Pavement coring and PAH testing

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A significant proportion of roads constructed or resurfaced in the UK before 1985 may contain some coal tar or pitch as a binder.

Coal tar and pitch comprise of Polycyclic Aromatic Hydrocarbons (PAHs) that are known to have carcinogenic effects if they are present in a significant concentration. The hazardous nature of coal tar to humans has mainly been studied in workers from the coal tar industry who have had long exposures to coal tar products at high levels. Laboratory studies have also been carried out. Coal tar and its derivatives are regarded as carcinogens in their own right. It is true that identifiable components of coal tar are also carcinogenic, but the overriding factor is the health hazard presented by coal tar itself. Road tar is also listed as a carcinogen because it is made mainly from high temperature distillate.

In considering risks to human health and the environment all pathways into the human body must be considered. For tar in road arisings (plainings) the most likely human pathways are inhalation and ingestion of dust at ambient temperatures. There is also a possibility of the release of PAH fumes at elevated temperatures. Skin exposure is also a pathway for tar products so that personnel should avoid handling road arisings. Road tar also contains substances that are toxic to aquatic life, in both fresh and salt water.

The Managing Reclaimed Asphalt – Highways and Pavements ADEPT Guidance note is intended as an aid to classifying and reusing arisings from bituminous bound road materials. The main aim of the document is to reduce the amount of hazardous or non-hazardous waste being sent to landfill or for incineration, allowing the industry to reuse as much of this valuable material as possible. The document also outlines the requirements and recommendations for sampling and testing strategies in order to correctly identify the arisings as product wherever possible. 

Most UK roads have been constructed and maintained in an ad-hoc way such that tar and bitumen bound materials may be found in close proximity in adjacent layers. In most cases any tar bound material will be below the surface and the start and end of the material (in terms of ‘chainage’) will not be precisely known. Often the amount of overlaying material will also vary resulting in the depth of the tar material changing along the road.

In some cases tar and bitumen were used interchangeably in the same course - even in relatively recently constructed roads - so the location of tar-bound material will be unpredictable. This can be a consequence of the material being supplied form more than one production location at a time when specifications did not impose the level of control that is now required.

Prior to any excavation operation for a highways project, it is recommended that an investigation is carried out to establish the location of any road tar contaminated material, the properties of the in-situ material and the anticipated properties of any excavated material. An investigation into existing road conditions and the assessment of the properties and quality of the arisings are essential to achieving the proper re-use of the excavated material.

Cores are recommended for investigating the bound construction, as they allow individual layers to be easily distinguished, while trial pits are useful for investigating the unbound layers.

To assess the nature of the arisings, the variability of the source material must be considered. Cores should be nominally 150mm in diameter and be taken between a range of 25 to 50m centres. A minimum of three cores should be taken unless the site is less than 30 metres squared, when one core will be adequate.

A road core sampling plan should be made and recorded, including the details of all decisions made.

Once the sampling has been completed road tar in core samples are screened with PAK marker spray. The identification is not quantitative and PAK spray is sensitive to quite low levels of tar, below the hazardous threshold. (PAK spray can give ‘false negative’ results.)

After the samples have been screened using PAK marker spray, any negative results can be further tested to quantify the levels of coal tar within the sample and classified accordingly. 

Currently the only reliable methods are odour, which may not be very specific, and PAK spray that is white but turns yellow if in contact with tar products. In reality both are useful:

· odour because there is some evidence that PAK spray may not work on very high viscosity binders

· PAK spray because it can usually identify individual layers

The identification is not quantitative and PAK spray is sensitive to quite low levels of tar, below the hazardous threshold. (PAK spray can give ‘false negative’ results.)

After the samples have been screened using PAK marker spray, any negative results can be further tested to quantify the levels of coal tar within the sample. When detecting tar products, two tests are usually carried out:

· Speciated PAH Analysis (PAH16)

· Phenols and cresols either by speciated analysis or by phenol index

To establish whether waste, or potential waste, is hazardous, it is only necessary to test the material in solid form.

From this testing the levels of PAH are determined and suitable treatment options can be selected. This includes:

· Short-term prevention (leaving tar contamination in place)

· Reusing plainings containing tar classed as hazardous waste (only in cold mix processes)

· Recycling (turning new waste into a new substance/product)

· Recovery processes (removing road tar from the aggregate using biological agents or via pyrolysis of the tar)

· Disposal (a last resort option – incineration and landfill may be available)

Any treatment chosen must meet the engineering demands of the specific road and the appropriate permits and licences obtained from the relevant regulators. Options include in-situ stabilisation, which can be used to repair roads containing tar, and ex-situ treatment, which involves the removal and treatment of asphalt by crushing, grinding and screening.