Tack Coat

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General Parameters & Advice

Distributor truck calibration is also vital to the application of a proper bond coat. Periodically, a trial bond coat application should be placed over a test area to verify correct nozzle operation and configuration. Distributors should be calibrated annually as a minimum. Increasingly, owner agencies are requiring a valid certification of calibration to ensure the proper functioning of the distributor and its components. ASTM D2995 (described below) provides guidance for the calibration procedure. The calculations needed to determine bond coat applications are rather straightforward. Most commonly, bond coat is specified in terms of volume (gallons/square yard). However, it might also be specified in terms of mass (pounds/square yard). The following steps can be applied with the volume method:

  1. Determine the distance traveled.
  2. Calculate the area sprayed = distance traveled X width sprayed and convert from sq. ft. to sq. yd. if needed.
  3. Calculate the gallons of material applied = beginning volume – ending volume. The volumes may be determined by using a dipstick calibrated to the truck’s tank or onboard meters. 
  4. Correct for temperature back to 60°F by applying correction factor.
  5.  Account for any dilution.
  6. Calculate residual asphalt by accounting for the water in the undiluted emulsion. 
  7. Calculate residual emulsion application rate, which is the gallons of residual emulsion applied divided by the area of application.

A formal method for determining application rates has been adopted by ASTM under their D2995 procedure. D2995 offers two methods: Use calibration pads that are pre-weighed and attached to the roadway surface. The pads should be attached both longitudinally and transversely. The truck being calibrated drives over the pads while spraying its material. The pads are quickly removed and reweighed. The application rate is then determined by taking the difference between the post-sprayed and pre-sprayed weights. Any dilution needs to be accounted for, as should the water in the undiluted emulsion to get the application rate in terms of residual material.

Cite the source FHWA Technical Bulletin on Tack Coat Best Practices 2016

Calibration Guidelines

Proper Nozzle Setup on the Spray Bar

During the calibration process check the bar nozzles.

  1. All nozzles should be the same size and appropriate size for shooting prime
  2. All nozzles should be on or off at the same time.  Fix nozzle components that prevent proper operation
  3. The fan angle should adjust to 15 to 30 degrees and all set the same, some contractors prefer to set end nozzles at a different angle.  

Proper asphalt distributor construction procedures are required to prevent streaking, allow proper application rates and uniform coverage. To prevent the spray of liquid asphalt from interfering with adjacent spray nozzles, the nozzles should be set at an angle of 15 to 30 degrees to the horizontal axis of the spray bar.

Proper Bar Height

The height of the spray bar should be set to allow for an exact single, double or triple overlap. For uniform application, proper spray bar height must be maintained during application. This requires that the spray bar height be adjustable to correct for the truck’s rear springs rising as the load lessens. The figure shows the effect of incorrect spray bar height and the proper spray bar heights for double and triple coverage


Material Temperature For Proper Spray Pattern

Adequate viscosity of the liquid asphalt is required for proper spray application. This is achieved by heating MC cutbacks and occasionally heating emulsions or diluting emulsions with water. Table 1 shows recommended application temperatures for typical bond coat materials. 

Table 1. Recommended spray temperature range for prime and tack coat

Type and Grade of Asphalt Temperature °C  Temperature °F
SS-1, SS-1h, CSS-1, CSS-1h 20-70 70-160
MS-1, MS-2, MS-2h, CMS-2, CMS-2h 20-70 70-160
MC 30 30+ 85+
MC 70 50+ 120+
MC-250 75+ 165+
AE-P 49-82 120-180
EAP&T 15-38 60-100