A bitumen road emulsion is a two-phase fluid in which bitumen is dispersed as droplets, typically 1 to 10 microns across, through water and held in suspension by an emulsifier. Emulsifying the bitumen lowers its viscosity, so it can be applied to a road at a far lower temperature than hot bitumen would need.
Road Emulsion Association, Technical Data Sheet No.1
Why bitumen is emulsified at all
Bitumen is produced by distilling crude petroleum, in a range of grades whose properties run from hard and brittle to soft and viscous at road temperatures. Many of those grades are used in road maintenance and construction, and to make them usable their viscosity has to be reduced temporarily.
There are four ways to do that: add solvents, apply heat, combine solvents and heat, or emulsify. Emulsification is the one that allows the material to go down cool.
Cationic and anionic: the difference that matters on a road
Emulsions are classified by the electrical charge carried by the bitumen droplets. Where the droplets are positively charged, the emulsion is cationic. Where they are negatively charged, it is anionic.
That distinction is not academic. Cationic emulsions have good adhesive properties with respect to the bond between the residual bitumen and mineral aggregates, and they are by far the most widely used type on UK roads. Anionic emulsions are used mostly in non-road, industrial applications.
The two also break differently. An anionic emulsion breaks through the substantial removal of water, mainly by evaporation but also by absorption into the surfaces it touches, particularly mineral aggregates.
How emulsions are specified in the UK
Historically, BS 434-1 defined the nomenclature and specification of bitumen emulsions in the UK. Cationic grades are now specified by BS EN 13808. Anionic grades continue to be specified in BS 434-1, because BS EN 13808 applies only to cationic emulsions.
A BS EN 13808 designation can be read straight off the label:
C69BP3
A cationic bituminous emulsion with a nominal binder content of 69%, produced from bitumen, containing polymers, with a Class 3 breaking value.
C60BF4 40/60
A cationic bituminous emulsion with a nominal binder content of 60%, produced from bitumen, containing more than 3% flux, with a Class 4 breaking value, produced using 40/60 penetration grade bitumen.
In principle an emulsion could be specified with any combination of classes, but that would produce grades nobody makes. The National Annex to BS EN 13808 gives further guidance on selecting and specifying realistic grades.
Polymer modified emulsions
The use of polymer modified bitumen emulsions has increased dramatically in road maintenance. They are specified by BS EN 13808, which provides a single specification capable of categorising these materials adequately.
Related
All REA technical datasheets — twelve datasheets covering emulsion types, handling, surface dressing, bond coating and more.
The REA is the UK trade association for producers and users of bitumen road emulsion, and publishes these datasheets through its Technical Committee.