You have probably seen waste concrete piled up at demolition sites. Some experts estimate that half of all the concrete ever used has been made in the last two decades. Recycling concrete helps solve many different problems.
Besides water, concrete is the most widely used material in the world. It combines both fine and coarse aggregate with some kind of cement. When it hardens, it becomes a very strong material. Ancient concrete structures thousands of years old still exist.
Although environmentally benign in its ingredients, producing it requires tremendous energy. As a consequence, it has high carbon dioxide emissions. And we make a lot of it––about 8 million cubic meters annually, or about one cubic meter for every person in the entire world’s population!
The ancients regarded their concrete structures as permanent. We hardly count anything as permanent. We build concrete buildings, roadways, bridges, dams, and other structures. Then we demolish them to build something else. Demolition also produces large amounts of brick and asphalt rubble. It can also be recycled.
What happens to all of the rubble? Formerly, it all went to landfills, an expensive way to dispose of it. Transporting such heavy material can cost as much as $25 per mile for every ton of waste. Tipping fees at landfills can be $100 per ton or more. Avoiding that cost only begins to explain the benefits of concrete recycling.
The processes of recycling concrete
A paddle crusher converts deteriorated barriers into one-inch bits. A paddle wheel inside spins and crushes the concrete, and afterward, the pieces are moved to a vibrating sorting box. These pieces of a once massive concrete slab are then recycled in roadwork, erosion, and dust control projects.
The same equipment that reduces boulders to gravel and sand can also crush used concrete. Double- and triple-deck screeners can produce and separate two or three different products simultaneously. Whether virgin or recycled aggregate.
Therefore, unlike other kinds of recycling, recycling concrete and similar materials does not require special equipment.
The equipment doesn’t have to be at a recycling facility. Smaller, portable crushers can do the job on site, making it possible to demolish, say, a sidewalk and use the debris to construct the replacement.
Compared to making virgin aggregate, recycling concrete presents some extra problems. It often contains dirt, plastic, wood, or other organic material. Reinforced concrete contains steel rebars. An increasing number of applications use fiber-reinforced concrete instead. The tiny fibers are more difficult to separate from the concrete than steel.
It is necessary to remove the contaminants before the rubble can be reused. Electromagnetic separators can remove structural steel. Air separators, water flotation, even picking contaminants out by hand can remove other materials. These inefficient methods produce a lot of dust, which is a health hazard for workers.
Recently, researcher have begun to explore electro-fragmentation to remove contaminants. The process involves immersing the concrete in water and subjecting it to very short flashes of lightning. With bursts of lightning less than 500 nanoseconds, the electricity actually travels through the solid more easily than through the water. It also goes through all the air pockets in the concrete with the force of a small explosion to separate concrete from contaminants.
So far, this method cannot process concrete waste fast enough to be commercially useful. But it appears to be the only one that works on fiber-reinforced concrete.
The benefits of recycling concrete
Garden terrace made from an old concrete basketball court
Recycling concrete and similar materials saves money—especially when done on site with portable crushers.
It also reduces emissions from transporting them to landfills—especially considering that many cities now must use landfills in another county. Some demolition companies that recycle concrete may have multiple locations.
Recycling extends the useful life of quarries and other natural reserves, too. Mining rock to make aggregate, like other mining operations, uses a lot of water and energy. And like other mined materials, the earth does not have an inexhaustible supply.
Gravel or sand recycled from construction rubble can be used in most of the same ways as virgin aggregate. It makes a lighter concrete that’s cheaper to transport. Or the crushers can leave coarser pieces suitable for such uses as sub-bases for highway construction.
Builders seeking LEED certification can receive points for using recycled concrete. In addition, reef restoration projects can use larger pieces of concrete as a foundation for coral.
As we use more and more concrete, we’re also recycling more and more of it. Without concrete recycling, construction projects cannot be sustainable.
Sources:
Used concrete, once for the landfill, now heads to recycling facilities / Tracey Schelmetic, Thomas Insights. October 31, 2012
Ways to recycle and reuse concrete / Juan Rodriguez. The Balance Small Business. January 7, 2019
Why not recycled concrete? / William G. Gilroy, Notre Dame News. February 8, 2016
Photo credits:
Concrete waste pile. Public domain from Wikimedia Commons
Paddle crusher. US Air Force photo by Matt Coleman-Foster
Garden terrace. Some rights reserved by brewbooks
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