Right Grade of Bitumen for Flexible Pavements: IndianPerspective

M.N. Nagabhushana, Scientist, Flexible Pavemments Division, Central RoadResearch Institute, New Delhi

Introduction

Indian road transportation infrastructure is rapidly expanding with the ambitiousdevelopment of road networks under National Highways Development Programme (NHDP),State Highways Improvement Programmes (SHIPs), Bharat Nirman, Pradhan Mantri GramSadak 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 ahigh quality pavement structure as the main corridors are required to cater to very heavytraffic-both in terms of number and axle loading. Road-laying under the GoldenQuadrilateral project and the North-South and East-West corridors project of NationalHighways Authority of India (NHAI) has been quantitatively significant. According to theoutcome 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 forcompletion in the present fiscal year. While these road development projects help in addingconsiderable infrastructural assets, their construction and subsequent maintenance phasesrequire huge amount of suitable pavement materials.

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

Significance of Properties of Bitumen

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

In bituminous pavements, stone aggregates and bituminous binder are the key ingredientsand 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 alongwith the proper proportioning with stone aggregates is the basic requirement to makeworkable layer mixes. Asphaltic bitumen is obtained by refining the petroleum crude. It isthe costliest and a very important component of the bituminous mix.

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

Bitumen and Bituminous Binders in Road PavingBitumen Demand and Its Usage

The average annual demand of bitumen in India is around 4MMT (million metric tonnes) andit is estimated that bitumen works alone cost about 45% of the overall road construction

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it is estimated that bitumen works alone cost about 45% of the overall road constructioncost. Of late, bitumen that is generated during the refining process of petroleum — adiminishing product, tends to become scarce while the demand is increasing. This hasresulted into a situation where the gap between supply and demand is fast increasing. Thebitumen supply scenario is not showing any sign of improvement since only countable crudeoil refineries manufacture bitumen while a small quantity(between 1–4% )of crude oil isused for bitumen. With the road construction industry outgrowing, local demand hasexceeded the bitumen production and supply. The completion targets of many road projectsthereby are adversely affected and hence the construction and maintenance require longspreading/deferring of works. The industry has also witnessed sharp cost escalations inbitumen binder over the years.

It is estimated that the global demand for bitumen will increase 2-3 percent annuallythrough 2009. Accelerating demand for bitumen, though will reflect gains in GDP growth,will result in increased expenditures on infrastructure in developing nations. Indiarepresents one of the largest and fastest growing bitumen markets. Continuing rapidindustrialisation and strong growth in building construction markets are driving demand forbitumen in the paving market, especially with the country undergoing the most dramaticgrowth in road building of any nation, which will spur demand for bitumen used in pavinggrade 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 willcontinue 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 alayered flexible pavement. They may be thick or thin, hot or cold, plant-mixed orsite-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 allthese are common, being stone aggregates and bituminous binder, the specificationrequirements 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 treatmentsincluding prime coat and tack coat between different thick layers for the purpose of bondingthe 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 isutmost 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 materialor product to result in desired output. In road pavements the requirement is a betterperforming structure to serve the road users with comfort, safety and at lower costs. Thespecifications 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 ofbitumen to be considered are the mechanical, adhesive and the durability properties.

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

Conventional tests used for bitumen characterisationPenetration, ductility, R&B softening point, flash point, Fraass breakingpoint…..

1.

Ageing characteristics:Thin Film Oven Test, Rolling Thin Film Oven Test, Pressure Aging Vessel…

2.

Rheological tests:Bending Beam Rheometer, Direct tension Test, Dynamic Shear Rheometer…..

3.

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

Issues in Selection of Right Paving BitumenDevelopments in Grading of Bitumen

After recognising the applicability and advantages of usage of bitumen in civil engineeringconstructions, especially roads, concerned efforts were started to examine, evaluate andclassify the naturally available as well as processed bituminous materials. To enable theproper handling of the related issues tests approach methods specifications and concepts

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proper handling of the related issues, tests, approach methods, specifications and conceptswere developed to grade the bitumen. Some of the important milestones may be enlistedas:

Beginning of specifications - Around early 20th century

Classification of asphalts(later called as bitumen) for commercial purposes in the firsthalf 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 wasthe 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 ofMaterials (ASTM) adopted the grading of bitumen through penetration testing at 25°C. Thelower penetration value indicated the harder bitumen, while the higher penetration valueindicated the softer bitumen. The penetration based grading system continued until 1970and the same continues even today in many countries including India. Thereafter in order toaddress various construction problems like tender mix resulted by 60/70 penetration gradebitumen (due to its low viscosity at 135°C) and performance related problems such asrutting at high pavement temperatures, viscosity based grading system was introduced inUS in the year 1970.

British Standard (BS: 12591) specifies 9 grades of bitumen based on penetration values. InBS: 12591, viscosity at 60°C and 135°C are incorporated as optional properties along withthe wax content, Frass Breaking point and Ageing Properties. Presently, ASTM: D-946specifies "Penetration Based Specification"of bitumen and ASTM:D-3381 specifies "ViscosityBased 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 thehybrid 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-64-16bitumen is suitable for a project location, where average 7 days maximum pavementtemperature 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 pavementtemperature of 63°C in summer and a minimum pavement temperature of -1°C in winterwill require PG-64-16 bitumen. Experimental studies at CRRI have shown that PG gradesare quite close to VG grades.

Indian Scenario and Associated Problems

In India, the bitumen grading is practised on the basis of penetration test, which isconducted 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 standardpenetration needle loaded with 100 grams is allowed to penetrate the bitumen (maintainedat 25°C) for five seconds. The penetration is measured in mm and it indicates the relativehardness of the bitumen. Higher the penetration, softer is the bitumen. A penetration of 0.6to 0.7 mm indicates 60/70 penetration grade bitumen.

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

Bitumen processed from different petroleum crude sources and/or refining processes mayhave the same penetration grade at 25°C but may exhibit significantly different hardness inthe 60-70°C temperature range. Those which are very soft (low viscosity) are more proneto 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 at25°C) through viscosity test was implemented in U.S. some 30 years ago. The viscosity ofthe bitumen is measured in poises at 60°C using a simple viscometer. Various viscositygrades of asphalt cement (bitumen) were evolved as AC-30 Grade, AC-20 Grade and AC-10Grade. These were implemented in North America during 1970s according to climatewherein higher viscosity grades were selected for hotter climate.

The bitumen specification used in India at the present time has a 60/70 penetration gradebut it was 80/100 and equivalent to an AC-10 viscosity grade until recently. The use ofsofter AC-10 grade is suited for the cold climate and is unacceptable for India, since it isvery soft and will simply increase the potential for rutting under heavy trucks. There is aneed to at least use an AC-30 grade in India to minimise the rutting potential of ourpavements, 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 laboratorymeasured properties of penetration grade bitumen and their performance in asphaltmixture on actual road sites Penetration softening point and ductility are the key

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mixture on actual road sites. Penetration, softening point and ductility are the keyproperties of bitumen used to evaluate bitumen's quality and to assess their contributions tothe 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 unitsand cannot be related to any of the required rheological properties. It is also establishedthat none of these parameters indicate viscous/elastic behaviour of bitumen at the testtemperature. A number of field studies have already been conducted to investigate therelationships between these properties and performance related aspects. Penetration valueupto 25 is acceptable for prevention of cracking, while a higher value tends to proceedtowards cracking at a higher rate. Similarly, a viscosity value of 22,000 Poise at 60°C foroxidized bitumen is acceptable but higher value accelerates the development of cracking.

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

Recently, Strategic Highway Research Program (SHRP) introduced performance basedbinder specification in total quality perspective based on the shear susceptibilityparameters, addressing each type of pavements failure. These failure modes are identifiedas critical pavement distress modes in which quality of binder plays a deciding role. The newbinder specification is based on the measurable engineering properties like dynamic shearmodulus 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 designatedas'Performance Grade (PG) Bitumen'and is classified mainly as traffic and temperatureresponsive. However, the significance of these tests and the interpretation of the results areyet to be recognised and adopted for Indian applications.

Latest Development in Indian Bitumen SpecificationsNeed for Change in Grading System

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

New Grading by Indian Standards

The grading of bitumen through penetration testat 25°C was first adopted in 1961 by Bureau ofIndian Standards and IS:73-1961 is the firstIndian standard on Paving Bitumen, which was inpractice until 1991. This standard recommendedfive 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 thisstandard was performing well until the end of seventies, when construction specificationslike surface dressing and premix carpet were largely in practice. With the increased use ofdesigned dense mixes, increased traffic volume and loading, higher road user's expectationsand changes in crude quality, revision of IS:73-1961 was felt essential in 1985 andsubsequently IS:73–1961 was revised to IS:73-1992 in 1992. In this, new tests likepenetration ratio, paraffin wax, Frass breaking point, Loss on heating by Thin Film OvenTest (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% isconsidered harmful as it produces tender mixesdue to the lowering of viscosity at 60°C. Indianpetroleum refineries are generally producing

only two grades of bitumen viz 80/100 and 60/70 penetration grades. The specified valuesof 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 expertsin the BIS forums and ultimately decided to switch over from Penetration based gradingsystem (IS:73-1992) to Viscosity based grading system (IS:73-2006). Thus, Bureau ofIndian Standards (BIS) initiated the revision of IS: 73-1992 and has published new grading

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