Bitumen, Asphalt and Tar: What the Terms Actually Mean

The British and international convention

Across the United Kingdom and Ireland, continental Europe, India, Australia, New Zealand, South Africa, most of Africa and the great majority of international trade documents, the split is clean and easy to hold in the head:

• Bitumen is the binder. It is the black, highly viscous petroleum-derived material that arrives in new steel drums, jumbo bags, a tank container or a bulk parcel. It contains no aggregate.
• Asphalt is the mixture. It is bitumen combined with graded mineral aggregate and filler, produced at a mixing plant and laid and compacted on the road.

The European standards are written exactly this way, which makes them the cleanest place to see the convention in print. EN 12597, titled Bitumen and bituminous binders — Terminology, defines bitumen as a virtually involatile, adhesive and waterproofing material derived from crude petroleum or present in natural asphalt, completely or nearly completely soluble in toluene and very viscous or nearly solid at ambient temperature. It defines asphalt as a mixture of bituminous binder with mineral aggregate. The mixture standards follow suit: the EN 13108 series carries the overall title Bituminous mixtures — Material specifications, and its parts are named Asphalt Concrete, Asphalt Concrete for Very Thin Layers, Soft Asphalt, Hot Rolled Asphalt, Stone Mastic Asphalt, Mastic Asphalt, Porous Asphalt and Reclaimed Asphalt. In that system asphalt is unambiguously something you lay. It is not something anyone ships across an ocean.

The North American convention

In the United States and Canada the same word denotes the binder, and it does so throughout the governing specifications:

• ASTM D946 covers penetration-graded material. Older editions of the standard are titled Penetration-Graded Asphalt Cement for Use in Pavement Construction; current editions read Penetration-Graded Asphalt Binder. Either way, the subject is the binder.
• ASTM D3381 covers viscosity-graded asphalt binder, with the familiar AC-2.5 to AC-40 designations, and AASHTO M 226 covers the same territory on the AASHTO side.
• AASHTO M 320 and ASTM D6373 cover performance-graded asphalt binder — PG 64-22 and the rest of that family.

The mixture is then given a different name entirely: asphalt concrete, or hot mix asphalt (HMA), or warm mix asphalt where production temperature is reduced. Superpave volumetric mix design sits under AASHTO M 323. So a North American engineer who writes asphalt means the binder, and reaches for a second word when the aggregate is in it.

The trap buried inside ASTM terminology

ASTM D8, Standard Terminology Relating to Materials for Roads and Pavements, does define bitumen — but not in the way a European reader expects. In the definition D8 has long carried, and which should be checked against the current edition before it is quoted in a document, bitumen is a class of black or dark-coloured cementitious substances, natural or manufactured, composed principally of high molecular weight hydrocarbons, of which asphalts, tars, pitches and asphaltites are typical. Asphalt is then the particular member of that class in which the predominating constituents are bitumens occurring in nature or obtained in petroleum processing.

Read that carefully. In ASTM usage bitumen is the broader word and asphalt is the narrower one — the exact inversion of European usage. Worse, it is a definition under which coal tar counts as a bitumen. This is why quoting a dictionary never settles the argument between an American engineer and a European one. Inside their own document sets they are both correct, and neither definition is a mistake that can be corrected away.

What the split actually costs

Three failures recur often enough to be predictable, and all three are expensive.

• The mis-scoped enquiry. A US buyer asking for 5,000 MT of asphalt is asking for binder and can be quoted immediately. A contractor in Europe or South Asia asking about asphalt supply may be asking about the output of a mixing plant, which no exporter can ship. The two enquiries look identical in an inbox. Answering the wrong one wastes a week.
• The ambiguous specification. A tender clause reading asphalt to be supplied by the contractor means the binder if the document was drafted to AASHTO practice and the finished mixture if it was drafted to EN or Indian practice. The commercial gap between those two readings is the entire cost of aggregate, plant, haulage and laying.
• The customs description. Petroleum bitumen classifies under HS 2713.20. Bituminous mixtures such as mastics and cutbacks classify under HS 2715, and natural bitumen under HS 2714. An invoice that says only asphalt gives a broker nothing to work with, and a heading argued after arrival is argued from the weakest possible position.

None of this is a language problem and none of it is solved by insisting on the correct word. It is solved by describing the material in a way that carries its own definition: a grade designation and the standard that defines that grade.

Where each one comes from

Bitumen is the heavy residue of crude oil. It is what remains at the bottom of the vacuum distillation column after the lighter fractions have been taken off, and it can also occur naturally in lakes, rock deposits and tar sands. Its chemistry is dominated by long aliphatic and naphthenic structures alongside aromatic and asphaltene fractions, and it is relatively hydrogen-rich.

Coal tar is the condensate produced when coal is carbonised — heated in the absence of air in a coke oven, or historically in a town gas retort. High-temperature carbonisation is conventionally described as running above roughly 900 °C; that figure is a description of normal coking practice, not a limit set by any material standard, and it is given here only to make the process difference visible. It was never a fuel product; it was a by-product of making metallurgical coke and gas. High-temperature carbonisation drives the chemistry hard towards condensed aromatic ring systems, so coal tar and the pitch distilled from it are rich in polycyclic aromatic hydrocarbons (PAH) at concentrations far above anything found in petroleum bitumen. That single compositional difference explains almost every practical consequence that follows.

The standards treat them as separate materials, and that is the proof

The clearest evidence that these are not interchangeable is that the standards bodies never wrote one specification to cover both.

• Roofing. ASTM D312 specifies asphalt used in roofing, in four numbered types graded by softening point and penetration, running from Type I at a softening point of 57–66 °C through to Type IV at 99–107 °C. Those limits are quoted from the standard and it is revised periodically, so confirm them against the current edition before writing them into a contract. Coal-tar pitch for roofing, dampproofing and waterproofing has its own separate document, ASTM D450. Two specifications, two materials. A product cannot satisfy both.
• Steelwork and pipeline coatings. Bitumen-based hot-applied protective coatings are covered by BS 4147. Coal-tar enamel for steel water pipelines is covered by AWWA C203. Buyers who ask for enamel without saying which family they mean get quoted the wrong one routinely.
• Softening point. Bitumen is measured ring and ball to ASTM D36 or EN 1427. Pitch is measured cube-in-air to ASTM D2319 or by the Mettler method to ASTM D3104. Different apparatus, different heating regime, and numbers that are not comparable across the two families.
• Solubility. A paving bitumen is expected to be almost entirely soluble, and that expectation is written into the product specifications rather than being a matter of custom: ASTM D946 and IS 73 each set a minimum of 99.0 % solubility in trichloroethylene, determined by ASTM D2042, and EN 12591 sets a 99.0 % minimum determined in toluene by EN 12592. Pitch is the opposite case: its insoluble content is a deliberately measured property, with toluene-insolubles by ASTM D4072 and quinoline-insolubles by ASTM D2318. A material engineered to carry insoluble carbon cannot pass a bitumen solubility line.

The health and regulatory position, stated precisely

This is where the distinction stops being academic. The classifications below are published positions, quoted as issued, and they should be checked against the current edition before being relied on.

• IARC classifies coal tar and coal-tar pitch in Group 1, carcinogenic to humans (Monographs Volume 100F). For bitumen the same programme reached different conclusions in Volume 103 (2013): occupational exposures to straight-run bitumens and their emissions during road paving are Group 2B, possibly carcinogenic; occupational exposures to oxidized bitumens and their emissions during roofing are Group 2A, probably carcinogenic; and occupational exposures to hard bitumens and their emissions during mastic asphalt work are Group 2B.
• Occupational exposure limits follow the same split. For coal tar pitch volatiles the OSHA permissible exposure limit is 0.2 mg/m³ as the benzene-soluble fraction (29 CFR 1910.1000 Table Z-1, with the term itself defined at 29 CFR 1910.1002), the NIOSH recommended exposure limit is 0.1 mg/m³ as the cyclohexane-extractable fraction, the NIOSH IDLH value is 80 mg/m³, and the ACGIH threshold limit value is 0.2 mg/m³ with an A1 confirmed human carcinogen notation. For asphalt (bitumen) fume the ACGIH TLV is 0.5 mg/m³ as the benzene-soluble inhalable aerosol with an A4 notation — not classifiable as a human carcinogen — and the NIOSH recommended exposure limit is 5 mg/m³ as a 15-minute ceiling.
• Creosote is restricted by name in the EU. Entry 31 of Annex XVII to REACH restricts creosote and creosote-containing substances derived from coal tar where benzo[a]pyrene is present at or above 0.005 % by mass, or water-extractable phenols at or above 3 % by mass, for wood treatment and for placing treated wood on the market for general public use.
• Coal tar changes the waste classification of a pavement. Under the European List of Waste (Commission Decision 2000/532/EC), reclaimed bituminous material is coded 17 03 02 and is not hazardous, whereas 17 03 01*, bituminous mixtures containing coal tar, and 17 03 03*, coal tar and tarred products, are hazardous entries carrying the asterisk. That distinction decides where planings can go, what they cost to dispose of and whether they can be recycled into new mixtures at all.

Tar in old pavements is a live commercial issue

Coal tar is no longer used as a general road binder in the United Kingdom, Europe or North America, and its use has been restricted or discontinued in many other countries on health grounds. The legacy remains in the ground. Pavements laid before the phase-out can still contain tar-bound layers, and when those layers are planed out the question of whether the arisings are hazardous waste is decided by their PAH content, not by their appearance.

A PAH indicator spray gives a colour change on tar-bound material and none on bitumen-bound material, which makes it a fast field screen at the excavation. It is a screen and nothing more: the classification decision belongs to a laboratory PAH determination against the applicable national threshold. Anyone buying, selling or recycling reclaimed asphalt should establish the tar question before the material moves, because the cost of getting it wrong is a hazardous waste consignment rather than a saleable feedstock.

Where tar terminology is still legitimate

None of this makes the word tar obsolete. Coal tar, coal-tar pitch and coal-tar enamel remain real commercial products with real standards behind them, used in coatings, refractories, carbon and aluminium electrode manufacture and a range of chemical processes. The error is not using the word. The error is using it as a loose synonym for bitumen, or accepting an enquiry that uses it that way without asking which material is meant.

The mixture names, market by market

In the European system the mixture types are enumerated in the parts of EN 13108: Asphalt Concrete (Part 1), Asphalt Concrete for Very Thin Layers (Part 2), Soft Asphalt (Part 3), Hot Rolled Asphalt (Part 4), Stone Mastic Asphalt (Part 5), Mastic Asphalt (Part 6), Porous Asphalt (Part 7) and Reclaimed Asphalt (Part 8). Every one of those names contains the word asphalt, and every one of them refers to a material with aggregate in it. That is the whole European convention in a single list.

In North America the same family is described as asphalt concrete or hot mix asphalt, designed volumetrically under Superpave (AASHTO M 323) or, in older and still widely used practice, by the Marshall method. Stone matrix asphalt is the North American spelling of stone mastic asphalt and is the same concept.

In the United Kingdom the pre-EN vocabulary has not fully died. Hot rolled asphalt (HRA), dense bitumen macadam (DBM), close-graded surface course and coated macadam still appear in specifications, maintenance contracts and site conversation. In India the mixtures are named in the road authority specifications as Bituminous Concrete (BC), Dense Bituminous Macadam (DBM) and Bituminous Macadam (BM), all of which use bitumen as the binder despite the historical word macadam in the name.

The practical rule is that a mixture name is a recipe: it implies a grading envelope, a binder content and a compaction regime. A binder name is a material: it implies a consistency band and a set of test limits. Confusing the two categories is the single most common source of a quotation that answers the wrong question.

Terms that describe a fraction, not a product

Bitumen is not a compound. It is a complex mixture, and the analytical convention is to divide it into four solubility and polarity classes, conventionally abbreviated SARA: saturates, aromatics, resins and asphaltenes. ASTM D4124 is the published method for separating a bitumen into four fractions, and ASTM D6560, equivalent to IP 143, determines the asphaltene fraction specifically as the heptane insolubles.

Three points follow, and they are worth stating plainly because enquiries for these materials arrive regularly.

• An asphaltene is defined by a solvent, not by a molecule. Change the precipitating solvent from n-heptane to n-pentane and the measured asphaltene content changes, because a different set of molecules stays in solution. Any asphaltene figure without its method and solvent is meaningless.
• Maltene is simply the complement. The maltene or petrolene fraction is everything that dissolves when the asphaltenes precipitate. It is a residue of a laboratory operation, not a grade.
• Neither is a commercial product in the bitumen trade. Asphaltene content is a diagnostic quantity used in research and in crude oil handling, where asphaltene precipitation fouls wells and pipelines. Buyers who write asking for asphaltene tonnage almost always want a paving or industrial bitumen grade, and the productive response is to ask which application it is for.

The adjective problem

Finally, note that bituminous is an adjective of very wide scope. In European standards it means belonging to the binder family. In the Harmonized System, heading 2715 covers bituminous mixtures based on natural asphalt, natural bitumen, petroleum bitumen, mineral tar or mineral tar pitch — a definition that deliberately spans both the petroleum and the coal-derived families. In geology, bituminous coal is a rank of coal and has nothing at all to do with the binder. Context carries the meaning every time, so a document that relies on the adjective alone has not specified anything.