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Much of the U.S. research focused on using crushed, hardened concrete as an aggregate in fresh concrete has been in highway paving. Work on this issue began with a major endeavor in the 1980s in Minnesota. Sorry to say, most of the exploration focused on using recycled concrete aggregate (RCA) as a foundation material to the pavement. But in other regions of the globe, many accept RCA as a valuable aggregate resource when appropriately intregated into the mix design procedure. For example, Japan has used RCA for more than 20 years in structural concrete applications. RCA can be used actually in structural concrete. Dr. A. Ghani Razaqpur, a professor and lead 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 conflicts the perception that concrete (plain or reinforced) made with RCA has inherently mediocre short- and long-term properties. He supported his allegation by highlighting the results contained in the document, "The Key to the Design and Fabrication 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 comparable reinforced concrete prepared with fresh structural concrete. The outcome of his effort is a novel mix-proportioning technique for concrete made with coarse recycled concrete aggregate, in which RCA is handled as a two-phase material comprising mortar and natural aggregate. The remaining mortar in RCA is considered part of the entire mortar (fresh plus residual mortar) in the mix. "By testing an broad number of specimens, we have demonstrated that the projected approach would result in generating high-quality, structural-grade concrete, with predictable results," said Razaqpur. Razaqpur hopes this new approach to mix proportioning will support using RCA in structural concrete. At the same event, Bill Palmer, senior engineer at Complete Construction Consultants, a Boulder, Col.-based consulting firm, offered several supplementary resources for information on using recycled aggregates in concrete. He listed a number of organizations that can provide guidance and technological information: The first time I'd seen one, was outside a restaurant where I lived and they did a pretty good job, using the parts of an aged sidewalk, productively. I never observed anything like this before and like most of us know, there is a first point for everything. Building concrete stairs by means of recycled materials got me thinking about other things that we may perhaps construct with recycled building products. People are not just using recycled concrete for stairways, they are using them for minor retaining walls. Recycled concrete retaining walls and stairways can be built from slight to big sections of destroyed sidewalks and driveways. Simply position the ruined pieces into attractive shapes, until you have something that functions as a stairway. Start from the substructure and work your way up, until you have produced a beautiful recycled concrete stairway. If you are planning on building a retaining wall out of used concrete, just stack these materials on top of each other, until you have shaped the retaining wall, you envisioned. I do not suggest building retaining walls higher than 24 ins with these kinds of materials.
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Recycled Concrete Reinforced Concrete
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