Concrete is a long--lasting material, but roads, buildings and other concrete structures can exhaust their usefulness. When these structures are demolished, the debris can get a new life through concrete recycling.
Meet the crusher
Concrete recycling begins by transforming large pieces of concrete into useable rock. The tool for this job is a crusher. Both portable and stationary models are available. The main difference between them is that stationary crushers use an impacter that is mounted in a fixed location, Eagle Crusher Company, Inc., marketing manager Jeff Hillis explains. Portable crushing systems can be towed between locations behind truck tractors, though it may take several loads to move all the parts. In addition to an impacter, each crusher system has a chassis with separating screens, a hopper, a feeder and conveyor belts to move the crushed material out.
Concrete waste is put into the hopper using a front--end loader or excavator. A vibrating feeder moves the material into a transition point where fine material is separated out to reduce wear on the crusher. The concrete then goes into the impacter. To make aggregate, a horizontal shaft impacter is required.
“This type is state--of--the--art for recycling concrete because it makes a cubicle product,” Hillis says. “If you’re going to use aggregate in concrete, asphalt or most approved applications, it has to be cubicle.” Many other crushers are cone and jaw type, creating elongated pieces that don’t lock together as cubicle product does.
“Our crushers feed the concrete in whole, rebar and everything,” Hillis says. “As the material is crushed, it goes up the belt and under an electromagnet which removes the metals off to one side. This is a significant part of the business, because if you had to remove the rebar first, it would not be economically feasible to crush concrete. The machine does it quite efficiently.” The metal is sold as scrap, adding another product to the recycler’s bottom line.
The resulting concrete product depends largely on the screening process. Triple deck screens make three products plus the fines while double deck screens yield two products plus fines. Hillis said most people make either 1 1/2--in. down to 1--in. material for use as road base.
While recycled aggregate can be used in new ready--mix, the resulting product is normally limited to curbing, pavement or barriers in the United States rather than for structural concrete because of quality concerns, notes Dr. Abdul Chini, professor and director of the Rinker School of Building Construction at the University of Florida.
Eagle’s smallest crusher can crush 60 tons of material per hour, while its largest can process more than 600 tons per hour. When the crushing process is completed, the recycled rock is put in conical product piles about 100 ft. high.
A portable crusher can go to a site where a building is being demolished and replaced, and the resulting crushed concrete can be used right there for fill. “This reduces costs because you don’t have to truck the material,” Hillis points out. “People who live in the area like it because there aren’t concrete trucks going up and down the street everyday.”
Little impact on operators
Although those working close to a crusher may need ear protection, the machines are surprisingly quiet. “The highest decibel sound on a crushing site is the backup alarm on a loader,” Hillis says. The second noisiest thing is the crusher’s vibrating screen.
“Generally dust is controlled by a water spray bar,” he notes. “That meets most requirements.” Foam and mist systems are also available, but most people use water at all of the transition points.
Economics of concrete recycling