Bitumen is a black, sticky, very viscous material obtained from crude oil, and it is the heaviest fraction left after a refinery has distilled off the fuels. It is used mainly as the binder that holds the stone together in road surfaces, and as a waterproofing material in roofing. In American English the same material is called asphalt.
Bitumen definition
To define bitumen in one line: it is the residue of the vacuum distillation of crude oil, a mixture of very large hydrocarbon molecules that is semi-solid at ambient temperature and flows when heated.
By weight, bitumen is typically about 80% carbon and 10% hydrogen, with up to 6% sulphur (US: sulfur). Chemists describe it as asphaltenes, the heaviest molecules, dispersed in a lighter oily phase called maltenes. The balance between the two governs how hard the bitumen is and how it responds to temperature.
Bitumen is not tar. Tar is made by heating coal or wood without air; bitumen comes from crude oil. The difference is set out in bitumen vs asphalt vs tar.
What are bitumen's sources?
Nearly all commercial bitumen is made in oil refineries. Natural deposits also exist: the Pitch Lake in Trinidad is the largest, estimated at 10 million tons, and the oil sands of Alberta in Canada hold very large volumes of natural bitumen mixed with sand.
World bitumen consumption was about 120 million tonnes in 2022, according to figures published by Eurobitume, the European producers' association.
How is bitumen made?
Crude oil is first distilled at atmospheric pressure, which takes off gas, naphtha, kerosene and gasoil. The residue from that column is then distilled again under vacuum. The material that boils above roughly 500°C does not vaporise and is drawn from the bottom of the vacuum column. This vacuum residue is bitumen, or the base from which bitumen grades are made.
Refineries then adjust it to grade in four ways:
- Distillation to grade. The vacuum column is run so that the residue comes out at the required hardness.
- Blending. Harder and softer residues are mixed in set proportions.
- Oxidation. Air is blown through hot bitumen, which makes it harder and less sensitive to temperature.
- Modification. Polymers or other additives are mixed in with high-shear blenders.
Not every crude will do. Eurobitume states that of the 1,300 or more types of crude oil, only about 10% can produce bitumen that meets current engineering requirements, and not every refinery makes it.
How is bitumen graded?
There are three grading systems in use. Which one applies depends on the country and the road authority.
| System | What is measured | Example grades | Typical standard |
|---|---|---|---|
| Penetration grade | Depth a 100 g needle sinks into the sample at 25°C, in units of 0.1 mm | 40-50, 60-70, 85-100, 120-150, 200-300 | ASTM D946 |
| Viscosity grade | Viscosity at 60°C, checked again at 135°C | AC-2.5, AC-5, AC-10, AC-20, AC-30, AC-40 | ASTM D3381 |
| Performance grade (PG) | A set of tests run at the pavement temperatures the binder must survive | PG 58-22 | AASHTO M 320 |
In penetration grading, a higher number means a softer bitumen. A 60-70 grade is one where the needle sinks 6.0 to 7.0 mm. Softer grades suit cold climates and harder grades suit hot ones.
In viscosity grading, the grade number tracks viscosity at 60°C. AC-5, for example, is 500 poise, plus or minus 100, at that temperature. The 135°C test represents mixing and laying temperatures.
In performance grading, used across the United States, the two numbers are temperatures in degrees Celsius. In PG 58-22, the 58 is the average seven-day maximum pavement temperature the binder is designed for, and the -22 is the minimum pavement design temperature. These are pavement temperatures, not air temperatures.
A contract should name the grade, the standard and its year. A "60/70" under one national standard is not automatically identical to a "60-70" under another.
Types of bitumen
- Paving grade bitumen. The straight refinery product, sold by penetration or viscosity grade, such as bitumen 60/70, bitumen 80/100 and viscosity grade bitumen.
- Oxidised (blown) bitumen. Air-blown to make it harder. Used in roofing, pipe coating and industrial waterproofing. See oxidised bitumen.
- Cutback bitumen. Bitumen thinned with a petroleum solvent so that it can be sprayed at lower temperatures. The solvent evaporates after application. Environmental rules have cut its use in many countries because the solvent is released to the air.
- Bitumen emulsion. Fine droplets of bitumen suspended in water with an emulsifying agent. The droplets are mostly 1 to 10 micrometres across. Emulsions are applied with little or no heating and are used for tack coats, surface dressing and slurry seals. See bitumen emulsion and our guide to tack coat and prime coat.
- Polymer modified bitumen (PMB). Bitumen blended with polymer to improve resistance to rutting and cracking on heavily trafficked roads. See polymer modified bitumen.
What is bitumen used for?
About 85% of bitumen goes into asphalt for roads, about 10% into roofing, and the remaining 5% or so into sealing and insulation. In a road mix the bitumen is the minor ingredient: a typical mix is about 5% bitumen and 95% aggregate by weight.
Other uses include waterproofing membranes for bridges, tunnels and reservoirs, pipe coatings, sound-deadening panels in vehicles, and the lining of dams and canals. Eurobitume counts more than 250 known applications.
How is bitumen shipped?
Bitumen has to be kept hot to stay pumpable. It is typically stored and shipped at around 150°C, which shapes the logistics:
- Bulk. Heated bitumen tankers load at a refinery or terminal and discharge into heated shore tanks. This is the lowest-cost route for a buyer with a terminal.
- Drums and bags. For ports without heated storage, bitumen is filled into steel drums or meltable bags and shipped in standard containers. See drummed bitumen.
- Emulsion and cutback. These are liquid at ambient temperature and move in drums, IBCs or tank containers.
The packing changes the landed cost and the handling at destination, so an enquiry should state it. Our earlier guide, How to buy bitumen: grades, specifications and Incoterms, covers the buying side in more detail.
How bitumen is traded
Bitumen is sold on a named grade and standard, per tonne, on FOB, CFR or CIF terms. The specification lines that decide acceptance are penetration, softening point, ductility, flash point and solubility, each with its test method.
CBIL is a commodity brokerage company. We broker bitumen between a named refinery or seller and a named buyer, against independent inspection by SGS, Intertek or a similar company at loading. The inspector samples the cargo, tests it against the contract specification and certifies the quantity loaded. The full range is on our bitumen page.
Request a bitumen quote with the grade, standard, quantity, packing and destination port.