Lightning, strike!

Today, I turned in the final chapter of my dissertation - and haven't a brain cell left to spare.  Fortunately, the world is full of brilliance, and my momentary dimness is easily replaced with the fascinating news from the other bright minds.  Thus with full acknowledgement, I am posting a research news released this month by the Frauenhofer institute.

what to do with the concrete canoe?

Whether the Pantheon in Rome or the German concrete canoe regatta, whether ultralight or decorative: concrete is unbelievably versatile and is the world’s most widely used material – next to water. It is made of cement, water and aggregate, a mixture of stone particles such as gravel or limestone grit in various sizes. However, the CO2 emissions, which are mainly the result of cement production, are problematic: the production of one ton of burned cement clinker of limestone and clay releases 650 to 700 kilograms of carbon dioxide. This means that every year 8 to 15 percent of global CO2 production is attributable to concrete manufacturing. And when it comes to recycling waste concrete, there is no ideal solution for closing the materials loop. In Germany alone the quantity of construction waste amounted to almost 130 million tons in 2010.

“This is an enormous material fl ow, but at the moment there is no effective recycling method for concrete rubble” explains Volker Thome from the Fraunhofer Institute for Building Physics IBP from the Concrete Technology Group in Holzkirchen. The current method is to shred the concrete, which produces huge amounts of dust. At best, the stone fragments end up as sub-base for roads. “This is downcycling,” explains Thome, in other words, simply the reutilization of raw materials, the quality of which deteriorates from process to process. On the other hand, if it were possible to separate the stone particles from the cement stone, the gravel could easily be reused as an aggregate in new cement – a first decisive step in the direction of recycling waste concrete. “The recovery of valuable aggregate from waste concrete would multiply the recycling rate by a factor of around ten and thereby increase it to 80 percent,” says Thome. If it were also possible to obtain a cement substitute from waste concrete, the cement industry’s CO2 emissions would be considerably reduced. To achieve these goals Thome revived a method that Russian scientists already developed in the 1940s then put on ice: electrodynamic fragmentation. This method allows the concrete to be broken down into its individual components – aggregate and cement stone.

Recycling valuable components

Using this approach, the researchers in Holzkirchen are unleashing a veritable storm of lightning bolts. “Normally, lightening prefers to travel through air or water, not through solids,” says Thomas. To ensure the bolt strikes and penetrates the concrete, the expert uses the Russian scientists‘ expertise. More than 70 years ago they discovered that the dieletric strength, i.e. the resistance of every fl uid or solid to an electrical impulse, is not a physical constant, but changes with the duration of the lightning. “With an extremely short fl ash of lightning – less than 500 nanoseconds – water suddenly attains a greater dielectric strength than most solids,” explains Thome. In simple terms, this means that if the concrete is under water and researchers generate a 150 nanosecond bolt of lightning the discharge runs preferably through the solid and not through the water.