Introduction of Bitumen

BITUMEN: is defined as “A viscous liquid, or a solid, consisting essentially of hydrocarbons and their derivatives, which is soluble in trichloroethyelene and is substantially nonvolatile and softens gradually when heated. It is black or brown in colour & possesses waterproofing and adhesive properties. It is obtained by refinery processes from petroleum, and is also found as a natural deposit or as a component of naturally occurring asphalt, in which it is associated with mineral matter.

Bitumen or bituminous binder available in India is mainly of the following types :

Penetration Grade :

• Bitumen 80/100 : The characteristics of this grade confirm to that of S 90 grade of IS-73-1992. This is the softest of all grades available in India. This is suitable for low volume roads and is still widely used in the country.
• Bitumen 60/70 : This grade is harder than 80/100 and can withstand higher traffic loads. The characteristics of this grade confirm to that of S 65 grade of IS-73-1992. It is presently used mainly in construction of National Highways & State Highways.
• Bitumen 30/40 : This is the hardest of all the grades and can withstand very heavy traffic loads. The characteristics of this grade confirm to that of S 35 grade of IS-73-1992. Bitumen 30/40 is used in specialized applications like airport runways and also in very heavy traffic volume roads in coastal cities in the country.

Viscosity Grade :

Paving grade bitumen is the bitumen obtained from refineries and conforms to IS 73. Recently, the third revision of Indian Standards for Paving Bitumen Specifications IS 73:2006 has been released by Bureau of Indian Standards. Three grades of Bitumen confirming to IS 73: 1992 are manufactured in India. In this third revision grading of Bitumen is changed from penetration grade to viscosity grade. To improve the quality of Bitumen, BIS revised IS-73-1992 Specifications based on viscosity grade (viscosity @ 60 deg. C) in July 2006. As per the Specifications, there are four grades VG-10, VG-20, VG-30 & VG-40.

With the current revision several key issues are addressed, like :

Performance at high temperatures by adopting a viscosity-graded bitumen specification (based on viscosity at 60 ºC), in place of the current penetration-graded specification (based on penetration at 25 ºC)

Issues relating to compaction, which the tender asphalt mixtures create as push and shove under the roller wheels, have also addressed by having a requirement of minimum viscosity at 135ºC, it will be helpful in minimizing the tender mix problems in the field.

Adoption of viscosity-graded paving bitumen specifications will also reduce the number of total tests to, without compromising the quality of bitumen and also no new tests are required in implementing this specification.

Viscosity grades Bitumen are categorized according to Viscosity (degree of fluidity) grading. The higher the grade, the stiffer the Bitumen. In Viscosity Grade, Viscosity tests are conducted at 60 deg. C and 135 deg. C, which represents the temperature of road surface during summer and mixing temperature respectively. The penetration at 25 deg. C, which is annual average pavement temperature, has been also retained in Specifications.

Viscosity Based System

The actual tests conducted are as follows:

Viscosity Test

Viscosity at 135 Deg.C is a fair indicator of the ability of bitumen to coat the aggregates properly. In order to get best coating the viscosity has to be optimum. Too viscous bitumen would result in inadequate and non-uniform coating of the aggregates. Very low viscosity would again result in inadequate coating as the bitumen will tend to bleed. Therefore viscosity at 135 Deg.C is a true reflection of the quality of bond that is likely to be formed with the aggregate. Various testing equipments like Capillary Viscometer, Cup Viscometer, Tar Viscometer, etc. can be used for testing the viscosity.

Viscosity at 60 Deg.C is a very good indicator of the resistance of bitumen to melting/flowing on the road. It is considered to be replacement test for Softening Point test. Some specifications have replaced softening point test with Viscosity at 60OC. However, at many :

places both the tests are carried out as both the tests are empirical and have their own limitations.

METHODOLGY FOR PROCESSIG OF BITUMEN VG10 TO VG40 :

The Theory of Air Blowing 

• Air blowing is essentially a de-hydrogenation process that convert lighter fraction of bitumen into heavier fraction uniformly.  
• The technique is used for conversion of lower viscosity grade bitumen into higher viscosity grade (increase of softening point from 40OC to > 55OC).
• The air blowing process does not involve any evaporation of lighter fraction from bitumen thus the composition of base bitumen remains unchanged.
• After air blowing, the recovery is very close to 100%. About 10MT softer grade bitumen is converted into 9.999 MT higher softening point and higher viscosity grade bitumen without appreciable weight loss. 
• The process increases the degree of un-saturation of hydrocarbon and the lighter oils and resins fractions of base bitumen are converted into more Asphaltenes contents, liberating heat and water.
• The process thus reduces the hydrogen ratio to carbon making the bitumen viscous and resulting into increase of softening point.
• Conversion of low viscosity grade bitumen into higher viscosity grade ensures a uniform bitumen input for modification and hence ensuring the consistency of the quality of end product which is otherwise not possible due to variation in crude and quality of base bitumen from refinery to refinery and batch to batch supply from refinery.

The Process of Air Blowing

Following procedural steps are involved for blowing process:

• VG 10 bitumen is received in tanker from refinery and the softening point and penetration value will be tested and recorded.
• The accepted bitumen is pumped into heating and mixing tank and is heated up to 200 OC ± 5 OC.
• The heated bitumen is transferred to the air blowing column.
• The compressed air is blown through specially designed air distributors at rate about 1800 cum per hour from the bottom of the blowing column.
• During this process, de-hydrogenation reaction takes place and the bitumen is converted to higher viscosity grade.
• The reaction is exothermic and the softening point of base bitumen is raised from 40OC to >55OC.
• The time of blowing depends on initial softening point of the bitumen and the process is usually completed in 2 hours.
• Samples are drawn from blowing column and the softening point, viscosity  of processed bitumen is tested and recorded. 
• After test and compliance, the processed bitumen is transferred from blowing column to storage tank and thoroughly tested to insure all parameters of VG-40.  

METHODOLGY FOR PROCESSIG OF BITUMEN VG30 TO CRMB60 :

• Bitumen VG30 is unloaded into the raw bitumen heating tank, where it is heated up to around 200°C.
• Then we add previously measured quantity of crumb rubber modifier into the tank with help of top funnel. 
• Two stirrers are continuously deployed to ensure we get a homogenous mix of bitumen and modifier. The mixing and stirring is carried on for around one hour. This gives enough time for reaction between the bitumen and crumb rubber. The reaction is not a chemical process but rather a diffusion process that includes the physical absorption of aromatic oils from the bitumen into the polymer chain of the rubber. The rubber particles swell as they absorb oils, which cause the viscosity of the CRMB to increase during the first hour
• We then check for viscosity, softening, penetration and elastic recovery parameters as per IRC SP 53; to check for the quality in our laboratory.
• After checking for the parameters, we can give delivery of the final modified bitumen product.
• To ensure consistent quality we check the processed quality of the product at all stages in our state of art laboratory to be provided on site. 
• This also helps the client and consultants to be assured of the quality of the product being used during execution at site.