An Overview Of The Viscosity Grading System Adopted In India For Paving Bitumen
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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: [email protected])
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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.
STIFFNESS
B
A
25
60
135
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) AC-5 AC-10 AC-20 AC-30 AC-40 (hardest)
250 +/- 50 500 +/- 100 1000 +/- 200 2000 +/- 400 3000 +/- 600 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 8
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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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 Specific gravity at 27 C, min Water, % by mass, max Flash point, C, min Softening point, C Penetration at 25 C Penetration ratio, min Ductility at 27 C, cm, min
Grade S45 0.99 0.2 175 50-60 40 to 50 40 75
Grade S65 0.99 0.2 175 45-55 60 to 70 35 75
Grade S90 0.99 0.2 175 40-55 80-100 30 75
Paraffin wax, % by mass, max Fraass breaking point, C, min Loss on heating, TFO, %, max Retained penetration after TFO, % of original, min Solubility in trichloroethylene, %, min Viscosity at 60 C, poises Viscosity at 135 C, cSt, min
4.5 -4 1 55
4.5 -6 1 52
4.5 -8 1 47
99
99
99
4000 +/- 800 400
2000 +/- 500 300
1000 +/- 250 250
TABLE 2. ASTM D3381(TABLE 2) SPECIFICATIONS FOR VISCOSITY GRADED BITUMEN Characteristics Viscosity at 60 C, poises Viscosity at 135 C, cSt, min Penetration at 25 C, min Flash point, C, min Solubility in trichloroethylene, %, min Testing of thin film oven residue: Viscosity at 60 C, poises, max Ductility at 25 C, cm, min
AC-10 1000 +/- 200
AC-20 2000 +/- 400
AC-30 3000 +/- 600
AC-40 4000 +/- 800
250
300
350
400
80
60
50
40
219 99.0
232 99.0
232 99.0
232 99.0
5,000
10,000
15,000
20,000
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 Absolute viscosity, 60 C, poises, min Kinematic viscosity, 135 C, cSt, min Flash point, C, min Solubility in trichloroethylene, %, min Penetration at 25 C Softening point, C, min Tests on residue from thin film oven test/RTFOT: Viscosity ratio at 60 C, max Ductility at 25 C, cm, min, after thin film oven test
VG-10 800
VG-20 1600
VG-30 2400
VG-40 3200
250
300
350
400
220 99.0
220 99.0
220 99.0
220 99.0
80-100 40
60-80 45
50-70 47
40-60 50
4.0 75
4.0 50
4.0 40
4.0 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 (VG)
General Applications
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
Lowest Daily Mean Air Temperature, C More than –10 C -10 C or lower
Highest Daily Mean Air Temperature, C Less than 20 C 20 to 30 C More than 30 C VG-10 VG-10
VG-20 VG-10
VG-30 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 15
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) (b) (c) (d) (e)
Calibrated Cannon-Manning viscosity tube Water or oil bath maintained at 60 C temperature Vacuum pump to apply partial vacuum up to 300 mm Hg Vacuum manometer and vacuum controller 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.
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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-ofthe-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
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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.
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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: [email protected])
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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.
STIFFNESS
B
A
25
60
135
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) AC-5 AC-10 AC-20 AC-30 AC-40 (hardest)
250 +/- 50 500 +/- 100 1000 +/- 200 2000 +/- 400 3000 +/- 600 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 8
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 Specific gravity at 27 C, min Water, % by mass, max Flash point, C, min Softening point, C Penetration at 25 C Penetration ratio, min Ductility at 27 C, cm, min
Grade S45 0.99 0.2 175 50-60 40 to 50 40 75
Grade S65 0.99 0.2 175 45-55 60 to 70 35 75
Grade S90 0.99 0.2 175 40-55 80-100 30 75
Paraffin wax, % by mass, max Fraass breaking point, C, min Loss on heating, TFO, %, max Retained penetration after TFO, % of original, min Solubility in trichloroethylene, %, min Viscosity at 60 C, poises Viscosity at 135 C, cSt, min
4.5 -4 1 55
4.5 -6 1 52
4.5 -8 1 47
99
99
99
4000 +/- 800 400
2000 +/- 500 300
1000 +/- 250 250
TABLE 2. ASTM D3381(TABLE 2) SPECIFICATIONS FOR VISCOSITY GRADED BITUMEN Characteristics Viscosity at 60 C, poises Viscosity at 135 C, cSt, min Penetration at 25 C, min Flash point, C, min Solubility in trichloroethylene, %, min Testing of thin film oven residue: Viscosity at 60 C, poises, max Ductility at 25 C, cm, min
AC-10 1000 +/- 200
AC-20 2000 +/- 400
AC-30 3000 +/- 600
AC-40 4000 +/- 800
250
300
350
400
80
60
50
40
219 99.0
232 99.0
232 99.0
232 99.0
5,000
10,000
15,000
20,000
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 Absolute viscosity, 60 C, poises, min Kinematic viscosity, 135 C, cSt, min Flash point, C, min Solubility in trichloroethylene, %, min Penetration at 25 C Softening point, C, min Tests on residue from thin film oven test/RTFOT: Viscosity ratio at 60 C, max Ductility at 25 C, cm, min, after thin film oven test
VG-10 800
VG-20 1600
VG-30 2400
VG-40 3200
250
300
350
400
220 99.0
220 99.0
220 99.0
220 99.0
80-100 40
60-80 45
50-70 47
40-60 50
4.0 75
4.0 50
4.0 40
4.0 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 (VG)
General Applications
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
Lowest Daily Mean Air Temperature, C More than –10 C -10 C or lower
Highest Daily Mean Air Temperature, C Less than 20 C 20 to 30 C More than 30 C VG-10 VG-10
VG-20 VG-10
VG-30 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 15
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) (b) (c) (d) (e)
Calibrated Cannon-Manning viscosity tube Water or oil bath maintained at 60 C temperature Vacuum pump to apply partial vacuum up to 300 mm Hg Vacuum manometer and vacuum controller 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.
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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-ofthe-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
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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.
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