The United States is hungry for concrete. According to the U.S. Geological Survey, in 2014 the U.S. produced 80.5 million tons of Portland cement and 2.2 million tons of masonry cement. Portland cement is a primary ingredient in concrete, and most of the cement produced domestically went to the production of concrete. Though concrete is a sustainable material the amount produced every year results in significant environmental impact, making the development of recycled aggregate sources of dire import.

Producing concrete in the U.S. requires mining billions of tons of natural aggregates (such as sand and gravel) each year, and mining for these aggregates isn’t environmentally friendly. Transporting the aggregates to concrete-production facilities after mining is costly, and it also produces an abundance of greenhouse gases as well. Using recycled aggregates for concrete production will not only help to conserve natural aggregate resources and limit greenhouse gas production, but will help the construction industry save money.

Why Do We Need Sustainable Aggregates?

     The construction industry tends to rely on “virgin” natural aggregates. In order to keep up with concrete production levels, U.S. companies need to mine billions of tons of raw natural aggregates each year, and while some might assume that aggregates like sand are nearly limitless, they are a finite resources. In some parts of the world, high-quality natural aggregates for use in concrete are beginning to run low. For example, in the United Kingdom, sand and gravel resources are being consumed twice as fast as they can be replenished. Additionally, mining for any natural resources, whether it’s coal or gravel, requires reshaping the land, stripping away natural flora, and displacing local fauna. In short, mining for natural aggregates damages nature. Construction companies in certain cities, such as in Toronto, Canada, are beginning to use up natural aggregate resources close to their bases of operation, which pressures the companies to travel further and further to find quality aggregates, causing transportation costs to gradually rise. 

     Traveling is also a major environmental issue; aggregates need to be transported to concrete-production facilities, which means loading these resources onto greenhouse-gas emitting vehicles and trains. For example, in California alone, from 1981 to 2010, construction companies in the state consumed roughly 180 million tons of construction-grade aggregates annually. The California Geological Survey estimates that if the aggregates were moved in 25-ton truckloads with an average of 50-mile round-trip deliveries, then during that time frame those trucks burned up to 47 million gallons of expensive diesel fuel, and they added 520,000 tons of CO₂ to the atmosphere each year. If the round-trip distances increased to 100 miles, then CO₂ emissions actually jumped to 1 million tons on an annual basis—and that’s just in California. The transportation costs are extremely high, and it is an area where costs can be cut. Employing readily available recycled aggregates can help cut down on these emission and expenses. 

The Solution: Recycled Materials

     The Construction and Demolition Recycling Association estimates that construction and demolition companies in the United States generate 325-million tons of recoverable construction-related materials each year. Some of these materials can be used as recycled aggregates for concrete. The average new home, for instance, uses120 tons of sand, gravel, and stone, and one mile of interstate highway in the U.S. uses 85,000 tons of aggregates, as well. When demolition companies demolish homes, roads, or other large structures, they should make an effort to recover some of the wasted concrete and aggregates that are inevitably left behind.

    Companies can use mobile processing facilities situated near demolition sites to recover left-behind aggregates and broken-up concrete to produce recycled aggregates. Not only does this cut back on having to mine natural aggregates for construction purposes, but it also cuts down on the costs and emissions associated with shipping the aggregates from a far-off mine to a concrete-production facility. The construction industry generates millions of tons of reusable waste each year. While much of this material is recycled or reused, there is still a significant amount of waste that is simply ignored and transported off to the nearest landfill. Instead of using up precious and limited natural resources, the construction industry should look to recycled aggregates—by doing this, they can cut expenses and protect the environmental simultaneously.

The Challenges

     There are some potential challenges to using recycled aggregates. While using recycled aggregates won’t hamper a batch of concrete’s strength, the Portland Cement Association notes that using recycled aggregates may affect a batch’s durability. Recycled aggregates have a higher absorption rate than natural aggregate concrete, which means that it contracts or “shrinks” a little differently when drying. However, this roadblock isn’t an impossible one to surmount—some construction companies have found that blending different ratios of recycled aggregates (such as a mix of fine, e.g. sand, and coarse aggregates) helps to increase durability. Other researchers have found that the differences can actually improve certain concrete durability properties. Despite these contradictory findings, recycled concrete aggregates show great promise for helping reduce the environmental impact of concrete materials.

     More experimentation and research is needed in order to fully understand how to use recycled aggregate concrete effectively. Comprehending the characteristics of this type of concrete will take time, which is why the construction industry needs to begin researching and using recycled aggregates immediately. The sooner the industry begins investing in recycled aggregates, the sooner it can start conserving finite and fragile resources, cutback on transportation emissions, and also slash expenses dramatically. Time is of the essence, and in order to protect the environment and save money, the construction industry needs to turn to recycled aggregates now.

Matthew P. Adams is an Assistant Professor of Civil and Environmental Engineering at New Jersey Institute of Technology. He is an internationally recognized expert on the use of recycled concrete as a replacement for natural aggregate in new concrete.