BITUMEN

Bitumen is a black, viscous substance that has applications in waterproofing and asphalt construction. There are various types of tar, each with its specific uses. Bitumen is derived from petroleum and is often produced in oil refineries.

Tar is a hydrocarbon material that is completely soluble in carbon disulfide and carbon tetrachloride. Bitumen is solid at room temperature. However, with increasing temperature, it becomes a paste-like consistency and then turns into a liquid. The important properties of tar that contribute to its significant applications are:

Tar extracted from specific types of oil or mineral rocks is called pure Bitumen and is classified based on its origin. Pure tars undergo additional processes to acquire desired properties for various applications, resulting in different types of tar (including blown Bitumen, cutback Bitumen, emulsion Bitumen, polymer-modified Bitumen, etc.).

Typically, Bitumen is obtained from the distillation of crude oil. This type of Bitumen is referred to as petroleum Bitumen or distillation Bitumen. Petroleum Bitumen is produced through two stages of distillation in a distillation tower. In the first stage, light materials such as gasoline and propane are separated from the crude oil. This process occurs at near atmospheric pressure. In the second stage, heavy compounds such as diesel and heavy oil are removed. This process takes place at a pressure close to vacuum. Finally, a mixture of very fine solid particles called asphaltene remains, which is immersed in a greasy fluid called maltene.

However, some types of tar are naturally formed over an extended period through the gradual transformation of crude oil and the evaporation of its volatile components. This type of Bitumen is called natural Bitumen and has a longer durability compared to petroleum Bitumen. Natural tar can exist in a pure form in nature (such as lake tar) like the Lake Behbahan tar in Iran and the Tar seeps of Trinidad in the United States, or it can be extracted from mines (mineral Bitumen). Natural Bitumen is also known as Uintaite.

Pure Tar:

The Bitumen that is extracted from the top of the distillation column using the above method is called pure Bitumen or Fentertion grade bitumen. By selecting appropriate distillation, various types of pure Bitumen with different viscosities can be prepared. For example, the following pure Bitumen are mentioned based on their penetration grade at 25 degrees Celsius:

10/20, 20/30, 30/40, 40/50, 50/60,
60, 70, 90, 110, 190, 210, 280, 320, 400, 500

Some of the above pure Bitumen are obtained by blending and returning softer tars with harder Bitumen or by dissolving softer Bitumen in higher-boiling point Bitumen to produce Bitumen with higher penetration grades. It is necessary to mention that pure tars for their application should always be heated. Usually, Bitumen and aggregates are separately heated, then mixed together while hot, and the resulting hot mixture is spread. For this reason, pure Bitumen are commonly used in asphalt pavements.

Mixed Bitumen:

In some road construction operations, for various technical and specific reasons, the use of pure solid Bitumen is not possible. Therefore, refineries supply mixed or reclaimed Bitumen, which are mostly in a liquid state at ambient temperatures. Some are also liquidized by overall heating. These mixed Bitumen have several advantages and are commonly used in various types of surface asphalts and macadam asphalts. Mixed Bitumen are prepared by extracting a Bitumen with an appropriate penetration grade using a solvent that is capable of extracting the desired Bitumen. Typically, in refineries, Bitumen with a penetration grade of 300-100 (Abadan refinery: 100-80) are mixed with petroleum solvents such as kerosene, coal tar pitch, anthracene oil, or their mixtures to produce various types of mixed Bitumen. As mentioned below, other types of petroleum solvents are also used.

In some cases, for example, when the aggregate materials are moist, in order to increase the adhesiveness between the materials and the Bitumen, the used mixed Bitumen should have an appropriate acid value. In this case, cresote is added as a solvent for mixed tar or as a part of the mixed Bitumen to increase the acidity of the Bitumen. The preparation of mixed Bitumen in refineries is done when they are in a hot state, and the viscosity of the mixed Bitumen and the setting time on the road depend on the specifications, type, and amount of petroleum solvents used in preparing the mixed Bitumen. After the spread of the mixed Bitumen on the road, due to air flow and heat, the solvent in the mixed Bitumen evaporates, leaving behind the pure tar with its specific properties. The American system classifies mixed Bitumen into the following categories:

A: Rapid Curing Mixed Bitumen (R.C)

B: Medium Curing Mixed Bitumen (M.C)

C: Slow Curing Mixed Bitumen

The aforementioned mixed Bitumen each consist of several mixed Bitumen, differing in their viscosity, which are identified by numbers ranging from 6 to 0. It should be noted that R.C.6 tar is the most fluid mixture, while S.C.5 Bitumen is the firmest and has a paste-like consistency. In any case, R.C. mixed Bitumen are obtained by blending pure 100/80 tar with diesel fuel, M.C. mixed Bitumen are obtained by blending or reclaiming cresote with pure 100/80 tar, and finally, S.C. mixed Bitumen are obtained by blending pure 100/80 tar with oil and gas. The general specifications of mixed Bitumen are mentioned here. M.C. slow-curing Bitumen are widely used in road construction operations, including maximum cold asphalts, red mix, and premix.

Penetration Index (PI) of Bitumen:

Penetration index refers to the depth of penetration of a cylindrical weight into the tested Bitumen at a specific temperature and time. The test weight is a cylindrical rod with a hemispherical base, weighing a total of 100 grams. The test duration is 5 seconds, and the temperature is 25°C. Each 1.0 mm of penetration equals one unit of penetration index. For example, when it is said that the Bitumen is 70/60, it means that the penetration rod penetrates 6 to 7 mm into it. Penetration index is a measure of Bitumen hardness, with a penetration index of 10 used for hard Bitumen, 15 to 40 for roofs, and up to 100 or more for applications such as moisture insulation.