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A sticky problem

Tar in roads is a legacy from Britain’s industrial past which still, perhaps surprisingly, is demanding careful management today. NCE reports.

Like many sectors, the UK highways industry is dealing with the consequences of its history. Tar has not been used as a binder in UK roads since the 1980s but, with significant quantities still embedded in the network, the financial burden and environmental impact of managing the material is coming to the fore as roads are planed out and resurfaced.

As a classified hazardous waste, tar now carries a landfill cost of £150 a tonne and, as it contains carcinogens, it is important to identify its presence in advance of resurfacing work, so that it does not pose a health and safety hazard and can be managed properly. 

Tar history

From the mid 1800s, tar derived from the high temperature distillation of coal in the production of domestic “town” gas was used on roads. While bitumen started to take market share from the early 1900s, tar continued to be used on roads as a binder in macadam and surface dressing until the late 1980s and is, therefore, likely to be present across the network in different parts of its construction.

With very few records documenting construction materials used on UK roads, local authorities still face major challenges to identify the material. According to Neil Thomas, technical manager at Tarmac National Contracting, councils need to start implementing strategies to deal with the issue.

Scale of problem

“Some local authorities have a well-managed approach to identifying and dealing with tar, but others are lagging behind and perhaps do not fully understand the true scale of tar in their roads. It makes sense to have this clarity, not only from a health and safety perspective, but also because it can help save money and cut landfill costs by identifying tar before work starts and looking at disposal alternatives,” says Thomas.     

Best practice

Following best practice ­guidance is essential. Under the Construction Management (Design and Management) Regulations 2007, any form of excavation in a bituminous pavement means that the designer or scheme compiler has a duty of care to determine whether there is hazardous waste and inform the contractor.

If there are no existing records to indicate tar in a pavement, designers can undertake coring trials or elect not to investigate and deal with the problem of tar if they encounter it.  Thomas continues: “Taking cores or fragments of bituminous material from each layer is the best approach to gather this information, so that informed decisions can be made. 

Safety and cost implications

“Just starting the work and testing the arisings after excavation has safety and cost implications. This approach breaches the legal requirement to inform the contractor that excavated materials may contain tar.

“It means that the removal process is not planned in advance to separate the tar and non-tar materials. And because testing is done after excavation, it risks delaying the scheme and doesn’t allow for informed decisions to be made regarding the reuse of arisings.”

Highways teams need to understand threshold levels of tar outlined in the Control of Substances Hazardous to Health Regulations. Individual threshhold levels will dictate if the material can be used as an unbound material or recycled in-situ as a bitumen bound material in a asphalt cold mix.


Tar treatment threshhold levels

If threshold levels meet the standards set out below, local authorities have an opportunity to maximise the benefits of recycling the material in a bitumen bound cold asphalt material.

  • The concentration of Benzo(a)pyrene is below 100ppm
  • The concentration of all other components of PAH17 is below 1,000ppm
  • The concentration of phenol in the leachate of a liquid to solid ratio of 10 litres per kg is below 1 mg/kg.

Tarmac can provide local authorities with help to identify tar in their roads and offers an energy-efficient foamed bitumen and emulsion technology capable of safely recycling  tar. Its FoamMaster system is an alternative to hot mix asphalt and an Environment Agency-approved method of eliminating tar disposal issues.
Tackling tar in a safe, responsible and cost-effective fashion is paramount. By gathering information on tar before work begins, authorities can warn contractors and provide them with the evidence to make informed decisions that may allow more tar to be recycled and costly landfill charges avoided. Understanding the history of our roads has never been so important.  

Readers' comments (1)

  • Totally disagree with this statement “Just starting the work and testing the arisings after excavation has safety and cost implications. This approach breaches the legal requirement to inform the contractor that excavated materials may contain tar"

    It doesn't breach anything if the designer "informs the contractor that the excavated material may contain tar"

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