Tack Coat is an asphalt emulsion that is applied between HMA pavements to promote surface bonding. It should be evenly applied across the pavement and provide a 90% coverage. Application rates of a tack coat will vary based on the condition of the pavement. The pavement surface needs to be clean, dry and free from any existing dirt or milled pavement. Thus sweeping and washing are needed to achieve maximum bonding. To cool and/or wet the surface we recommend using emulsified tack coat minerals. However, this may increase the length of time for curing. Too little Tack will result in inadequate bonding between layers. Too much can create a lubricated slippage plane between layers. Excess tack may also cause it to be drawn into an overlay which could negatively effect mix properties or even create the potential for bleeding in thin overlays.
Proper settings and adjustments are critical to achieve maximum tack coat placement. The nozzle pattern, spray bar height and distribution pressure must work in unison to achieve a uniform tack application. The best application results are from a double or triple-lap coverage sprayer. Double/Triple lap means the nozzle spray patterns overlap one another so every portion of the pavement receives spray from exactly two/three nozzles. Spray patterns should be exact to apply evenly, otherwise you get streaking.
As Tack is applied the truck will become lighter causing the distributor bar to rise. The vehicles computer should be able to compensate for this to ensure an even application.
Temperatures in the distributor need to be maintained between 75 and 130 degrees Fahrenheit. Excessive heating might cause the emulsion to break while still in the distributor.