Tack coat application description and what factors are considered based on application. As a continuing effort posting the various specifications gives a view of how various state agencies approach and descriptions for construction purposes. In reading them you can see what factors are important to produce a long lasting pavement. The State of Missouri Department of Transportation State specifications for Tack Coat. This has been edited for brevity in order to show what they consider but not contract measurement items. Tack Coat This item is used for impervious surfaces such as portland cement concrete, asphaltic concrete or bituminous surfaces prior to resurfacing to improve the bond between the existing surface and the roadway pavement. In contracts the grade of emulsified asphalt is selected by the contractor. If the tack coat is too heavy or too light, an inadequate bond is created and the mat may slip during compaction, which results in shoving and checking. In addition, a slippage failure (cracking) may occur after the pavement has been subjected to traffic. Bleeding may also occur if the tack coat is too heavy. The tack coat must be uniformly applied to the existing surface at the rate specified in the contract. Generally, the application rate between the existing pavement and the first lift should be in the range of 0.05 to 0.10 gal/yd2. It is strongly recommended to apply a light tack coat between each lift of bituminous pavement. The application rate between lifts should be in the range of 0.02 to 0.05 gal/yd2. Depending on the condition of the existing surface, the application rate may need to be adjusted from the specified rate. This is acceptable as long as approval is obtained from the RE and the rate is within the applicable range mentioned above. For example, an open textured surface, such as an SP250 mix, requires more tack than a surface that is tight or dense, such as an SP125 mix. A milled surface requires additional tack because of the increased surface area (from the grooves left by the cutting teeth on the milling machine). A dry, aged pavement also requires a heavier tack coat than a newer pavement. These conditions, and any other possibilities, must be considered when a tack coat is applied. The tack coat must be allowed to break (cure) prior to spreading mix. Shortly after the tack coat has been applied, its color will change from brown to black as the water in the emulsion begins to evaporate. The emulsion also becomes “tacky” to the touch. The amount of time it takes for the tack coat to break depends on the type and grade of emulsion used, the application rate, the temperature of the existing surface, and the environmental conditions. The tack coat is said to have “set” once all of the water in the emulsion has evaporated. If there is reason to believe that the tack coat is being diluted with a material other than water or that the dilution rate is other than what has been reported, a sample should be taken and shipped to the Central Laboratory. Tack It is extremely important that the tack coat remains on the existing surface in order to create an adequate bond between the existing surface and the mat. The tack coat should be allowed to set before it is subjected to construction traffic. Otherwise, the vehicle tires will pick up the tack. At the very least, the amount of construction traffic, including haul trucks, should be minimized. The tacked surface should be covered with mix the same day. If this is not possible because of equipment problems (plant or paver breakdowns), sand must be lightly distributed over the tacked surface before opening to traffic. This will “blot” the tack, preventing it from being picked up by the traffic, and provide friction to the surface. When paving resumes, the excess sand must be removed before the mix is spread. Maintenance Tacking or Tack Coating Maintenance uses tacking or tack-coating by applying a light spray application of cutback or emulsified asphalt to impervious surfaces such as Portland cement concrete, asphaltic concrete, or bituminous surfaces prior to resurfacing. Purpose The purpose of the tack coat is to improve the bond between the existing surface and the roadway pavement. A tack coat shall be applied to provide a bond between old and new wearing courses. The best results occur when the tack is applied to a dry and clean surface, free of loose material. Application Rates The application rate will vary from 0.03 to 0.15 gal/yd2, depending on the condition of the old pavement surface. The pavement course surface should be evaluated to determine the amount of tack to be applied. A viscous material should be used because very little penetration of the asphaltic oil into the pavement surface is expected. The tack coat should be allowed to become tacky or sticky before the surface course is laid. Emulsions are recommended for tacking on heavily traveled routes. Too much tack can create a slippage plane between the old pavement and the new pavement overlay as the tack coat acts as a lubricant instead of an adhesive. Also too much tack could result in bleeding of the tack through to the new overlay surface, which can produce a slick pavement condition. After application of the tack, time must be allowed for the tack to break. Breaking is the phenomenon when the asphalt and water in the emulsion separate, beginning the curing process (brown to black color). Traffic should be kept off the tacked area. The original content from the Missouri Department of Transportation can be found at the following link.http://epg.modot.org/index.php?title=Category:407_Tack_Coat
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