A tack coat is essentially a type of industrial glue that is used in the pavement construction trade to bind together layers of asphalt on roadways. It is an emulsion that is a liquid-liquid mixture of asphalt binder, water, and emulsifying chemicals that act to mix the ingredients together in a colloidal suspension. This asphalt emulsion is applied in a thin layer between hot mix asphalt (HMA) layers as a roadway surface is built, and serves to create an adhesive bond between them. The tack coat has to cover 90% of the road surface or more to prevent slippage and, if incomplete coverage is done, the roadway will be more prone to long-term degradation effects such as the formation of alligator cracking, rutting, and potholes.
The main ingredient in tack coatings is a bituminous liquid which is a liquified hydrocarbon version of the solid asphalt surface itself. The emulsion may also contain a dilution of water, which is common in certain US applications of the product and serves to provide a more even application over the road surface. In European nations, a more rapid-setting type of tack coat is used that doesn't require the evaporation of water from the emulsion, but, nevertheless, has a low viscosity or thickness to it. In some instances, tack coat is also used that contains a liquid polymer base that serves as a latex binder. These variations create coating materials that can range from slow to rapid setting and low to high viscosity depending on the needs of the road surface being worked on and the local climate.
The thickness for the layer of tack coat that is applied as well as the volume of material that is needed to do an adequate job has been precisely quantified as of 2001 based on the type of road surface on which is being worked. When the road surface is a new construction of HMA, 0.03 to 0.04 gallons per square yard (0.14 to 0.18 liters per square meter) are required. These levels rise slightly if the surface is oxidized HMA, and nearly double for milled HMA at 0.06 to 0.08 gallons/yd2 (0.27 to 0.36 liters/m2).
Tack coat is also used to seal together layers of precast concrete (PCC). Where layers of milled PCC require the same amount of tack coat as milled HMA does, normal PCC requires the same amount of material as oxidized HMA. The amount of tack coat also increases significantly if it is diluted with water beforehand to aid in thorough spreading over the road surface, with the volume of material used being three to four times higher than the usual volume of asphalt binder needed.
Some cases of liquid asphalt application also require the use of a prime coat beforehand, which can reduce dust on the surface. If the road surface is not clean of dust when tack coat is applied, it will bind to the dust instead of to the upper and lower layers of asphalt, which leads to failure of its adhesive properties known as delamination. In some countries, a cutback layer is applied instead of actual tack coat, which is a mixture of asphalt that has been dissolved in hydrocarbons that act as solvents like kerosene, and it is then applied between layers of asphalt to bind them together.