What’s the world’s most widely used material? Water, of course. What’s the world’s second-most widely used material? It’s concrete. The composite of cement, water and aggregate — usually crushed limestone, granite or possibly sand – has been used for at least 3,000 years. Its relatively harmless contents don’t seem to make it a green villain — but it’s not the contents that pose the problem. It’s the energy intensity of the production of concrete, which is high in emissions. In fact, about 8 to 15 percent of the world’s carbon dioxide emissions are attributed to concrete manufacturing.
About 8 billion cubic meters of concrete are manufactured each year, equaling a staggering one cubic meter for every person on Earth.
Waste concrete is everywhere — in every building demolition and replaced bridge and roadway — which is why interest in concrete recycling is rising. In years past, used concrete was simply dumped into landfills. Today, many construction projects have turned to concrete recycling not only to improve environmental conditions, but also to save money.
Thanks to mobile, on-site processing equipment that can crush concrete at up to 600 tons per hour, concrete recycling (some call it “downcycling” since it’s not actually returned to its pure raw materials) can allow some of the old material to be reused, lowering the need to buy new materials, and can help the construction industry avoid the high costs of haul-off, which can run as high as $25 per ton per mile, as well as landfill fees, which can exceed $100 per ton.
Recycled concrete has a number of uses. It can be included in new asphalt pavement as a substitute for virgin aggregate in laying a base for road work. It has been used in building slabs and foundations, sidewalks and curbs, residential streets, commercial buildings, storm drains, curbs, airport tarmacs and in concrete paving, according to the Construction Materials Recycling Association (CMRA). It can also be used for soil stabilization, pipe bedding and landscape materials such as rock walls, retaining walls and water features.
The use of recycled concrete has been endorsed by most government agencies that regulate building and building materials. The American Society for Testing and Materials (ASTM) and the American Association of State Highway and Transportation Officials (AASHTO) have accepted recycled concrete as a good source of aggregate for new concrete, and these groups have set quality standards for recycled concrete. The U.S. Department of Transportation’s (DoT) Federal Highway Administration has ruled that recycled concrete poses no threat to the air, soil and water, and advises that with careful design, engineering and application, recycled concrete can reduce or eliminate the need to search for and extract new, virgin materials. According to the DoT, 38 states use recycled concrete aggregate for road sub-base and 11 states recycle it into new concrete.
Recycled concrete aggregate not only meets applicable standards, it makes a very high-quality product and can be used in concrete and asphalt products with better performance over comparable virgin aggregates (not to mention lower costs). It’s also lighter: It weighs 10 to 15 percent less than virgin products. Finally, it’s one of the fastest routes to reducing landfill waste streams and minimizing environmental impacts, according to the CMRA.
That’s not to say it doesn’t have drawbacks. The process of creating recycled concrete is more than a little labor intensive. For starters, the waste chunks of concrete must be crushed. After that, it’s pre-sized, sorted, screened and removed of any contaminants.
The leftover chunks of used concrete are crushed and screened with primary jaw crushers or impact crushers, cone crushers or impactors that can take pieces that are up to four feet wide. Depending on how the recycled material is going to be used, a secondary cone or impactor may be required, followed by one or two screenings of the resulting material.
Often, magnets are used to remove rebar from the concrete. Something called a “scalping screen” removes dirt and foreign particles. Finally, a fine harp deck screen separates the fine material from the coarse aggregate. Beyond this, further cleaning may be required to ensure the end-product is free of dirt, clay, wood, plastic and organic materials. There are a number of methods to accomplish this, including water flotation, hand-picking, air separators and electromagnetic separators.
Not only are these methods complex, they’re also inefficient and produce dust, which can be harmful to human health. As a result, industry researchers have been looking for a better, easier way, and recently some German researchers may have found one.
“This [old concrete] is an enormous material flow, but at the moment there is no effective recycling method for concrete rubble,” said Dr. Volker Thome of the Fraunhofer Institute for Building Physics IBP from the Concrete Technology Group in Holzkirchen Germany.
The process involves inflicting lightning strikes on the old concrete while it’s immersed in water.
“Normally, lightning prefers to travel through air or water, not through solids,” Thome said, noting that the process uses inspiration from the work of Russian scientists who discovered 70 years ago that the dieletric strength (the resistance of every fluid or solid to an electrical impulse) is not a physical constant, but one that changes with the duration of the lightning.
“With an extremely short flash of lightning – less than 500 nanoseconds – water suddenly attains a greater dielectric strength than most solids,” said Thome. By immersing the concrete under water and hitting it with a 150-nanosecond bolt of lightning, the researchers can ensure that the electricity runs through the solid concrete and not the water. The lightning, of course, follows the path of least resistance and goes through the tiny air pockets between the components and particles of the cement, weakening the bond between the components and easily separating them.
“The force of this pressure wave is comparable with a small explosion,” said Thome. The slabs of concrete are broken apart and the material returns to its basic components. The output is impressive for a project still in the lab: The researchers say that they can currently process one ton of concrete waste per hour. The scientists’ goal is to refine the method so that it can process about 20 tons per hour. They believe the method could be ready for commercialization in about two years.
“The recovery of valuable aggregate from waste concrete would multiply the recycling rate by a factor of around 10 and thereby increase it to 80 percent,” said Thome.
By stepping up concrete recycling, the world’s building industries could greatly save energy compared with the more traditional processes of mining, processing and transporting new, virgin aggregates. It would also greatly lower the concrete industry’s carbon footprint while at the same time keep millions of tons of unsightly broken concrete out of the world’s landfills. And all it might take is a little lightning.