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Concrete Reuse Gets Bolt from the Blue

Thursday, March 7, 2013

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A flash of inspiration has led researchers to a remarkable new lightning-laced advance in recycling concrete.

With the aid of lightning bolts, researchers at the Fraunhofer Institute for Building Physics' Concrete Technology Group in Holzkirchen, Germany, have developed a method to zap concrete into its components—cement and aggregate.

lightning strike

l∞senut / Flickr

Fraunhofer Institute’s Concrete Technology Group has developed a new concrete recycling method with help from lightning and some research from the 1940s.

This jolting method of recycling is much better than crushing it, according to the scientists' research announcement.

Reducing CO2

Concrete manufacturing accounts for eight to 15 percent of global carbon dioxide production, according to the researchers. One ton of burned cement clinker of limestone and clay releases 650 to 700 kilograms of CO2.

“And when it comes to recycling waste concrete, there is no ideal solution for closing the materials loop,” the team said. Germany alone generated 130 million tons of construction waste in 2010, the scientists note.

Although various groups are pursuing concrete recycling, the current processes produce huge amounts of dust; at best, the stone fragments end up as sub-base for roads, not building material, the German team said.

concrete recycling
© Fraunhofer IBP

The new method can break down concrete into its constituent parts.

“This is downcycling,” explains Volker Thome, a researcher from the institute.

To curb some of the harmful carbon emissions and efficiently reduce concrete into workable ingredients for new construction, the team revived a method that Russian scientists developed in the 1940s and “put it on ice,” they said.

Pulsed Lightning

The new process involves using “electrodynamic fragmentation”—very short pulses of induced lightning—to separate concrete into aggregate and cement materials.  

“Normally, lightning prefers to travel through air or water, not through solids,” said Thome. To ensure that the bolt strikes and penetrates the concrete, the experts used the Russian scientists’ expertise.

water

Malene Thyssen / Wikimedia Commons

Lightning prefers to travel through water rather than a solid.

More than 70 years ago, they discovered that dieletric strength (the resistance of every fluid or solid to an electrical impulse) is not a physical constant, but changes with the duration of the lightning, the scientists said.

“With an extremely short flash of lightning—less than 500 nanoseconds—water suddenly attains a greater dielectric strength than most solids,” explains Thome.

'A Small Explosion'

Fraunhofer researchers learned that when concrete immersed in water is hit with a 150-nanosecond bolt of lightning, the discharge runs through the concrete and weakens it.

“In the concrete, the lightning then runs along the path of least resistance, which is the boundaries between the components, i.e. between the gravel and the cement stone,” the researchers said.

Explained Thome: “The pre-discharge which reaches the counter-electrode in our fragmentation plant at first, then causes an electrical breakdown.

“At this instant, a plasma channel is formed in the concrete, which grows within a thousandth of a second, like a pressure wave from the inside outwards. The force of this pressure wave is comparable with a small explosion.”

Currently, the laboratory fragmentation plant can process one ton of concrete waste per hour. Thome said the researchers have a goal of at least 20 tons per hour, which could be market-ready in less than two years.

   

Tagged categories: Concrete; Construction; Recycled building materials; Research

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