|what to do with the concrete canoe?|
“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.