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Much of the U.S. investigation focused on using crushed, hardened concrete as an aggregate in fresh concrete has been in mainline paving. Work on this subject started with a major endeavor in the 1980s in Minnesota. Regrettably, most of the exploration focused on using recycled concrete aggregate (RCA) as a foundation material to the pavement. But in other parts of the globe, many accept RCA as a significant aggregate source when appropriately intregated into the mix design process. For instance, Japan has used RCA for more than 20 years in structural concrete applications. RCA can be used for practical purposes in structural concrete. Dr. A. Ghani Razaqpur, a professor and chair of the Civil Engineering Department at McMaster University in Hamilton, Ontario, gave a presentation at the 2008 Concrete Technology Forum, sponsored by NRMCA in May in Denver. Razaqpur disputes the belief that concrete (plain or reinforced) made with RCA has inherently second-rate short- and long-term components. He supported his statement by highlighting the outcome contained in the paper, "The Key to the Design and Manufacture of High Quality Structural-Grade Recycled Aggregate Concrete." Razaqpur described how his team examined 14 different mix designs using RCA. They examined fresh and hardened components (slump, fresh and hardened density, elastic modulus, compressive strength, stress-strain correlation, creep, and shrinkage) and compared the outcome to related reinforced concrete made with fresh structural concrete. The outcome of his effort is a unusual mix-proportioning method for concrete prepared with coarse recycled concrete aggregate, in which RCA is treated as a two-phase material comprising mortar and natural aggregate. The residual mortar in RCA is considered part of the total mortar (fresh plus residual mortar) in the mix. "By testing an extensive number of specimens, we have demonstrated that the projected practice would result in producing high-quality, structural-grade concrete, with predictable outcome," said Razaqpur. Razaqpur hopes this new approach to mix proportioning will promote using RCA in structural concrete. At the same event, Bill Palmer, senior engineer at Complete Construction Consultants, a Boulder, Col.-based consulting firm, presented several supplementary resources for information on using recycled aggregates in concrete. He listed numerous organizations that can give help and technological information: The first time I'd seen one, was outside a restaurant where I lived and they did a pretty first-class job, using the pieces of an aged sidewalk, resourcefully. I never seen something like this before and like most of us know, there is a first time for everything. Building concrete stairs by means of recycled materials got me thinking regarding other things that we could build with recycled building products. People are not just using recycled concrete for stairways, they're using them for undersized retaining walls. Recycled concrete retaining walls and stairways can be constructed from slight to big sections of broken sidewalks and driveways. Simply position the damaged pieces into attractive shapes, until you have something that functions as a flight of stairs. Start from the substructure and work your way up, until you've formed a beautiful recycled concrete stairway. If you are planning on building a retaining wall out of used concrete, merely stack these materials on top of each other, until you've created the retaining wall, you envisioned. I don't recommend building retaining walls higher than 24 inches with these types of materials.
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Recycled Concrete Reinforced Concrete
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