Recycled Concrete: Rocky Road to Growth


LAS VEGAS—Recycling the world's most common building material is certainly "green," but using recycled concrete in buildings still faces obstacles, both real and perceived.

It’s green from the environmental and economic standpoints, says materials engineer and consultant Dr. Kevin MacDonald. But there are risks as well.

MacDonald, from Beton Consulting Engineers LLC, of Mendota Heights, MN, shared some of his expertise about recycled concrete’s risks, rewards, properties and processes Wednesday (Feb. 6) with a room full of contractors, engineers, designers and manufacturers at the 2013 World of Concrete in Las Vegas.

The annual four-day international event provides the commercial concrete and masonry construction industry first-hand knowledge of the latest in concrete technology.

Concrete Exposed

“Close to 50 percent of municipal solid waste is concrete, asphalt and masonry rubble,” MacDonald said in his presentation, "Recycled Materials in Concrete: Projects, Specifications, Risks and Rewards."

Rechanneling that amount of concrete debris is therefore benefiting limited landfill space, among other things.

“We are aware that we all leave our imprint, mark on the world, whether we like it or not,” he said.

New York
Alfred Hutter / Wikimedia Commons

Depleting aggregate sources near larger cities, such as New York City, suggest that a different method to making concrete should be explored.

Concrete manufacturing has a high carbon footprint, and using “a little less new material” in the process will help  curb that, he said. For example, using concrete aggregate, supplementary cementitious materials like fly ash or slag to replace some Portland cement and recycled mix water would reduce the carbon footprint, he said.

Moreover, MacDonald said, depleting aggregate sources near larger cities around the country also suggest that a different method should be explored.

Concrete recycling is an expanding business that is “in its infancy,” according to the Construction Materials Recycling Association. To date, recycled concrete has been used as an aggregate base for roads, landscape materials, ready mix concrete, asphalt pavement, soil stabilization and pipe bedding, according to the association.

Benefits Explored

Various benefits exist from the use of recycled concrete, MacDonald said.

“Less energy is required to produce crushed concrete, compared to mining aggregate from a quarry,” according to his presentation. And its use helps to conserve virgin aggregate supplies.

In addition, owners are interested in using local, recycled materials in projects to achieve green-building certifications, and using more recycled aggregates would help achieve those goals.

However, restrictions that prohibit the use of recycled materials without technical merit need to be removed, standards need to be developed, and other obstacles must be addressed before its use can make a significant impact moving forward, MacDonald noted.

ASTM C33, Standard Specification for Concrete Aggregates, permits the use of crushed concrete as a source of aggregate materials, he said.

Properties and Studies

MacDonald cited two studies in his presentation. One, conducted by the School of Mines at the University of Colorado, indicated that concrete made from crushed concrete aggregates was 10 percent lighter and more porous than that made with virgin aggregates.

“Since the aggregate was lighter, the concrete was easier to place and strike off,” according to the presentation materials.

Kevin MacDonald

MacDonald is a materials engineer and consultant who specializes in concrete materials and construction.

That study also found that finishing with steel trowels and magnesium floats was the same compared to virgin aggregate, and finishers reported that the concrete was more “creamy” or “fat” and “easier to finish,” according to the presentation.

Another study conducted by the National Ready Mixed Concrete Association found that compared to virgin aggregate, crushed concrete aggregate had “lower specific gravity, higher absorption, higher percentage of 200 fines, and lower aggregate weathering potential as measured by the sulfate soundness test.”

“Concrete made with recycled materials have (sic) properties that are as good, or better than, conventional concrete,” MacDonald said.   

Risks Noted

However, certain risks are inherent in the concrete-recycling process. For example, attention should be paid to the demolished concrete; if it is in good condition, there is reason to believe that it will perform well in the future, MacDonald said.

Also, a number of precautions must also be taken with regard to stockpiles of crushed concrete. Those include quality control measures like keeping the pile moist, conducting absorption and relative density tests weekly, looking out for slump-retention issues, and others.

Moreover, concrete with recycled aggregate may require more engineering to achieve the required workability and strength, he noted.

Despite the risks, MacDonald concludes that concrete made with recycled materials has far more economical and environmental benefits.

About the Presenter

MacDonald is a Licensed Professional Engineer in Minnesota and Ontario, Canada, and is a fellow of the American Concrete Institute (ACI).

He is co-chairman of Committee 130 B (Sustainability); vice-chairman of ACI 306 (Cold Weather Concreting); and a member of Committee 620 (Laboratory Technician Certification) and ACI 302 (Construction of Concrete Slabs on Ground).

He has published a number of papers on concrete durability and practical aspects of the production of high-performance concrete using large quantities of recycled materials. He holds a bachelor's degree in chemical engineering and a master's degree and Ph.D. in engineering materials from the University of Windsor in Ontario.


Tagged categories: Building materials; Concrete; Conferences; Good Technical Practice; Green building; Green design; Recycled building materials; World of Concrete

Join the Conversation:

Sign in to our community to add your comments.