Right Grade of Bitumen for Flexible Pavements: Indian Perspective

Indian road transportation infrastructure is rapidly expanding with the ambitious development of road networks under National Highways Development Programme (NHDP), State Highways Improvement Programmes (SHIPs), Bharat Nirman, Pradhan Mantri Gram Sadak Yojana (PMGSY) etc. Also, other category roads and airports are largely expanded. The fast growing Indian economy will further demand for road transport network with a high quality pavement structure as the main corridors are required to cater to very heavy traffic-both in terms of number and axle loading. Road-laying under the Golden Quadrilateral project and the North-South and East-West corridors project of National Highways Authority of India (NHAI) has been quantitatively significant. According to the outcome budget of the Ministry of Shipping, Road Transport and Highways, Govt. of India, 5,694 km of road-laying and four-lining out of a total of 5,846 kilometres were targeted for completion in the present fiscal year. While these road development projects help in adding considerable infrastructural assets, their construction and subsequent maintenance phases require huge amount of suitable pavement materials.

Currently, majority of the Indian roads are flexible pavements, the ones having bituminous layer/s. earlier, there used to be scarcity of cement and India went for flexible pavements with bituminous toppings. Now, flexible pavement are preferred over cement concrete roads as they have a great advantage that these can be strengthened and improved in stages with the growth of traffic. Another major advantage of these roads is that their surfaces can be milled and recycled for rehabilitation. The flexible pavements are less expensive also with regard to initial investment and maintenance.

Significance of Properties of Bitumen

The durability and the long term satisfactory performance of pavements are always influenced and affected to a greater extent by the employed pavement ingredient materials and their inherent properties. The upper layers of a pavement structure are vital in taking care of load/stress alleviation and protecting the structure. They are conceived as layers of superior quality materials in the structure and are constructed accordingly.

In bituminous pavements, stone aggregates and bituminous binder are the key ingredients and hence are desired to be of good quality, making their selection an important task, which is often paid less attention. The applicability and adhesive properties of bitumen along with the proper proportioning with stone aggregates is the basic requirement to make workable layer mixes. Asphaltic bitumen is obtained by refining the petroleum crude. It is the costliest and a very important component of the bituminous mix.

It is very much pertinent to consider the properties of bituminous binders and the bitumen content in a mix while attempting for enhancing the performance characteristics of bituminous mixes. The construction sector is interested in using a right type of bitumen for obtaining durable pavements with longevity of 10-15 years especially for BOT (build, operate, and transfer) type projects.

Bitumen and Bituminous Binders in Road Paving
Bitumen Demand and Its Usage

The average annual demand of bitumen in India is around 4MMT (million metric tonnes) and it is estimated that bitumen works alone cost about 45% of the overall road construction cost. Of late, bitumen that is generated during the refining process of petroleum — a diminishing product, tends to become scarce while the demand is increasing. This has resulted into a situation where the gap between supply and demand is fast increasing. The bitumen supply scenario is not showing any sign of improvement since only countable crude oil refineries manufacture bitumen while a small quantity(between 1–4% )of crude oil is used for bitumen. With the road construction industry outgrowing, local demand has exceeded the bitumen production and supply. The completion targets of many road projects thereby are adversely affected and hence the construction and maintenance require long spreading/deferring of works. The industry has also witnessed sharp cost escalations in bitumen binder over the years.

It is estimated that the global demand for bitumen will increase 2-3 percent annually through 2009. Accelerating demand for bitumen, though will reflect gains in GDP growth, will result in increased expenditures on infrastructure in developing nations. India represents one of the largest and fastest growing bitumen markets. Continuing rapid industrialisation and strong growth in building construction markets are driving demand for bitumen in the paving market, especially with the country undergoing the most dramatic growth in road building of any nation, which will spur demand for bitumen used in paving grade bitumen, modified bitumen, bitumen emulsions and other paving applications (interface treatments). Paving materials will remain the dominant market for bitumen, accounting for 85% of demand in 2009. The greatest growth potential for bitumen will continue to be found in new paving grade products.

Applications of Bitumen

Bitumen bound layers are normally used in wearing, surfacing, base and binder courses of a layered flexible pavement. They may be thick or thin, hot or cold, plant-mixed or site-mixed etc. Some examples are Bituminous Macadam (BM), Premix Carpet (PC/PMC), Dense Bituminous Macadam (DBM), Bituminous Concrete (BC/DBC), Built Up Spray Grout (BUSG), Bitumen Penetration Macadam (BPM) etc. The basic essential ingredients in all these are common, being stone aggregates and bituminous binder, the specification requirements and proportioning may only vary. However, till date, hot bituminous mixes (premixed) are most widely used for new constructions as well as maintenance treatments. Bitumen and bituminous materials are also required in the thin interface treatments including prime coat and tack coat between different thick layers for the purpose of bonding the individual layers. Providing a bituminous layer for pavements generally involves:

• Selection of layer Specification/s
• Identification of suitable materials
• Preparation and production of mix
• Transport to laying site
• Laying and compacting operations

It may be appreciated here that the properties of binder implied by the specifications is utmost important in deriving maximum benefits at economical costs.

Specifications for Paving Bitumen and Related Tests

Specifications describe and envelope the required properties and functions of any material or product to result in desired output. In road pavements the requirement is a better performing structure to serve the road users with comfort, safety and at lower costs. The specifications for a bituminous binder are expected to:

• specify properties that directly reflect bitumen's behaviour
• express these properties in physical units
• provide limits for those properties to exclude poor performing products
• provide information from which the service performance can be predicted

While addressing these issues through proper specifications, the important properties of bitumen to be considered are the mechanical, adhesive and the durability properties.

Based on the past experience or established performance indicators, certain tests and their outcome are always used as demarking pointers to decide the suitability of a material or otherwise. There are age old traditional/conventional tests as well as new generation tests to help in framing the specifications for bituminous binders, such as:

1. Conventional tests used for bitumen characterisation
   • Penetration, ductility, R&B softening point, flash point, Fraass breaking point…..
2. Ageing characteristics:
   • Thin Film Oven Test, Rolling Thin Film Oven Test, Pressure Aging Vessel…
3. Rheological tests:
   • Bending Beam Rheometer, Direct tension Test, Dynamic Shear Rheometer…..

It is imminent to select an array of above tests to assist in the selection of right quality of bitumen for pavement applications.

Issues in Selection of Right Paving Bitumen
Developments in Grading of Bitumen

After recognising the applicability and advantages of usage of bitumen in civil engineering constructions, especially roads, concerned efforts were started to examine, evaluate and classify the naturally available as well as processed bituminous materials. To enable the proper handling of the related issues, tests, approach methods, specifications and concepts were developed to grade the bitumen. Some of the important milestones may be enlisted as:

• Beginning of specifications - Around early 20th century
• Classification of asphalts(later called as bitumen) for commercial purposes in the first half of the century - Exclusively conventional tests
• First grading test - Penetration at 25°C
• 1960's in USA: grading test - Viscosity at 60°C
• Canada: hybrid - Penetration/viscosity
• 1990's in USA : SHRP's Superpave specification - Performance grade

Global Practices

Stiffness of bitumen is the ultimate phenomenon for bitumen, while chewing in mouth was the earliest mode of testing bitumen; hence the test temperature at that time was 37°C (which is the temperature of human body). In 1903, American Society for Testing of Materials (ASTM) adopted the grading of bitumen through penetration testing at 25°C. The lower penetration value indicated the harder bitumen, while the higher penetration value indicated the softer bitumen. The penetration based grading system continued until 1970 and the same continues even today in many countries including India. Thereafter in order to address various construction problems like tender mix resulted by 60/70 penetration grade bitumen (due to its low viscosity at 135°C) and performance related problems such as rutting at high pavement temperatures, viscosity based grading system was introduced in US in the year 1970.

British Standard (BS: 12591) specifies 9 grades of bitumen based on penetration values. In BS: 12591, viscosity at 60°C and 135°C are incorporated as optional properties along with the wax content, Frass Breaking point and Ageing Properties. Presently, ASTM: D-946 specifies "Penetration Based Specification"of bitumen and ASTM:D-3381 specifies "Viscosity Based Grading System."Similarly, AASHTO specification also specifies two grading system, making the grading system into a hybrid system. Some states in the USA also adopted the hybrid grading system in 1970, before switching over to the viscosity based grading system. It is reported that this hybrid system in US did not work due to the following two reasons.

• Bitumen suppliers were not able to satisfy the two criterion simultaneously
• Better quality could not be reflected by performance

Super Pave performance grade bitumen is based on the climate. For example, PG-64-16 bitumen is suitable for a project location, where average 7 days maximum pavement temperature is as high as 64°C and the minimum pavement temperature is as low as -16°C. Therefore, a project location in India which has a maximum 7 days pavement temperature of 63°C in summer and a minimum pavement temperature of -1°C in winter will require PG-64-16 bitumen. Experimental studies at CRRI have shown that PG grades are quite close to VG grades.

Indian Scenario and Associated Problems

In India, the bitumen grading is practised on the basis of penetration test, which is conducted at a temperature of 25°C, and 60/70 penetration grade bitumen is widely used. The empirical penetration test was developed over 100 years ago in which a standard penetration needle loaded with 100 grams is allowed to penetrate the bitumen (maintained at 25°C) for five seconds. The penetration is measured in mm and it indicates the relative hardness of the bitumen. Higher the penetration, softer is the bitumen. A penetration of 0.6 to 0.7 mm indicates 60/70 penetration grade bitumen.

The most common problem in the performance of bituminous concrete roads (50 mm or thicker) throughout the world including India is rutting during hot summer. The bitumen becomes soft in the 60 to 70°C temperature range (typical road surface temperature on a hot summer day) and starts to push and shove under loaded truck tyres leading to rutting and corrugations in the wheel tracks of the roadway.

Bitumen processed from different petroleum crude sources and/or refining processes may have the same penetration grade at 25°C but may exhibit significantly different hardness in the 60-70°C temperature range. Those which are very soft (low viscosity) are more prone to rutting/corrugations compared to those which are not as soft (high viscosity). Therefore, it is quite obvious that the consistency (viscosity) of the paving bitumen at high temperature (such as 60°C) is to be invariably determined to know if it is likely to cause rutting or not. Because of this problem, a requirement to test and grade the bitumen at 60°C (and not at 25°C) through viscosity test was implemented in U.S. some 30 years ago. The viscosity of the bitumen is measured in poises at 60°C using a simple viscometer. Various viscosity grades of asphalt cement (bitumen) were evolved as AC-30 Grade, AC-20 Grade and AC-10 Grade. These were implemented in North America during 1970s according to climate wherein higher viscosity grades were selected for hotter climate.

The bitumen specification used in India at the present time has a 60/70 penetration grade but it was 80/100 and equivalent to an AC-10 viscosity grade until recently. The use of softer AC-10 grade is suited for the cold climate and is unacceptable for India, since it is very soft and will simply increase the potential for rutting under heavy trucks. There is a need to at least use an AC-30 grade in India to minimise the rutting potential of our pavements, and even a harder grade like an AC-40 grade for southern India.

Limitations/Shortfalls of Current Grading Practices

Over the years, researchers have investigated relationships between the laboratory measured properties of penetration grade bitumen and their performance in asphalt mixture on actual road sites. Penetration, softening point and ductility are the key properties of bitumen used to evaluate bitumen's quality and to assess their contributions to the performance of asphalt mixtures on roads, on which early part of research had relied. These parameters are empirical in nature as these cannot be expressed in engineering units and cannot be related to any of the required rheological properties. It is also established that none of these parameters indicate viscous/elastic behaviour of bitumen at the test temperature. A number of field studies have already been conducted to investigate the relationships between these properties and performance related aspects. Penetration value upto 25 is acceptable for prevention of cracking, while a higher value tends to proceed towards cracking at a higher rate. Similarly, a viscosity value of 22,000 Poise at 60°C for oxidized bitumen is acceptable but higher value accelerates the development of cracking.

Investigations carried out at CRRI in 1980 concluded the terminal penetration value of 20 for the fail condition. These use different loading rates and different loading modes than a pavement normally experiences in the field. However, several studies indicate that the ductility measured at certain penetration level is a good indicator of pavement performance. Satisfactory performance of bitumen on roads can be ensured if four properties can be controlled. These are rheology, cohesion, adhesion and durability. The rheology of bitumen at service temperature is adequately demonstrated by the value of penetration index. By investigations of relationship between the field performance and measured properties, Shell Global Solution has developed a set of laboratory tests to assess the quality of bitumen. The set of these tests include six tests on the bitumen and three tests on the mixes.

Recently, Strategic Highway Research Program (SHRP) introduced performance based binder specification in total quality perspective based on the shear susceptibility parameters, addressing each type of pavements failure. These failure modes are identified as critical pavement distress modes in which quality of binder plays a deciding role. The new binder specification is based on the measurable engineering properties like dynamic shear modulus and stiffness modulus etc. Four types of tests typical to SHRP specification are:

• Flow at mixing temperature by rotational viscometer
• Dynamic shear modulus
• Bending Beam rheometer to measure properties at lowest pavement temperature
• Direct tension testing at lowest pavement temperature

The resulting bitumen binder grading in SHRP-Superpave concept is designated as'Performance Grade (PG) Bitumen'and is classified mainly as traffic and temperature responsive. However, the significance of these tests and the interpretation of the results are yet to be recognised and adopted for Indian applications.

Latest Development in Indian Bitumen Specifications
Need for Change in Grading System

India consumes about four million metric tons of paving grade bitumen in a year, as mentioned earlier. Paving grade bitumen accounts for 90% of the total binder requirements. Till now the quality of bitumen to be used is evaluated to conform to a penetration based grading system. These standards however, of late, have been found to be obsolete and unsuitable, keeping in view the ambitious National Highways Development Programmes currently underway in the country, costing over US $50 billion. Ideally, the performance based SHRP specifications are needed to be adopted. However, in view of facilities for testing of bitumen as per SHRP specification not being available at a large scale in the country, it became essential to formulate at least the reliable viscosity based grading system. In the first instance, this viscosity based designation is accepted worldwide, as it demonstrates fairly good relationships with the performance of bituminous mixes laid on roads and has also shown excellent performance records in the United States and in other countries too.

New Grading by Indian Standards

The grading of bitumen through penetration test at 25°C was first adopted in 1961 by Bureau of Indian Standards and IS:73-1961 is the first Indian standard on Paving Bitumen, which was in practice until 1991. This standard recommended five grades of bitumen viz. S-35, S-45, S-65, S-90 and S-200. The key requirements for bitumen were the specific gravity, penetration, water content, flash point, softening point, ductility, loss on heating test and solubility in trichloroethylene. Bitumen conforming to this standard was performing well until the end of seventies, when construction specifications like surface dressing and premix carpet were largely in practice. With the increased use of designed dense mixes, increased traffic volume and loading, higher road user's expectations and changes in crude quality, revision of IS:73-1961 was felt essential in 1985 and subsequently IS:73–1961 was revised to IS:73-1992 in 1992. In this, new tests like penetration ratio, paraffin wax, Frass breaking point, Loss on heating by Thin Film Oven Test (TFOT) and absolute viscosity at 60°C and 135°C were introduced. Penetration ratio is an indicative test for temperature susceptibility, while the wax content in excess of 4.5% is considered harmful as it produces tender mixes due to the lowering of viscosity at 60°C. Indian petroleum refineries are generally producing only two grades of bitumen viz 80/100 and 60/70 penetration grades. The specified values of viscosity for these two grades of bitumen are minimum 750 and 1500 Poise respectively, which are not considered adequate to control rutting at high pavement temperatures (say, 60-70°C). Therefore, viscosity based specifications were extensively deliberated by experts in the BIS forums and ultimately decided to switch over from Penetration based grading system (IS:73-1992) to Viscosity based grading system (IS:73-2006). Thus, Bureau of Indian Standards (BIS) initiated the revision of IS: 73-1992 and has published new grading system, IS: 73-2006, which is viscosity based grading system for paving bitumen with the requirements as given in Table 1.

The revised grading system is yet to be adopted / implemented both by manufactures and by highway construction industry in the country, though the choice of viscosity grades is simple since the following criterion of equivalency holds good on engineering logic.

Summary of Advantages of Viscosity Grading Concept:

• Viscosity is fundamental property; testing is independent of test system and sample size
• Viscosity is tested at 60°C which is regarded as maximum pavement temperature in summer
• Temperature susceptibility can be controlled by viscosity test results at 600 and 135°C.

Concluding Remarks

The quality of bitumen used for construction and maintenance of bituminous roads is an issue of vital concern to the highway professionals. The need to predict the long term performance and translate measured properties into the standards and specifications drew attention of professionals due to the requirements of improved pavement performance and economics in road construction and maintenance. Therefore, laboratory investigations were undertaken for paving grade bituminous binders from different sources to determine their quality and performance.

The quality of road building materials in all layers of the pavement structure is of great importance to achieve desired performance during the design life of roads. Also, maintenance treatments applied during its service life need bitumen of requisite quality and, therefore, specification of bitumen gains paramount importance for ensuring the performance of bituminous surfaces. Thus, the perspective of quality of bitumen in terms of addressing the performance of roads needs to be incorporated in the specification of bituminous materials based on the rational considerations rather than empirical parameters.

Acknowledgment

The author wishes to express sincere thanks to the Director, CRRI for permitting to publish this paper. Thanks are also due to Sh. B.M. Sharma, Head and Dr. P. K. Jain, Scientist 'F', Flexible Pavement Division, CRRI for their encouragement in preparation of this paper.

References

• Kandhal, P.S., "Selection of Bitumen for Paving Highways in India", Indian Highways (2005), pp 17.
• Bureau of Indian Standards, 'Paving Bitumen – Specifications, IS: 73-2006'.
• P.K. Jain and B.M. Sharma (CRRI), Theme Paper for Panel Discussion On "Implementation of IS–73-2006 and Road Map Thereof" BIS-CRRI Workshop on 'Advancements in Bitumen Technologies' September 1^st, New Delhi.