1

AN OVERVIEW OF THE VISCOSITY GRADING SYSTEM ADOPTED IN

INDIA FOR PAVING BITUMEN

By

Prof. Prithvi Singh Kandhal*

[This paper was published in the Indian Roads Congress, “Indian Highways”, Volume 35, No. 4, April

2007]

ABSTRACT

Viscosity graded system has recently been adopted in India for paving bitumen based on

IS 73:2006 Paving Bitumen – Specification (Third Revision) issued in July 2006. The

viscosity grading system replaces a 100-year old penetration grading system. This system

specifies paving bitumen by viscosity at 60 C rather than penetration at 25 C.

The paper reviews the history of grading paving bitumen over the last 125 years. The

progression of grading systems: grading by chewing, penetration grading, viscosity

grading, and Superpave performance grading, has been discussed with their respective

advantages and disadvantages.

The paper gives an overview of the newly adopted viscosity grading system to acquaint

highway engineers, contractors, suppliers, and consultants with it to make the necessary

change over from penetration grades to viscosity grades in practice. The new viscosity

graded (VG) bitumen specification has been compared with the old penetration graded

bitumen specification. Selection criteria for VG grades based on climate and traffic has

been recommended. Recommendations have also been made to implement the VG grades

as soon as possible to improve the consistency, quality, and durability of bituminous

pavements in India. Information about the viscosity test such as testing equipment,

testing procedure, and estimated cost of the equipment has been given.

1. INTRODUCTION

India has now adopted a viscosity grading system for paving bitumen. The Bureau of

Indian Standards has issued IS 73:2006 Paving Bitumen – Specification (Third

Revision)4 in July 2006, which specifies paving bitumen by viscosity at 60 C rather than

the old system3 of grading paving bitumen by penetration at 25 C. The Ministry of Road

Transport and Highways (MORTH) is expected to revise its highway specifications

related to bituminous materials and pavements soon to reflect this significant change in

grades of paving bitumen.

______________________________________________________________________

*Associate Director Emeritus, US National Center for Asphalt Technology, Karanpura

House, 50 Raj Bhawan Road, Civil Lines, Jaipur (Email: pkandhal@eng.auburn.edu)

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This paper gives an overview of the newly adopted viscosity grading system to acquaint

highway engineers, contractors, suppliers, and consultants with it to make the necessary

change over from penetration grades to viscosity grades in practice.

At the present time, we are at a critical point in the history of road building in India.

Intense road construction activity is under way in building our national highways (Golden

Quadrilateral, East-West Corridor, and North-South Corridor), state highways, and rural

roads. Most of these highways will be paved with asphalt mixtures of which bitumen is

an important and expensive ingredient. It is of paramount importance that we select

bitumen which is suited for Indian climate and traffic conditions and whose physical

properties are based on rational rather than empirical parameters. This has now been

achieved by adopting the viscosity grading system for selecting paving bitumen in India.

Before giving an overview of the new system, it will be prudent to review the history of

grading bitumen over the last 125 years. This review will assist us in knowing how far we

have come and where we should be going in the long term so far as grading and selecting

bitumen are concerned.

2. HISTORY OF GRADING BITUMEN

Bitumen is a thermoplastic material, that is, its stiffness is dependent on its temperature

as shown in Figure 1. Its stiffness decreases as its temperature is increased. This

temperature versus stiffness relationship is different for different bitumens based on the

origin of the petroleum crude and/or method of refining. This is quite evident in Figure 1

wherein Bitumens A and B have different relationships.

Therefore, it is obvious we should define a test temperature at which the grading will be

done and bitumens compared. If we compare Bitumens A and B, Bitumen B is stiffer

than Bitumen A at 25 C whereas the situation is reversed at 60 C. Although the stiffness

of bitumen at low temperatures is also important to resist thermal cracking, Figure 1

shows three temperatures at which the stiffness of bitumen has significance as follows:

Stiffness at 135 C: The temperature of 135 C is near the temperatures used for mixing

and compacting asphalt mixtures during construction. It is useful to determine the

stiffness (measured in terms of kinematic viscosity) of the bitumen to establish proper

mixing and compaction temperatures for asphalt mixtures.

Stiffness at 60 C: The temperature of 60 C is near the maximum bituminous pavement

temperature on a hot summer day, when rutting is likely to occur as shown in Photo 1. It

is useful to determine the stiffness (in terms of viscosity) of the bitumen at 60 C so that

we can specify its minimum stiffness to ensure adequate resistance to rutting during hot

summer. Rutting of asphalt pavements is the most prevalent problem in the world

including India.

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Stiffness at 25 C: The temperature of 25 C is near the average annual temperature of an

asphalt pavement during a year. It is useful to determine the stiffness (in terms of

penetration) of the bitumen at 25 C so that we can specify its maximum stiffness

(minimum penetration) to resist pavement raveling and/or fatigue cracking (Photo 2)

resulting from aged/brittle bitumen after 5-10 years in service. Unlike rutting which may

occur right after construction, raveling/fatigue cracking usually occurs after 5-10 years in

service, especially if the pavement is structurally inadequate.

25 60 135

STIFFNESS

A

B

Temperature, °C Figure 1. Temperature versus stiffness relationships of different bitumens

Photo 1. Rutting occurs at maximum pavement temperatures during summer

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Photo 2. Fatigue cracking may occur after bitumen has aged in service

2.1 Grading by Chewing

Chewing in mouth was the first mode of testing to determine stiffness (hardness) of

bitumen during the late 19th century. Experienced bitumen inspectors used the technique

for testing and accepting bitumen for paving. Obviously, the test temperature was 98.6 F

(37 C) equal to the average human body temperature.

2.2 Penetration Grading

Grading of bitumen by penetration test at 25 C was adopted by the American Society for

Testing and Materials (ASTM) Committee D04 on Road and Paving Materials in 1903,

about 100 years ago. Figure 2 shows the schematic of the penetration test, in which a

needle loaded with 100 grams is allowed to penetrate the bitumen maintained at 25 C

temperature in a water bath, for 5 seconds. The resulting penetration is measured in mm;

1 penetration unit = 0.1 mm. The greater the penetration, the softer is the bitumen. ASTM

Standard D946 specified five penetration grades for bitumens:

40-50 (hardest bitumen grade)

60-70

85-100

120-150

200-300 (softest bitumen grade)

So far, the 100-year old penetration grading system has been used in India with 60-70

penetration grade being most widely used. The softer 80-100 penetration grade has been

used for low-volume roads and spray applications (such as in surface dressing) in India.

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Penetration Test

Figure 2. Schematic of penetration test

The penetration grading system has the following disadvantages:

1. It is based on an empirical test and not on a fundamental test.

2. It cannot be used effectively for polymer-modified bitumens.

3. Similitude at 25 C is very deceptive to performance at higher and lower service

temperatures. This is evident in Figure 3 wherein three 60-70 grade Bitumens A,

B, and C have the same penetration of 65 but different stiffness values at higher

(say 60 C) and lower (say 0 C) service temperatures. For example, Bitumen C is

more prone to rutting compared to Bitumens B and A, since its stiffness

(viscosity) at 60 C is lower.

4. No bitumen viscosity is available near asphalt mixing and compaction

temperatures for the guidance of contractors.

5. Penetration grading does not control the temperature-susceptibility (slope of

temperature versus stiffness line) of bitumens. Highly temperature susceptible

bitumens (with steep slopes) are not desirable because they are very soft at high

service temperatures and very stiff at low service temperatures.

2.3 Viscosity Grading

Viscosity grading at 60 C was introduced in the Unites States during 1970s to address

construction problems (tender mixes which could not be rolled without the mix pushing

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Figure 3. Three 60-70 penetration grade bitumens with different stiffnesses

at high and low service temperatures

and shoving under the roller) and high temperature performance (rutting during hot

summer) as mentioned earlier. The 60-70 penetration grade bitumen most widely used in

the US prior to 1970s was significantly variable in terms of resistance to rutting. Some

60-70 penetration bitumens also had very low viscosity at 135 C, which caused tender

mix problems. Viscosity grading is based on a fundamental, scientific viscosity test,

which is conducted at 60 C (near the maximum pavement temperature during summer)

and its measurement unit is poise.

The test equipment for measuring viscosity both at 60 C and 135 C is simple (Photo 3)

and is already available in most bitumen testing laboratories in India because these tests

were already specified in IS 73:1992.

Six asphalt cement (AC) viscosity grades were established as follows:

GRADE VISCOSITY at 60 C, poises

AC-2.5 (softest) 250 +/- 50

AC-5 500 +/- 100

AC-10 1000 +/- 200

AC-20 2000 +/- 400

AC-30 3000 +/- 600

AC-40 (hardest) 4000 +/- 800

Low viscosity grades such as AC-2.5 and AC-5 were used in cold climate of Canada.

AC-10 was used in the northern tier states of the US, AC-20 was used in most of the US,

and high viscosity AC-30 was used in southern states such as Florida, Georgia, and

Alabama with hot climate and rainfall similar to that of India.

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Photo 3. Vacuum capillary viscosity testing equipment

Figure 4 depicts the graphical representation of viscosity graded AC-30 bitumen. The

following advantages resulted from adopting the viscosity grading system for bitumen:

1. Unlike penetration grades, same viscosity grade bitumens gave similar rutting

performance in hot summer.

2. Minimum penetration values were retained in the viscosity grading system to

maintain acceptable performance (in terms of resistance to fatigue cracking) at

yearly average service temperature of 25 C.

3. Minimum specified values of kinematic viscosity at 135 C helped minimize the

potential of tender mixes during construction.

4. Minimum specified penetration at 25 C and minimum specified kinematic

viscosity at 135 C established the maximum allowable temperature susceptibility

(slope of temperature versus stiffness line).

5. Viscosity grades bitumens were suitable for a wide range of temperatures: 25 C

for raveling/fatigue cracking; 60 C for rutting; and 135 C for construction.

6. Since the viscosity values are measured at two temperatures, bitumen suppliers

could provide to the users rational and accurate asphalt mixing and compaction

temperatures (corresponding to bitumen viscosity of 170 and 280 centistokes,

respectively).

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Figure 4. Graphical representation of viscosity-graded AC-30 (VG-30) bitumen

2.4 Superpave Performance Grading

The viscosity grading system gave excellent performance results in the US for over 20

years. However, the viscosity grading system, although more rational than the penetration

grading system, was still based on experience. A 50-million dollar, 5-year Strategic

Highway Research Program (SHRP) was undertaken from 1987 to 1992 to develop a

performance based grading system for bitumen, which was based on engineering

principles to address common asphalt pavement distress problems. The so-called

Superpave performance grading system includes new bitumen tests and specifications

with the following salient features 1,16,18

:

1. Tests and specifications are intended for bitumen “binders”, which include both

modified and unmodified bitumens.

2. The physical properties measured by Superpave bitumen tests are directly related

to field performance by engineering principles rather than just the experience.

3. A long-term bitumen aging test, which simulates aging of bitumens during 5-10

years in service, was developed and included for the first time.

4. Tests and specifications are designed to eliminate or minimize three specific types

of asphalt pavement distresses: rutting, fatigue cracking, and thermal cracking.

Rutting typically occurs at high temperatures, fatigue cracking at intermediate

temperatures, and thermal cracking at low temperatures.

5. As shown in Figure 5, the entire range of pavement temperatures experienced at

the project site is considered. New testing equipments were developed/adopted for

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testing bitumens for this purpose. A rotational viscometer is used to measure the

bitumen viscosity at 135 C. A dynamic shear rheometer is used to measure the

viscoelastic properties of the bitumen at two temperatures: high temperature

corresponding to the maximum 7-day pavement temperature during summer at the

project site, and intermediate temperature corresponding to the average annual

temperature of the pavement at the project site. A bending beam rheometer and a

direct tension tester are used to measure the rheological properties of the bitumen

at the lowest pavement temperature during winter at the project site.

Figure 5. Superpave performance grade bitumen testing is conducted over the entire

range of temperatures experienced at the project site

The Superpave performance grade (PG) bitumen is based on climate. For example, a PG

64-22 bitumen is suitable for a project location, where the average 7-day maximum

pavement temperature is as much as 64 C, and the minimum pavement temperature is

–22 C.

The high temperature grades are PG 52, PG 58, PG 64, PG 70, PG 76, and PG 82. The

low temperature grades are –4, – 10, -16, -22, -28, -34 and so forth. Both high and low

temperature grades are in increments of 6 Celsius degrees.

Example: A project location in Rajasthan has a maximum record 7-day pavement

temperature of 70 C in summer and a minimum record pavement temperature of –3 C. A

PG 70-4 bitumen will be specified for paving that project.

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3 SELECTION OF BITUMEN FOR INDIA

3.4 Viscosity Grading System for India

There is no question the 100-year old penetration grading system used in India was

outdated, inadequate, and unsuitable for the ambitious highway construction programme

underway. Ideally, the latest Superpave performance grading system should be used.

However, it involves complex and expensive testing equipment. The highway specifying

agencies would also need to be trained for its implementation. Therefore, it was realistic

and practical to adopt the 30-year old viscosity grading system, which was initiated and

recommended by the author in year 20055,6. This system had an excellent performance

history in the United States including the southeastern states, which have hot climate and

heavy rainfall similar to India. A vast storehouse of published papers is also available on

the development, use, and experience with viscosity graded bitumens 7-14,19,20

. The

adoption of Superpave performance grading system can be considered as a long-term

goal or for use on very important, large paving projects if needed at the present time.

Table 1 gives the former Indian Specifications for bitumen (IS 73:1992) for three

penetration grades: 40-50 (S45), 60-70 (S65), and 80-100 (S90). Besides the penetration

range, the Indian Specifications also included a viscosity range at the bottom. Some states

in the US adopted this hybrid specification approach during the 1970s when switching

over from penetration grading to viscosity grading. However, their attempt failed due the

following reasons and those states subsequently adopted pure viscosity grading:

1. Simultaneous penetration grading and viscosity grading is not justified technically

and practically.

2. It is an unnecessary double jeopardy for the bitumen suppliers, who have to meet

both penetration and viscosity ranges.

3. It does not necessarily ensure a better quality bitumen. In fact, some good quality

bitumens (those with high viscosity and high penetration) are unnecessarily

eliminated by this double requirement.

In view of the preceding problems, it was recommended in the US to switch over

completely from this hybrid grading to simple viscosity grading. The change from

penetration to viscosity grades did not result in any significant price increase for paving

bitumen in the US. The same is expected in India.

Table 2 gives ASTM D3381 specification values (refer to table two of the specification)

for four viscosity grades AC-10, AC-20, AC-30, and AC-40, which are also generally

suitable for Indian climatic conditions. “AC” stands for asphalt cement, which is the term

used in the US for paving bitumen. AC-10 means asphalt cement with a target viscosity

of 1000 poises at 60 C (1000 has been abbreviated to 10). Similarly, AC-20, AC-30, and

AC-40 mean asphalt cements with target viscosity of 2000, 3000, and 4000 poises,

respectively.

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TABLE 1. FORMER BITUMEN SPECIFICATIONS IN INDIA

Characteristics Grade S45 Grade S65 Grade S90

Specific gravity at 27 C, min 0.99 0.99 0.99

Water, % by mass, max 0.2 0.2 0.2

Flash point, C, min 175 175 175

Softening point, C 50-60 45-55 40-55

Penetration at 25 C 40 to 50 60 to 70 80-100

Penetration ratio, min 40 35 30

Ductility at 27 C, cm, min

75 75 75

Paraffin wax, % by mass, max 4.5 4.5 4.5

Fraass breaking point, C, min - 4 - 6 - 8

Loss on heating, TFO, %, max 1 1 1

Retained penetration after TFO,

% of original, min

55 52 47

Solubility in trichloroethylene, %,

min

99 99 99

Viscosity at 60 C, poises 4000 +/- 800 2000 +/- 500 1000 +/- 250

Viscosity at 135 C, cSt, min 400 300 250

TABLE 2. ASTM D3381(TABLE 2) SPECIFICATIONS FOR VISCOSITY

GRADED BITUMEN

Characteristics AC-10 AC-20 AC-30 AC-40

Viscosity at 60 C,

poises

1000 +/- 200 2000 +/- 400 3000 +/- 600 4000 +/- 800

Viscosity at 135 C,

cSt, min

250 300 350 400

Penetration at 25 C,

min

80 60 50 40

Flash point, C, min 219 232 232 232

Solubility in

trichloroethylene, %,

min

99.0 99.0 99.0 99.0

Testing of thin film

oven residue:

Viscosity at 60 C,

poises, max

5,000 10,000 15,000 20,000

Ductility at 25 C, cm,

min

75 50 40 25

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It should be noted that the following tests and requirements given in the former

penetration-graded Bitumen Specifications IS 73:1992 (Table 1) are not there in the

recommended viscosity-graded specifications in Table 2, because they are simply

redundant and unnecessary owing to the following reasons:

Softening Point: The softening point is an empirical test, which was used in the past to

control high temperature performance of the bitumen. With the viscosity grading at 60 C,

the softening point has become redundant.

Penetration Ratio: The penetration ratio requirement was used in the past to control

temperature susceptibility (slope of temperature-stiffness line) of bitumen. With a

minimum penetration requirement at 25 C, a viscosity range at 60 C, and a minimum

viscosity at 135 (as shown in Figure 4) the temperature susceptibility of the bitumen is

adequately controlled. Therefore, the penetration ratio has become redundant.

Paraffin Wax Content: The presence of excessive paraffin in the bitumen makes it

highly temperature susceptible (low stiffness at high temperatures and high stiffness at

low temperatures). For the reasons mentioned previously for penetration ratio, this test is

now redundant and unnecessary for viscosity-graded bitumens. The viscosity-graded

bitumens preclude bitumens with excessive paraffin wax because those bitumens will not

satisfy viscosity requirements at both 60 C and 135 C.

Fraass Breaking Point: First of all, the empirical Fraass breaking point has significance

in controlling thermal cracking in very cold climate countries such as those in Europe. It

is not relevant or necessary in most of India. Moreover, highly temperature susceptible

bitumens are likely to fail the Fraass breaking test. Since the temperature susceptibility is

controlled in the viscosity-graded bitumen specifications, the Fraass breaking point test is

redundant now.

Therefore, the adoption of viscosity graded bitumen specification will reduce the total

number of tests to 8 only compared to penetration graded bitumen specification, which

used 14 tests. This will reduce the time and cost of testing bitumen without

compromising its quality

It would have been ideal to adopt a truly, time-tested specification for viscosity graded

paving bitumen in India similar to ASTM. However, deliberations in Sectional

Committee PCD 6 of the Bureau of Indian Standards resulted in some adjustments in the

adopted specification (as given in Table 3), which came into effect in July 2006. Some

adjustments were made to facilitate the transition from penetration grades to viscosity

grades. It is hoped as suppliers and user agencies get more acquainted with the viscosity

grading system, these specifications will be fine tuned and made more rational in the near

future.

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TABLE 3. CURRENT VISCOSITY GRADED (VG) BITUMEN SPECIFICATION

IN INDIA (IS 73:2006, THIRD REVISION)

Characteristics VG-10 VG-20 VG-30 VG-40

Absolute viscosity, 60 C,

poises, min

800 1600 2400 3200

Kinematic viscosity, 135 C,

cSt, min

250 300 350 400

Flash point, C, min 220 220 220 220

Solubility in

trichloroethylene, %, min

99.0 99.0 99.0 99.0

Penetration at 25 C 80-100 60-80 50-70 40-60

Softening point, C, min 40 45 47 50

Tests on residue from thin

film oven test/RTFOT:

Viscosity ratio at 60 C, max 4.0 4.0 4.0 4.0

Ductility at 25 C, cm, min,

after thin film oven test

75 50 40 25

The following points should be noted in IS 73:2006 Paving Bitumen –Specification

(Table 3) issued in July 2006 in comparison to the ASTM Specification given in Table 2.

1. Since the term AC or asphalt cement is not used in India, VG (viscosity grade)

has been substituted for AC. For example, AC-10, AC-20, AC-30, and AC-40

ASTM viscosity grades are being called VG-10, VG-20, VG-30, and VG-40,

respectively in IS 73:2006.

2. In place of a viscosity range (target viscosity value +/- 20%) specified in ASTM,

only the minimum viscosity at 60 C has been specified. For example, VG-30

should have a viscosity of 3000 +/- 600 poises, but only the lower value of 2400

poises has been specified in IS 73:2006. As suppliers gain more experience with

their products, it will be possible and desirable to specify the upper limit as well.

3. Instead of specifying a minimum penetration value for each viscosity grade like in

ASTM Specification (Table 2), the current Indian specification gives a

penetration range. For example, VG-30 has a penetration range of 50 to 70. This

was done so that the practicing engineers can relate the new viscosity grades to

the old penetration grades. However, it is hoped that the upper value of the

penetration will be deleted in the near future because with identical viscosity at 60

C, a higher penetration gives a better paving bitumen in terms of fatigue

resistance compared to low penetration.

4. Minimum softening point values have been retained so that the user agencies can

check the high temperature consistency of the paving bitumen until they acquire

the viscosity testing equipment.

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3.2 Recommended Bitumen Grades for India

It is recommended to use viscosity-graded VG-30 paving bitumen in lieu of 60-70

penetration grade in most parts of India. Equivalent AC-30 grade has been used in

building excellent highways in southeastern United States, which has climate similar to

India (Figure 6 shows the overlap of the two countries along same latitudes).

Similar Latitudes

Figure 6. Southeastern Unites States aligned with India following same latitudes

illustrating closer climate justifying the use of AC-30 (VG-30) grade bitumen

Tables 4 and 5 give recommended guidelines for selection of viscosity grade (VG) of

paving bitumen in India. Table 4 gives the general guidelines including the equivalent

penetration grades. Table 5 gives the selection criteria based on climatic conditions.

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TABLE 4. VISCOSITY GRADED (VG) BITUMENS AND THEIR GENERAL

APPLICATIONS

Viscosity Grade General Applications

(VG)

VG-40 Use in highly stressed areas such as those in intersections, near toll

booths, and truck parking lots in lieu of old 30/40 penetration grade

VG-30 Use for paving in most of India in lieu of old 60/70 penetration grade

VG-20 Use for paving in cold climatic, high altitude regions of North India

VG-10 Use in spraying applications such as surface dressing and for paving in very

cold climate in lieu of old 80/100 penetration grade

TABLE 5. SELECTION CRITERIA FOR VISCOSITY-GRADED (VG) PAVING

BITUMENS BASED ON CLIMATIC CONDITIONS

Highest Daily Mean Air Temperature, C

Lowest Daily Mean

Air Temperature, C

Less than 20 C 20 to 30 C More than 30 C

More than –10 C VG-10 VG-20 VG-30

-10 C or lower VG-10 VG-10 VG-20

Ideally, selection of bitumen grade should be based on high and low pavement

temperatures. However, it will be too complex for the field engineers to convert air

temperatures to pavement temperatures. Therefore, from practical considerations

selection should be based on air temperatures. In the Superpave performance grading

(PG) system for bitumen, the 7-day maximum pavement temperature (based on more

than 20 years’ weather data) in the region is used for high temperature grade. The closest

to that approach which we can use in India is the highest daily mean air temperature in a

year in the region. This data is available from the Indian Meteorological Organization

(IMO). For every weather station in India, the IMO has records on the highest and lowest

daily mean temperature for all 365 days in the year based on several years’ weather

records. When it is reported in the newspaper that the high temperature on a particular

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day was 3 C below normal, the normal is that day’s highest mean temperature.

Fortunately, the highest daily mean air temperature in India (usually in May-June)

generally ranges from 31 to 42 C only from Punjab to Kanya Kumari and from Gujarat to

Assam. For this range of high temperatures, a VG-30 grade is suitable based on

experience in other countries. Softer viscosity grades such as VG-20 and VG-10 are

recommended for regions with highest daily mean air temperatures of 30 C and lower

(such as cold climatic high altitude regions of North India). It is not advisable to use the

highest air temperature ever recorded because rutting is not caused during one hot day but

during sustained hot days.

The lowest daily mean air temperatures (which are also fairly close to pavement

temperature) occur in India in January. They range from –2 C to 21 C from Kashmir to

Kanya Kumari. Viscosity graded VG-30 bitumen is suitable down to –10 C (due to its

controlled temperature susceptibility). At temperatures lower than –10 C we can use

softer grades such as VG-20 and VG-10.

Tables 4 and 5 have been formulated in view of the above discussion, Indian

Meteorological Organization climatic data, and practical considerations such as

minimizing temperature ranges and viscosity grades. It should be noted that VG-40 has

not been recommended in Table 5 for normal paving. There is no practical experience

with the use of neat, unmodified AC-40 (equivalent of VG-40) in the US or other

countries. It is probably considered too hard for paving unless this grade is produced with

polymers.

4. IMPLEMENTATION OF VISCOSITY GRADING SYSTEM IN INDIA

Since IS 73:2006 (Third Revision) issued in July 2006 specifies viscosity grades in lieu

of penetration grades, it should be implemented as soon as possible to improve the

consistency, quality and durability of bituminous pavements in India. The following steps

are necessary for implementing the viscosity grading system.

1. MORTH’s highway specifications related to bituminous materials and bituminous

pavements should be revised by substituting viscosity grades (VG) in lieu of

penetration grades. For example, VG-30 should replace 60-70 penetration grade.

2. Bitumen suppliers should start supplying VG grades in place of penetration

grades and must report bitumen viscosity values at 60 and 135 C for the

information of the user agencies. They should also recommend asphalt mixing

and compaction temperatures to the contractors, which can be determined

corresponding to viscosity values of 170 and 280 centistokes at 135 C as

described in the Asphalt Institute Manual Series MS-2, Sixth Edition.

3. User agencies should at least acquire the vacuum capillary viscometer conforming

to IS Test Method IS 1206 (Part 2) to verify the bitumen viscosity at 60 C and

thereby to verify the VG grade of the supplied paving bitumen. As shown in

Photo 3, the viscosity testing equipment mainly consists of the following:

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(a) Calibrated Cannon-Manning viscosity tube

(b) Water or oil bath maintained at 60 C temperature

(c) Vacuum pump to apply partial vacuum up to 300 mm Hg

(d) Vacuum manometer and vacuum controller

(e) Thermometers, stop watch, etc.

Calibrated Cannon-Manning viscosity tubes have to be imported through an Indian

distributor at the present time and will cost about Rs. 9,800.The remaining test equipment

is readily available in India. The total quoted price15 in 2006 of the complete vacuum

viscometer equipment including the imported viscometer tube and imported vacuum

pump is Rs.58,100. The complete breakdown of price for various components is as

follows: imported viscosity tube (Rs. 9,800); viscometer bath with digital temperature

controller (Rs. 18,500); vacuum pump oil free imported (Rs. 19,500); manometer with

electronic controller (Rs. 7,500); and accessories such as stop watch, oil, etc.(Rs. 2,800).

Obviously, the total cost will be higher if more components of the testing equipment are

also imported.

Conducting a viscosity test in accordance with IS Test Method 1206 (Part 2) or ASTM

D2171-01 is very easy and a laboratory technician can be trained in a day to conduct this

test. The following is a brief description of the test procedure for determining bitumen

viscosity at 60 C.

Figure 7. Vacuum capillary viscometer tube

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A vacuum capillary tube viscometer shown in Photo 3 and Fig. 7 is used to perform the

viscosity test at 60 C. The viscometer tube is charged with hot bitumen through the large

side until the level of the bitumen reaches the filling line. The viscometer is then mounted

in a thermostatically controlled, constant temperature water or oil bath, which is

maintained at 60 C. After the filled viscometer tube is kept in the bath for a prescribed

period of time to obtain an equilibrium temperature of 60 C, a partial vacuum is applied

to the small side of the viscometer tube to cause the bitumen to flow upwards. It is

necessary to apply partial vacuum because the bitumen is too viscous to flow at 60 C. A

vacuum pump and a vacuum control device are needed.

After the bitumen starts to flow, the time (in seconds) required to flow between two

timing marks on the tube is measured with a stopwatch. The measured time in seconds is

multiplied by the calibration factor (supplied with each viscosity tube) to obtain the value

for viscosity in

poises, which is the standard unit for reporting viscosity. Manufactures calibrate their

viscosity tubes with standard oils and furnish the calibration factor with each tube.

5. SUMMARY

Viscosity graded system has recently been adopted in India for paving bitumen based on

IS 73:2006 Paving Bitumen – Specification (Third Revision) issued in July 2006. The

viscosity grading system replaces a 100-year old penetration grading system. This paper

has reviewed the history of grading paving bitumen over the last 125 years. The

progression of grading systems: grading by chewing, penetration grading, viscosity

grading, and Superpave performance grading, has been discussed with their respective

advantages and disadvantages.

The new viscosity graded (VG) bitumen specification has been compared with the old

penetration graded bitumen specification. Selection criteria for VG grades based on

climate and traffic has been recommended. Recommendations have also been made to

implement the VG grades as soon as possible to improve the consistence, quality, and

durability of bituminous pavements in India. Information about the viscosity test such as

testing equipment, testing procedure, and estimated cost of the equipment has been given.

6. REFERENCES

1. Anderson, D.A. and T.W. Kennedy, “ Development of SHRP Binder

Specifications”, Journal of the Association of the Asphalt Paving Technologists,

Volume 62, 1993.

19

2. ASTM D2171 Standard Test Method for Viscosity of Asphalts by Vacuum

Capillary Viscometer. ASTM Volume 04.03, Road and Paving Materials, 2005.

3. Indian Standard IS 73:1992 for Paving Bitumen – Specification (Second

Revision), Bureau of Indian Standards, February 1992.

4. Indian Standard IS 73: 2006 for Paving Bitumen – Specification (Third Revision),

Bureau of Indian Standards, July 2006.

5. Kandhal. P.S., “Selection of Bitumen for Highways in India”, Presentation at the

65th Annual Session of the Indian Roads Congress , Bangalore, January 8-11,

2005.

6. Kandhal, P. S., “ Selection of Bitumen for Paving Highways”, Indian Roads

Congress, Indian Highways, July 2005.

7. Kandhal, P.S., L.D. Sandvig, and W.C. Koehler, “ Asphalt Viscosity Related

Properties of In-Service Pavements in Pennsylvania”, ASTM Special Technical

Publication 532, 1973.

8. Kandhal, P.S. and M.E.Wenger, “ Asphalt Properties in Relation to Pavement

Performance”, TRB, Transportation Research Record 544, 1975.

9. Kandhal, P.S. and W.C. Koehler, “ Significant Studies on Asphalt Durability:

Pennsylvania Experience”, TRB, Transportation Research Record 999, 1984.

10. Kandhal, P.S., “ Low-Temperature Ductility in Relation to Pavement

Performance”, ASTM Special Technical Publication 628, 1977.

11. Kandhal, P.S., “Low-Temperature Properties of Paving Asphalts”, TRB State-of-

the-Art Report 7, 1988.

12. Kandhal, P.S. and W.C. Koehler, “Effect of Rheological Properties of Asphalts

on Pavement Cracking”, ASTM Special Technical Publication 941, 1987.

13. Kandhal, P.S., L.D. Sandvig, and M.E. Wenger, “ Shear Susceptibility of

Asphalts in Relation to Pavement Performance”, Proc. Association of Asphalt

Paving Technologists, Volume 42, 1973.

14. Kandhal, P.S., “Evaluation of Low-Temperature Cracking on Elk County

Research Project”, TRB, Transportation Research Record 777, 1980.

15. Kandhal, P. S. Quotations for Supply of Vacuum Viscometer Equipment from

Indian Suppliers, 2006.

16. McGennis, R.B., S. Shuler, and H.U. Bahia, “Background of Superpave Binder

20

Test Methods”, FHWA Report no. FHWA-SA-94-069, July 1994.

17. Roberts, F.L., P.S. Kandhal, E.R. Brown, D.Y. Lee, and T.W. Kennedy, “Hot Mix

Asphalt Materials, Mix Design, and Construction” Asphalt Textbook, NAPA

Education Foundation USA, Second Edition, 1996.

18. Superpave Performance Graded asphalt Binder Specification and Testing. Asphalt

Institute, Superpave Series No. 1 (SP-1), 1994.

19. Terrel, R.L., J.A. Epps, and C. Crawford, “Making the Most of Temperature/

Viscosity Characteristics”, National Asphalt Pavement Association, Information

Series 102, 1988.

20. Welborn, J.Y., E.R. Oglio, and J.A. Zenewitz, “A Study of Viscosity-Graded

Asphalt Cements”, Proc. Association of Asphalt Paving Technologists, Volume

35, 1966.