ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 323

emulsifying agent. There are many types of emulsifying agents; basically they are a
soap material. The emulsifying molecule has two distinct components, the head
portion, which has an electrostatic charge, and the tail portion, which has a high
affinity for asphalt. The charge can be either positive to produce a cationic emulsion
or negative to produce an anionic emulsion. When asphalt is introduced into the
water with the emulsifying agent, the tail portion of the emulsifier attaches itself to
the asphalt, leaving the head exposed. The electric charge of the emulsifier causes a
repulsive force between the asphalt globules, which maintains their separation in the
water. Since the specific gravity of asphalt is very near that of water, the globules have
a neutral buoyancy and, therefore, do not tend to float or sink. When the emulsion is
mixed with aggregates or used on a pavement, the water evaporates, allowing the as-
phalt globs to come together, forming the binder. The phenomenon of separation be-
tween the asphalt residue and water is referred to as breaking or setting . The rate of
emulsion setting can be controlled by varying the type and amount of the emulsify-
ing agent.
Since most aggregates bear either positive surface charges (such as limestone) or
negative surface charges (such as siliceous aggregates), they tend to be compatible
with anionic or cationic emulsions, respectively. However, some emulsion manu-
facturers can produce emulsions that bond well to aggregate-specific types, regard-
less of the surface charges.
Although emulsions and cutbacks can be used for the same applications, the
use of emulsions is increasing because they do not include hazardous and costly
solvents.

9.2 Uses of Asphalt

The main use of asphalt is in pavement construction and maintenance. In addition,
asphalt is used in sealing and waterproofing various structural components, such as
roofs and underground foundations.
The selection of the type and grade of asphalt depends on the type of con-
struction and the climate of the area. Asphalt cements, also called asphalt binders,
are used typically to make hot-mix asphalt concrete for the surface layer of asphalt
pavements (See Figures 9.3 and 9.4). Asphalt concrete is also used in patching and
repairing both asphalt and portland cement concrete pavements. Liquid asphalts
(emulsions and cutbacks) are used for pavement maintenance applications, such as
fog seals, chip seals, slurry seals, and microsurfacing (See Figures 9.5 and 9.6) (The
Asphalt Institute 1989, Mamlouk and Zaniewski 1998). Liquid asphalts may also
be used to seal the cracks in pavements. Liquid asphalts are mixed with aggregates
to produce cold mixes, as well. Cold mixtures are normally used for patching
(when hot-mix asphalt concrete is not available), base and subbase stabilization,
and surfacing of low-volume roads. Table 9.1 shows common paving applications
for asphalts.