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Reclaimed Concrete Material
Reclaimed concrete material (RCM) can be used as coarse and/or fine aggregate in granular base. The properties of processed RCM generally exceed the minimum requirements for conventional granular aggregates. Being a 100 percent crushed material, processed RCM aggregates "lock up" well in granular base applications, providing good load transfer when placed on weaker subgrade. The lower compacted unit weight of RCM aggregates compared with conventional mineral aggregates results in higher yield (greater volume for the same weight), and is therefore economically attractive to contractors. For large reconstruction projects, on-site processing and recycling of RCM are likely to result in economic benefits through reduced aggregate hauling costs.
Reclaimed concrete material (RCM) is sometimes referred to as recycled concrete pavement (RCP), or crushed concrete. It consists of high-quality, well-graded aggregates (usually mineral aggregates), bonded by a hardened cementitious paste. The aggregates comprise approximately 60 to 75 percent of the total volume of concrete.
RCM is generated through the demolition of Portland cement concrete elements of roads, runways, and structures during road reconstruction, utility excavations, or demolition operations.
In many metropolitan areas, the RCM source is from existing Portland cement concrete curb, sidewalk and driveway sections that may or may not be lightly reinforced. The RCM is usually removed with a backhoe or payloader and is loaded into dump trucks for removal from the site. The RCM excavation may include 10 to 30 percent subbase soil material and asphalt pavement. Therefore, the RCM is not pure Portland cement concrete, but a mixture of concrete, soil, and small quantities of bituminous concrete.
The excavated concrete that will be recycled is typically hauled to a central facility for stockpiling and processing or, in some cases (such as large reconstruction projects), processed on site using a mobile plant. At the central processing facility, crushing, screening, and ferrous metal recovery operations occur. Present crushing systems, with magnetic separators, are capable of removing reinforcing steel without much difficulty. Welded wire mesh reinforcement, however, may be difficult or impossible to remove effectively.
Recycling
Reclaimed concrete material can be used as an aggregate for cement-treated or lean concrete bases, a concrete aggregate, an aggregate for flowable fill, or an asphalt concrete aggregate. It can also be used as a bulk fill material on land or water, as a shore line protection material (rip rap), a gabion basket fill, or a granular aggregate for base and trench backfill.
Aggregate Substitute
The use of RCM as an aggregate substitute in pavement construction is well established, and includes its use in granular and stabilized base, engineered fill, and Portland cement concrete pavement applications. Other potential applications include its use as an aggregate in flowable fill, hot mix asphalt concrete, and surface treatments.
To be used as an aggregate, RCM must be processed to remove as much foreign debris and reinforcing steel as possible. Reinforcing steel is sometimes removed before loading and hauling to a central processing plant. Most processing plants have a primary and secondary crusher. The primary crusher (e.g., jaw crusher) breaks the reinforcing steel from the concrete and reduces the concrete rubble to a maximum size of 75 mm (3 in) to 100 mm (4 in). As the material is conveyed to the secondary crusher, steel is typically removed by an electromagnetic separator. Secondary crushing further breaks down the RCM, which is then screened to the desired gradation. To avoid inadvertent segregation of particle sizes, coarse and fine RCM aggregates are typically stockpiled separately.
Information provided by www.tfhrc.gov/hnr20/recycle/waste/rcc3.htm
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