15 May 2001
c. Application rate. Prime coats are usually applied in quantities of 0.38 to 1.13 liters per square
meter (0.10 to 0.25 gallons per square yard) of residual asphalt. The optimum amount of prime is highly
dependent on the plasticity index of the base material, the amount of fines in the base, the nature of
fines, the tightness of the surface, and the moisture content of the base. Therefore, the optimum
amount of prime required should be determined by field trial. Test sections at various application rates
are recommended for determining the optimum amount of prime. After 48 hours of curing, if there is free
or excess bitumen on the surface or if the base continues to appear shiny, the base is probably
overprimed. Generally, most of the prime should be absorbed into the base within 2 to 3 hours. When
excessive prime is used, the surplus can be absorbed into the overlying asphalt layers. In turn, the
absorption of the excess may contribute to pavement slippage or rutting. Where excessive prime is
applied, the excess must be blotted with an application of clean fine sand or mineral dust. The ideal end
result is to obtain maximum penetration without leaving free prime on the surface.
d. Placement. Surfaces to be primed which contain appreciable amounts of loose material or are
dusty should be lightly broomed. A dusty surface will sometimes cause prime to "freckle," that is, have
small areas with no prime and adjacent areas with drops of excess prime. A light application of water
just before applying the prime will aid in reducing "freckles" and getting good distribution of the prime.
Priming should be uniformly applied with a pressure distributor at the required application rate and at the
proper temperature for the asphalt used. Minimum curing time will vary according to the grade and type
of asphalt being used, the nature of the base, temperature, and humidity, but generally curing should
take place within 48 hours.
e. Control. Since most prime is applied with a pressure distributor, the distributor must be
calibrated and checked for the specified application rate before applying the prime. ASTM D 2995 offers
a method for determining the application rate of bituminous distributors. In addition, all nozzles should
be free and open, the same size, and to the same angle in reference to the spray bar to produce a
uniform fan of prime. The height of the spray bar above the surface is important because a bar too high
or too low will give an unequal application in the middle of the spray fan and at the ends, causing
streaking. The height of the spray bar should be such that a double or triple lap of the spray fan is
a. General. A tack coat is a light application of a asphalt material to an existing pavement or
asphalt base course immediately prior to placing the next pavement layer or course. The purpose of the
tack coat is to provide a bond between the two pavement layers. The tack coat is applied by pressure
distributor to cleaned surfaces. The tack coat must be applied in a light and uniform application.
(1) Emulsions are the most common types of asphalt material used as tack, but cutbacks and
asphalt cements may be used in some situations. The correct spraying viscosities (temperatures) need
to be obtained for the type of material used. Recommended tack coat materials and spray application
temperatures are shown in table 3-2.
(2) The cutbacks and emulsions can be sprayed at relatively low temperatures, but the asphalt
cements may require considerable heating to reach a viscosity suitable for spraying.
(3) In cold weather, the cutbacks can be used with less concern than emulsions which contain
water. However, environmental restrictions limit the use of cutback materials, making them unavailable
at many locations. The use of emulsions for tack coats may require that the emulsion be diluted with
water so that a light tack is applied, and its use also requires that special consideration be given to