by V. Bhasya and B. H. Bharatkumar
American Concrete Institute ACI Materials Journal March 2018
Newswise — 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.
Solid waste generation
Construction and demolition waste consists of inert materials including concrete, plaster, metal, glass, wood, and plastics. These wastes, which are usually dumped in landfills, affect the landfills and significantly impact the environment; about 200 to 300 million tons of solid waste is generated annually in the United States alone. In other countries, such as India, the Central Pollution Control Board has estimated solid waste generation at about 48 million tons per year, of which 25 percent is from the construction industry.
Recycled concrete aggregates from construction and demolition wastes consist 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. Similarly, heating treatment can be used to remove the total attached mortar content by heating the RCA from 932ºF to 1382ºF (500º to 750ºC). The authors found, however, that only a few laboratory studies have been reported to evaluate the physical and mechanical properties of RCA treated with heat, and there is no available literature on the mechanical and durability properties of concrete. 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.
Studies, therefore, focus 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. Physical and mechanical properties of RCA were improved after heating and rubbing treatment (Fig. 1). The 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 was performed in Chennai, India in 2017 to research the economic feasibility of reusing construction waste. Findings indicated the treated RCA cost was higher when compared to NAs, mostly because of the high labor costs in Chennai. However, RCA and treated recycled aggregate could result in remarkably 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 landfills charges. Environmental benefits include less landfill space use, less soil and groundwater contamination, improved public image, and addressed environmental concerns.
The research can be found in a paper titled “Mechanical and Durability Properties of Concrete Produced with Treated Recycled Concrete Aggregate,” published by ACI Materials Journal.
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