Grading of Bitumen, although various standards are in place, the property whether Penetration or Viscosity has to be determined for the effective selection of the material for construction of roads is discussed here.
The grading of bitumen through penetration test at 25°C was first adopted in the early Sixtees on Paving Bitumen, which was in practice till today . 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 , 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, was felt essential lately . Hence new standards were developed 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 5% is considered harmful as it produces tender mixes due to the lowering of viscosity at 60°C. 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 to Viscosity based grading system .
The revised grading system is yet to be implemented both by manufactures and by highway construction industry , though the choice of viscosity grades is simple since the following criterion of equivalency holds good .
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 60 and 135°C.
However, all said, refineries are mostly producing / specifying in terms or Penetration grades till now.