Frequently Asked Questions

Q. What is the difference between Penetration & Viscosity grades?

Ans: Penetration Grade classifications are based on the Penetration value (degree of hardness) (Test conditions: 250C, 100 gm, 5 sec) while VG system is based on absolute Viscosity (degree of Flow Resistance) of the Bitumen samples measured in Poise (Test conditions: @ 600C, 300 mm Hg vacuum). It also includes Kinematic Viscosity measured in cst @ 1350C.

Q. What are the benefits / advantages of VG Bitumen over Penetration Grade?


VG system is based on fundamental engineering parameter (not empirical)

Viscosity is measured at 600C and 1350C, which takes care of both low and high temperature susceptibility of the binder, which is not possible with Penetration value @250C. Hence, pavement engineers, contractors & consultants can have a better understanding about the binder’s performance in the field.

Any two samples of same Viscosity Grade would give similar rutting performance in hot summer unlike Penetration Grade.

Greater ease in handling for users as Viscosity Value at two different temperatures (@ 600C and @ 1350C) is available, which would enable users to measure accurate mixing and compaction temperatures.

Minimum specified Kinematic Viscosity value @ 1350C helps minimise the potential of tender mixes during construction.

Viscosity Graded Bitumen is suitable for a wide range of temperatures:
250C for raveling / fatigue cracking, 600C for rutting and 1350C for construction (mixing and compaction).

IS 73-2006 has only seven tests to evaluate a sample compared to 14 tests in Penetration Grade system. This reduces time and cost of testing without sacrificing its quality.

Q. What are the limitations of Penetration Grade?


This gradation is based on an empirical test and not a fundamental test; it doesn’t have any relevance to field performance of the sample.

Two samples having same Penetration value may show different behaviour at high and low temperatures.

No Bitumen Viscosity is available near Bitumen mixing and compaction temperatures for the guidance of end-users.

Penetration grading doesn’t control the temperature susceptibility of Bitumen. Highly thermal susceptible Bitumen grades are not desirable because they are soft at high service temperature and very stiff at low service temperature.

It cannot be used effectively for Polymer Modified Bitumen.

Q. Why does pavement made of VG Bitumen have longer durability than PG Bitumen?

Ans:The pavement made from VG Bitumen will have better performance, because Viscosity value measured at 600C correlates well with rutting behavior and Viscosity value at 1350C gives sufficient idea about the mixing and compaction temperature and as a result pavement life is improved.

Q. Can we use VG 30 Bitumen in high temperature zones where the critical highway temperature is >60oC?

Ans:Yes, VG 30 can be used in high temperature zones as it has good thermal susceptibility.

Q. Why is a minimum limit to absolute Viscosity @ 600C prescribed? Is it OK to keep the minimum limit?

Ans: The temperature of 600C is the near maximum Bituminous pavement temperature on a hot summer day, when rutting is likely to occur. It is useful to determine the stiffness (in terms of absolute Viscosity) of Bitumen at 600C so that we can specify its minimum stiffness to ensure adequate resistance to rutting during hot summer. Pavement rutting is the most prevalent problem in India.

Q. What is the relevance of Ductility Test @ 250C on residue of TFOT?


Thin Film Oven Test (TFOT) is nothing but the simulation of aging condition during mixing and compaction. If the material shows good ductile characteristics after TFOT, it implies that the binder can be laid nicely on the road and will not age (deteriorate) much during mixing and compaction.

Q. The number of tests for VG Bitumen is less than that of Penetration Grade. How would this give assurance about the quality of Bitumen?

Ans:Some of the tests given in old Penetration Grade specification are the repetition of checking one parameter by different methods and some are redundant. For example, ductility measurement after TFOT ensures ductile property; there is no need to check it before TFOT. Penetration ratio, Paraffin Wax content and Fraass breaking point tests are redundant as these properties have been taken care in new Viscosity Grade specifications.

Q. Do we have a ready-made chart to use various Bitumen Grades as per the temperature zones?

Ans:Ideally, selection of Bitumen grades should be based on high and low pavement temperatures (climatic conditions). For practical consideration, selections need to be based on air temperatures. Weather data can be obtained from IMO (Indian Meteorological Organisation) for the purpose of understanding regionwise requirement of binder grades. Selection criteria for VG paving Bitumen based on climatic conditions is tabulated below:

S.No Lowest Daily Mean Air Temperature (0C) < 250C 20 to 300C > 300C
1. More than – 100C VG-10 VG-20 VG-30
2. – 100C or lower VG-10 VG-10 VG-20

Q. What is the effect of using VG-10 Bitumen in hot climate areas? What is the right grade to be used in this area?

Ans: Due to high temperature in hot climatic areas, use of VG-10 would not provide good rutting resistance. Based on the highest daily mean air temperature, which generally ranges from 300C to 440C, VG-30 Bitumen can be used in this area.

Q. Is there any difference in the manufacturing process for VG Bitumen over Penetration Grade?

Ans: Yes, process parameters need to be modified to produce VG Bitumen. It is produced by blowing bitumen with air.

Q. How to measure viscosity at 600C? What type of equipment and manufacturers do you recommend?

Ans: A vacuum capillary tube viscometer is used to perform the Viscosity test at 600C. Viscosity test equipment consists of i. Calibrated Cannon-Manning Viscosity tube, ii. Oil bath maintained at 600C, iii. Vacuum pump and iv. Vacuum gauge, controller, thermometer, stop watch. Viscosity tube to be imported through an Indian distributor and remaining items are easily available in India. Generally Cannon Manning vacuum capillary viscometer, Cannon fenske viscometer.