Coal tar is an increasingly significant concern across the waste and construction industries, particularly where the removal or remediation of historic road surfaces is concerned. With over 2.5 million road and pavement excavations carried out annually by utility companies alone, identifying and safely managing coal tar in asphalt is a challenge that continues to grow.
When and why did coal tar use cease in road construction?
Prior to the 1980s, coal tar was commonly used as a binder in road surfacing. Derived as a by-product of coal gas production, coal tar was eventually replaced with petroleum bitumen (a safer, non-hazardous alternative made from crude oil). Unlike coal tar, bitumen can be recycled and reused more easily within modern infrastructure systems.
Today, under WM3 guidance, any asphalt waste containing more than 0.1% coal tar, or any material showing more than 50mg/kg of Benzo[a]pyrene, is classified as hazardous. This classification introduces strict compliance and handling requirements.
Why is coal tar hazardous?
Coal tar is categorised as a Class 1 carcinogen, meaning it poses a risk of causing cancer. When bound within road surfaces, it is largely inert and not immediately harmful. However, when roads degrade or are excavated, this encapsulation is disturbed, potentially releasing harmful substances into the environment and posing health risks to workers and the public.
While older roads are most likely to contain coal tar, newer roads may also be affected—particularly where layers of bitumen have been laid over pre-existing coal tar substrates.
Identifying coal tar in asphalt
The first step in managing coal tar is accurate identification. Core sampling, removing a sample of road surface for laboratory analysis, remains the most reliable method, although results can take several weeks.
An alternative, faster method is the use of PAK spray, which reacts with hydrocarbons found in coal tar. The spray changes colour from white to brown or yellow upon detection. While not a replacement for lab analysis, it provides an efficient preliminary indication of presence across various layers of a surface.
Managing and treating coal tar
Once coal tar has been identified, there are a limited number of treatment or disposal options:
- Leave in situ: In certain scenarios, it may be safe to leave coal tar undisturbed, particularly where it remains securely bound within the asphalt.
- Overlaying: A new surface layer can sometimes be applied above the affected area, although this is dependent on site-specific conditions.
- Bituminous Foam Treatment: This technique stabilises the material, reclassifying it as Cold Recycled Bound Material (CRBM), which can then be reused as a base layer.
- Excavation and Disposal: Where no in situ option is viable, affected asphalt must be removed and treated as hazardous waste in line with regulatory standards.
Where is coal tar most likely to be found?
Coal tar is commonly found in roads, car parks, and driveways constructed before the early 1980s. These areas should be prioritised for investigation, particularly if redevelopment or maintenance work is planned.
