Recycled Concrete Slab gains importance because it protects geological resources

Much of the U.S. exploration focused on using crushed, hardened concrete as an aggregate inside fresh concrete has been in road pavement. Work on this issue started with a major effort in the 1980s in Minnesota. Sorry to say, the majority of the research focused on using recycled concrete aggregate (RCA) as a foundation material to the pavement.

But in other areas of the globe, many accept RCA as a helpful aggregate source when appropriately intregated into the mix design development. For instance, Japan has used RCA for more than 20 years in structural concrete applications.

RCA can be used effectively 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 disputes the perception that concrete (plain or reinforced) made with RCA has inherently low-grade short- and long-term components. He supported his statement by highlighting the outcome contained in the essay, "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 results to comparable reinforced concrete prepared with fresh structural concrete.

The outcome of his work is a novel mix-proportioning procedure for concrete made with coarse recycled concrete aggregate, in which RCA is handled as a two-phase material comprising mortar and organic aggregate. The residual mortar in RCA is considered part of the overall mortar (fresh plus remaining mortar) in the mix. "By testing an extensive number of specimens, we have demonstrated that the projected procedure would result in producing high-quality, structural-grade concrete, with predictable results," 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 Total Construction Consultants, a Boulder, Col.-based consulting firm, offered a number of supplementary resources for information on using recycled aggregates in concrete. He listed a number of organizations that can give assistance and technological information:

The first time I'd seen one, was outside a restaurant where I lived and they did a pretty fine job, using the portions of an old sidewalk, productively. I never seen something like this before and like most of us know, there's a first point for everything. Building concrete stairs by means of recycled materials got me thinking regarding other things that we might build 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 designed from small to big sections of ruined sidewalks and driveways.
Simply position the damaged pieces into decorative designs, until you have something that functions as a flight of stairs. Start from the substructure and work your way up, until you have fashioned a beautiful recycled concrete stairway.

If you're planning on building a retaining wall out of second-hand concrete, purely stack these materials on top of each other, until you've shaped the retaining wall, you envisioned. I do not suggest building retaining walls higher than 24 inches with these types of materials.

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