Recycled concrete aggregate as road base: Leaching constituents and neutralization by soil Interactions and dilution Nautasha Gupta, Matt Kluge, Paul A. Chadik, Timothy G. Townsend ⇑ Department of Environmental Engineering Sciences, University of Florida, P.O. Box 116450, Gainesville, FL 32611-6450, USA article info Article history: Received 24 July 2017 Revised 31 October 2017 Accepted 6 November 2017 Available online xxxx Keywords: Recycled concrete aggregate Road base pH Soil Soil acidity Heavy metal leaching abstract Recycled Concrete Aggregate (RCA) is often used as a replacement for natural aggregate in road construc- tion activities because of its excellent mechanical properties, and this trend should increase as more transportation departments include RCA in specifications and design manuals. Concerns raised by some engineers and contractors include impacts from leachate generated by RCA, both from transport of metals to water sources and the impact of a high pH leachate on corrosion of underlying metal drainage pipes. In this study, RCA collected from various regions of Florida exhibited pH ranging from 10.5 to 12.3. Concentrations of Al, Ba, Cr, Fe, Mo, Na, Ni, Sb, and Sr measured using batch leaching tests exceeded appli- cable risk-based thresholds on at least some occasions, but the concentrations measured suggest that risk to water supplies should be controlled because of dilution and attenuation. Two mechanisms of pH neu- tralization were evaluated. Soil acidity plays a role, but laboratory testing and chemical modeling found that at higher liquid-to-solid ratios the acidity is exhausted. If high pH leachate did reach groundwater, chemical modeling indicated that groundwater dilution and carbonation would mitigate groundwater pH effects. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction The US Federal Highway Administration (FHWA) defines recy- cled concrete aggregate (RCA) as reclaimed portland cement con- crete (PCC) produced by crushing concrete pavement, bridges, sidewalks, curbing, and other concrete structures (FHWA, 2007). RCA is produced at mobile and fixed crushing operations, where crushed concrete is screened to produce products of desired size gradation. The aggregate retained on the 4.75 mm (No. 4) screen is typically referred to as ‘‘coarse aggregate” and the material pass- ing that screen is referred to as ‘‘fine aggregate” (FHWA, 2004). RCA has been shown an effective alternative to natural aggregate to meet the increasing demand for road construction materials such as concrete and asphalt pavement aggregate, and for use as road base or subbase, as it provides necessary mechanical and perfor- mance properties including bearing capacity, resilient modulus, and specific gravity (Bennert & Maher, 2008; Bozyurt et al., 2012; Wen et al., 2015). The US Geological Survey (USGS) reported that about 21.8 million tons of RCA were sold in the US in 2014 (USGS, 2016). Road base has been reported as the most common RCA construction application, with uses such as asphalt pavement aggregate, concrete aggregate, rip-rap, and general fill providing lesser markets (USGS, 2000). Since RCA is a waste-derived product and has been in contact with possible contaminants during its life cycle, some environmen- tal concerns from using RCA as unencapsulated road base have been raised (FHWA, 2004; Reiner, 2008). Percolation of rain water through an RCA road base (which should be small in the case of well-maintained paved roads) results in leachate, and the transport of inorganic pollutants to the surrounding environment has been reported as a potential environmental risk. Heavy metals occur in relatively small amounts in RCA, a result of their occurrence in nat- ural aggregates and waste products added during the production of concrete (e.g., fly ash, slag; Aydilek, 2015; Mullauer et al., 2015), as well as contact with chemicals during the life of the concrete. Total elemental analysis of RCA has shown the presence of aluminum (Al), arsenic (As), antimony (Sb), barium (Ba), calcium (Ca), cad- mium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), magnesium (Mg), molybdenum (Mo), nickel (Ni), potassium (K), selenium (Se), sodium (Na), strontium (Sr), vanadium (V), and zinc (Zn) (Aydilek, 2015; Chen et al., 2012, 2013; Engelsen et al., 2006; Garrabrants et al., 2004). The magnitude of RCA metal concentrations (mg/kg) differs depending on the RCA source and the demolition and recycling process utilized to produce the crushed aggregates. Total element concentrations are less of a concern than leachable concentrations as direct exposure should https://doi.org/10.1016/j.wasman.2017.11.018 0956-053X/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: ttown@ufl.edu (T.G. Townsend). Waste Management xxx (2017) xxx–xxx Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman Please cite this article in press as: Gupta, N., et al. Recycled concrete aggregate as road base: Leaching constituents and neutralization by soil Interactions and dilution. Waste Management (2017), https://doi.org/10.1016/j.wasman.2017.11.018