Concrete, one of the most widely used construction materials in the world, is made up of 65 to 70 percent by volume of fine and coarse aggregates including sand, gravel and crushed rock. Global consumption of natural aggregate (NA) is estimated at 10 to 11 billion tons each year, of which approximately eight billion tons is used annually to make concrete. At this rate of use, there will be a critical shortage of NA in the future unless an alternative material for natural coarse aggregate is used. One source for alternative aggregate is recycled concrete aggregate (RCA) from construction and demolition waste.

Construction and demolition waste consists of inert materials including concrete, plaster, metal, glass, wood Heat-treated aggregate. Source: American Concrete InstituteHeat-treated aggregate. Source: American Concrete Instituteand plastics which are usually dumped in landfills. RCA derived from such waste consists of NA and attached cement mortar. This old attached cement mortar gives recycled aggregates their poor physical and mechanical properties, limiting their use to no more than 30 percent replacement in concrete for structural applications and making it necessary to remove them before use.

Four treatment methods are used to remove old attached cement mortar from RCAs: (1) mechanical; (2) acid; (3) microwave; and (4) thermal – all with varying outcomes. Only 10 to 20 percent of attached mortar can be removed using the mechanical method. Although acid treatment is difficult to apply in field applications, this method can be evaluated in the laboratory for total mortar content of RCA. Microwave treatment removes the attached mortar; however, for satisfactory results this application requires expertise and the ability to understand and moderate effects like uneven heating or thermal runaway. Also, for microwave treatment, field application is difficult, especially with large quantities. Thermal treatment can be used to remove the total attached mortar content by heating the RCA from 932ºF to 1382ºF (500º to 750ºC). As with the other treatments, there are difficulties in the field with heating treatment, specifically achievement of the high temperatures required to remove the mortar content.

Researchers from the CSIR-Structural Engineering Research Centre in Chennai, India focused on a combination of heating and mechanical treatment and the mechanical and durability properties of concrete. With this method, recycled aggregate was heated to a temperature of 482ºF (250ºC) and then dry-mixed in a pan mixture to remove the attached mortar. This method removed 70 to 80 percent of the attached mortar from the RCA, and addition of either fly ash or silica fume in concrete produced with treated RCA showed significant improvement in mechanical and durability properties when compared with conventional concrete.

A cost-benefit analysis performed in Chennai, India of the economic feasibility of reusing construction waste indicated the treated RCA cost was higher when compared to NAs, mostly because of high regional labor costs. However, RCA and treated recycled aggregate could result in markedly less expensive concrete than NA concrete when the cost of concrete is reported using only the necessary expenses to eliminate environmental impact. These include cost-savings from waste transportation and landfill charges. Environmental benefits include less landfill space used and reduced risk of soil and groundwater contamination.

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