• The history of coal tar production and use
• Why it is considered a health risk
• How it can be disposed of safely and sustainably
Coal tar is a potentially hazardous material, which prior to 1980 was commonly used in road construction and maintenance processes in this country. It is now classified as carcinogenic, due to the high concentration and type of polynuclear aromatic hydrocarbons (PAHs) within its make-up. Coal tar has also been identified as being potentially hazardous when combined in other materials such as asphalt waste containing coal tar (AWCCT).
This has significant implications whenever road or surface materials are dug up, for instance on playgrounds, pavements, highways or car parks. It is therefore essential that coal tar testing on these materials is carried out professionally to:
Disposing of materials containing coal tar to landfill is generally expensive and potentially unsustainable. This article describes a process which the site investigator can follow to assess the road materials they propose to excavate, and then develop ways to reuse or recycle the material dug up (the arisings) thus avoiding disposal to landfill, reducing the burden on primary aggregates and encouraging a circular (sustainable) economy.
From the mid-1800s coal tar was produced as a by-product derived from the pyrolysis of coal during the production of domestic “town” gas. The coal tar was used on highways, pavements and the like as a binder for the aggregate. Coal tar continued to be used on UK highways until the late 1970s/early 1980s until it became increasingly scarce due to the closure of town gas works. With the discovery and implementation of North Sea natural gas, the town gasworks were phased out and as such the production and use of coal tar declined significantly.
Simultaneously, concerns began to be expressed regarding its possible carcinogenic nature. The International Agency for Research into Cancer (IARC) has since classified coal tar as a Group 1 carcinogen. Due to this, partly, the use of coal tar dwindled and by the early/mid 1980s bitumen became the sole binder used for both macadam mixes and for surface dressing. It is possible, however, that some recycled aggregates from road planings (waste derived from scraping off the top layer of roads) may also contain coal tar.
It is important to distinguish between coal tar and bitumen in terms of their health risk. Coal tar has now been classified as carcinogenic, while numerous studies have found bitumen to be a Group 2B carcinogen in general (possibly causing cancer) and Group 2A (probably carcinogenic to humans for roofing bitumen products).
Coal tar was commonly used in road production before 1980 and may still be in recylced aggregate from planings
The difference between the degree of hazard posed by coal tar and bitumen arises from the different concentrations and types of PAHs present. Some PAHs, such as benzo(a)pyrene, are known to have carcinogenic effects and concentrations of these can be very high in the case of coal tar, but extremely low in bitumen.
Coal tar contains in excess of 4,000 different individual compounds including volatile organic compounds, catechols, cresols, naphthols, phenols, carbazoles and PAHs.
European Directive 91/689/eec, the subsequent Hazardous Waste Regulations 2005 (HWR) and the List of Wastes Regulations 2005 (LoWR) classify certain materials containing coal tar as “hazardous waste” or potentially hazardous waste with regard to Waste Management Document 2 (WM2, Environment Agency, 2006). Within the document, “Bituminous Mixtures Containing Coal Tar” (ie AWCCT) is a mirror entry which means that if coal tars are present in excess of a threshold then the material is classified as hazardous. This classification may then invoke restrictions on methods of reuse and disposal of these materials. For example, transport of hazardous waste requires a consignment note.
Maintenance works on roads or other areas where surface materials are present, such as school playgrounds, may involve excavation of materials containing coal tar (AWCCT). Analysis of the materials via site investigation (desk based and/or intrusive) can give an indication of how the project is to proceed, likely savings from disposal and potential for reuse on site. With the ever increasing costs of landfilling, especially with reference to hazardous materials, it makes sense to delineate and map out areas of potentially hazardous and non-hazardous materials prior to excavation utilising any desk information that may be available.
A combined desk and sensibly designed site investigation can save a project manager time and money in characterising the materials on site and determining their fate. Furthermore, if the material can be re-used as a secondary aggregate (say, in a cold binder) with the permission of the Environment Agency (EA) (including any treatment permits that will be necessary) and Local Planning Authority (LPA) then it may also create revenue.
The presence of coal tar can have major implications to a construction project due to the health and safety issues posed by handling the material as well as the additional cost that comes with the removal and disposal of a potentially hazardous waste. The early identification of such material can therefore be vital in reducing project costs and delays and implementing suitable solutions. Clearly with some projects, a materials management plan (MMP) or site waste management plan (SWaMP) are required. If some work and tests are carried out in advance, there is a clear advantage in the preparation of these plans.
Early identification by a site investigation team appointed by the contractor enables:
If there is to be any form of excavation then there is a duty under the Construction (Design and Management) Regulations 2007 to determine whether or not any of the materials encountered could be hazardous waste. This is to minimise construction hazards and also inform of any remaining hazards.
If any historic records about the construction on site show that no coal tar is present in the layers, or that the layers to be excavated were laid after the mid 1980s further investigation may be unnecessary. However, if there is doubt about whether material to be excavated contains coal tar, several choices exist.
An investigation is recommended before repairs to roads are carried out
It could be accepted that the excavated material may contain tar and the requirements for materials containing tar can be followed. This may be appropriate and cost effective for small scale work such as patching.
For larger scale work it will be more cost effective to determine whether all or some of the layers to be excavated contain tar. In this case, cores should be taken or fragments of bituminous material should be taken from each layer encountered in a trial pit (a small excavation of the ground). If it is known that removal will mix the layers, a combined sample from all the layers should be analysed.
Coring or trial pitting would also provide valuable information of layer thickness to assist in design and in calculating quantities. The number of cores taken or pits excavated will depend on the extent of the scheme, but there should be sufficient to identify the number of layers present and the extent of any material containing tar. (Knowledge of the previous maintenance will be helpful in assessing what materials may be present).
Basically, the contractor is responsible for site works, which includes organising the site investigation/testing. Each job is different – sometimes a contractor will subcontract the site investigation out to a company which then may well subcontract it out further still.
It may be that the specialist decides not to take cores prior to the works commencing, but rather to test the arisings. This has several disadvantages:
These examples are from the same site and highlight the problems coal tar can pose. Above left: core showing thick base course and two distinct wearing course layers. Above right: Core showing thicker base course and a visually different and thicker wearing course.
After analysis, an assessment of the results is required. Assessment can include the option for reuse (which requires additional analytical parameters) but will certainly classify the material as either hazardous, if the threshold is exceeded, or non-hazardous. Whether classified as hazardous or not the material may still be applicable for reuse depending on the solution employed. However, the relevant environmental permit and assessment will be required from the Environment Agency.
WRAP – the government’s recycling body — considers that material containing coal tar can be recycled by in-situ or
ex-situ methods assuming there is agreement with the Environment Agency and the local planning authority. Further site-specific assessment for suitability may be requested depending on the sensitivity of the site’s end use or its proximity to environmentally sensitive receptors.
WRAP is also currently investigating an End of Waste Protocol for AWCCT. (An End of Waste Protocol for AWCCT means where the material is no longer deemed as waste by the Environment Agency.)
Recycling/reuse for use as a construction material should always be considered as the first option. This encourages sustainable development, e.g. retaining material on site and developing sensible site solutions. It is important that the Environment Agency is consulted about any dealings with hazardous waste or potentially hazardous waste and any necessary permits or exemptions for the operations are obtained in good time for its reuse and as such further work may be required (such as risk assessments) prior to use in particularly sensitive areas.
If it is decided that the material could be reprocessed (at a permitted facility) then it can be used as aggregate in a bitumen bound material such as cold mix asphalt, cement bound material or a hydraulically bound material such as a structural material for reinstatement (SMR), assuming that any other specifications for that final product are met such as British Standards or Specification for Highways Works.
As mentioned above, the Environment Agency and WRAP are currently considering a waste protocol for asphalt waste containing coal tar under the European Pathway to Zero Waste initiative, which could change the regulations. However, at the moment all treatments of AWCCT (including mobile plant) require an environmental permit from the Environment Agency. Even when treated, the use of the materials requires an environmental permit.
However, the EA, according to its regulatory position, will not pursue an application for an environmental permit for AWCCT where:
The relevant objectives of the Waste Framework Directives are met, ie “ensuring that waste management is carried out without endangering human health, without harming the environment and without risk to water, air, soil, plants or animals; or without causing a nuisance through noise or odours; and without adversely affecting the countryside or places of special interest”.
Assessment of materials on site prior to excavation can help to characterise the materials (hazardous/non-hazardous) on site and provide a streamlined rationale for disposal and reuse of the materials by preparation of any treatment and reuse permits that are required along with any additional assessments that may need to be carried out.
Reuse of any materials must always adhere to their applicability to the final specification for use, such as Highways Works Specifications, as well as any permitting restrictions or additional assessments that may be required.
Russell Corbyn is an environmental consultant at site investigation specialist Kiwa CMT Testing www.cmt-ltd.co.uk