavements team, which is always looking for innovations that may enhance the network, give road users a better experience and provide value for money,” he adds.

 

The next step for the researchers is to further refine the geometry of each capsule and better understand how they degrade over time. They also want to develop a suite of capsules of differing strengths that can break under different types of stresses and which suit a range of asphalt mixes and road conditions.

 

In due course the team will experiment with a range of vegetable oils and look to see if waste cooking oils from restaurants could be put to good use in asphalt capsules.