“Bitumen” and “asphalt” are two common terms in road construction. People often use these words to mean the same thing. But even though bitumen and asphalt are related, they differ in several ways. In this article, we will consider the difference between bitumen and asphalt.
What is Bitumen?
Bitumen is a hydrocarbon substance mostly made of heavy molecular weight components such as asphaltenes, resins, and aromatics. It can be found in nature or taken from crude oil.
What is Asphalt?
Asphalt (or asphalt concrete) is a bituminous material. Asphalt is a combination of aggregate (small rocks) and bitumen (also called asphalt binder). It is produced by mixing these ingredients and is mostly used in road construction.
The difference between bitumen and asphalt is in their production, physical properties, chemical properties, uses, and classification methods. You will read about these differences in more detail in the following text.
Differences in Production of Asphalt and Bitumen
Bitumen and asphalt are different in how they are made. Asphalt production has more steps than bitumen production.
Bitumen Production
Bitumen comes from two origins:
- Nature (natural bitumen): It comes from lakes or mines (Gilsonite).
- Oil refineries (refinery bitumen): Crude oil is heated up to 300 °C in the oil distillation column. By increasing the temperature, hydrocarbon molecules are separated from oil. Bitumen is made of heavy molecules such as asphaltene and resin that are left.
Asphalt Production
Bitumen is mixed with aggregates to make asphalt concrete. This mixing occurs in an efficient manufacturing plant.
First, the rocks are sorted by size. Then, they’re heated using a dryer to remove water and make it easy for bitumen to stick to them.
The bitumen is kept hot in a tank, and then the pug mill mixes the bitumen with the aggregate. The mixing time is important since the bitumen can be oxidized during mixing. However, it should be long enough to have a uniform mixture. After mixing, the asphalt is put in trucks.
Differences in Physical Properties of Asphalt and Bitumen
Bitumen and asphalt are different in physical properties:
- Material state: Bitumen is a viscous liquid or semi-solid material, while asphalt is a semi-solid mixture.
- Texture: Bitumen is smooth and sticky. However, the asphalt texture is rough and granular because of the aggregate in it.
- Density: The typical density of bitumen is 1.01-1.06 g/cm3. Asphalt density is higher due to aggregate presence and is 2.2-2.4 g/cm3.
Differences in Chemical Properties of Asphalt and Bitumen
Bitumen and asphalt are different in chemical properties, such as:
Composition:
Bitumen is made of high-molecular-weight compounds such as asphaltene and resin. Its composition depends on the crude oil from which it is derived.
On the other hand, the asphalt mixture is a mixture of aggregate and bitumen. In general, about 5-10% (by weight) of the total asphalt mixture is bitumen.
Aggregate is made 90-95% (by weight) and 75-85% (by volume) of asphalt.
Oxidation (bitumen combined with oxygen) :
Bitumen can oxidize and become hard over time. Asphalt mixtures can also oxidize, but the oxidation rate is lower because of the aggregate.
Below table is a comparison of physical and chemical properties of bitumen and asphalt.
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Comparison of Physical and Chemical Properties of Bitumen and Asphalt | ||
property | Bitumen | Asphalt |
state | Viscous liquid/ semi-solid | Semi-slid mixture |
texture | Smooth, sticky | Rough, granular |
Density | 1.01-1.06 | 2.2-2.4 |
Composition | Heavy hydrocarbon molecules | 5-10% bitumen, 90-95% aggregate (w/w) |
Oxidation | High oxidation rate | Lower oxidation rate |
Difference in Uses of Asphalt and Bitumen
Another difference between bitumen and asphalt is related to how they are used.
Bitumen is a valuable material used in various industries. It can be used in road construction, roofing, flooring, waterproofing, and coating.
Asphalt is mainly used in road construction. However, it is sometimes used in parking lots, railroad construction, and roofing.
Bitumen and asphalt are used in different ways in road construction. Asphalt mixture is used in the base, binder, and surface course of road layers. However, bitumen is used to help these layers stick together or seal the road. Bitumen is used in tack coat and prime coat.
(You can read about the types of bitumen used in road layers on this page.)
Differences in the Classification of Asphalt and Bitumen
Bitumen is categorized based on penetration, viscosity, and composition. For example, cutback bitumen consists of pure bitumen and petroleum solvent. In contrast, bitumen emulsion is a combination of bitumen, water and an emulsifying agent.
Different types of bitumen are:
- Penetration grade bitumen
- Viscosity grade bitumen
- Performance grade bitumen
- Oxidized bitumen
- Cutback bitumen
- Bitumen emulsion
- Polymer-modified bitumen
(Read this article to learn more about bitumen types and grades.)
On the other hand, the classification of asphalt mixture is based on production methods, the aggregate size and the bitumen type.
In general, asphalt has three types:
- Hot mix asphalt (HMA): bitumen and aggregate are mixed at high temperatures.
- Warm mix asphalt (WMA): It is made at a lower temperature by adding additives.
- Cold mix asphalt (CMA): Aggregates have not been heated and are produced at low temperatures.