Bitumen is a petroleum-based product with a specific gravity between 0.97 and 1.02 which is a mixture of heavy organic molecules.
It is a semi-solid viscous material that is brown to black. Bitumen has no unique composition formula. As a result, it is mostly classified based on its physical properties such as hardness and viscosity.
In this article, a closer look will be taken at some of the important bitumen properties.
General Characteristics of Bitumen
Bitumen contains two main groups of molecules named asphaltene and maltene, which determine the properties of bitumen. Maltenes are the reason for the adherence property of bitumen. Asphaltene molecules affect the hardness and viscosity of the bitumen.
Bitumen is a thermoplastic material which means it is sensitive to temperature changes.
Bitumen doesn’t have specific melting, boiling, and freezing points due to the wide range of organic compositions.
Bitumen is known as a chemically inert material since it is incapable of conducting electricity.
Like many hydrocarbon materials, it doesn’t dissolve in water. Moreover, the viscosity and adhesive properties of bitumen make it a good waterproof and anti-rust material.
Use of Bitumen in Waterproofing
The chemical reaction of bitumen with oxygen which is known as oxidation takes place slowly. It is a key feature in bitumen’s long-lasting life.
The above-mentioned features were the general characteristics of bitumen. In addition to these properties, there are physical properties that are used to classify bitumen types.
In the following, some of the important characteristics are discussed in detail.
Hardness or Penetration of Bitumen
The penetration value of bitumen is defined as bitumen hardness. It is measured by applying a needle into the bitumen surface. The needle carries out 100 grams loads for five seconds. The depth of moving the needle into the bitumen is the penetration value. The unit of this value is deci-millimeter (dmm) which means one-tenth of a millimeter.
Read about Penetration Test of Bitumen
Low penetration values indicate harder bitumen whereas higher values denote softer bitumen. It is possible to categorize bitumen based on its hardness. So bitumen with penetration higher than 100 deci-mm is suitable for waterproofing projects and bitumen with penetration values between 40 to 100 deci-mm is useful in road construction such as 60/70 and 80/100 penetration grades.
Also, the hardness of bitumen is a determining factor that shows which type of bitumen will perform better in the weather of the construction region. Soft bitumen like 80/100 penetration grade is suitable for cold temperatures and hard bitumen like 30/40 grade is proper for hot weather.
Bitumen can become harder if it is exposed to UV rays for a long time or if its volatile substances are released over time.
Also, lowering the temperature in most cases leads to bitumen hardening, which of course is resolved by increasing the temperature.
Adhesion of Bitumen
The adhesive property of bitumen is the factor that enables it to act as a strong binder. In road construction, bitumen is used to make asphalt because of its strong adhesive properties to stick the aggregates together. Before applying bitumen, it is important to ensure that the surface is dry, as it can’t adhere to wet surfaces.
Click to read about different types of bitumen are used in each layer of asphalt.
Softening Point of Bitumen
Bitumen is a mixture of various heavy organic molecules. As a result, bitumen does not have a specific boiling point. For this reason, the softening point is defined for it.
The temperature at which bitumen becomes softer to turn into liquid is called the softening point. It is measured with the ring and ball test method.
For road construction, the temperature of asphalt, at the desired environment, must be lower than the softening point of bitumen, otherwise, the bitumen will melt and begin to flow and lose its adhesive properties.
In hot weather, bitumen with a high softening point is suitable for use because it melts more slowly and the bitumen efficiency is maintained.
Viscosity and Fluidity of Bitumen
Viscosity is a property that indicates the fluid resistance to flow. The higher the viscosity, the harder it moves. This property can be measured with different types of viscometers.
Viscosity is very important to have a uniform asphalt. For this reason, the optimum viscosity amount should be chosen to stick the aggregates well together.
Bitumen with very low viscosity acts like a lubricant and its adherence property is low. On the other hand, bitumen with a high viscosity value cannot cover the aggregates uniformly because its fluidity is low.
For this reason, one of the grading systems for choosing road binder bitumen is viscosity.
Bitumen viscosity is a temperature sensitive property. Increasing air temperature leads to the softening of the bitumen binder. It causes bitumen adhesion decreases as well as it comes out of the asphalt surface. Therefore the lower viscosity of bitumen makes it suitable for hot weather since it softens slower at higher temperatures.
Decreasing in temperature leads to the hardening of the bitumen binder. Also, It causes a decrease in the viscosity and durability of the bitumen against the traffic load. It leads to the cracking of the asphalt.
For this reason, bitumen with high viscosity is suitable for cold weather conditions since it hardens slower.
There are different definitions for bitumen viscosity and each of them is measured in different ways.
Read our article about “Viscosity Test of Bitumen“.
Ductility of Bitumen
Bitumen has ductility and viscoelastic properties. This unique feature leads to asphalt being a flexible layer. In other words, when bitumen is deformed by traffic load, it can return to its original shape without damaging or cracking.
If ductile bitumen is used as a binder, the formation of the bituminous film for coating aggregates will be flexible and uniform.
To ensure the existence of this property in the produced bitumen, the ductility test is conducted.
In this test, bitumen is poured into the briquette and stretched at a temperature lower than the softening point. The length at which bitumen string breaks is reported as bitumen ductility.
