Pavement Preservation Journal - Fall 2017 - 34
Asphalt Emulsion Bond Coats
Optimize UTBO, Thinlay Performance
BY TOM KUENNEN
pavement perform better when thoroughly
bonded to each other.
Whether for thin or conventional hot-,
warm- or cold-mix asphalt overlays,
asphalt emulsions are ideal for bonding
the layer, compared to straight or "neat"
liquid asphalt, or cutback asphalt.
"The most widely used tack coat material in the world is emulsified asphalt,"
according to 2012's NCHRP Report 712,
Optimization of Tack Coat for HMA
Placement. A survey in the report indicated that approximately 92 percent of
tack coat applications worldwide utilized
Why does the world prefer asphalt emulsions for bond coats? Straight or "neat"
liquid asphalt must be kept heated to stay
liquefied, and that poses burn hazards
and other safety problems as the distributor moves around on the site. And use of
volatile cutback asphalt poses air pollution
problems, resulting in curtailed use from
coast to coast.
Asphalt emulsion, also called emulsified asphalt, is a nonflammable liquid
substance produced by combining water,
liquid asphalt and emulsifier and blending
them in a powerful colloid mill, and storing
prior to use.
Because oil and water don't mix, the
emulsifier-an engineered surfactant
(detergent) or surface-active agent-
maintains the microscopic asphalt droplets in a stable suspension, keeping
them from recombining. The amount
IMAGE CREDIT: ROADTEC INC.
sphalt emulsion bond or "tack"
coats-done with the right
material, application rate and
full-width of paving-are
essential to long-term performance of any
But because of the thin depth of wearing course placed, they're absolutely
essential to-and play a special role in-
performance of ultra-thin bonded asphalt
overlays (UTBOs) and thin asphalt overlays
(Thinlays) used for pavement preservation.
A bond coat adheres one layer of asphalt
to another layer of pavement, whether bituminous or portland cement concrete. The
same way that fragile, thin veneers of wood
are glued to each other to form a robust
sheet of plywood, research shows layers of
For ultrathin bonded overlay, asphalt emulsion bond coat from paver-mounted tank is placed on existing pavement directly in
front of asphalt paver screed for hot-on-hot paving
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