MnDOT: Recycled concrete aggregate saves money, promotes sustainability

Using recycled pavement as aggregate in new concrete mixes can save money and promote environmental sustainability. New design methods allow engineers to create more durable mixes from recycled aggregate than in the past, reducing the need for virgin aggregate, a diminishing and expensive resource.

“This report shows that a lot can be done with recycled aggregate,” says Matt Zeller, Executive Director, Concrete Paving Association of Minnesota. “We can get the strength up to that of concrete with virgin aggregate by bumping our mix design and lowering our water-to-cement ratio.”

“Concrete pavement made with RCA can be beneficial both economically and environmentally,” says Farhad Reza, Professor, Minnesota State University, Mankato, Department of Mechanical and Civil Engineering.

Reza served as the project’s principal investigator.

When pavements are due for reconstruction, the old pavement is frequently crushed to aggregate-sized particles and used as the base course for new pavement. In the 1980s, MnDOT and other state transportation agencies began using such recycled aggregate in the concrete course as well. But this latter practice was discontinued by the early 1990s due to mid-slab cracking observed in pavements constructed with such concrete. Using recycled concrete aggregate (RCA) in the base course has continued, however.

Newer mechanistic-empirical design methods and performance engineered mixtures have led to improved RCA mixtures. For example, concrete mixtures now have lower water-to-cement ratios. These advances present an opportunity to re-evaluate the use of recycled aggregates in concrete mixes, which aligns with two important trends: the diminishing availability of virgin, high-quality aggregate, and the growing federal emphasis on sustainable design. Using recycled concrete as aggregate fulfills the three basic principles of sustainability: performance, environmental stewardship and cost-effectiveness.