Bitumen emulsion is
heterogeneous, two-phase systems consisting of two immiscible liquids, bitumen
and water, stabilized by a third component, the emulsifier. The bitumen is
dispersed throughout the continuous aqueous phase in the form of discrete
droplets, typically 0.5 to 5 microns in diameter, which are held
in suspension by electrostatic charges. Emulsified Bitumen usually consists of
bitumen droplets suspended in water. This dispersion under normal circumstances
would not take place since everyone knows that oil and water don’t mix, but if
an emulsifying agent is added to the water the asphalt will remain dispersed.
Most emulsions are used for surface treatments. Emulsions enable much lower
application temperatures to be used. Application temperatures range
from 45°C to 70°C. This is much lower than
the 150°C to 190°C used for hot mix asphalt cement. The lower
application temperatures will not damage the asphalt and are much safer for
field personnel. Bitumen emulsions are basically an O/W (Oil on Water
solution). A dispersion of bitumen particles on water, stabilized with the
addition of surfactants (Surface active agents) or most commonly known as
emulsifiers, that will permit the bitumen to be diluted in water. They are
primarily used for tack coats for use in between hot mix asphalt
layers and prime coats for thin hot mix surfacing layers or a chip seal pavements. The common test method which is
typically used is ASTM D 2397M.The bitumen emulsion is classified into two types.
Base on surface charge
emulsions can be divided into two classes which are cationic and anionic. The terms cationic bitumen emulsion, anionic
bitumen emulsion refer to the overall particle charge on the bitumen
droplet imparted by the emulsifier which is related to The type (chemistry) of
the emulsifying agent used.
In these kind of bitumen the overall particle charge of bitumen is positive. The positive charged drops (cationic emulsion) were applied to negative charged surface. The bitumen drops are immediately attracted to the surface and begin to break.
Bitumen emulsion can be divided into 4 classes based on time of setting:
Rapid-setting (RS) emulsions set quickly in
contact with clean aggregates of low-surface area, such as the chippings used in chip seals
(surface dressings). Medium-setting (MS) emulsions set sufficiently less quickly that they can be
mixed with aggregates of low surface area, such as those used in open-graded mixes.
The QS (quick-setting) and CQS (cationic quick setting) designations for quick-setting emulsions have been introduced for emulsions intermediate in reactivity between MS and SS, which do not need to pass the cement mix test, and are used primarily in quick-set slurry surfacing applications.
Slow setting (SS) emulsions will mix with reactive aggregates of high surface area.