Better Roads Staff
That’s why as an engineered material, RCA must be tested and analyzed in a lab before being included in a structure or mix. In particular, the physical and mechanical properties of RCA vary with the quality and quantity of reclaimed mortar, which may affect the design of the structure or concrete mixture. These effects can be significant when making reclamation simpler by including lots of mortar, or minimal when efforts are made to eliminate as much reclaimed mortar as possible.
Late last year, the Michigan DOT produced a definitive guide for use of RCA. Prepared by Applied Pavement Technology of Urbana, Ill., for the Michigan Tech Transportation Institute, Using Recycled Concrete in MDOT’s Transportation Infrastructure: Manual of Practice (download by Googling the title) is an essential guide.
Michigan refers to RCA, the most widely used term, as crushed concrete aggregate (CCA), and observes how it’s different from virgin aggregates. “CCA has different properties than natural aggregate, largely because the resultant crushed material is composed of both the original natural aggregate and reclaimed mortar, which significantly affects the properties and behavior of materials produced with CCA unless specific steps are taken to account for it in the design and construction process,” the report says. “Moreover, the composition of CCA can be highly variable, and in addition to aggregates and reclaimed mortar may contain contaminates such as soil and clay balls, joint sealant, and asphalt or other construction waste.”
Freshly processed RCA/CCA also is highly alkaline and may contain chlorides that may limit is use or applicability, Michigan DOT says “Nevertheless, when its characteristics are properly considered and accounted for, CCA can be used effectively in a number of transportation infrastructure applications.”
Data collected from 2009 indicate that concrete pavements are recycled for transportation infrastructure applications in at least 41 states; moreover, about 140 million tons of CCA are produced in the United States per year, according to the American Concrete Pavement Association. “The material has been used in applications ranging from placement in various paving layers (surface, base, subbase) and as fill and embankment material,” MDOT says.
A major concern regarding the use of CCA in base layer applications is related to leachates, MDOT says. “CCA contains calcium hydroxide from the original cement hydration reaction,” according to the manual. “It is water-soluble, and when water flows through a CCA base, some calcium hydroxide will dissolve into the water. Subsequently, it interacts with atmospheric carbon dioxide to form calcium carbonate, precipitating out of solution and leaving deposits where the water flows. This is problematic if the precipitate clogs up elements of a pavement drainage system, such as filter fabrics, drainage pipes, and outlets.”
Some environmental concerns exist regarding the use of RCA/CCA as base material, primarily because of its alkalinity. However, the alkalinity rapidly decreases with time, and is not considered a major concern although some vegetation may be destroyed where runoff is discharged directly from a CCA base.
CCA contaminants may be encountered during the recycling process, including HMA overlays and patches, joint sealant, reinforcing steel, dowel and tie bars, and soils and foundation materials, the manual states. “Other contaminants may be present within the concrete itself, such as alkalis and chlorides from deicing salts. Efforts should be made to minimize the potential for introducing contaminants, especially if the CCA is to be considered for use in new concrete.”
RCA/CCA will exhibit lower specific gravity, which decreases with increasing amount of reclaimed mortar; higher absorption, which increases with increasing amount of reclaimed mortar; greater angularity; and increased abrasion loss, which increases with increasing amount of reclaimed mortar.
“In addition, CCA may contain unhydrated cement, which may alter its behavior and complicate stockpiling, especially the fine material,” according to the 2011 manual. “Finally, the fines produced during the crushing operation (those passing the No. 4 sieve) are coarse and angular, which tend to make CCA concrete mixtures very harsh and difficult to work.”
RCA/CCA in Pavements
RCA/CCA can be used with confidence in asphalt pavements. “Although not common, CCA can be used as an aggregate in asphalt paving layers,” the DOT says. “As with the application in base courses, CCA can produce a stable mixture because of its high angularity. And, because the asphalt cement forms a film around the aggregate, leaching and other complications from water interacting with the CCA are minimized.”
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