Note: This section is largely a synthesis of the following two tack coat publications:
- Flexible Pavements of Ohio. (May 2001). Technical Bulletin: Proper Tack Coat Application. Flexible Pavements of Ohio. Columbus, OH.
- Texas Department of Transportation (TxDOT). (2001). Technical Advisory: Proper Use of Tack Coat. Technical Advisory 2001-1. Texas Department of Transportation. Austin, TX.
A tack coat is a thin bituminous liquid asphalt, emulsion or cutback layer applied between HMA pavement lifts to promote bonding. Adequate bonding between construction lifts and especially between the existing road surface and an overlay is critical in order for the completed pavement structure to behave as a single unit and provide adequate strength. If adjacent layers do not bond to one another they essentially behave as multiple independent thin layers – none of which are designed to accommodate the anticipated traffic-imposed bending stresses. Inadequate bonding between layers can result in delamination (debonding) followed by longitudinal wheel path cracking, fatigue cracking, potholes, and other distresses such as rutting that greatly reduce pavement life (TxDOT, 2001).
Tack coats should be applied uniformly across the entire pavement surface and result in about 90 percent surface coverage. In order for this uniformity to be consistently achieved, all aspects of the application must be considered and carefully controlled. Specific aspects are:
- The condition of the pavement surface receiving the tack coat
- The application rate
- Tack coat dilution
Condition of the Pavement Surface Receiving the Tack Coat
The pavement surface receiving the tack coat should be clean and dry to promote maximum bonding. Emulsified tack coat materials may be applied to cool and/or damp pavement, however, the length of time needed for the set to occur may increase (Flexible Pavements of Ohio, 2001). Since existing and milled pavements can be quite dirty and dusty, their surfaces should be cleaned off by sweeping or washing before any tack coat is placed, otherwise the tack coat material may bond to the dirt and dust rather than the adjacent pavement layers. This can result in excessive tracking of the tack coat material. Construction vehicles and equipment pick up the tack-dirt mixture on their tires and leave the existing roadway with little or not tack coat in the wheelpaths (Figure 4).Slippage cracking and delamination are distresses typically seen when cleanliness is lacking (Flexible Pavements of Ohio, 2001).
Tack coat application should result in a thin, uniform coating of tack coat material covering approximately 90 percent of the pavement surface (Flexible Pavements of Ohio, 2001). To achieve this result, application rate will vary based on the condition of the pavement receiving the tack coat. Too little tack coat can result in inadequate bonding between layers. Too much tack coat can create a lubricated slippage plane between layers, or can cause the tack coat material to be drawn into an overlay, negatively affecting mix properties and even creating a potential for bleeding in thin overlays (Flexible Pavements of Ohio, 2001). Table 1 shows recommended application rates from Flexible Pavements of Ohio (2001).
Table 1. Recommended Emulsion Tack Coat Application Rates from Flexible Pavements of Ohio (2001)
|Existing Pavement Condition||Application Rate * (gallons/sy)|
|New Asphalt||0.03 – 0.04||0.05 – 0.06||0.10 – 0.12|
|Oxidized Asphalt||0.05 – 0.06||0.08 – 0.09||0.15 – 0.18|
|Milled Surface (Asphalt)||0.05 – 0.06||0.08 – 0.09||0.15 – 0.18|
|Milled Surface (PCC)||0.04 – 0.05||0.06 – 0.08||0.12 – 0.15|
|PCC||0.04 – 0.05||0.06 – 0.08||0.12 – 0.15|
Residuals: The application rate of just the asphalt binder content of the emulsion
Undiluted: The application rate of the undiluted emulsion
Diluted 1:1: The application rate of an emulsion diluted 1:1 with water
Basic application rate considerations are:
- Roughness of the pavement surface receiving the tack coat. Rough surfaces require more tack coat than smooth surfaces. For instance, milling produces a rough, grooved surface, which will increase the existing pavement’s surface area when compared to an ungrooved surface. The surface area increase is dependent on the type, number, condition and spacing of cutting drum teeth but is typically in the range of 20 to 30 percent, which requires a corresponding increase in tack coat (20 to 30 percent more) when compared to an unmilled surface (TRB, 2000).
- Distributor vehicle. Several vehicle-related adjustments and settings are critical to achieving uniform tack coat placement. Essentially the nozzle patterns, spray bar height and distribution pressure must work together to produce uniform tack coat application. Generally, the best applications results from a “double lap” or “triple lap” coverage. “Double/triple lap” means that the nozzle spray patterns overlap one another such that every portion of the pavement receives spray from exactly two/three nozzles (Figure 5). Specific guidance follows:
- Nozzle spray patterns should be identical to one another along a distributor spray bar. Differing coverages will result in streaks and gaps in the tack coat.
- Spray bar height should remain constant. As tack coat is applied, the vehicle will become lighter causing the spray bar to rise. The tack coat application vehicle should be able to compensate for this. Excessively low spray bars result in streaks, while excessively high spray bars cause excessive nozzle overlap resulting in an excessive application rate.
- Pressure within the distributor must be capable of forcing the tack coat material out the spray nozzles at a constant rate. Inconsistent pressure will result in non-uniform application rates.
- Temperature within the distributor should be maintained between about 24°C (75°F) and 54°C (130°F). Excessive heating may cause the emulsion to break while still in the distributor.
Figure 5. Tack coat spray patterns: good and bad.
Sometimes emulsified asphalt tack coats are diluted with water to increase the total volume of liquid while maintaining the same volume of asphalt binder within the emulsion. This can help achieve a more uniform application without applying excessive amounts of asphalt binder. Dilution should be avoided if possible due to the various problems that can result from improper dilution (e.g., emulsion is excessively diluted and dilution causes premature emulsion break). Other methods such as adjusting nozzle opening size or tack coat application pressure should be investigated before attempting dilution. If dilution must be done, Flexible Pavements of Ohio recommends/notes the following (2001):
- Dilution should be accomplished by the emulsion supplier and not the contractor.
- Dilution will increase emulsion break and set time.
- Dilution should be done on a 1:1 basis.
- Dilute only by adding water to the emulsion and not vice versa, which could cause the tack to break.
- Diluted emulsions should be applied at a greater rate than undiluted emulsions to obtain the same residual asphalt coverage. Residual asphalt is the asphalt binder left over after the emulsion has set.
Other Tack Coat Aspects
Although not critical for to the uniform application of tack coat or its subsequent bonding ability, there are several other noteworthy aspects to tack coats.
Generally, a tack coat should be allowed enough time to break and set (emulsion) or cure (cutback) before applying the next layer of HMA.
Tracking is the pick-up of tack coat material by vehicle tires. Tracking deposits tack coat material on adjacent pavement surfaces. Although this material is unsightly, it generally has little effect and wears away quickly. In extreme cases, tracking may deposit enough tack coat material to distort pavement surfaces or hinder a driver’s ability to navigate (Flexible Pavements of Ohio, 2001). Rubberized tack coats have an especially high propensity to stick to vehicle tires. Allowing tack coats to set (emulsions) or cure (cutbacks) before driving on them can substantially reduce tracking.
Traffic on Tack Coats
Generally, traffic should not be allowed on tack coats. When a tacked road surface is exposed to traffic, the potential exists for reduced skid resistance, especially during wet weather (Flexible Pavements of Ohio, 2001). When tack coat surfaces must be opened to traffic, they should be covered with sand to provide friction and prevent pick-up. A typical rate for applying sand cover aggregate is 4 to 8 lbs/yd2 (Flexible Pavements of Ohio, 2001).
Tack Coat Survey from VDOT May 28, 2008
- Does your DOT require a tack coat material to be placed prior to overlay on an unmilled surface? If yes, what tack coat materials are used? Do you specify a bond strength or an application rate?
- Does your DOT require a tack coat material to be placed prior to overlay on a milled surface? If yes, what tack coat materials are used? Do you specify a bond strength or an application rate?
- Has your DOT done any research on bond strength? If so, are you willing to share this research?
- Contact person if other than e-mail respondent (Name, e-mail address, phone number)?
- Texas Department of Transportation (TxDOT). (2001). Technical Advisory: Proper Use of Tack Coat. Technical Advisory 2001-1. Texas Department of Transportation. Austin, TX.↵
- Flexible Pavements of Ohio. (May 2001). Technical Bulletin: Proper Tack Coat Application. Flexible Pavements of Ohio. Columbus, OH.↵
- Transportation Research Board (TRB). (2000). Hot-Mix Asphalt Paving Handbook 2000. Transportation Research Board, National Research Council. Washington, D.C.↵