The bitumen components are classified into the following four:

• saturates, saturated hydrocarbons, the % saturates correlates with a softening point of the material
• Naphthenic aromatics, consisting of partially hydrogenated polycyclic aromatic compounds.
• Polar aromatics, consisting of high molecular weight phenols and carboxylic acids
• Asphaltenes, consisting of high molecular weight phenols and heterocyclic compounds

Aromatic compounds with naphthenic and polar groups are typically the main constituents. Furthermore, natural bitumen contains an average of 4% sulfur, as it contains organosulfur compounds.

Physical Properties

When the temperature increases, bitumen becomes softer and harder, so it is thermoplastic and semi-solid at room temperature. At elevated temperatures, the viscosity decreases as the temperature goes up. In order to handle and apply bitumen in its intended use, it must be heated. Bitumen is also a visco-elastic material, which means it behaves like a solid when loaded quickly and as fluid when loaded slowly. A polar molecule in bitumen contributes to bitumen's affinity for aggregates, making it both adhesive and waterproof.

The specification for bitumen is based on physical properties, not chemical composition since it is a product of engineering. In addition to determining the suitability of the bitumen for a given application, physical properties also dictate the conditions under which it must be handled in order to be placed in the structure in which it will be used.

Temperature Susceptibility

A variety of ambient temperatures require products with reduced temperature susceptibility, in order to perform effectively. Methods are available for determining the change in properties of bitumen with a temperature that relates to changes in physical properties, such as stiffness and penetration values. Europe uses the Penetration Index (PI) for this purpose.

Bitumen oxidation modifies penetration-softening point relationships, reducing the temperature susceptibility of the material and increasing the PI of the oxidized substance. Due to this, PI can serve as an indicator of oxidation. Temperature susceptibility can also be determined by other methods.

Bitumen Components - Chemical composition of bitumen

Bitumen's chemical composition is generally similar but can vary based on the original crude oil and refining/blending processes. Bitumen is a complex mixture of hydrocarbons with a high molecular weight containing a variety of chemical compounds. Bitumen's molecular weight distribution is uncertain. During bitumen manufacturing, the 'cut point' of the distillation process is used to determine the smallest size. The largest size of bitumen molecules has not been determined yet; previous research suggested that molecules with up to 10000 Dalton are present, while some research indicates that very few if any, molecules larger than 1500 Dalton are found in bitumen.

A bitumen's functionality is determined by how molecules interact with each other or with other materials, such as aggregate surfaces. Sulfur, nitrogen, oxygen, and metals are found in some molecules, causing them to have a slight polarity. Molecules containing heteroatoms have significance in bitumen chemistry because of their ability to form molecular associations, which strongly influence the physical properties and performance of bitumen. In crude bitumen obtained from different sources, heteroatomic compounds can vary in content and characteristics.

Chemical characterization of bitumen

Since bitumen is a viscoelastic material, chemical polarity is an important property to measure. In terms of stiffness (modulus), bitumen is strongest in its most polar components. In asphalt, the least polar components provide flexibility and low-temperature properties, while in bitumen; the intermediate polarity components provide compatibility between the least polar and most polar components. As bitumen contains a continuous range of molecules, it is impractical to analyze each compound individually. Consequently, bitumen is usually divided into four broad, increasingly polar fractions: saturates, aromatics, resins, and asphaltenes (SARA). In most cases, solvent precipitation is used to separate asphaltenes, while chromatography is used to define the remaining fractions. There are several standard methods for separating bitumen into these four fractions, and the naming of the fractions is not indicative of their chemical composition, which is subject to change.