Distributor truck calibration is also vital to the application of a proper prime coat. Periodically, a trial prime 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 prime coat applications are rather straightforward. Most commonly, prime 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:
A formal method for determining application rates has been adopted by ASTM under their D2995 procedure. 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
Proper Nozzle Setup on the Spray Bar
During the calibration process check the bar nozzles.
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. A double overlap is recommended for most prime applications. 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 emulsions or diluting emulsions with water. Table 1 shows recommended application temperatures for typical prime coat materials.
Table 1. Recommended spray temperature range for prime and tack coat
|Type and Grade of Asphalt
|SS-1, SS-1h, CSS-1, CSS-1h
|MS-1, MS-2, MS-2h, CMS-2, CMS-2h