Modified Bitumen are bitumen with additives. These additives help in further enhancing the properties of bituminous pavements. Pavements constructed with Modified Bitumen last longer which automatically translates into reduced overlays. Pavements constructed with Modified Bitumen can be economical if the overall lifecycle cost of the pavement is taken into consideration.
The new method of grading the product has now rested on the viscosity of the Bitumen (at 600C and 135 0C). The new grades have thus evolved with nomenclature:
Paving grade bitumen which has three different grades like bitumen 60/70 , bitumen 80/100 and bitumen 40/50 are 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 (bitumen 60/70 , bitumen 80/100 and bitumen 40/50) 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 7 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.
The properties of Bitumen can be defined in terms analogous to the Modulus of Elasticity of solid materials. In case of solids, Modulus of Elasticity E is defined by Hooke’s law Bitumen is a Visco-elastic material. At high temperatures it behaves like a liquid & hence liquid flow properties like Viscosity are exhibited. However, at low temperatures bitumen behaves like a solid and hence solid properties like stress & strain become relevant.
Similarly, for shorter loading time bitumen behaves like a solid whereas for longer loading times bitumen behaves like a liquid. The properties that bitumen exhibits in the intermediate temperature range and loading time are of great relevance as this range is very long and bitumen is handled in this temperature range most of the times. Due to the visco-elastic nature of bitumen, there is always a phase lag in stress & strain in case of repetitive loadings. For purely elastic material the phase lag is 00 and for purely viscous material the phase lag is 900. In case of bitumen since it is neither a liquid nor a solid at most temperatures hence the phase lag is always between 00 to 900. The above theory is extremely useful in studying fatigue characteristics, properties of creep & also tensile strength of bitumen. Bitumen price is specified based on these characteristic by bitumen suppliers .
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