Road Science: The Chemistry of Asphalt Emulsions.
by Tom Kuennen, Contributing Editor
of Road Building
Asphalt emulsions mix
oil with water
s pavement preservation techniques tighten their
beachheads in state, county and municipal road
agencies, new attention is focused on surface treatments, thin asphalt surfacings and the asphalt emulsions
that make them possible.
Asphalt emulsions – mixtures of liquid asphalt and water
– make expensive liquid asphalt go farther, covering far
more square yardage and aggregate surface area for asphalt
pavement preservation surfacings than possible with thicker
lifts of hot or warm mix asphalt, all things being equal.
While they don’t add structural value, these thin surfacings aren’t intended to. Instead they protect aging asphalt
pavements, seal cracks, retain aggregate, waterproof pavement structure, enhance friction, and with rejuvenators,
give new life to an aged, oxidized asphalt surface. Emulsions used in full depth reclamation (FDR), though, can
add structural value to a pavement structure.
“Asphalt emulsions are essential to all aspects of pave-
18 June 2012 Better Roads
ment preservation, be they recycling, micro surfacing, chip
seals, or crack sealing,” says Mike Buckingham, director of
pavement preservation, COLAS, Inc., and 2012 president,
FP2 Inc. “Almost everything involved in pavement preservation is somewhat tied to emulsions.”
Asphalt emulsions make all this possible, yet they have a
physio-chemical basis that in some ways is more complicated than plain bituminous pavements.
Everyone knows oil and water don’t mix. That is, unless
they are emulsiﬁed, and the chemistry of the necessary
emulsiﬁers is key to the successful application of asphalt
An asphalt emulsion is a homogeneous mixture of two
insoluble substances, oil and water. In it particles of liquid
asphalt (in the dispersed phase) are surrounded by molecules of water (the continuous phase).
They are an emulsion is not a solution, which is a homogeneous mixture of two substances that are soluble with
each other. Instead an emulsion is much like a solution, but
as the two substances won’t dissolve into each other, small
particles of one substance must be created that will become
surrounded by the other substance.
Milk is one example of a liquid emulsion, in which
globules of milk fat are suspended in liquid. Smoke is an
example of an emulsion in which solid particles (carbon)
are suspended in a mixture of gases (air).
Believe it or not, liquid asphalt is an emulsion. “Asphalt
binder itself is an emulsion, even before you emulsify it,”
says Andy Bickford, applications chemist, MeadWestvaco
Corp., at the Asphalt Emulsion Technologies Workshop held in St.
Louis in November 2011 by the Asphalt Emulsion Manufacturers Association. “Asphalt is composed of small crystal
particles called asphaltenes, which are suspended in a continuous oily liquid phase generally classiﬁed as maltenes, meeting
the deﬁnition of an emulsion. It’s the balance of the hard
crystalline particles vs. the oil phase that determines things
like asphalt grade and hardness.”
Why Emulsify Asphalt?
Because asphalt generally is solid at ambient temperatures,
its viscosity must be lowered in order for it to be used.
Energy can be added to the material to heat and liquefy it;
solvents can be added to liquefy it; or it can be emulsiﬁed.
“When you emulsify asphalt, you are creating a vehicle
that will transport asphalt without having to heat it, or use