Asphalt’s miraculous cure

Etienne Jeoffroy looks through his protective glasses, deep in concentration. With a small spoon, he stirs a copper-colored powder into a test tube. What looks like copper powder are actually iron oxide nanoparticles. When working with these nanoparticles, Jeoffroy has to put his hands in an ‘autoclave’, a glass chamber filled with inert gases on the inside. Normally, scientists from André Studart‘s group at ETH Zurich use this laboratory to carry out research into what are referred to as “bio-inspired materials”. Today, the workstation is occupied by the PhD student from Empa’s Road Engineering / Sealing Components lab: Jeoffroy wants to try to use the iron nanoparticles to revolutionize road construction.

In order to produce asphalt, a material called bitumen is used in road construction that is obtained from processing oil. Bitumen is jetblack, extremely viscous and sticky. It works like a kind of glue in the pavement of the road, holding the small stones and sand together. This glue crumbles due to wear and tear, differences in temperature or chemical substances like oxygen in the air. Cracks form in the asphalt over time. Initially they are microscopic, but they gradually increase under the constant weight of the traffic on the road. This eventually leads to the need for repairs on whole sections of road, which in turn results in high costs, construction sites and traffic jams.

To help avoid this in future, Jeoffroy developed a type of ‘healable’ bitumen. The idea is to close the cracks in the road surface while they are still small. “If you can see the cracks with the naked eye, it‘s already too late”, he explains. To make the material healable, Jeoffroy mixes iron oxide nanoparticles into the bitumen. When exposed to a magnetic field, they heat up. The heat is transferred to the bitumen, which reduces its viscosity significantly at temperatures of between just 50 and 100 degrees. As a result, it slowly begins to soften again, and closes the small cracks in the road surface. Maintenance for these types of roads would involve driving over them at intervals of about a year using a special vehicle that creates a magnetic field. This would repair the micro-cracks in the asphalt again and again, and the road surface would last considerably longer.

A few years ago, specialists at the lab already pursued a similar approach under the leadership of Manfred Partl. Instead of nanoparticles, the researchers used steel wool fibers at the time. But there were three weaknesses with that method. Firstly, it proved difficult to mix the fibers with the bitumen. Instead of spreading evenly, lumps formed in different places. This is what Jeoffroy calls the “ball of wool” effect. It led to localized overheating when the fibers heated up under the effect of a magnetic field. There was a risk of damage to the road surface. Another problem was corrosion on the fibers. Because they were made from steel wool, after a while rust formed on the surface.

But the main difficulty was that metal fibers of that size needed a long time to heat up in a magnetic field. This meant that it would have taken several minutes for half a meter of road to be repaired. If you extrapolate this figure for a 12-kilometer long section of road (roughly the length of the Northern bypass for Zurich), it could take between one and two months for the healing process to complete. And because a road has to be blocked in order to treat it with the magnetic field vehicle, such a long maintenance period would simply not be realistic.
So Jeoffroy looked at various approaches to resolve the weaknesses associated with the old method. First of all he used millimeter-sized steel particles instead of steel wool. This allowed him to get around the “ball of wool” effect, but it did not solve the problem of rust. And even though the time for heating up was a little shorter, it was still way too long.

Eventually Jeoffroy came across a solution in the field of medicine: magnetic hyperthermia is a procedure that has been used for some years now in treating cancerous tumors. It involves injecting magnetic iron oxide nanoparticles directly into the tumor and, like with the tarmac, exposing them to a magnetic field that heats them up. The aim is to ‘burn out’ the tumor from the inside and destroy it in this way.

Inspired by this method, Jeoffroy reduced the scale of the particles from millimeters to nanometers and replaced the metallic steel particles with magnetic nanoparticles. “The smaller the particles, the faster they heat up”, Jeoffroy explains. And he did in fact succeed in reducing the heating-up time to just a few seconds with the help of iron oxide nanoparticles used in cancer treatment. At the same time, the nanoparticles also solved the rust problem. Iron oxide is not a metal, and you can‘t have rust without a metal.

The tests with the nanoparticle bitumen were all very promising. “With asphalt that uses this kind of bitumen, cracks in the road surfaces will never again be the reason for needing to replace a section of road”, he announces. He also explains that the method is not harmful to human health. In any case the iron oxide nanoparticles are biocompatible, but they are so strongly bound in the bitumen that they virtually never escape again.

At present, the only catch with Jeoffroy‘s method is the price. The nanoparticles he uses are currently much too expensive for practical use. But Jeoffroy thinks he might also have a solution to this problem: he has found similar nanoparticles in a whole different industry that he believes are also suitable for use with his method. They are slightly bigger than the nanoparticles used in the treatment of cancer, but much cheaper. This makes the bitumen marketable.

Jeoffroy has already filed a patent for his development but still has to wait for it to be confirmed. He intends to use that time to try out the new particles and test his method in practice. “Everything worked in the lab. Now it all has to work in practice, too”, he says. So Jeoffroy will soon cast his lab coat aside, as it’s time for the PhD student to leave the research rooms behind and put his asphalt to the test on the road.