What is Bitumen? | Types and Tests of Bitumen

Bitumen is a dense, highly viscous, petroleum-based hydrocarbon that is found in deposits such as oil sands and pitch lakes (natural bitumen) or is obtained as a residue of the distillation of crude oil (refined bitumen).

It is generally composed of asphaltene resin and other petroleum compounds. 

Different compositions of bitumen result in other properties.

Origin of bitumen

Bitumen is naturally occurring and can also be extracted from crude oil.

1. Bitumen of natural origin

Millions of years ago, living organisms such as microscopic algae (diatoms) were buried under sedimentary rocks, swamps, and other aquatic habitats.

 These residues were converted to natural bitumen under the pressure of the upper sedimentary layers and temperatures above 50 °C.

Bitumen is now found in hills and oil lakes. Most of the bitumen resources are in Canada, Venezuela, and Oman. There are also natural bitumen mines in western Iran. It has different ash content.

Natural bitumen, also known as Gilsonite, contains many impurities, including sulfur and heavy metals. The cost of extracting and using this bitumen is higher than that of the bitumen obtained from crude oil.

It is usually used as an additive for waterproofing coating, mud drilling, and road construction.

2. Crude oil distillation

Crude oil is the primary source of bitumen for the industry. Crude oil is a mixture of different compounds that is separated by a distillation tower. The remnants of the distillation tower, known as the vacuum bottoms, are transferred for processing and converted into bitumen.

Different types of bitumen, including penetration or oxidised bitumen, are produced through a distinct process on the vacuum bottom in the refinery. The main application of this bitumen is in road construction.

What is the difference between bitumen, tar, and asphalt?

Many people are unaware of the differences between bitumen, asphalt, and tar. Due to their similarities, these three terms can be used interchangeably. They are black, sticky, and suitable for use in road construction. However, they have distinct features that need to be considered before application.

In what follows, we provide a comprehensive guide to each product’s description. We also elaborate on the properties and uses of bitumen, asphalt, and tar.

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How is Bitumen Produced?

Bitumen is a by-product of crude oil. After taking crude oil to an atmospheric distillation column in refineries, we heat it to temperatures between 300 °C and 350 °C. As a result, lighter fractions of crude oil separate from non-boiling components. This process leaves a sticky substance at the bottom of the column, known as the vacuum bottom, and utilizes it as the raw material for producing refined bitumen.

Refined bitumen is not the only type of bitumen. Before the processing of crude oil, natural sources of bitumen have been in use worldwide. For thousands of years, human beings extracted bitumen from lakes and rocks.

The bitumen that comes from lakes contains 40 to 70 % pure bitumen. To purify the bitumen, we boil it until all the water has completely evaporated and the impurities are separated. Rock bitumen, also known as oil sands, is a sedimentary source that contains 5 to 15 % bitumen. The primary procedure for extracting bitumen from rocks involves crushing, heating, and separating the bitumen from other materials.

What are the different forms of bitumen?

The forms of bitumen are generally five types as follows.

• Penetration Grade Bitumen
• Viscosity Grade Bitumen
• Cutback Bitumen
• Bitumen Emulsion
• Oxidized Bitumen

Penetration Grade Bitumen: These grades of bitumen are classified due to their level of hardness. Penetration grades of bitumen are bitumen 30/40, 60/70, 80/100, and 100/120.

To identify various grades of penetration bitumen, we insert a standard needle into our sample and measure the depth it can penetrate within a given time.

Viscosity Grade Bitumen: These types of bitumen are the result of a highly accurate system of classifying bitumen. After producing bitumen, we test it and measure its viscosity level.

This testing system introduces four VG grades, including VG10, VG20, VG30, and VG40. The higher the number, the more complex the bitumen in the viscosity grading system.

Cutback Bitumen: Cutback bitumen is produced by adding solvents to bitumen to decrease the viscosity level. This procedure involves the addition of controlled amounts of petroleum extracts such as kerosene, naphtha, diesel oil, and furnace oil to bitumen. Cutback bitumen doesn’t need heating. Cutback bitumen can be used as paint in cold weather conditions.

Bitumen Emulsion: This type of bitumen is just like cutback bitumen, with one significant difference. Refinery workers reduce the viscosity of bitumen emulsions with water, not petroleum additives. When we apply emulsions on the surface, water evaporates, and the mix starts to set. As a result, there is no need to heat the mixture.

Oxidised Bitumen: To make bitumen more resistant to temperature, we can blow air into bitumen and produce oxidised bitumen.

Known as blown bitumen, this type of bitumen is ideal for various industrial purposes, including roof insulation, flooring, industrial mastics, pipe coating, and paints.

What is asphalt?

In American English, people use the term asphalt (or asphalt cement) instead of bitumen. Outside the United States, however, asphalt is a mixture of bitumen with sand or gravel. For road construction, we primarily use asphalt, mainly due to its durability and flexibility as a pavement material.

Asphalt is produced in two different ways as follows.

• Natural asphalt
• Residual asphalt

Natural Asphalt

Natural asphalt is obtained directly from nature, primarily from two resources: lakes and rocks. The lake asphalt contains 40 to 70% pure bitumen, which is boiled in the tank, where the water content evaporates and impurities are separated. The final product is called asphalt, which can be used for laying roads and other purposes.

Rock asphalt contains 10 to 15% pure bitumen and calcareous matter. These rocks are crushed, heated, and consolidated by sudden cooling. This asphalt is used for paving tiles and other purposes.

Residual Asphalt

This residual asphalt is obtained artificially through the distillation of crude petroleum oil with an asphaltic base.

Forms of Asphalt

The available forms of asphalt are:

• Cutback asphalt
• Asphalt emulsion
• Asphalt cement
• Mastic asphalt

Cutback Asphalt: is in a liquid state. Asphalt is dissolved in a volatile solvent to get this cutback asphalt. It is used for manufacturing bituminous paint and repairing roofs, among other applications.

Asphalt Emulsion: is obtained by adding 50 to 60% water to the asphalt in the presence of a 1% emulsifying agent. The emulsifying agent forms a waterproof film when water evaporates. So, it can be used in cold conditions.

Asphalt cement is subjected to high-pressure air at high temperatures to produce asphalt cement. Asphalt cement is a plastic-like material used for flooring, roofing, and as a waterproofing material, among other applications.

Mastic Asphalt is obtained by heating natural asphalt with sand and mineral fillers. It is impermeable matter and does not contain any voids.

The hardness and melting point of mastic asphalt can be controlled during the heating process. It is in a solid state, and by heating, it can be brought to a liquid state. It is used for damp-proofing courses.

Difference between bitumen and asphalt

In various countries, people use these two terms interchangeably. To avoid mistakes, we need to understand that bitumen is a sticky petroleum product suitable for binding other materials.

This substance possesses special adhesive properties that make it ideal for mixing with sand to produce asphalt. Asphalt, therefore, is a mixture of sand and bitumen. We use asphalt for paving roads.

What are bituminous roads?

In some parts of the world, it is possible to use bitumen alone for paving roads. In this method of road construction, we lay a layer of bitumen and cover it with aggregates. We then repeat this process to create a two-coated seal. We can also use bitumen to seal the various layers of a road.

What is Tar?

Tar is a highly viscous liquid that contains a high amount of carbon. It is used for laying surface layers of pavements, preserving wood, and other purposes.

Forms of Tar

General forms of tar are as follows.

• Coal tar
• Wood tar
• Mineral tar

Coal Tar

Coal tar is the by-product obtained during the production of coal gas. It is used for preserving timber and laying macadam roads, among other applications.

The process involves heating coal in closed iron vessels, and the resulting evaporated gases are collected in tubes. These tubes are circulated with cooling water.

So, some matter is deposited in these tubes, and it is nothing but coal tar. It is a dense and strong-smelling liquid. It is black in colour.

Wood Tar

Wood tar is obtained by the distillation of resinous woods like pines, etc. The distillation of resinous timber provides creosote oil, which has excellent preservative properties. So, wood tar is well-suited for preserving wood.

Mineral Tar

The exploitation of kerogens produces mineral tar. Kerogens are generally obtained from bituminous shale, which is a type of rock. The volatile content of mineral tar is very low.

Difference between bitumen and tar

Tar is a viscous, black liquid distilled from coal or wood. We use tar because of its excellent properties as a waterproofing and sealing agent.

Bitumen and tar are similar in appearance, but their properties differ significantly. In comparison to bitumen, tar has a higher carbon content, is a better adhesive, and becomes more fluid when heated. However, few people use tar instead of bitumen in road construction.

Tar has a wide variety of applications, including waterproofing, coating timbers, and skin care.

Types of bitumen and their applications

Since various types of crude oils yield different types of bitumen with distinct properties, grading bitumen is crucial.

For grading bitumen based on its properties, refineries employ a range of standard tests to measure the viscosity level, penetration value, performance, and consistency of the bitumen.

The result of these tests is bitumen penetration grades and viscosity grades.

Other grades of bitumen, including cutback bitumen, oxidized bitumen, and bitumen emulsions, are classified based on their production process.

1. Gilsonite (Natural bitumen):

Gilsonite is natural bitumen with a shiny, smooth, and solid appearance. Types of Gilsonite are classified based on their ash contents and the solubility in organic solvents.

Since it is a complex and brittle material, it is mainly used in powder form. The Gilsonite composition is similar to refinery bitumen. 

In the road construction industry, the combination of Gilsonite with refined bitumen is sometimes used. Although it has a higher resistance to water penetration, its degree of penetration and deformation is low.

Applications of Gilsonite

1. Printing Industries
2. Painting industry
3. Drilling mud and cement
4. Asphalt and road construction
5. Casting sand additive
6. Chemical products
7. Improving the quality of bitumen

2. Penetration grade bitumen

Penetration bitumen is the most widely used bitumen for road construction. 

After production in the refinery, it is classified by penetration test using a penetrometer apparatus. The deeper the needle penetrates the bitumen surface, the softer it becomes and the more suitable it is for colder weather. The most widely used bitumen for road construction is penetration bitumen of 60/70 grade, followed by the 80/100 grade. Furthermore, 60/70 grade is widely used as a waterproofing material in buildings.

To use this bitumen effectively, you should consider the average temperature of the area and the level of traffic.

In the table below, you can see the different grades of this penetrating bitumen, along with their respective applications.

3. Viscosity grade bitumen

Viscosity grade bitumen is classified based on its viscosity after production of the bitumen in the refinery. This classification is newer than the penetration classification. The highest consumption of this bitumen is in India.

Viscosity bitumen has four different grades: VG 10, VG 20, VG 30, and VG 40. They have other applications.

The reason for using viscosity grading instead of penetration is the change in the viscosity of bitumen at high temperatures. 

Here are some advantages of VG bitumen to Pen grades:

• This grading system is designed to increase the accuracy of bitumen in hot temperatures. A specific grade of pen bitumen may show various results at a particular temperature, but VG bitumen always behaves the same.
• When there is a need to predict bitumen behaviour in the hottest time of the year, as well as its properties in normal weather conditions, VG bitumen is a better choice. Since viscosity testing is based on experimenting with bitumen’s properties at three different temperatures (25° C, 60° C, and 135° C), all grades of VG bitumen come with a more straightforward application.

4. Emulsion bitumen

Bitumen emulsion derives its name from the method of its production. This type of bitumen is a chemical combination of bitumen and water. 

The industry sought to avoid the environmental damage associated with cutback bitumen. Water is a reliable alternative to organic solvents for diluting bitumen sufficiently for special applications.

Emulsion bitumen is beneficial for waterproofing, spraying, and tack coats between layers of asphalt pavement.

Transporting, handling, and storing bitumen emulsion are all easy, economical, and safe.

Bitumen emulsions are classified based on two main factors: 

• Particles’ electrostatic charge
• Breaking or setting time

According to their charge, bitumen emulsions can be either anionic (with a negative charge) or cationic (with a positive charge).

The second grading system is based on the time required for a bitumen emulsion to lose water and become sticky enough to bind aggregates. 

Various grades of bitumen emulsions have different properties; therefore, each type is suitable for a particular kind of application.

Three factors —viscosity level, setting time, and electronic charge of bitumen particles —have an influence on where we use each grade and type of bitumen emulsion.

Bitumen emulsions are less viscous than other grades of bitumen because they are mixed with water and emulsifiers. As a result, we use this type of bitumen for spraying applications. 

Bitumen emulsions are also ideal for pavement maintenance and the renovation of old roads.

Significant applications of bitumen emulsions in today’s bitumen industry are tack coats, chip seals, slurry seals, and micro surfacing.

5. Cutback bitumen

This bitumen is used in industries that require low-viscosity bitumen. Cutbacks are also used in cases where bitumen can not be preheated to make asphalt.

To prepare cutback bitumen, penetration bitumen is mixed with a specific amount of organic solvents. These organic solvents include gasoline, naphtha, diesel oil, and furnace oil. 

This type of bitumen is suitable for cold climates. The solvents evaporate when bitumen is used.

The cutback is suitable for primer sealing and sprayed sealing.

The only concern was the harm that the cutback bitumen had to the environment. The volatile solvents were hazardous to the environment, humans, and nature.

6. Oxidized bitumen

Oxidized bitumen is obtained in the refinery by blowing hot air into the penetration bitumen. The bitumen obtained from this process is chemically stable and has high durability. It is also impermeable to water and is suitable for insulation. 

Oxidized bitumen has a lower degree of penetration and a higher softening point compared to pure bitumen. Also, it is classified based on these two properties. 

Applications of oxidized bitumen include waterproofing in dam construction projects, buildings, and other structures. 

Additionally, it is applicable in road construction when used in combination with different types of bitumen. In other industries, especially oil and gas, it is used to cover pipelines to prevent corrosion.

7. Performance Grade bitumen (PG bitumen)

Bitumen Performance Grade is a type of bitumen grading system according to its performance at different temperatures.

A bitumen that can endure a wide range of temperatures is more suitable.

Classification of PG bitumens is done at high and low temperatures. 

The primary issue for high-temperature performance is rutting, which typically develops over time. It takes some time for rutting to disappear.

At low temperatures, the probability of cracking increases.

To evaluate bitumen’s performance, the average maximum and minimum environmental temperatures at which asphalt will be made are chosen.

The bitumen is then exposed to these two temperatures for a week. 

A 6 °C increase raises the average high and low temperatures. 

The high temperature in these grades is in the range of 46 to 82 °C, and the low temperature is in the range of -46 to -10 °C.

Grading bitumen is based on a positive number and a negative number, which indicate the maximum and minimum temperatures.

For instance, bitumen PG 64-10 meets the performance factor at an average seven-day maximum pavement temperature of 64 °C and also at a minimum pavement temperature of -10 °C.

If the sum of high and low-temperature numbers is more than 90, it will be polymer-modified bitumen, and if it is less than 90, it will be unmodified polymer bitumen.

For example, a PG 76-22 is considered a polymer-modified binder since the sum of the components is 98. Another example is PG 64-10, which is an unmodified binder.

Thermal cracking is less likely to occur in PG grades 58-34, 64-22, 64-28, and 76-22. Additionally, the PG 70-22, 70-28, 76-28, and 82-22 grades exhibit greater resistance to rutting.

Note: Due to the black colour of asphalt and more heat absorption, the asphalt temperature can be 20 °C higher than the ambient temperature.

Performance grade bitumen is commonly used in the construction and reconstruction of pavements, sealing edges, and sealing cracks in both dense- and open-graded Hot Mix Asphalt (HMA).

Bitumen tests

Bitumen quality tests are a criterion for diagnosing the quality and classification of bitumen in the industry. These tests are also conducted to control the quality and ensure the bitumen load issued and imported at the ports of origin and destination.

According to SGS standards, bitumen quality control tests, or bitumen tests, in summary, include:

1. Penetration test: 

The penetration test is used to measure the hardness and softness of bitumen. Penetration bitumen obtained from the refinery is classified based on this test. The degree of penetration of bitumen is directly related to its ductility, indicating that asphalt has a higher loading capacity.

High-penetration bitumen is suitable for cold climates, and low-penetration bitumen is ideal for hot climates.

To perform this test, a needle is penetrated the bitumen surface with a load of 100 grams for 5 seconds. The degree of penetration is equal to the amount of needle penetration. Its unit of measurement is 0.1mm.

2. Viscosity test: 

The viscosity test is used to determine the adhesion of bitumen to asphalt aggregates. Bitumen must have an optimal viscosity value.

This means that very viscous or very low viscosity bitumens are not suitable for road construction. In addition, viscosity is highly sensitive to temperature increase, and with increasing temperature, the amount of viscosity decreases; as a result, its cohesion is lost.

For this reason, viscosity is measured at temperatures of 60 °C and 135 °C. By measuring these two viscosities, we can obtain sufficient information about the optimal temperature for mixing and compacting bitumen with aggregates.

To measure the viscosity of the viscometer apparatus, the time required to remove a certain amount of bitumen from the viscometer is calculated and multiplied by the correction factor of the device. The number obtained is equal to the viscosity of the bitumen.

A schematic is shown below, together with a cup viscometer.

3. Softening point test: 

Bitumen, as a combination of different hydrocarbon materials such as asphaltene and resin, does not have a specific melting point. By heating the bitumen, it becomes soft and melts slowly. 

The temperature at which bitumen starts to melt/soften is called the softening point. This point is essential for engineers because it indicates at what temperature the bitumen loses its adhesive properties.

The laboratory method of measuring the softening point is the ring and ball method.

To perform the test, the bitumen sample is taken in two brass rings. Two steel balls are placed on the bitumen samples. The assembly is then placed in a water bath and heated.

The softening point is the temperature at which the steel ball coated with bitumen hits the bottom of the glass beaker.

4. Ductility test:

As long as the bitumen has good ductility, it will be resistant to road traffic as well as the expansion and contraction of bitumen due to temperature changes, and the possibility of asphalt cracking will decrease, thereby increasing asphalt life.

To measure this property, bitumen is poured into a special mould and the mould is pulled from both sides at a constant speed of 5 cm per minute. The amount of bitumen stretched to the moment of rupture is reported as ductility. The longer the length, the greater the ductility.

5. Specific gravity test 

Specific gravity is the ratio of the mass of bitumen to the mass of water, both of which have the same volume. 

Each bitumen has a specific gravity. As a result, this property is used to measure the number of impurities in the bitumen. It is also used for calculations.

We use a pycnometer to measure this property. First, weigh the pycnometer, then fill it with bitumen and weigh it again. Similarly, fill the pycnometer with water and weigh it. We subtract the weight of the pycnometer from the readings of the pycnometer filled with water and oil. The ratio of bitumen weight to water weight is equal to the specific gravity of bitumen.

6. Solubility in TCE

This test is performed to measure the purity of bitumen. This test detects non-oil impurities. For this purpose, 2 grams of bitumen is dissolved in 125 ml of trichloroethylene. The solution is weighed and then filtered through Whatman paper. Impurities are washed, dried, and finally weighed. The percentage of impurities should not exceed 1%, as this indicates a low-quality bitumen. High impurities in bitumen will result in low adhesion strength.

Insoluble matter (%) = mass of insoluble materials/mass of bitumen sample

7. Flash and Fire point test

There are also risks in transporting and storing flammable materials. As a result, it is more important to notice the flash and fire point of bitumen.

Heat causes liquid materials to evaporate. If the liquid is flammable, it will catch fire if an ignition source, such as a flame, is present. 

Two devices are used to obtain the flash point and fire point:

• Pensky-martens
• Cleveland open cup tester

The procedure for the test involves pouring bitumen into the cup of the apparatus and then heating it. 

During heating, the flame passes over the liquid surface and through the vapours once every 2 seconds. 

The temperature at which an instant flame appears on the surface of bitumen is known as its flash point.

To determine the flash point, heat is continued, and the flame is allowed to pass over the surface of the liquid until a momentary flame reappears on the liquid surface. 

Compared to the previous flame, this one lasts longer. Its temperature is reported as the fire point.

8. Loss on heating test

The loss on the heating test measures the amount of volatiles in the bitumen. Volatiles are compounds that have a lower boiling point than water and evaporate at a lower temperature. 

The higher the percentage of volatile materials in the bitumen, the harder it becomes for the asphalt during the heating process, and it loses its elasticity. A suitable bitumen binder should contain less than 1% volatiles.

To perform this test, 50 g of the bitumen sample is heated in the oven at 163 °C for 5 hours. After that, the bitumen sample is weighed.

The percentage of loss on heating is calculated as follows:

Loss on heating (%) = [(Initial weight – Final weight)/Initial weight]*100

9. Spot test of bitumen

The spot test of bitumen is used to indicate that the bitumen is not damaged during the refinery’s overheating. 

In a refinery, if the heating is too high, cracking occurs in bitumen. Cracked bitumen is not sticky enough to hold aggregates together. Moreover, it can not tolerate the preheated process of road construction.

To perform the test, 10 mL of a solvent is added to a flask containing 2 grams of the bitumen sample. The solution is rotated and placed in boiling water until it is dissolved completely.

After it cools to room temperature, put a drop on filter paper.

If the trace of the drop is brown, the test is negative. Inversely, if the drop is brown with a black centre, the test is positive and the bitumen is damaged.

10. Drop in penetration after heating test of bitumen

Drop-in penetration tests measure the hardness and volatile content of bitumen. 

When bitumen is heated for construction, the volatile components evaporate, causing the material to harden. When bitumen has a more volatile content, its hardness increases after heating.

To perform the drop-in penetration test after the heating test, apply a 100-gram load to the needle for 5 seconds on the bitumen surface. Note the amount of penetration at 0.1 mm.

Then, the bitumen sample is placed in the oven at 163°C for 5 hours.

After removing the sample, it is allowed to cool. In the next step, the penetration test is repeated.

Drop in penetration value (%) = 

[ difference between 2 amounts of penetration/initial amount ] * 100

Bitumen manufacturers

According to global statistics, China is the largest producer (and also consumer) of bitumen in recent years, with a production of 22 million tons per year, and the United States is in close competition with an annual output of nearly 21 million tons.

Russia ranks third with a production of almost 7.8 million tons per year. Iran, India, and South Korea follow closely behind with an annual output of about 5 million tons, 

It is essential to mention that the most critical exporters are South Korea, Iran, Singapore, Russia, Canada, and Germany, respectively.

In the bitumen market, competition is easier for manufacturers and sellers than in other markets.

There are no major players in this market. The top 5 largest producers and marketers of bitumen are:

• Exxon Mobil Corporation
• Royal Dutch Shell Plc
• bp p.l.c.
• Nynas AB
• China Petroleum & Chemical Corporation (SINOPEC)

Comparison Between Asphalt, Bitumen, and Tar Properties

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