Environmental suitability of recycled concrete aggregates used in geotechnical applications

摘要

Solid waste and by-products have been widely recycled as construction or geotechnical materials. They may contain several inorganic toxic chemicals such as heavy metals. Thus, a testing framework to evaluate their leaching behaviours and the relevant environmental impact needs to be developed. A parametric study was conducted using FE analysis to calculate the transport of the contaminant of concern (COC) in the surrounding subsurface to discuss the effects of influential factors such as the leaching concentration, infiltration rate, and flow rate of aquifer. In addition, a series of leaching tests was conducted for recycled concrete aggregates (RCAs) to determine the leaching potential/concentration of COC for the environmental impact assessment. Three different acceleration tests; wetting-drying, freezing-thawing and abrasion, which actually occur in field conditions, were conducted respectively to evaluate the effect of the exposure to these conditions on the leaching of hexavalent chromium (Cr(VI)) contained in the RCA, since physical and chemical properties of cement based secondary materials, such as RCA, may change under environmental conditions due to the loss of soluble constituents and/or the surface wearing. Exposure to these accelerated conditions, particularly the wetting-drying, promoted the leaching of Cr(VI). Leaching amounts of granular RCA exposed to these conditions are no more than that with the conventional batch leaching test, which is conducted for crushed RCA with < 2 mm in grain size. This finding suggests that the Cr(VI) leaching amount in the field can be conservatively estimated by the conventional batch leaching test.