Cold roads ahead
Research proving it’s possible to make chip sealed roads without using dangerous hot bitumen will be key to the future of roading.
At a glance
Currently many chip sealed roads are laid using potentially explosive hot bitumen and kerosene..
Roading expert Road Science has been searching for a way to surface roads with a completely cold bitumen emulsion.
But it needed to know how the cold substance would flow in standard spraying equipment.
Extensive flow modelling by Callaghan Innovation scientists proved that the new technique could work and demonstrated what type of equipment would be needed.
An explosive risk
You’d be hard pressed to find anyone who thinks exploding bitumen tankers are an acceptable risk.
Yet this is a hazard the Kiwi roading industry faces every day.
Because New Zealand has an extensive roading network yet light traffic compared with more populated countries, most of our roads are surfaced with cost-effective chip seal – a hot cutback bitumen layer sprayed at 180 degrees which is covered with stone chips.
Kerosene is added to the hot cutback bitumen to make it runnier and stickier so that the chip adheres well. With roading tankers heating the bitumen to 180 degrees and kerosene having a flash point of 38 degrees it doesn’t take a scientific genius to realise that creates a danger.
“Even though there is a code of practice for working with hot cutback bitumen there’s a potential explosive risk pretty much every time it’s used,” says Nik Vishwanath, a Process Engineer with Road Science’s Technical Development Team.
The worst happens around once a year during the peak sealing time. Aside from the paramount health and safety concerns each explosion means the loss of a sprayer or tanker worth around $700,000. “If you think about the amount of kit that’s being destroyed just because we haven’t switched over to a better alternative, it’s staggering,” he says.
Road Science has found that alternative. A sister company to the Downer group, over the last 10 years the road surfacing expert has developed bitumen emulsions which can be sprayed on the road at around 80 degrees. “We’ve always been advocates for spraying bitumen emulsion because it’s so much safer for people in the field,” Nik says.
It had developed the technology to the point where it was able to spray the emulsion at 80 degrees. But the next challenge was figuring out how to spray it totally cold, he says.
Modelling experts to the rescue
There are numerous advantages to using bitumen emulsion, Nik says:
- It adheres to the chip better because it’s water-based.
- The emulsion gets drawn into the stone more effectively.
- It saves on the energy bills.
- It can be laid in hotter and colder weather.
- It emits 50 percent less CO2 over its life cycle.
- There are many vital safety benefits.
Road Science had done some work on how to spray bitumen emulsion completely cold, but quickly realised greater expertise was required, Nik says. It sought the help of Callaghan Innovation’s Advanced Materials team which did initial testing. Then the Materials team handed the baton over to Alan Caughley, Team Leader Mechanical Engineering.
“We needed further development, and they suggested Alan as a scientist who could do some modelling so we could better understand our emulsions and the spraying equipment we might need,” Nik says.
Alan’s work has allowed Road Science to understand how the product performs at different temperatures, and the restrictions on its current equipment if it was going to be spraying at lower temperatures.
“It highlighted that if we were spraying cold bitumen emulsion we would definitely need new specialist equipment,” Nik says.
The work involved rheology, Alan Caughley says – the study of flow properties. “We made a viscosity model of the fluid and calculated what happens as it flows through things,” he says.
The Callaghan Innovation team then validated their work. They got a spray truck test rig and tried the cooler emulsion out, matching their results with Road Science videos.
“We were able to see what conditions produce spray and which don’t,” Alan says. “We could say ‘this is what’s happening, this is what it means, and these are the parameters’.”
The information Alan provided was “above and beyond”, Nik says. “This really shows when we present the work to customers and internally within the Downer group. They’re amazed that someone’s actually looking at it in this level of detail.”
“We were extremely impressed and happy with the work. It’s been one of those projects where almost everything has gone to plan,” Nik says.
Nik and Alan co-presented the results at the annual Fluids in New Zealand conference earlier this year.
The research is potentially ahead of its time but will be key for the roading sector going forward, Nik says.
The stalling point is achieving the level of investment in the new equipment required to spray cold emulsions. “Currently the industry is in two minds about whether to do 100 per cent bitumen emulsion or whether to stay with using a combination of emulsion and hot cutback bitumen.”
He believes that the sector will move towards 100 percent emulsion, however.
“It’s a no-brainer when it comes to health and safety, so I’m pretty confident that’s going to be the case eventually,” Nik says.
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