In 1986, Illinois rebuilt 4.14 miles of I-57 using recycled concrete; 20 years later, load transfer efficiency across transverse cracks still averaged over 90%

Picture a section of highway being excavated, crushed into gravel-sized aggregates, and re-buried into the ground as fresh pavement. Sounds like the sort of short-term solution one would expect to decay to dust within a season, but in the mid-1980s, the state's transportation department actually did just this along a stretch of Interstate 57 near Effingham. And twenty years later, engineers returned to the site to see how well their experiment had borne up.

As per a 2009 Illinois Center for Transportation research report by Jeffrey Roesler and J. Gregory Huntley, things were going swimmingly. Between 1986 and 1987, the Illinois Department of Transportation (IDOT) excavated a four-and-a-tenth-mile stretch of an older, faulted concrete roadway, repurposing the broken-up aggregate as the primary component in a ten-inch-thick continuously reinforced concrete pavement (CRCP).

After two decades of wear and tear, researchers took core samples of the material, tested its deflection under standard 9,000-pound test loads, and assessed its general load-sharing qualities across the transverse cracks in the pavement: a sort of structural examination of the long-forgotten cousin of the more familiar practice of recycling asphalt pavement. Recycling concrete at this scale is no small matter: a 2004 FHWA national overview of recycled concrete aggregate use projected that U.S. aggregate production, then around two billion tons a year, would grow to more than 2.5 billion tons annually by 2020, and it put building-demolition waste alone at roughly 123 million tons a year: a volume state transportation agencies have increasingly looked to crushed concrete aggregate to help absorb as a substitute for virgin rock aggregate.


What the study shows twenty years later

While the researchers' primary finding (as stated in this press release for the study) is that the section of the road excavated and repurposed as a CRCP was doing fine under normal traffic loading conditions, it does highlight one significant weakness that is likely to constrain the technique's wider adoption. Specifically, while the pavements displayed excellent load distribution characteristics across the transverse cracks, with load transfer efficiencies averaging above 90 percent and minimal reductions in these efficiencies across twenty years of service, but those transverse cracks also formed much closer together than normal: about 1.5 feet apart on average, versus the 4-to-6-foot spacing typical of Illinois concrete pavements.

This appears to be due to the recycled aggregate's greater shrinkage upon drying, lower tensile strength than normal aggregate, or a combination of both, but the pavement's main structural issue has been longitudinal cracks along the pavement length. Separately, the section’s single biggest maintenance problem over its life has been longitudinal cracking: cracks running parallel to the direction of travel, above the steel reinforcement bars. That distress has been traced to the “tube feeding” method IDOT once used to place the reinforcing steel, a construction technique the agency has since discontinued, rather than to the recycled aggregate itself.

The bigger picture for green infrastructure

That said, the report’s authors are careful to note that this isn’t necessarily the pavement’s final chapter: by the time of their 2009 study, the section was already more than 20 years old, showing increasing signs of distress, and an asphalt concrete overlay was programmed to be placed within the next couple of years.

A 2023 University of Texas (Center for Transportation Research) report titled “Increase the Allowable Content of Recycled Crushed Concrete Aggregate in Class P Concrete” found that the United States currently produces well over 140 million tons of recycled concrete aggregate (RCA) each year. That same report describes its own successful field trial: a CRCP section built with recycled fine and coarse aggregate on I-10 near Sealy, Texas, as evidence that high levels of recycled aggregate can perform well in structural pavement; it doesn’t reference the Illinois I-57 project. For an industry producing recycled concrete at that scale, building smarter, more durable roads matters, and so does using materials more efficiently, reusing old concrete in new roads rather than sending it to a landfill.