605-1 DESCRIPTION
A tack coat is the spray application of low viscosity asphalt to an existing asphalt or concrete surface to promote bond between old surfaces and the overlying pavement structure.
605-2 MATERIALS
All tack coat materials shall meet the Standard Specification requirements and will be either Asphalt Binder Grade PG 58-28 or PG 64-22 or Asphalt Emulsion, Grade RS-1H, CRS-1H, CRS-1, HFMS-1 or CRS-2 unless otherwise approved by the Engineer. Asphalt Emulsions shall not be diluted with water. Different grades of tack coat must not be intermixed in a tanker or distributor since this can cause the material to break in the tank and become almost impossible to spray.
Since most contractors elect to use an asphalt emulsion for tack coat, the application temperature should not much higher than 150 degrees Fahrenheit. If the distributer must be heated above this temperature, there is a chance the asphalt emulsion has separated in the tank where the residual asphalt has collected at the bottom of the tank and the water and emulsifier is floating above.
Emulsions that have become separated or “broken” cannot be mixed back together without a colloidal mill.
605-4 SURFACE PREPARATION
The
4 essential requirements of a good tack coat operation are:
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The surface to be tacked must be thoroughly cleaned.
Remove grass, dirt, and other materials from the edge of the existing pavement prior to the placement of tack coat.
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It must be applied at the proper rate. (See
Article 605-7 for application rates and temperatures.)
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It must uniformly cover the entire area to be resurfaced.
It must be allowed to fully break (evaporate the moisture away, leaving the residual asphalt binder in place) before placing the asphalt layer.
605-7 APPLICATION RATES AND TEMPERATURES
Tack coats should be applied in a thin coat and uniformly cover the entire surface, including all vertical surfaces of joints and structures. Too little tack coat can cause debonding and too much tack coat can cause slippage.
A loss of bond between HMA layers can cause crescent-shaped slippage cracks or debonding to occur, leading to reduced pavement life.
Apply only as much tack coat material as can be covered with base, intermediate, or surface course material during the day's operation, unless no traffic is on the facility. When no traffic is present, apply only as much tack coat that can be covered within the next day's operation.
The tack coat beneath an open-graded friction course should be either a PG 58-28, Grade PG 64-22 tack coat material, or an approved non-tracking hot-applied (NTHA) asphalt tack coat material. The rate of application to be used on the project will be specified in accordance with
Section 650. Open-graded friction course applications are thin layers of asphalt, where the shear stresses at the pavement interface are a concern, thus the more stringent tack coat application is desired.
605-9 PROTECTION OF TACK COAT
If possible all traffic should be kept off the tack coat.
ADDITIONAL RESOURCES
