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Todd Loesch/MEDILL

You'll never see them, but carbon nanotubes hold the promise of concrete that's virtually crack-free. Potholes like these could go the way of the horse and buggy.

Paving a smarter, greener road

by Todd Loesch
Jan 28, 2010

A road that heals

The road you drive on could one day become green and self-healing, all thanks to nanotechnology.

Surendra Shah, a professor in Northwestern University’s engineering department, says that nanotechnology could soon become a part of America’s concrete industry. And that would not only make our roadways more environmentally friendly, it would also make them smarter.

When cement and water mix to create concrete, carbon dioxide, a key greenhouse gas, is released. Currently, almost 12 billion tons of concrete are used annually, which translates into a lot of CO2.

But Paul Tennis, from the Portland Cement Association, said nanotech advances could greatly reduce the industry’s carbon footprint. Carbon nanotubes, for example, extend concrete’s lifespan, meaning that less concrete would need to be made.

Shah also says nanomaterials could help concrete a green substance. Fly ash, which is a byproduct of coal plants, can be used in concrete. But the problem is that fly ash slows the chemical reactions down.

“When you’re constructing a building, you don’t want to wait months before you can remove the form,” Shad says. Adding carbon nanotubes speeds up those reactions, making fly ash a more practical ingredient.

Nanotechnology could also make concrete ‘smart’. Shah says other labs are looking into nano-sensors. When a large enough crack formed, a special type of bacteria would be added, forming a bioceramic seal.

Potholes, rough roads and heel-catching cracks in the sidewalk may soon be a thing of the past, as researchers at Northwestern University apply nanotechnology to create relatively crack-proof concrete.

Northwestern engineer Surendra Shahand and his team are adapting cutting-edge technology to improve a decidedly low-tech substance by infusing concrete with carbon nanotubes. Carbon nanotubes are strong, flexible pipe-like arrangements of carbon atoms too small to be seen by most microscopes.

“Normally when you think of cement and concrete, you think high-rise buildings and big structures,” Shah said, “And the connection with nano is not often very clear.”

Concrete is one of the most common substances produced on the planet, used in everything from roads and sidewalks to the new 168-story Burj Khalifa tower in Dubai.

“We use 2 tons per capita per year of concrete, can you imagine that?” Shah said. Worldwide, that translates into nearly 12 billion tons of concrete per year. And, as countries such as China and India continue to develop, that amount will almost certainly go up.

But concrete needs to be replaced, as any driver will tell you, and it also poses an environmental dilemma. Essentially, concrete is a mixture of cement, water and aggregates such as sand or gravel. During the process of creating concrete, carbon dioxide is naturally produced.

That’s where Shah’s work comes in. At the atomic scale, concrete looks like a bunch of tennis balls packed together. The chemical reactions that take place between cement and water create nanovoids, or spaces, between the balls. This means that chips, cracks and potholes actually start at the nanoscale.

“If you want to prevent, or delay or make concrete more durable, then you want to make sure that those nanoscale-cracks are prevented,” Shah said. Adding nanotubes reinforces the concrete and makes the packing more efficient. Construction workers today use the same idea by reinforcing the concrete with metal bars, but nanotechnology offers those nanoscale tubes instead.

Paul Tennis, who works with the Portland Cement Association in Skokie, said that problems with traditional concrete occurs during the notorious freeze-thaw cycle in the winter months.

“Concrete is porous, it’s a porous material,” he said, “A common analogy is to think of it as a sponge.”
Water seeps into the pores, and when that water freezes, it expands, which can cause cracks. This gets worse during the winter months, when salt is used to keep roads from freezing. Salt works itself into the concrete and corrodes the steel reinforcing bars, further damaging the structure.

Using carbon nanotubes would make the concrete nearly impenetrable, greatly extending the lifespan of roads, bridges and buildings.

“If you can make concrete very impermeable, so that salt doesn’t go through, then you can extend the life to a hundred years rather than 20,” Shah said.

Not surprisingly, this new technology isn’t cheap. But when that higher price tag is spread out over a much longer lifespan, it could become cost-effective. And this doesn’t factor in the reduced cost to the environment.

“We have to include all of this, not only the material cost,” Shah said.

Tennis also said that cheaper isn’t always better.

“We can make concrete at cheap as possible, but in an Illinois-type environment it may only last four five years,” he said.

Shah hopes that nanotube-reinforced concrete will make its way into roads and buildings in five years, but acknowledges that technological advances are hard to predict.

“I hope faster,” he said. “But it takes time.”