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Coal tar-derived roading material contains over 1000 times more polycyclic aromatic hydrocarbons (PAHs) than equivalent bitumen pavements and has been identified as a major source of PAHs in both Christchurch and Auckland aquatic receiving environments. Many old streets containing coal tar will soon require reconstruction, and therefore the excavation and potential disposal of contaminated road construction layers represents a significant financial and environmental problem. To address this problem, we evaluated in-situ foamed bitumen (FB)/cement stabilisation as an environmental acceptable method to reuse the contaminated tar road material.

Based on contaminant leaching and toxicity, the reuse of tar-contaminated roads as compacted stabilised base material represents minimal risk to the environment. FB decreased PAH leachate concentrations by ca 4–6, although algal toxicity was correlated to leachate copper, which was increased by the co-use of alkaline hydraulic binders. Despite this, the low potential for harm from the leachates combined with a reuse application (ie road base) that limits environmental exposure/risk via being: i) capped with a waterproof seal layer; and ii) located beneath carriageway and thus not being reused in an environmentally significant ‘compartment’. A limitation of the method is that many of the older tar-contaminated streets may not be suitable for in-situ FB recycling without additional makeup aggregate being applied.

Keywords: asphalt, cement, coal tar, contaminants, copper, foamed bitumen, immobilisation, leaching, metals, mobilisation, PAHs, recycling, roads, seal, stabilisation, toxicity