Viscosity Grade Bitumen (Asphalt) is a Bitumen grade mostly used as a paving grade and it is suitable for road construction and for asphalt pavements producing premier attributes. VG Bitumen is usually used in the production of hot-mix asphalt. Bitumen Viscosity Grade is classified into VG–10, VG–20, VG–30, and VG–40. Viscosity Grade Bitumen is petroleum-grade bitumen produced from fractional vacuum bottom which comes from the distillation of crude oil where the appliance and behavior change according to its temperature. ASTM International, formerly known as the American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services. Some 12,575 ASTM voluntary consensus standards operate globally. It is specified by ASTM Standard D3381–09 and AASHTO M226–80 (2008). VG Bitumen specifications cover by Viscosity at 60 ºC (140 ºF).

We have two methods of grading:

1. Standard Viscosity Grade Bitumen (AC–Grades), in which the Viscosity of the standard bitumen (asphalt) is measured at 60 ºC (140 ºF).
2. RTFOT Viscosity Grade Bitumen (AR–Grades), in which the Viscosity of bitumen (asphalt) is measured at 60 °C (140 °F) after the roll-on thin film oven test.

Viscosity-grade bitumen has a thermoplastic feature that causes the material to soften at high temperatures and harden at lower temperatures. This temperature viscosity relevance is significant when specifying the performance parameters such as the adhesion, rheology, durability, and application temperatures of bitumen. In the Viscosity Grade Bitumen specifications, the most important factor is based on the Bitumen ductility.

Advantages of Viscosity Grade Bitumen:

1. Unlike penetration grades, the same Viscosity Grade Bitumen gave a similar rutting performance in hot weather.
2. Minimum penetration values were retained in the viscosity grading system to maintain acceptable performance (in terms of resistance to fatigue cracking) at the yearly average service temperature of 25 ºC (77 ºF).
3. Minimum specified values of kinematic viscosity at 135 ºC (275 ºF) help to minimize the potential of tender mixes during construction.
4. The minimum specified penetration at 25 ºC (77 ºF) and the minimum specified kinematic viscosity at 135 ºC (275 ºF) established the maximum allowable temperature susceptibility (slope of temperature versus stiffness line).
5. Viscosity grade bitumen is suitable for a wide range of temperatures: 25 ºC (77 ºF) for raveling/fatigue cracking; 60 ºC (140 ºF) for rutting; and 135 ºC (275 ºF) for construction. Temperature susceptibility (the change in asphalt binder rheology with temperature) can be somewhat determined because viscosity is measured at three different temperatures (Penetration only is measured at 25 °C).
6. Since the viscosity values are measured at two temperatures, bitumen suppliers could provide the users with rational and accurate asphalt mixing and compaction temperatures (Corresponding to bitumen viscosity of 170 and 280 centistokes, respectively).

Application of Viscosity Grade Bitumen (Asphalt):

Viscosity Grade Bitumen (Asphalt) is a standard grade Bitumen usually used as a Paving Grade Bitumen suitable for road construction and for the production of asphalt pavements with superior properties. This grade of Bitumen is mainly used in the manufacture of hot mix asphalt for bases and wearing courses and possesses characteristics and qualities that are unique and quite different from other agents. They achieve very flexible and tenacious connections with other materials due to the Viscoelastic response of bitumen whose behavior depends on how fast charges are applied. Viscosity Grade Bitumen is petroleum-grade bitumen manufactured from fractional / vacuum distillation of crude oil whose practical appliance and behavior vary according to its temperature. The Bitumen supplied is produced from the vacuum residue (short residue) feedstock. Viscosity Grade bitumen is specified by the methods described in ASTM Standard D3381–09 and AASHTO M226–80 (2008). Viscosity Graded specifications cover bitumen (asphalt) graded by Viscosity at 60 ºC (140 °F).

According to viscosity (degree of fluidity) grading, the higher the grade, the stiffer the Bitumen. Tests are conducted at 60 ºC (140 ºF) and 135 ºC (275 ºF), which represent the temperature of road surface during summer (hot climate, similar to northern parts of India) and mixing temperature respectively. The penetration at 25 ºC (77 ºF), which is the annual average pavement temperature, is also retained.

VG–10 BITUMEN: VG–10 is widely used in spraying applications such as surface dressing and paving in very cold climates in lieu of the old 80/100 Penetration grade. It is also used to manufacture Bitumen Emulsion and Modified Bitumen products.

VG–20 BITUMEN: VG–20 is used for paving in cold climates & high-altitude regions.

VG–30 BITUMEN: It is a kind of bitumen, which is produced during the process of oxidation of the vacuum bottom in a distillation tower and is categorized based on its viscosity grade. The main common usage of bitumen VG–30 is primarily used to construct extra heavy-duty Bitumen pavements that need to endure substantial traffic loads. Also, VG–30 is in road construction, waterproofing, building construction industries, and also in cutback bitumen production. It can be used in lieu of a 60/70 Penetration grade. It is the most suitable for use in hot and rainy weather conditions and suitable to be used in mild regions.

VG–40 BITUMEN: VG–40 is used in highly stressed areas such as intersections, near toll booths, and truck parking lots in lieu of the old 30/40 Penetration grade. Due to its higher viscosity, stiffer Bitumen mixes can be produced to improve resistance to shoving and other problems associated with higher temperatures and heavy traffic loads.

STANDARD SPECIFICATIONS FOR VARIOUS GRADES OF BITUMEN VISCOSITY – VG/10, 20, 30 & 40

Bitumen is a thermoplastic material and its stiffness is dependent on temperature. The temperature–vs.–stiffness relationship of bitumen is dependent on the source of crude oil and the method of refining. 

The Bureau of Indian standards (BIS) introduced paving grade bitumen specifications (IS: 73–1950) for the first time in the year 1950 and classified it on penetration. The specifications were revised in the years 1962 and 1992. To improve the quality of bitumen, BIS revised IS–73–1992 specifications based on viscosity (Viscosity at 60 ºC) in July 2006. As per these specifications, there are four grades VG/10, VG/20, VG/30 & VG/40. A few qualification tests like specific gravity, water content, ductility, loss on heating & Fraass Breaking Point were removed from IS: 73–1992 specifications as these tests do not have any relationship either with the quality or performance of the product.

Indian oil commenced marketing of bitumen as per viscosity grade specifications confirming to IS: 73–1992 from all its refineries from August 2009. Therefore, the penetration grades have been replaced by viscosity-grade bitumen. According to viscosity (Degree of fluidity) grading, the higher the grade, the stiffer the bitumen. These are conducted at 60 ºC and 135 ºC, which represent the temperature of road surface during summer (hot climate, similar to Northern parts of India) and mixing temperature respectively. The penetration at 25 ºC, which is the annual average pavement temperature, is also retained.

Different grades of Bitumen marketed by Indian oil:

VG – 10 Bitumen: VG/10 is widely used in spraying applications such as surface–dressing and paving in a very cold climate in lieu of the old 80/100 penetration grade. It is also used to manufacture bitumen emulsion and modified bitumen products.

VG – 20 Bitumen: VG/20 is used for paving in cold climates & high-altitude regions.

STANDARD SPECIFICATIONS FOR VARIOUS GRADES OF BITUMEN VISCOSITY – VG/10, 20, 30 & 40

VG – 30 Bitumen: VG/30 is primarily used to construct extra heavy-duty bitumen pavements that need to endure substantial traffic loads. It can be used in lieu of a 60/70 penetration grade.

VG – 40 Bitumen: VG/40 is used in highly stressed areas such as intersections, near toll booths, and truck parking lots in lieu of old 30/40 penetration grade. Due to its higher viscosity, stiffer bitumen mixes can be produced to improve resistance to shoving and other problems associated with higher temperatures and heavy traffic loads.

Table: Viscosity grade (VG) bitumen specification as per IS 73:2006