Generally, there exist two types of pavement- flexible pavement and rigid pavement. Flexible pavements are made of bitumen. Different bitumen types are used for different layers of roads.
The flexible pavement is made of aggregates and different bitumen types and grades. This type of pavement consists of different road layers: surface course, tack coat, binder course, prime coat, base course and sub-base course.
The below-attached infographic from Infinity Galaxy summarises the different types of bitumen used in flexible pavement construction and maintenance.
Surface Course
The surface course is the top layer on the road surface. It is directly exposed to the traffic load. Hot asphalt can be used on the surface course. It consists of aggregates and pure bitumen. Pure bitumen has two different forms- penetration grade bitumen & viscosity grade bitumen. Each of these two bitumen types has different grades that can be used in different weather conditions and different ranges of temperatures.
Penetration grade bitumen: The bitumen is classified into different grades as per its’ penetration and softening point under the standard test conditions. The bitumen is graded as x/y grade by conducting a penetration test on it. The ‘x’ represents the minimum penetration of a 100-gram needle at 25 degrees centigrade and ‘y’ represents the maximum penetration under similar conditions. One of the most common grades is bitumen in 60/70.
Viscosity grade bitumen: Gradation of bitumen is also done by measuring the viscosity of the bitumen under standard test conditions. The representation of viscosity grade bitumen is VG-10, VG-20, VG-30 & VG-40.
Also, cold asphalt can be used on this layer. Cold asphalt consists of aggregates and liquid bitumen. There are two types of liquid bitumen:
- Bitumen emulsion: It is a mixture of bitumen and water. As the bitumen & water don’t mix, it is prepared by mechanical shearing in a colloid mill.
- Cut-back bitumen: It is a mixture of bitumen and kerosene. It has a lower viscosity than pure bitumen and has higher penetration.
Tack Coat
A tack coat is a layer which binds the binder course and the surface course. Different grades of bitumen emulsion which are CRS-1 and CRS-2 can be used in the tack coat layer.
Other than this, MC30, the medium curing cut-back bitumen, is also used in this layer.
Binder Course
The binder course is a layer which is placed between the surface course and the base course. This layer distributes the entered loads from the surface to the bottom layers.
Different grades of pure bitumen can be used in the binder course. These bitumen grades are as below:
- Penetration grade bitumen: 30/40, 40/50, 60/70, 80/100, 120/150
- Viscosity grade bitumen: VG10, VG20, VG30, VG 40
Bitumen 60/70 and VG30 are the highly demanded bitumen grades in road construction.
Prime Coat
The surface of the base course is sandy. It is better to pour a thin layer of liquid bitumen on this surface before asphalting it. This thin layer of liquid bitumen is called a prime coat.
Different grades of liquid bitumen that can be used in the prime coat are as below:
- Bitumen emulsion: All grades of cationic and anionic bitumen emulsion
- Cut-back bitumen: MC30, SC70, MC70, RC70, SC250, MC250, RC250
Base Course
The base course transfers the loads which are applied from the surface course to the sub-base and subgrade layer. This layer consists of hard aggregates and gravel.
Sub Base Course
Sub-base course is made of crushed aggregates or gravel. This layer is placed under the base course. This layer should be hard enough to tolerate the upper layers and the tensions applied to it.
Protective Asphalts
Protective asphalts protect the surface layer from damage and cracking. Generally, different grades of bitumen emulsion and polymer-modified bitumen can be used in protective asphalts.
Different types of protective asphalts are as below:
- Fog seal: It is a specially formulated asphalt emulsion and it is sprayed over the existing surface. The process of applying a fog seal is known as fogging and hence the name fog seal. It seals the narrow cracks. Bitumen emulsions used in fog seal are SS-1, SS-1h, CSS-1 or CSS-1h.
- Chip seal: It is a process of applying asphalt with fine aggregate in one or more layers. It is used on lower-traffic roads for their repair. Bitumen emulsions used in chip seal are CRS-2, RS-2, HFRS-2 and PMB.
- Seal coat: A seal coat protects the asphalt surface from oil, water etc. and improves the appearance. Bitumen emulsions used in seal coat are CSS-1, CSS-1h, SS-1 and SS-1h.
- Slurry seal: A slurry seal is similar to a fog seal except it has fine aggregates mixed with water, bitumen emulsion and chemical admixture. Bitumen emulsions used in slurry seal are CSS-1, CSS-1h, SS-1, SS-1h and CQS-1h.
- Micro-surfacing: Roads with narrow cracks and low to moderate stress are treated with micro-surfacing. It is the application of bitumen emulsion, fine aggregates, water and chemical admixtures. Bitumen emulsions used in micro-surfacing are PMQS-1, PMCQS-1h, and CQS-1P.
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