UFC 3-250-03
15 May 2001
evaporated) before the overlying layer is placed. A properly cured surface will feel tacky. Work should
be planned so that no more tack coat than is necessary for one day of operation is placed on the
surface. All nonessential traffic should be kept off the tack coat so that dust, mud, or sand will not be
tracked onto the surface.
e. Control. To insure that the tack coat is applied as specified, the asphalt distributor should be
calibrated and inspected. ASTM D 2995 offers a method for determining the application rate of asphalt
distributors. In addition, all nozzles must be free and open, the same size, and at the same angle in
reference to the spray bar to produce a uniform spray of tack. Spray bar height above the surface is
also important for uniform application. A bar too high or too low will give a variable application in the
middle and at the edges. The spray bar should be adjusted to a height that provides a double or triple
lap.
4.
FOG SEALS.
a. General. A fog seal is a very light spray application of a diluted emulsified asphalt to an existing
asphalt pavement surface. The fog seal is used to maintain old pavements, reduce raveling, waterproof,
and in general, extend the life of the existing pavement. Fog seals are especially good for treating
pavements which carry little or no traffic. However, there are several considerations when using fog
seals.
(1) The pavement skid resistance can be reduced.
(2) The pavement air voids or permeability can be reduced.
(3) The pavement should be closed to traffic for 12 to 24 hours to allow for proper cure of the
seal material.
recent years the materials used are emulsions and rejuvenators. The emulsions most often used are
SS-1, SS-1h, CSS-1, and CSS-1h. There are several products marketed as rejuvenators, they are
proprietary products (see paragraph Rejuvenation of this chapter).
c. Application rate.
(1) The proper application and dilution rate for fog seal will vary with the absorption
characteristics of the existing pavement surface. Field test sections should be placed to determine the
best application rate for the existing pavement. The application rate should be adjusted so that the
pavement does not become slick or unstable nor have an excess of free material on the surface after
curing 12 to 24 hours.
(2) The amount of dilution must be evaluated for each job. Asphalt emulsion can be applied at
full strength or can be diluted as much as 1 part emulsion to 10 parts water. Normal application dilution
is in the range of 1 to 4 parts water. When highly diluted fog seals are used, a small amount of surface
residue is obtained and the skid resistance is slightly reduced.
d. Placement. Only a pressure distributor which has been calibrated to deliver the fog seal at the
specified rate should be used to apply the seal material. All surfaces to which the seal is applied must
be clean. The fog seal should be applied when the ambient temperature is above 4.5EC (40EF), but
warmer temperatures are desired because the material will break and cure faster. The seal material
may be applied to a damp pavement if the dilution material is water, but the pavement must not be too
3-5
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