By Srinivas Chowdary Sunkara // petrobazaar
Bitumen - Grading Methodology
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.
The Key Requirements for Bitumen
The grading of bitumen through penetration test at 25 degrees C was first adopted in the early sixties 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.
New Standards of Bitumen
With the increased use of designed dense mixes, increased traffic volume and loading, higher road user's expectations and changes in crude quality, the revision of grading system 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 degrees C and 135 degrees C were introduced.The revised grading system is yet to be implemented both by manufacturers and by highway construction industry, though the choice of viscosity grades is simple since the following criterion of equivalency holds good.
Penetration grading - Viscosity grading
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.
Viscosity Grading Concept
Viscosity Grade Bitumen is petroleum grade bitumen, manufactured from fractional / vacuum distillation of crude oil, which practical appliance and behaviour varies according to its temperature
Advantages of Viscosity Grading Concept
1. Viscosity is fundamental property; testing is independent of test system and sample size.
2. Viscosity is tested at 60 degrees C which is regarded as maximum pavement temperature in summer.
3.Temperature susceptibility can be controlled by viscosity test results at 60 and 135 degrees C.
Methods of Viscosity Grading of Bitumen
1. Standard Viscosity Grade Bitumen (AC-Grades), in which the Viscosity of the standard bitumen (asphalt) is measured at 60 degrees C (140 degrees F).
2. RTFOT Viscosity Grade Bitumen (AR-Grades), in which the Viscosity of bitumen (asphalt) is measured at 60 degrees C(140 degrees F) after the roll on thin film oven test.