15 May 2001
(b) A test section is another method to evaluate the construction techniques and the
application rates required for surface treatment. At least one test section should be constructed before
allowing surface treatment applications on a full scale.
a. General. A slurry seal is a mixture of asphalt emulsion, well-graded fine aggregate, water, and
mineral filler. These materials are combined in the proper proportions to produce a homogeneous, fluid-
like slurry. The consistency of the slurry must be such that it can be squeegeed over an existing
pavement surface. A thick, sealed surface results after evaporation of the water and curing of the mix.
When properly designed, constructed, and cured, the slurry seal should improve the qualities of an
existing pavement surface, but the structural strength of the pavement structure is not significantly
improved. Slurry seals are used to protect worn, weathered, or cracked pavements from the adverse
effects of weather conditions and traffic wear. With proper use of aggregates, the slurry seal can also be
used to reduce skid or slipperiness problems. Slurry seals have application to roads and streets,
parking lots, and bridge decks. This type of seal coat is best suited for pavements that are not subjected
to heavy traffic. Because aircraft can cause a rapid deterioration of the slurry seal, slurry seals should
not be applied to airfields.
b. Equipment. Various types of equipment are needed on a slurry seal project, but the basic
pieces of equipment required include a truck-mounted continuous-mix slurry machine, spreader box,
power broom, front-end loader, distributor, and pneumatic-tired roller. The truck-mounted continuous-
mix slurry machine which serves as a portable mixing plant is the most important piece of equipment. It
is the only type of mixing equipment recommended for mixing a slurry seal. A slurry seal machine is
used to mix aggregate, filler, asphalt emulsion, and water in the correct proportions and to uniformly
apply the material to the surface to be sealed. The slurry seal machine generally contains storage for
the aggregate, filler, emulsion, and water. Before the machine is used, it must be calibrated and set to
deliver the job materials in the correct proportions. The machine manufacturer's instructions usually
offer the best guidance for calibrating the slurry machine. However, a calibration method based on a
revolution counter is applicable to all machines. By attaching a revolution counter to any shaft that is
mechanically interlocked with the emulsion pump, water pump, fines feeder, and aggregate conveyor,
the relative quantities of each of these components per revolution can be determined for various gate
openings, metering valve openings, or sprocket sizes. The materials are mixed and deposited into a
squeegee box, which applies the slurry seal onto the surface at a thickness approximately equal to the
maximum aggregate size.
c. Material requirements.
(1) Emulsion. The binder used in a slurry seal is asphalt emulsion. The emulsion is often
either slow-set anionic (SS-1 or SS-1h) or slow-set cationic (CSS-1 or CSS-1h). The slow-set emulsions
are best suited for slurry seals, but some quick-set emulsions are specifically designed for slurry seal.
The use of quick-set emulsions requires that an experienced slurry seal contractor perform the job
because of the small amount of time available for handling the slurry seal before it cures. Slow-set
cationic emulsions cure faster than slow-set anionic emulsions because the curing process is partly a
chemical reaction that expels some of the water from the mix. Anionic emulsions cure primarily by
evaporation of the water from the mix; therefore, they are greatly influenced by weather conditions. Low
temperatures, high humidity, or rain can slow or stop the curing process. Sometimes an emulsion will
break, that is, the asphalt will separate from the water upon contact with certain types of aggregates. If a
break occurs, either the emulsion or aggregate type must be changed.