Concrete Recycling Bolts Into the Future

German scientists are experimenting with sending quick electrical impulses – i.e., lightning bolts – through submerged concrete to break it down into its component stone. Future "fragmentation plants" could process hundreds of tons of concrete waste daily, expanding possible usage of recycled material, which totals millions of tons worldwide.

A metaphorical lightning bolt often accompanies a great idea. German researchers, however, are applying literal lightning bolts to an old idea – one with the potential to revolutionize the re-use of concrete.

One of the most widely used building materials in the world, concrete is made up of cement, water and aggregate. Aggregate is a mixture of stone particles, such as gravel or limestone grit, mixed into cement in various sizes.

“This is an enormous material flow, but at the moment there is no effective recycling method for concrete rubble,” explains Volker Thome, from the Fraunhofer Institute for Building Physics IBP, at the Concrete Technology Group in Holzkirchen, Germany.

If the stone particles could be separated from the cement stone, the gravel could be reused as an aggregate in new cement – a huge step forward in recycling waste concrete.

“The recovery of valuable aggregate from waste concrete would multiply the recycling rate by a factor of around ten, and thereby increases it to 80 percent,” said Thome. If it becomes possible to develop a cement substitute from waste concrete, industry emissions of CO2 would also be substantially reduced.

A ‘Striking’ Solution
To achieve these goals, Thome revived a method that Russian scientists developed in the 1940s: electrodynamic fragmentation, which breaks concrete down into its individual components – aggregate and cement stone – by sending an electric impulse through it.

Russian scientists discovered more than 70 years ago that dielectric strength, i.e. the resistance of fluids or solids to an electrical impulse, is not a physical constant, but fluctuates with the duration of the lightning.

Given an extremely short flash of lightning – less than 500 nanoseconds – water attains a greater dielectric strength than most solids, said Thome. In simple terms, this means that if the concrete is under water and researchers generate a 150-nanosecond bolt of lightning, the discharge will run though the solid and not the water.

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

The “pre-discharge” impulses weaken the material mechanically, and cause an electrical breakdown when they reach the counter-electrode in the fragmentation plant. A plasma channel grows in the concrete within a thousandth of a second, creating a pressure wave “comparable [to] a small explosion,” said Thome.

The concrete breaks apart and returns to its basic components. The research team can currently process one ton of concrete waste per hour in the laboratory fragmentation plant. “To work efficiently, our goal is a throughput rate of at least 20 tons per hour,” said Thome.

An efficient fragmentation plant could be ready for market launch in as little as two years.