Leaching of Alkaline Substances and Heavy Metals from Recycled Concrete Aggregate Used as Unbound Base Course

Abstract

There is an increasing trend toward using recycled concrete aggregate (RCA) in construction. However, concerns have been expressed in laboratory studies about the highly alkaline leachate, high-pH leaching patterns, and release of heavy metals from RCA (e.g., chromium, lead, and arsenic). In this study, RCA materials from a wide geographical area were investigated. Physical, chemical, and mineralogical properties were characterized and laboratory column leaching tests were conducted. Two field test sites were installed and instrumented, one at the Minnesota Road Research (MnROAD) facility in Minnesota and a second at a surface parking facility at the University of Wisconsin–Madison. The MnROAD site showed a neutral leachate pH (between 6.5 and 8.0) after 7 months of service life [1.2 pore volumes of flow (PVFs)]; however, the pH from column leaching tests ranged between 11.0 and 12.5 and showed no decline over 100 PVFs. Because of the large difference between the field and laboratory results, two samples of RCA in Wisconsin, one from a recent demolition of concrete pavement (WR-F) and the other from an RCA stockpile with an age of 5 to 10 years (WR-SP), were used in the Wisconsin field site to verify the Minnesota data and investigate the long-term leaching of alkaline compounds and heavy metals. WR-F RCA started with a high leachate pH (12.6) and remained constant for the first 5 PVFs; however, WR-SP RCA started at a lower pH (7.3) but showed a gradually increasing trend, up to pH = 11.9 for the first 2 PVFs. The pHs from both RCAs were higher than the U.S. Environmental Protection Agency drinking water standard. Arsenic, chromium, lead, and selenium in the RCA field leachate were observed to be higher than the maximum contaminant level for the U.S. drinking water standard.