Leaching Characteristics of Recycled Materials Used for Roadbed Construction.

Large quantities of aggregates are needed for use as base or sub-grade materials in road construction. With ever increasing cost of mining virgin aggregates as well as the cost of disposing waste materials, there is a strong interest in recycling wastes in road construction provided they are mechanically and hydraulically suitable and they do not impact the surrounding environment. A study was undertaken to characterize the suitability of four recycled materials in terms of water retention, hydraulic conductivity, shear strength, resilient modulus, and leaching characteristics relative to that of virgin aggregates. The recycled materials were reclaimed asphalt pavement (RAP), reclaimed concrete material (RCM), fly ash (FA), and foundry sand (FS). They were mixed in with virgin aggregates to create seventeen mixtures. In this paper, we report the results of heavy metal leaching from these mixtures under both saturated and unsaturated conditions. Leaching characterization was done with 0.01 M LiBr solution on specimens that have been compacted to a maximum density with a Gyratory compactor. There was no presence of arsenic in the leachate from any of the materials. Barium, copper, iron, cadmium, and zinc concentrations in the leachate from all seventeen mixtures were lower than the EPA drinking water standard. Lead and aluminum concentrations in the leachate from RCM mixtures and aluminum concentration in the leachate from all FA mixtures were higher than the drinking water standards. Chromium was generally not present in the leachate of RAP or FS mixtures and its concentration in the leachate from RCM mixtures was generally below the EPA drinking water standard. Chromium concentration in the leachate from FA increased with an increase in its proportion in the mixture. We discuss the implications of these results in identifying recyclable mixtures that may be suitable for use in road construction.