LEACHING OF ALKALINE SUBSTANCES AND HEAVY METALS FROM RECYCLED CONCRETE AGGREGATE USED AS UNBOUND BASE COURSE

摘要

There is an increased trend towards use of recycled concrete aggregate (RCA) inconstruction. However, concerns have been expressed about the highly alkaline leachate, high-pHleaching patterns, and heavy metal release from RCA (e.g., Cr, Pb, and As) in laboratory studies. In thisstudy, RCA materials from a wide geographical area were investigated. Physical, chemical, andmineralogical properties were characterized and laboratory column leaching tests were conducted. Twofield test sites were installed and instrumented, one at the MNROAD facility in Minnesota and a second atsurface parking facility of the University Wisconsin-Madison. The MNROAD site showed a neutralleachate pH (between 6.5 and 8.0) after 7 months of service life (1.2 pore volumes of flow, PVF);however, pH from column leaching tests ranged between 11.0 and 12.5 and showed no decline over 100pore volumes of flow (PVF). Due to the large difference between the field and laboratory results, twosamples of RCA in Wisconsin, one from a recent demolition of concrete pavement (WR-F) and the otherfrom an RCA stockpile with an age of 5-10 years (WR-SP), were used in the Wisconsin field site to verifythe Minnesota data and investigate the long-term leaching of alkaline compounds and heavy metals. WR63F RCA started with a high leachate pH (12.6) and remained constant for the first 5 PVF; however, WR64SP RCA started at a lower pH (7.3) but with a gradually increasing trend, up to pH = 11.9 for the first 2PVF. The pH from both RCAs were higher than the USEPA drinking water standard. Arsenic, chromium,lead, and selenium in the RCA field leachate were observed to be higher than the maximum contaminantlevel (MCL) for the USEPA drinking water standard.