Bitumen is defined as “A viscous liquid, or a solid, consisting essentially of hydrocarbons and their derivatives, which is soluble in trichloro-ethyelene and is substantially nonvolatile and softens gradually when heated”. It is black or brown in colour & possesses waterproofing and adhesive properties. It is obtained by refinery processes from petroleum, and is also found as a natural deposit or as a component of naturally occurring asphalt, in which it is associated with mineral matte.
Bitumen has the following five characteristic properties.
- Bitumen Adheres
- Bitumen is Elastic
- Bitumen is Plastic
- Bitumen is Viscoelastic
- Bitumen Ages
- Bitumen Hardens
1. Bitumen Adheres
Bitumen has excellent adhesive qualities provided the conditions are favorable. However in presence of water the adhesion does create some problems. Most of the aggregates used in road construction possess a weak negative charge on the surface. The bitumen aggregate bond is because of a weak dispersion force. Water is highly polar and hence it gets strongly attached to the aggregate displacing the bituminous coating.
2. Bitumen is Elastic
When one takes a thread of bitumen from a sample and stretches or elongates it, it has the ability to return to a length close to its original length eventually. For some bitumens this process may take longer than others. This property is referred to as the elastic character of bitumen.
3. Bitumen is Plastic
When temperatures are raised, as well as when a load is applied to bitumen, the bitumen will flow, but will not return to its original position when load is removed. This condition is referred to as plastic behavior. Applying a load means that you put a weight on the bitumen in order to subject it to stress. This could be in a lab or in the bitumens final position in the road and it is done to assess the bitumens reaction to the load.
4. Bitumen is Viscoelastic
Bitumen has a Viscoelastic character. Its behavior may be either viscous or elastic depending on the temperature or the load it is carrying. At higher temperatures there is more flow or plastic behavior, while at lower temperatures and short duration loading, the bitumen tends to be stiff and elastic. At intermediate temperatures it tends to be a combination of the two.
5. Bitumen Ages
Aging refers to changes in the properties of bitumen over time, which is caused by external condition. These changes are visible as cracks or crumbling areas. When bitumen is exposed to atmospheric conditions, the bitumen molecules react with oxygen, which results in a change of the structure and composition of the bitumen. This process of combining with oxygen, called oxidation, causes the bitumen to become brittle and hard and to change colour from dark brown or black to grey. This change is usually referred to as oxidative hardening or age hardening. This form of ageing occurs more frequently in warmer climatic or during warm seasons, causing older pavements to crack more easily. The condition can also occur where the surface films of bitumen are thin, or if there has been inadequate compaction during construction.
Also Read: Factors Affecting Aging of Bitumen
6. Bitumen Hardens
Exposure to ultraviolet (UV) rays and the evaporation of volatile compounds can cause bitumen to harden. A volatile material is a material that can change in to a gas very quickly. There are two kinds of hardening:
- Physical hardening
- Exudative hardening
Physical hardening occurs when waxy crystals form in the bitumen structures, or when asphaltenes agglomerates clump together. This condition can be reversed if the temperature is raised.
Exudative hardening is caused by the absorption of oily components in the bitumen.