A five-year European research study into a paving material made from rubber tire crumb, using polyurethane as a binder instead of bitumen, has proven it dramatically lowers road noise, but also highlights more work needs to be done.
Dr. Luc Goubert of the Belgian Road Research Centre (BRRC) says the premise was to investigate whether the paving formula would reduce road noise, not to develop it as a replacement for traditional asphalt.
"It costs much more than asphalt so you would only use it in places where road noise is an issue and in that case it is cost effective when you consider the cost of noise barriers," he says. "For example, we found in our tests on roads that tire noise was reduced by up to 10 decibels. To effect that change with noise barriers you would have to build them about six metres high which is very expensive."
The BRRC is a non-profit organization involved in the construction, management and operation of road infrastructure in Belgium. Previous research has identified many components of road noise, such as tire vibrations from unbalanced wheels or imperfections in the tire materials, road surface issues, vibrations from the tread types and even resonance in the air-filled space of the tire itself.
The challenge then was to work with established data to formulate a paving material that was elastic but durable.
What the researchers came up with is a "poroelastic" material made from granulated tire rubber, which has the added bonus of being a useful way to recycle used tires. The granules are mixed with stone and then bonded with polyurethane. The key factor is the elasticity which acts to reduce noise, but there is also an issue with durability and that’s where the team has focused its efforts.
They dubbed the project PERSUADE — PoroElastic Road SUrface: an innovation to Avoid Damages to the Environment.
Tests were conducted across Europe by paving sections of roads with the material and then measuring the results. Nations included Sweden, Denmark, Poland and Slovakia.
"This is not rubber asphalt," says Goubert. "That has been around for a long, long time. This is without bitumen so it is completely different but has also been around since the 1970s in Sweden."
He says in -40 C conditions in Sweden researchers found the road surface actually offered more friction and traction to tires than traditional asphalt. It also wears differently and reduces dust creation.
Ironically, the research program started when bitumen was probably at its highest price point, with oil costing more than US$100 a barrel in 2011. It has since fallen to below US$30, making the new material even less cost-effective compared to traditional asphalt.
Pricing, however, is cyclical and asphalt is a relatively small cost factor in the traditional asphalt formula. The real driver was noise.
Unlike previous incarnations of asphalt with tire crumb as an extender, there were no issues with surface water creating treacherous conditions or with freezing, Goubert explains.
"The material is porous so it drains away," he says. "In fact, when we did the safety tests for fire we had trouble getting the car to burn. We poured 20 litres of gasoline under the car and normally with asphalt we’d have huge flames within seconds. With this material though it all drained away and we couldn’t get it to burn."
Despite those positive findings, the negative results will send the researchers back to their laboratories to find a better way to bind the stone and bind the mix to the sublayer.
"What we found is that the material is not as durable as asphalt and that it tends to start separating with the stones getting loose," he says. "This is a particular problem with lorries (trucks) which really tear it up. One lorry passing is like 20,000 cars."
Different methods of laying the paving down were tried, he says. First it was spread with a traditional roadbuilding machine and then compacted as traditional asphalt would be. This worked but the long-term results showed the top layer delaminated from the substrate. The other method was to create one-metre-by-half-metre slabs with the material bonded to a concrete base with epoxy glue. These "tiles" were then laid onto the road bed.
It worked well enough but it’s an awkward and somewhat impractical roadbuilding technique, Goubert says.
Also, he adds, while the butt joint edges of the "tiles" seemed to hold up, there are concerns how they will wear and age over time.
The team has just released its technical reports and executive summaries, having wrapped up the five-year project in August 2015. The next phase will be to investigate which epoxies might work to bond the surface layer to the substrate and also to look at different formulations of polyurethane to better bond the aggregate.
"We have been using commercially available polyurethane but we will have to look at what else is available," says Goubert. "We’re also looking at epoxies."