4.6.1 Conventional Bitumen
Bitumen is a viscous product obtained from the refining of crude oil. Bitumen is temperature sensitive, i.e. the material is fluid when heated allowing it to be pumped and sprayed. When cold, it is a plastic solid providing sufficient stiffness to retain aggregate and resist the forces of traffic. Flow or deformation properties are important when considering selection and application.
Other key characteristics of bitumen include:
- adheres strongly to most materials
- water resistant
- flexible and ductile
- low toxicity
- low cost.
This combination of properties makes bitumen an effective binder for sprayed seals.
Bitumen is generally supplied and applied at elevated temperatures, as this is usually the most economic form of binder delivery system. The technology involved is well developed, widely understood and supported by industry investment in equipment and other resources. Bitumen can also be applied as a bitumen emulsion or foamed bitumen, and these delivery systems have certain advantages and disadvantages compared to conventional bitumen delivery systems at more elevated temperatures.
Classification and properties of bitumen
The properties of bitumen for roadmaking are specified in AS 2008: Residual Bitumen for Pavements. This Australian Standard classifies and specifies the properties of conventional pavement-grade bitumens most widely used in Australia.
The following classes are most commonly used in sprayed sealing works in Australia:
- Class 170: the most commonly used class for sprayed sealing
- Class 240: higher viscosity binder (compared to C170) used by some road agencies in warm climates
- Class 320: higher viscosity binder (compared to C170 and C240) sometimes used for sprayed sealing in areas where pavement temperatures or traffic stresses were high, but has been largely superseded by PMBs and multigrade bitumen.
In New Zealand, the sealing grade bitumens are characterised by penetration grade and consist of:
- 180/200 penetration: generally used in sprayed seal work (the approximate equivalent of Class 50 bitumen which was a softer class of bitumen than Class 170 used in Australia in the past)
- 130/150 penetration: a slightly harder grade used as an alternative to 180/200 (approximate viscosity between that of Class 50 and Class 170 bitumen)
- 80/100 penetration: generally only used in sprayed seals in hotter areas or when resealing fatty or bleeding seals (the approximate equivalent of Class 170 bitumen).