Coal tar is an increasingly significant concern across the waste and construction industries, particularly where the removal or remediation of historic road surfaces, such as tarmac and pavements, 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.
Introduction: What is Coal Tar and Why Does it Matter?
Coal tar is a dark, viscous by-product formed during the production of coal gas, historically used as a binder in road surfaces across the UK. Its widespread use in road construction continued until the 1980s, when it was largely replaced by petroleum bitumen—a non hazardous material derived from crude oil. While bitumen offers a safer and more sustainable alternative, many older road surfaces still contain coal tar, which can pose significant risks if disturbed.
The presence of coal tar in road surfaces is a concern because it is a complex mixture of hydrocarbons, some of which are hazardous to both human health and the environment. When maintenance work, excavation, or redevelopment takes place, these materials can be exposed, potentially releasing harmful substances into the environment. Understanding what coal tar is, how it was produced, and why it was used as a binder in road construction is essential for anyone involved in managing, maintaining, or redeveloping infrastructure that may contain coal tar. As a result, identifying and safely handling materials containing coal tar remains a priority for contractors, local authorities, and environmental agencies.
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. The hazardous classification process is governed by specific rules set out in the WM3 regulations, which define the thresholds and procedures for determining whether waste is hazardous. This classification introduces strict compliance and handling requirements. Several factors influence whether material is classified as hazardous, including its chemical composition, the concentration of hazardous substances, and the results of specific testing.
Why is coal tar hazardous?
Coal tar is categorised as a Class 1 carcinogen, meaning it poses a risk of causing cancer. It is also considered hazardous under current regulations due to its significant health and environmental risks. 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 to identify whether coal tar is present in the material. Core sampling, which involves removing a sample of road surface for laboratory analysis, remains the most reliable method to identify coal tar, although results can take several weeks.
An alternative, faster method is the use of PAK spray, which helps identify coal tar by reacting with hydrocarbons found in coal tar. The spray changes colour from white to brown or yellow upon detection, indicating coal tar is present. While not a replacement for lab analysis, it provides an efficient preliminary indication of presence across various layers of a surface.
Environmental Concerns and Impacts of Coal Tar
Coal tar in road surfaces presents a range of environmental concerns that must be carefully managed to protect both people and the environment. When materials containing coal tar are disturbed, there is a risk of contaminating soil and groundwater with hazardous hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs). These substances can persist in the environment and pose long-term health risks.
Disposal of coal tar hazardous waste is strictly regulated, and improper handling can lead to significant environmental pollution. Local authorities and contractors must consider the cost implications of dealing with coal tar, as specialized disposal and treatment methods are often required. Bituminous foam treatment is one method that can help encapsulate coal tar, reducing its environmental impact and allowing some materials to be reused safely.
Before any maintenance work or redevelopment, further investigation is essential to determine the presence and concentration of coal tar in road surfaces. Core testing and the use of PAK spray paint are valuable tools for identifying coal tar, helping to inform the best approach for managing the material. By taking these steps, local authorities and contractors can minimize environmental risks, ensure compliance with regulations, and make informed decisions about the most cost-effective and responsible way to deal with coal tar in road construction projects.
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. This method supports the recycling of bituminous mixtures containing coal tar, subject to regulatory approval.
- 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. Materials containing coal tar must be disposed of as hazardous waste, following all regulatory requirements. These materials are disposed of in accordance with environmental regulations to ensure safe handling and legal compliance.
Coal tar in road surfaces must be properly dealt with to mitigate health and environmental risks. Recycling options are available for some bituminous mixtures containing coal tar, depending on regulatory approval.
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.
