ConExpo attendees also had the opportunity to hear Charles Luedders of the Federal Highway Administration provide a summary of a five-state study the agency conducted on advanced uses of recycled concrete aggregate (RCA).
The 2004 study (summarized in “Choice Cuts,” January/February 2005 Construction & Demolition Recycling, pgs 20-24) examined how and to what extent RCA was being used in California, Michigan, Minnesota, Texas and Virginia.
The study determined that Texas is “a large user of RCA materials,” with private industry and municipalities consuming more than 60 percent of the RCA produced in the Lone Star State.
The Texas Department of Transportation (TxDOT) has approved the use of RCA in its highways for 10 years, finding engineering, economic and environmental benefits.
The agency acknowledges some initial hurdles that it had to overcome, including “problems with mix workability,” according to Luedders. “Contractors overcame this hurdle by [modifying] their process control program.”
The FHWA report notes that TxDOT has “performed training and continually presents information to its districts concerning the performance of the projects it has completed around the state.”
The Virginia Department of Transportation (VDOT) has worked “to provide a neutral playing field for recycled and virgin aggregates,” according to the report. Additionally, VDOT has established an income tax credit on the purchase price of recycling machinery, although it is restricted to fixed plants and not mobile equipment.
The use of RCA is permitted by Michigan Department of Transportation (MDOT) specifications, with the agency’s strategy being to permit use “if it enhances or equals the performance of virgin material in the final product.”
Permissible uses in Michigan for RCA include as aggregate within Portland cement concrete poured for curbs and gutters, sidewalks, barriers, driveways, temporary pavement, interchange ramps and shoulders. In hot-mix asphalt, the material can be used as aggregate for base course, surface course, shoulders, approaches and patching.
The Minnesota Department of Transportation (Mn/DOT) requires a 60-year pavement design life on its high-volume freeways and a 35-year design life on other highways. “The factors have contractors shying from [RCA’s] use in the concrete pavement, since the belief is aggregate washing would be required to produce usable aggregates,” the report notes.
According to information provided by the California Department of Transportation (CalTrans), “Most of the concrete pavement removed from existing highways and streets in California is processed and used as aggregate base throughout the state.”
Using RCA as part of a concrete pavement mix appears to have little historical backing from CalTrans. The report notes that the City of San Francisco is developing a specification that permits RCA’s use in concrete curbs, gutters and sidewalks.
The study has concluded that 38 states are using RCA as an aggregate base, but just 11 are putting RCA back into Portland cement concrete.
The study’s conclusion hails Texas as “a leader in the use of RCA as an aggregate in concrete pavement” and says the Lone Star state provides “an illustration of how recycled concrete aggregate can be re-used as an aggregate in a quality concrete pavement surface.”