Bitumen is a sticky, black, semi-solid form of petroleum that is commonly used in road construction and other applications. It is made from the residual products of petroleum refining, which are left over after the more valuable fractions, such as gasoline and diesel, have been removed.

Bitumen is composed of a complex mixture of hydrocarbons and other organic compounds, including long-chain molecules known as asphaltenes. These asphaltenes give bitumen its characteristic sticky consistency and make it resistant to water and weathering. Bitumen is also rich in sulfur, which gives it its distinctive smell.

Asphaltenes are a class of long-chain, high-molecular-weight organic molecules that are found in bitumen. Asphaltenes are known for their strong chemical bonds and their ability to dissolve in certain solvents, such as aromatic and naphthenic hydrocarbons. They give bitumen its characteristic sticky consistency and make it resistant to water and weathering. Asphaltenes also contribute to the chemical stability and thermal properties of bitumen.

Asphaltenes are typically found in higher concentrations in heavy crude oils and asphaltic residues, such as those produced from the refining of petroleum. They are also found in smaller amounts in lighter crude oils and in various refined products, such as gasoline and diesel.

In addition to their use in bitumen, asphaltenes have been explored for use in a variety of other applications, including in the production of plastics, adhesives, and fuel additives. They are also being researched for their potential use as a feedstock for the production of biofuels.

Bitumen is commonly used in road construction as a binder, or binding agent, to hold together the various layers of a pavement. It is mixed with aggregate, such as sand and gravel, to create asphalt, which is the main component of most paved roads. Asphalt is used to create the surface layer of the road, as well as the base and subbase layers, which provide structural support for the pavement.

There are several different grades of bitumen available, each with its own specific properties and uses. Some common grades of bitumen include:

1. Penetration grade bitumen: This is the most common type of bitumen used in road construction, and it is produced by heating the bitumen to a high temperature and allowing it to cool under controlled conditions. Penetration grade bitumen is classified by the depth to which a standard needle will penetrate the material at a specific temperature. It is used in the production of asphalt for roads and other surfaces.
2. Performance grade (PG) bitumen: This type of bitumen is used in areas with extreme weather conditions, such as high temperatures or heavy rainfall. It is formulated to have a specific range of performance characteristics, such as stiffness and flexibility, at different temperatures.
3. Cutback bitumen: This type of bitumen is used in cold weather conditions, as it is easier to work with at lower temperatures than other grades of bitumen. It is produced by blending bitumen with a solvent, which reduces its viscosity and makes it easier to spread.