Q. We are in the early planning stage of a major commercial project. During site preparation, the general contractor will require us to demolish and crush a large quantity of existing concrete used in the structure and pavement. In fact, looking through our records, it appears we supplied the concrete on the original project.
The owner's engineer has asked us to develop a proposal in which we offer a concrete mix design that includes using this crushed concrete as aggregate. Are there any guidelines or resources that can help?
A. Much of the U.S. research focused on using crushed, hardened concrete as an aggregate in fresh concrete has been in mainline paving. Work on this topic began with a major effort in the 1980s in Minnesota. Unfortunately, most of the research focused on using recycled concrete aggregate (RCA) as a base material to the pavement.
But in other parts of the world, many recognize RCA as a valuable aggregate source when properly intregated into the mix design process. For example, 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 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 inferior short- and long-term properties. He supported his claim by highlighting the results contained in the paper, “The Key to the Design and Production 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 properties (slump, fresh and hardened density, elastic modulus, compressive strength, stress-strain relationship, creep, and shrinkage) and compared the results to similar concrete made with fresh structural concrete.
The outcome of his work is a novel mix-proportioning method for concrete made 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 proposed method would result in producing high-quality, structural-grade concrete, with predictable results,” said Razaqpur.
Razaqpur hopes this new approach to mix proportioning will encourage using RCA in structural concrete.
At the same event, Bill Palmer, senior engineer at Complete Construction Consultants, a Boulder, Col.-based consulting firm, offered some additional resources for information on using recycled aggregates in concrete. He listed several organizations that can provide assistance and technical information:
Both presentations and papers, along with 74 other papers and presentations which were presented at the 2008 Concrete Technology Forum: Focus on Sustainable Development are included on a CD-ROM (item number 2PCTF08) available at www.nrmca.org.