PAH testing of asphalt: expert guide for road materials

This guide explains why Polycyclic Aromatic Hydrocarbons are tested in asphalt, how a sample is analysed, how to read the results and which precautions to take before deciding on recycling or an appropriate waste route.

What are PAHs?

Polycyclic Aromatic Hydrocarbons are a family of organic compounds consisting of several aromatic rings. They can be formed during incomplete combustion and may occur in various petroleum-derived or coal-derived products. In the road sector, PAH testing is particularly important where older pavements may contain historical materials or binders.

Some PAHs have properties of toxicological concern. The analytical requirement is therefore not simply to detect their overall presence, but to identify and quantify the compounds specified in the reference method.

Why are older pavements concerned?

Road construction techniques have evolved. Older pavements may contain layers of different types and ages. Some may include tar-bound products or materials resulting from historical practices that are no longer used. During road planing, these layers may be mixed if they have not been properly characterised beforehand.

Representative testing provides quantitative data before reclaimed asphalt planings are produced. The result can then be assessed against the proposed reuse, technical recommendations and acceptance criteria of the intended recovery or waste route.

Which 16 PAHs are tested by BJL?

BJL's current Cofrac accreditation scope for waste bituminous asphalt covers 16 PAHs: acenaphthene, acenaphthylene, anthracene, benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[ghi]perylene, benzo[a]pyrene, chrysene, dibenz[a,h]anthracene, fluoranthene, fluorene, indeno[1,2,3-cd]pyrene, naphthalene, phenanthrene and pyrene.

These compounds are quantified by GC-MS in accordance with NF EN 17503 for the matrix covered by the scope. For analytical details, see our method for analysis of the 16 PAHs by GC-MS.

From sample to result: the analytical process

1. Sample identification: reference, origin, type, layer and testing request.
2. Acceptance check: mass, packaging, required information and prior asbestos verification.
3. Pretreatment: crushing to below 4 mm, homogenisation and quartering in accordance with NF EN 15002.
4. Dry matter: gravimetric determination in accordance with NF EN 15934.
5. Extraction: solid/liquid extraction with agitation.
6. GC-MS: chromatographic separation, identification and quantification of PAHs in accordance with NF EN 17503.
7. Report: delivery of the analytical results.

Why representative sampling is essential

A laboratory analysis can only describe the sample received. A pavement is often stratified and may include a surface course, binder course, localised repairs and materials from different periods. Mixing heterogeneous layers may produce an average value that does not accurately represent any of them.

The sampling plan should therefore reflect the project and its homogeneous areas. Layers that can technically be separated should be identified. For BJL instructions, see how to collect and send an asphalt sample.

Why asbestos must be addressed before PAH testing

BJL's currently published conditions require an asbestos test report for the asphalt, issued by an accredited laboratory, to be available before PAH testing. If asbestos is confirmed, PAH testing is not carried out. This sequence avoids chemical preparation of a material in which the presence of asbestos has been confirmed.

See our dedicated page on asbestos and PAH testing of asphalt.

How should a PAH concentration be interpreted?

Three levels of interpretation should be distinguished:

• the analytical result produced by the laboratory;
• acceptability for a use or process, which depends on the applicable guidance and criteria;
• regulatory waste classification, which must not be inferred automatically from a single sum-of-PAHs value.

French technical references use a value of 50 mg/kg of dry matter for certain asphalt acceptance and recycling scenarios. More recent Cerema documents also provide for a value of 500 mg/kg in the specific case of cold recycling for certain uses. These values must always be read within the scope of the relevant document. See our summary of PAH thresholds.

PAHs and coal tar: avoid oversimplification

A high PAH concentration may be consistent with the presence of historically tar-bound materials. However, stating that a PAH result “detects coal tar” is an oversimplification. The analysis provides concentrations of defined compounds. Interpretation of the binder's origin depends on the technical and historical context of the sample.

When should PAH testing be requested?

• before planing an older pavement;
• before reusing or recycling reclaimed asphalt;
• when characterising road construction waste;
• before sending material to a platform or receiving facility that requires characterisation;
• as part of a preliminary assessment for a roadworks project.

Which sample should be sent?

BJL's Rev.22 sample submission form specifies a minimum mass of 200 g for PAH testing. The sample must be identified unambiguously and the layers to be tested must be specified. The current instructions in the quality form take precedence over any general information published on the website.

What does GC-MS provide?

Gas chromatography separates compounds in a mixture. Mass spectrometry provides information that enables their identification and quantification. GC-MS is suited to targeted determination of the PAHs covered by the method. The quality of the result also depends on sample preparation, extraction and analytical quality controls.

How should a BJL report be read?

The reader should check the sample identification, matrix, individual results, unit and any comments in the report. A result must not be interpreted outside the matrix and scope of the test. See how to read a PAH asphalt test report.

Frequently asked questions

Must every layer of a core be tested?

Not necessarily in the same way, but each technically distinct layer should be considered in the characterisation plan to avoid an unrepresentative average.

Can the laboratory choose the waste route on behalf of the client?

The laboratory provides the analytical results. The choice of route depends on the project framework, the contracting party, regulations and the acceptance criteria of the receiving facility or recovery process.

Can rapid on-site screening replace GC-MS?

No. Rapid methods may assist with preliminary identification, but they do not provide the same level of quantification and selectivity as a laboratory analytical method.