Tack coat refers to asphalt cement binder product that is applied to the asphalt base or intermediate course prior to the application of the next lift of asphalt. It is produced from an asphalt binder base stock and emulsified with water to allow for spray application. The purpose of Tack Coat is to encourage bonding between asphalt lifts and promotes achievement of maximum strength of the asphalt structure and should cover approximately 90% of the surface after application. Tack coat also decreases chances of delamination happening. Applying too much tack coat can be detrimental for 2 reasons; 1. it will act as a lubricant creating slippage, 2. too much material can also actually bleed into the lifts above and below affecting the delicate makeup of the mix design.
A rapid setting tack includes polymer modified emulsions and is often abbreviated with an RS. Slow setting Tack Coat emulsions are typically abbreviated with an SS prefix. Cleanliness of the substrate is critical for bonding of the Tack material, an unclean surface can lead to development of a slippage plane, slippage cracks, and tearing of the lifts of asphalt. Slow setting emulsions are typically not used in cold weather, at night, or when there is limited amount of time for construction to occur. The paving crew will need to wait until the tack has ‘broken’ before they should pave. This is typically indicated from a color change of the material from brown to black. Also it is important to wait for the Tack to Set before foot traffic or vehicle traffic enter the surface as the material can be tracked onto existing pavement by way of tires and foot traffic. Rubberized Tack is commonly used on concrete base to promote a better adhesion between the 2 dissimilar materials. Tack coat is typically applied with an Asphalt ‘distributer’ trailer or more commonly trucks. The distributer truck is equipped with an insulated tank with a heating system, a control system, and a spray bar. The spray bar is in the rear of the machine and has several nozzles pointing downward towards the pavement. The spray bar often requires self heating or heating with a propane torch to open up the nozzles and to produce good flow.
Tack coat application has really become an exact science that requires sophisticated elements working in equilibrium for proper application and coating. Most distribution trucks are equipped with hand sprayers to get to hard to reach locations. It is important to have the correct nozzle size installed to correspond with the correct grade and type of asphalt product. The height of the spray bar must be just right so that gravity and wind distortion does not affect the application rate. A common application rate for Tack Coat is 0.04 Gallons Per Square yard for a new pavement surface and 0.07 Gallons Per Square Yard for a milled pavement surface.