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F-1 F-1 Appendix F Tack Coat Training Manual

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F-3 F-3 Contents F-5 The Purpose of a Tack Coat F-5 Types of Tack Coat Materials Performance Graded Asphalt Tack Coat Materials, F-5 Asphalt Emulsion Tack Coat Materials, F-5 Cutback Asphalt Tack Coat Materials, F-7 F-7 Conditions of the Existing Pavement Surface Dust and Dirt, F-7 New Pavement Surface, F-8 Old, Aged Asphalt Concrete Pavement Surface, F-8 Texture of the Pavement Surface, F-8 Milled Asphalt Concrete Surface, F-9 Bleeding Surface, F-9 Portland Cement Concrete Surface, F-10 F-10 Pavement Conditions and Residual Tack Coat Rate Dusty or Dirty Pavement Surface, F-10 New Asphalt Pavement Surface, F-10 Old, Aged Pavement Surface, F-10 Surface Texture, F-10 Open-Graded Asphalt Pavement Surfaces, F-11 Milled Asphalt Pavement Surface, F-11 Bleeding Surface, F-11 Portland Cement Concrete Pavement Surface, F-11 F-11 Application Rate Versus Residual Asphalt Binder F-13 Asphalt Distributors Parts of an Asphalt Distributor, F-13 Asphalt Tank, F-13 Tack Coat Material Temperatures, F-13 Cleaning the Distributor Tank, F-13 Distributor Pump, F-13 Spray Bar Nozzle Angle, F-14 Spray Bar Height, F-16 Spray Bar Nozzle Size, F-16 Distributor Truck Inspection, Calibration, and Certification, F-18 Blocked Nozzles, F-18 Hand Wand Application, F-18 Summary, F-19 F-20 Tack Coat Break Time and Set Time Type of Tack Coat Material, F-20 Factors Affecting the Break and Set Times, F-21

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F-4 F-22 Construction Problems Uniformity of Tack Coat Application, F-22 Pick Up of Tack Coat Material on Truck Tires, F-24 Paving Over an Unbroken Emulsion, F-26 Spray Pavers, F-26 F-26 Types of Tack Coat Failures Inadequate Bond, F-27 Delamination of the Pavement Layers, F-27 Sliding Failures, F-28 F-28 Measuring Tack Coat Material F-28 Characterization of the Interface Shear Strength F-29 Summary F-30 Closure F-30 References

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F-5 The Purpose of a Tack Coat PG binder incorporated into the asphalt mixture. For exam- ple, if PG 64-22 is used in the mix, the same grade of material The primary purpose of a tack coat is to enhance the bond is typically used for the tack coat material. between two asphalt concrete pavement layers. As used in If a polymer-modified asphalt, such as a PG 76-22, is this manual, the term asphalt concrete is applied to both hot required in the asphalt mixture, in most cases, the tack coat mix asphalt (HMA) mixtures and warm mix asphalt (WMA) will be a different PG material, which is not polymer-modified. mixtures. A tack coat also serves to ensure acceptable bond This is done primarily to reduce cost. In this example, the tack when a new asphalt pavement layer is placed over a Portland coat material would most likely be PG 64-22 in lieu of the Cement Concrete (PCC) surface. PG 76-22. A good bond between pavement layers is necessary in A polymer-modified PG asphalt is sometimes used as a tack order for traffic loads applied to the pavement surface to be coat material. In most cases, this use is related to pavement loca- transmitted down through the whole pavement structure. tions where there is substantial stopping or turning traffic applied If the surface layer is not properly bonded to the underly- to the new asphalt concrete pavement surface. Polymer-modified ing pavement layer, horizontal shear forces at the interface between the layers will increase the tendency for cracking, binders are often used as a tack coat for thin lift asphalt con- debonding, and fatigue failure to occur in the upper portion crete pavement surface layer construction. of the pavement structure. The tack coat and the bond cre- ated between the layers allow the various courses within the Asphalt Emulsion Tack Coat Materials pavement structure to act as a whole. If a proper bond is not established between the existing pave- Types of Emulsions: Asphalt emulsions are divided into ment surface and the new asphalt pavement layer, delamina- three categories. Those three categories are anionic, cationic, tion may occur between the layers. This will result in a slippage and nonionic. An anionic emulsion has a negative electrical or sliding failure of the new mix on top of the existing pave- charge and a cationic emulsion has a positive electrical charge ment surface. Thus, in order to construct a durable, long- in a zeta potential test. If the letter "C" is placed in front of lasting asphalt concrete pavement, it is very important to apply the emulsion grade, the emulsion type is cationic. If the letter the proper type and amount of tack coat between the new and "C" is not shown in front of the emulsion grade, the emulsion old pavement layers. type is anionic. Nonionic emulsions are not generally used for pavement construction. For use as tack coat, the selection of anionic or cationic emulsion is generally not significant Types of Tack Coat Materials due to the relatively very small amount of emulsion applied Three basic types of asphalt materials can be used for a tack to the existing pavement surface. coat. Those three materials include asphalt emulsion, perfor- Emulsions are divided into three additional categories mance graded (PG) type asphalt cement binder, and cutback depending on how quickly the asphalt will coalesce or revert asphalt. By far, the most common type of material used for back to the form of an asphalt cement. Those three additional tack coats is asphalt emulsion. Cutback asphalts, which are categories are rapid set (RS), medium set (MS), and slow set combinations of asphalt cement and a petroleum-based dilu- (SS) emulsions. MS emulsions can additionally be classified ent (cutter stock) material, such as naphtha or kerosene, are as "HF" or high-float. In HF emulsions, the emulsifier forms rarely used today in the US due to environmental consider- a gel structure in the asphalt residue. The thicker asphalt ations related to the evaporation of the cutter stock material. film allows HF emulsions to perform in a wider temperature PG asphalt binder is used in some jurisdictions for tack coat. range. Further, some emulsions are graded with the letter "h" following the emulsion classification. The "h" means that harder base asphalt has been used in the emulsion. Performance Graded Asphalt Tack Asphalt emulsion consists of a blend of three different Coat Materials materials. The majority of the emulsion is asphalt cement, PG type asphalt binders consist of one hundred percent typically between 55 and 70 percent of the total weight of the asphalt cement without any added water or diluent material. emulsion. Water is the second largest ingredient, typically, Thus, if a PG asphalt is employed as the tack coat material, all from 44 to 29 percent of the total weight of the emulsion. The of the material that is applied to the existing pavement sur- remaining material is the emulsifying agent. face is useful in achieving the bond between the old and new SS emulsion is most often used as tack coat. SS-1, SS-1h, layers. For PG type asphalt, therefore, the residual asphalt CSS-1, and CSS-1h are four types of slow set emulsions. For binder rate and the application rate are the same. anionic asphalt emulsions, the minimum required amount When the tack coat consists of a PG asphalt, the grade of of residual asphalt binder in the emulsion is given in ASTM the PG material used is usually the same as the grade of the Standard Specification D 977. The minimum residual asphalt

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F-6 amount is 57 percent for both SS-1 and SS-1h emulsions. For a particular pavement surface is required to be 0.06 gallons cationic asphalt emulsions, the minimum amount of resid- per square yard, the application rate of the diluted asphalt ual asphalt binder is found in ASTM Standard Specification emulsion would need to be 0.18 g/sy. For a 1:1 diluted emul- D 2397. That residual asphalt amount is also 57 percent for sion, the application rate is 3.0 times greater than the residual both CSS-1 and CSS-1h emulsions. rate of the emulsion, or 0.06 g/sy times 3.0 = 0.18 g/sy. For a Other types of asphalt emulsions are sometimes used as a 1:1 diluted asphalt emulsion, if the required residual amount tack coat. Those emulsions include rapid setting (RS) emul- of asphalt binder in the tack coat is intended to be 0.04 g/sy, sions: RS-1, RS-2, as well as CRS-1 and CRS-2. It is noted the application rate for that emulsion would be 0.12 g/sy (or that the minimum binder content required by the ASTM 0.04 3.0 = 0.12). standards are shown to be 55 percent for RS-1, 63 percent for Most often, dilution of the asphalt emulsion occurs at RS-2, 60 percent for CRS-1, and 65 percent for CRS-2. the terminal of the emulsion supplier. This is the preferred location since the dilution rate can be carefully controlled. On occasion, a contractor will purchase an undiluted emul- Calculation of the Application Rate for Emulsion sion and add the water to the undiluted emulsion when that It is the residual asphalt binder that creates the bond material is in the tank of the asphalt distributor. Although between the pavement layers. To calculate the application this can be done, it is important that the proper amount of rate for an asphalt emulsion tack coat material, the starting water be added to the undiluted emulsion so that the appli- point is the required residual tack coat amount. The calcula- cation rate and the residual rate of the diluted emulsion are tions must work backward from the residual amount of tack correct. Therefore, it is important that a sample of the emul- coat to arrive at the application rate for the same tack coat sion, whether undiluted or diluted, be taken from the asphalt material. Based on a constituent ratio of 2/3 asphalt binder distributor prior to first use to assure that the proper residual and 1/3 water, the required application amount of asphalt in amount of asphalt binder material is actually present on the an asphalt emulsion will be 1.5 times greater than the residual pavement surface once the water in the emulsion evaporates. amount. For example, if the residual amount of asphalt binder on Polymer-Modified Asphalt Emulsion an existing pavement surface is 0.06 gallons per square yard (g/sy), the application rate of the asphalt emulsion will need If a polymer-modified asphalt emulsion is to be used as tack to be 0.09 gallons per square yard. For an undiluted asphalt coat, the residual tack coat rate will be the same as for a non- emulsion, the application rate is 1.5 times greater than the polymer-modified asphalt emulsion. The use of a polymer- residual rate of the emulsion, or 0.06 g/sy times 1.5 = 0.09 g/sy. modified emulsion may be justified for pavement locations For an undiluted asphalt emulsion, if the required residual where there is a substantial amount of stopping and/or turn- amount of asphalt in the tack coat is required to be 0.04 g/sy, ing traffic applied to the new asphalt pavement surface. the application rate for that emulsion would be 0.06 g/sy (or 0.04 1.5 = 0.06). Trackless Tack Coat Emulsion A polymer-modified asphalt emulsion has been developed Diluted Asphalt Emulsions which incorporates a hard base asphalt binder (low penetra- Many SS asphalt emulsions are diluted with additional tion asphalt cement) as part of the emulsion. Hard base asphalt, water before they are sprayed onto the existing pavement combined with the polymer additive, reduces the amount of surface as a tack coat. The primary reason for diluting emul- tracking that might occur on the tires of the haul trucks as well sion is to provide for a more uniform application of the tack as the tire or tracks of the asphalt paver. The residual asphalt coat material. The greater volume of the diluted emulsion binder in the trackless tack coat material is similar to that of provides a more consistent and uniform spray pattern from a standard asphalt emulsion. Thus, the application rate for the nozzles on the distributor. The most common dilution this material would be similar to that of a normal, undiluted rate is a 1:1 (50% : 50%) ratio of SS asphalt emulsion and asphalt emulsion. additional water. This results in a material that is one part It is noted that the trackless tack coat material typically asphalt emulsion and one part additional water. breaks and sets faster than a standard asphalt emulsion. This Based on the assumption that an undiluted emulsion con- change in both the break time and the set time of the trackless sists of 2/3 asphalt binder and 1/3 water, an asphalt emulsion tack coat can significantly reduce the amount of tracking that that is diluted 1:1 with additional water will have residual occurs on the tires of the construction traffic. (Note: Break asphalt binder that is only 1/3 of the weight of the diluted and set times are defined in the section titled, "TACK COAT emulsion. Thus, if the residual amount of asphalt binder on BREAK TIME AND SET TIME.")

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F-7 During the research that led to this manual, trackless tack would be approximately 0.07 g/sy (or 0.04 1.7 = 0.068 g/sy, products performed exceptionally well in that they provided rounded to 0.07 g/sy). excellent adhesion and strong shear resistance between the tacked layers. Conditions of the Existing Pavement Surface Cutback Asphalt Tack Coat Materials The application rate of a tack coat should vary depending on Types of Cutback Asphalt Materials the conditions of the pavement surface being overlaid. What is really important is not the application rate of the tack coat Cutback asphalt is a combination of asphalt binder and a material, but the residual rate of the tack coat or the amount diluent material, sometimes called petroleum distillate or cut- of asphalt binder that remains after the water has evaporated ter stock. The three primary types of cutback asphalt are dif- out of the asphalt emulsion or the diluent has evaporated out ferentiated by the relative speed of evaporation of the dilutent of the cutback asphalt material. The actual application rate used: rapid curing (RC), medium curing (MC), and slow cur- must be back-calculated starting from the residual rate. ing (SC). RC materials typically contain gasoline or naphtha The objective is to apply a sufficient quantity of tack coat, as the diluent material. MCs use kerosene. SCs contain diesel which results in a thin, uniform coating of asphalt binder or fuel oil. material over the existing pavement surface. Coordinating Cutback materials have occasionally been used for tack coat the residual tack coat rate, and thus the actual application applications. Typically, RC-70, MC-30, MC-70, or SC-70 is rate, of the tack coat to the conditions of the pavement sur- employed. For tack coat use, cutback asphalts are not diluted face is extremely important. and are thus used full strength. Due to the low flash point (thus The residual rate of tack coat needed, and thus, the actual fire danger) and environmental concerns (volatile organic application rate, depends on the conditions of the existing compound emissions), cutbacks are not recommended. For pavement surface including: these reasons, many state DOTs have prohibited the use of cutback asphalts. 1. Dusty or dirty pavement surface. 2. New pavement surface. Residual Amount of Binder in a Cutback Asphalt 3. Old, aged pavement surface. 4. Texture of the pavement surface. The minimum amount of asphalt binder required by ASTM 5. Milled asphalt pavement surface. Specification D 2028, Standard Specification for Cutback 6. Bleeding surface. Asphalt (Rapid Curing Type) for a RC-70 cutback is 55 per- 7. Portland cement concrete surface. cent. Typically, RC-70 will consist of approximately 60 percent asphalt and 40 percent cutter stock. Dust and Dirt If the pavement surface is dusty or dirty, it must be cleaned Calculation of the Application Rate in order to prevent the new asphalt pavement surface from slid- for Cutback Asphalt ing or delaminating at the dusty/dirty interface. Tack coat must Similar to the calculations for asphalt emulsion, to deter- be applied to a clean surface. Further, when using either a PG mine the application rate for cutback asphalt, the starting asphalt tack coat or a cutback asphalt tack coat, the pavement point is the required residual tack coat amount. Calcula- surface must be dry. Cleaning operations can be accomplished tions must work backward from the residual amount of tack either by mechanical brooming or by flushing the existing sur- coat to arrive at the application rate for the same tack coat face with water or blowing off debris using high-pressure air. material. If the asphalt pavement or PCC pavement surface is dusty or If the residual amount of asphalt tack coat on a particular dirty, there will be a tendency for the new asphalt concrete sur- pavement surface should be 0.06 gallons per square yard, the face to slide or slip (delaminate) at the dusty interface. This type application rate of the cutback asphalt will need to be approx- of bond failure is shown in Figure 1. Sliding failures occur most imately 0.10 gallons per square yard. For RC-70, the applica- often at locations where traffic decelerates, such as stop signs or tion rate is about 1.7 times greater than the residual rate of traffic signals. Sliding failures also occur where traffic acceler- the emulsion, or 0.06 g/sy times 1.7 = 0.102 g/sy, rounded ates or where traffic makes tight turning maneuvers. to 0.10 g/sy. If the residual amount of asphalt tack coat is The residual tack coat application rate should not be changed required to be 0.04 g/sy, and cutback asphalt is employed in order to compensate for a dusty or dirty pavement surface. for the tack coat material, the application rate for the RC-70 A heavier residual tack coat application rate may increase a

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F-8 layer of asphalt mixture was placed yesterday and the next layer is placed over that surface today, a tack coat is sometimes thought to be unnecessary between the two new layers of mix. This recommendation, however, assumes that the underlying surface is clean when the overlay is placed. If traffic, includ- ing construction traffic, travels over the bottom layer and the bottom layer becomes dusty or dirty for some reason, it will be necessary to clean the surface of the bottom layer and apply a tack coat to the cleaned surface. Results from NCHRP Project 9-40 indicated that tack coat is needed between two new asphaltic concrete pavement layers. If a tack coat is applied, the residual rate should be reduced to compensate for the lack of absorption of the tack coat material into the new asphalt concrete pavement layer. In most cases, the residual rate should be approximately one-half the amount applied to an old, oxidized pavement surface. Old, Aged Asphalt Concrete Pavement Surface If the asphalt concrete pavement surface contains an exten- sive number of cracks, a portion of the tack coat material may flow into the cracks and not be available to create the bond between the pavement layers. Significant flow of tack coat into cracks is usually a problem only when diluted emulsion is used. In this situation, the residual tack coat rate may need to be increased slightly in order to account for the loss of tack material into the pavement cracks. To avoid the potential for tack coat flow, consider the use of undiluted emulsion or PG Figure 1. Delamination type of sliding failure. asphalt, being sure to utilize the appropriate nozzle size to ensure proper coverage. Care must be taken that the amount of the residual tack coat in the non-cracked areas is not so potential sliding failure problem. The dust coating will create heavy as to create a slip plane or bleeding at those locations. a slip plane and any added, excess residual tack coat will fur- If the existing asphalt concrete pavement surface is highly ther weaken the bond between the layers and thus make the oxidized and brittle, a slightly higher residual tack coat rate problem worse. The only remedy for a dusty or dirty pavement may be needed. surface is to clean that surface and remove all loose dust or dirt. A small amount of moisture on the pavement surface Texture of the Pavement Surface from a passing shower probably will not be detrimental to the long-term function of a tack coat. If the amount of the As demonstrated by the results of NCHRP Project 9-40, moisture is minimal, this moisture should be flashed off by the surface texture of the existing pavement has a significant the subsequent hot asphalt mixture overlay. However, if the effect on the residual amount of tack coat needed. If that sur- pavement surface layer is saturated with water and the exist- face has a relatively fine texture, less residual tack coat will be ing pavement surface is damp, the ability of the tack coat required. If that surface has a relatively coarse texture, more material to provide adequate bond between the existing and residual tack coat will be needed. the new pavement layers may be significantly compromised. A pavement surface that has raveled will normally have a rougher surface texture. An old, aged pavement surface will also normally have a rougher surface texture. In both New Pavement Surface cases, it will be necessary to increase the residual tack coat A common perception is that a tack coat may not be needed application rate in order to account for the rougher surface between two new asphalt concrete pavement layers. If one texture.

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F-9 Milled Asphalt Concrete Surface A common perception is that a tack coat may not be needed when a new asphalt concrete pavement layer is placed on top of a milled asphalt pavement surface. It has been suggested that the surface texture of the clean milled surface will provide the amount of roughness and bond necessary between the old pavement and the new asphalt concrete overlay (Figure 2). This roughness may prevent the new asphalt mixture from sliding on the milled pavement surface and thus permit the applied traffic loads to be transmitted from the new overlay to the original, milled, pavement layers. However, results from NCHRP Proj- ect 9-40 indicated that the amount of bond generated between the milled surface and the new asphalt concrete overlay used in that study with no tack coat material was not sufficient to pro- vide an adequate level of shear strength at the interface between the milled surface and the new asphalt concrete overlay. Figure 3. Tack coat and debris picked up by construction traffic from a dirty milled/tacked surface. It is necessary, however, for the dust that is created during the milling operation to be removed to ensure that the milled surface is free of dust before the new overlay is placed, particu- larly the dust in the bottom of the grooves. Aggressive broom- grooves and collecting in the bottom of the grooves. If this ing followed by flushing of the milled surface with water or use situation occurs, the degree of bond achieved will obviously of compressed air is needed to assure that all of the dust has vary from the top of the grooves to the bottom of the grooves been removed prior to tack coat application. If the dust is not and, therefore, decrease the strength of the bond instead of removed from the grooves in a milled surface and a tack coat increasing the strength of the bond with the new asphalt con- is applied, the tack coat material can cause the dust to become crete layer. Use of diluted emulsion as tack coat will exacer- sticky and adhere to the tires of the construction equipment. bate this problem. The sticky material may build up on the tires to the extent that it may be carried off on the tires and become unavailable to Bleeding Surface provide the bond between existing and new pavement layers (Figure 3). Care must be taken when a tack coat is applied to an exist- If a tack coat is applied to a milled asphalt concrete sur- ing pavement surface that is flushed or bleeding. In this situ- face, the residual tack coat rate should be reduced in order to ation, the tack coat application rate must be reduced in order prevent the tack coat material from draining into the milled to take into account the amount of asphalt material already Figure 2. Clean milled surface.

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F-10 on the pavement surface. In addition, the tack coat appli- Dusty or Dirty Pavement Surface cation rate may have to be adjusted for different pavement There is no recommended residual tack coat rate for a surface conditions transversely across a traffic lane. Less tack dusty or dirty existing pavement surface. A dusty or dirty coat may be needed, for example, in the wheel paths of an surface must be cleaned and all of the dust or dirt removed existing pavement surface that is bleeding compared to the before any type of tack coat material is applied to that surface. amount of tack coat needed between the wheel paths and Cleaning can be accomplished using a mechanical broom or along the outside edges of the lane. by flushing the surface with water or using compressed air. A change in the residual tack coat rate across the width of the pavement lane being tacked will necessitate a change in the size of the nozzles on the distributor spray bar at different New Asphalt Pavement Surface locations along the length of the spray bar. In order to apply different amounts of tack coat (and thus different amounts A new asphalt concrete pavement surface will typically not of residual tack coat) at different transverse locations, the absorb a significant amount of tack coat material. A simple test amount of bleeding across the width of the pavement lane can be used to determine the amount of tack coat that might being tack coated must be consistent. If the bleeding areas be absorbed into the new surface. The test consists of pouring a vary longitudinally in width or severity, it will be impossible small amount of water onto the new asphalt pavement surface. to apply the correct amount of residual tack coat in the bleed- If the water simply "beads up" or just runs off, the new surface ing areas compared to the non-bleeding areas. In this case, it will not absorb any significant amount of tack coat material. In would be advisable to mill the surface to remove the bleeding such a case, placement of a tack coat between the new underly- areas and to provide a uniform pavement texture. ing asphalt concrete layer and the new asphalt concrete overlay is probably not necessary. If the water penetrates into the new surface (due to Portland Cement Concrete Surface improper compaction of the new mix, for example), then it The amount of residual tack coat applied to an existing can be assumed that a portion of the tack coat material would PCC pavement surface will depend on two primary factors. penetrate into the new asphalt mixture surface. Because of For most PCC surfaces, the amount of residual tack coat will the tightness of the new surface and the amount of asphalt be the same as for an asphalt concrete pavement surface that binder in that surface, the amount of residual asphalt binder is in relatively good condition. In general, no increase in the in the applied tack coat normally needs to be significantly less residual tack coat rate is required to account for the joints or than the amount needed for an old, aged pavement surface. cracks in the PCC surface. For a new, clean asphalt concrete pavement layer placed one If the PCC surface has been diamond ground, a slight day, and a second layer to be placed within a day or two, the increase in the tack coat residual rate may be necessary due residual tack coat rate on the pavement surface should be in to the increased texture of the diamond ground surface. If the range of 0.03 to 0.04 g/sy. the PCC surface has been milled, the milled surface should be cleaned, as described above, for the milled asphalt concrete Old, Aged Pavement Surface surface, and a tack coat should be applied. An old, aged, oxidized asphalt pavement surface will nor- mally absorb a significant amount of the applied tack coat Pavement Conditions and Residual material. This is particularly true when using a diluted asphalt Tack Coat Rate emulsion but not normally an issue when using PG asphalt as Table 1 presents a summary of the range of the residual tack coat. In order to have enough tack coat remaining on the asphalt binder application rates for the various pavement pavement surface to create an adequate bond between the old surface types. A detailed discussion for each pavement sur- and new pavement layers, the residual tack coat rate will have face type is given below. to be increased. In general, the residual amount of tack coat material should be in the range of 0.04 to 0.06 g/sy, Table 1. Table 1. Typical residual asphalt binder for tack coats. Surface Texture Condition of the Existing Pavement Surface Residual Asphalt Binder Dusty or Dirty Clean the Surface An asphalt concrete pavement surface that has a fine surface New Asphalt 0.03 to 0.04 g/sy Old, Aged Asphalt 0.04 to 0.06 g/sy texture will normally require a lower residual tack coat rate than Milled Asphalt 0.03 to 0.05 g/sy an asphalt concrete pavement surface that has a coarser surface Portland Cement Concrete 0.04 to 0.06 g/sy texture. Further, if the existing pavement surface is raveled, a

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F-11 greater residual tack coat rate will be needed to compensate Bleeding Surface for the increase in the surface area due to the rough texture. In Rarely is the amount of bleeding or flushing that occurs on addition, if the existing pavement surface is extensively cracked, the surface of an asphalt concrete pavement uniform either a greater amount of residual tack coat will be needed. in the transverse direction or the longitudinal direction. In Because the variation of surface texture of an existing asphalt most cases, the amount of bleeding is much greater in the pavement surface can be significant due to a wide range of sur- wheel paths of the roadway as compared to the pavement areas face issues, the range of residual tack coat rates is also greater. between the wheel paths or outside the wheel paths. This sig- In general, the residual tack coat range would be from 0.04 to nificant difference in the condition of the pavement surface at 0.08 g/sy. different locations makes it extremely difficult to determine the amount of residual tack coat material that is needed to pro- Open-Graded Asphalt Pavement Surfaces vide the proper bond between the new overlay and the exist- ing bleeding pavement surface across the width and down the A new open-graded asphalt pavement (OGAP) surface is length of the roadway. generally much more open than the surface of an OGAP sur- If the existing pavement surface is bleeding, the best approach face that has been used by traffic for a number of years. In addi- is to mill that surface and remove the excess binder material. tion, some OGAP surfaces have been clogged with dust and dirt If milling is not needed to correct the grade or cross slope of with time and traffic, and the air void content of the mix has the existing pavement structure, the depth of milling can be been reduced significantly due to the amount of dust and dirt minimal--the depth of the milling can be limited to inch that may have accumulated in the pores of the mix. It is, there- or even less. If the asphalt concrete mix is unstable, however, fore, very difficult to predict the amount of residual tack coat and is the cause of the bleeding, the deficient layer should be material that is needed to create the bond between the existing entirely removed. OGAP surface and the new asphalt concrete overlay. Thus, no It is possible to use different size nozzles on the asphalt residual tack coat material rate is suggested in this manual. distributor spray bar to apply different amounts of tack coat More importantly, it has been found that overlaying an material at different transverse locations across the width of the OGAP surface with a dense-graded asphalt pavement layer pavement lane being overlaid. This is feasible, however, only if has led to early failure of the new overlay due to the amount the bleeding areas are consistent in width and length. In the vast of water that may accumulate in the now underlying open- majority of the cases related to the overlay of an existing asphalt graded layer. Water can enter the open-graded layer both from pavement that is bleeding, the proper solution to the problem above and below. Many overlaid OGAP layers have experi- is to mill off the bleeding surface rather than attempt to apply enced significant stripping when overlaid with a dense-graded the "correct" amount of residual asphalt tack material to all of asphalt concrete mixture. It is generally recommended, there- the surface locations. Due to significant variation in the amount fore, that the OGAP surface be removed prior to the placement of bleeding that can occur on a pavement surface, it is basically of another asphalt concrete layer. impossible to provide a typical range for the residual asphalt binder in this guide. Milled Asphalt Pavement Surface Portland Cement Concrete Pavement Surface A common perception is that a tack coat may not be needed in order to create a bond between a milled surface of an asphalt In most cases, the residual amount of asphalt binder needed concrete pavement and the new asphalt concrete overlay and for a tack coat applied to a PCC surface is essentially the same that the roughness and exposed new asphalt surface created as that for an old, aged asphalt concrete pavement surface. by the milling operation provides the necessary bond between In general, the residual tack coat rate will be at the lower end the old and new layers. However, results from NCHRP Proj- of the range, usually between 0.04 and 0.05 g/sy. If the PCC ect 9-40 indicated that the roughness of the milled surface was surface has been diamond ground and has relatively high tex- not sufficient to provide the required shear strength at the ture, the residual asphalt tack coat rate should be in the range interface. Thus, a tack coat material will normally be needed. of 0.05 to 0.06 g/sy. If a tack coat is applied to a milled surface, the residual amount of the tack coat should be reduced compared to that Application Rate Versus Residual amount used for an old, aged pavement surface. A typical resid- Asphalt Binder ual tack coat rate for a clean, milled asphalt pavement surface should be in the range of 0.03 to 0.05 g/sy. Excessive residual As discussed above, the residual asphalt binder is the amount tack coat might actually reduce the bond achieved between the of material that actually provides the bond between two differ- milled surface and the new asphalt concrete overlay. ent pavement layers. Thus, the application rate of the asphalt

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F-12 Table 2. Typical application rates using PG asphalt. Condition of the Existing Pavement Surface Applied PG Asphalt Binder Rates New Asphalt 0.03 to 0.04 g/sy Old, Aged Asphalt 0.04 to 0.06 g/sy Milled Asphalt Mixture 0.03 to 0.05 g/sy Portland Cement Concrete 0.04 to 0.06 g/sy APPLICATION RATES USING UNDILUTED ASPHALT EMULSIONS Condition of the Existing Pavement Surface Applied Undiluted Asphalt Emulsion Rates New Asphalt 0.04 to 0.06 g/sy Old, Aged Asphalt 0.06 to 0.09 g/sy Milled Asphalt 0.04 to 0.07 g/sy Portland Cement Concrete 0.06 to 0.09 g/sy APPLICATION RATES USING 1:1 DILUTED ASPHALT EMULSIONS Condition of the Existing Pavement Surface Applied Diluted Asphalt Emulsion Rates New Asphalt 0.09 to 0.12 g/sy Old, Aged Asphalt 0.12 to 0.18 g/sy Milled Asphalt 0.09 to 0.50 g/sy Portland Cement Concrete 0.12 to 0.18 g/sy APPLICATION RATES USING RC-70 CUTBACK ASPHALT Condition of the Existing Pavement Surface Applied Cutback Asphalt Rates New Asphalt 0.05 to 0.07 g/sy Old, Aged Asphalt 0.07 to 0.10 g/sy Milled Asphalt 0.05 to 0.09 g/sy Portland Cement Concrete 0.07 to 0.10 g/sy material must be back-calculated from the desired residual For RC-70 cutback, using an estimated asphalt content of rate of the asphalt binder. 60 percent, the application rate would need to be approxi- In Table 2, the application rates for the four types of materials mately 1.6 times greater than the residual binder rate. Thus, listed are determined based on the following relationships: For for a RC-70 cutback asphalt that is applied to a new asphalt the PG asphalt, the application rate is the same as the residual pavement surface, for a residual application rate of 0.03 to rate. Based on the residual rate for a new asphalt concrete 0.04 g/sy, the required application rate for the cutback asphalt pavement surface of 0.03 to 0.04 gallons per square yard is in the range of 0.05 to 0.07 gallons per square yard. (Table 1), the application rate is the same, as shown in Table 2. It is very important to realize that the tack coat application For the undiluted asphalt emulsion material, the applica- rate MUST be determined by starting at the desired residual tion rate is approximately 1- times more than the residual application rate for the type of asphalt material being used for rate (based on a ratio of the emulsion being 2/3 asphalt binder tack coat and working backward to calculate the actual appli- and 1/3 water, which is a useful approximation). Thus, for cation rate. Table 3 provides a summary of the multiplication an undiluted asphalt emulsion applied to a new asphalt factors that need to be used to determine the application rate pavement surface, for a residual application rate of 0.03 for the four common types of tack coat materials--PG type to 0.04 g/sy, the required application rate for the undi- asphalt binder, undiluted asphalt emulsion, 1:1 diluted asphalt luted asphalt emulsion is in the range of 0.04 to 0.06 g/sy, emulsion, and RC-70 cutback asphalt. or approximately 1- times more than the residual binder. For an emulsion that is diluted 1:1 with water, the applica- tion rate is approximately three times more than the required Table 3. Typical residual rate--application rate residual rate (based on the diluted asphalt emulsion being multiplication factors. approximately 1/3 asphalt binder and 2/3 water). Thus, for a 1:1 Type of Tack Coat Material Multiplication Factor diluted asphalt emulsion applied to a new asphalt pavement PG Type Asphalt Binder 1.0 surface, for a residual application rate of 0.03 to 0.04 g/sy, the Undiluted Asphalt Emulsion 1.5 required application rate for the diluted asphalt emulsion is in 1:1 Diluted Asphalt Emulsion 3.0 RC-70 Cutback Asphalt 1.6 the range of 0.09 to 0.12 gallons per square yard.

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F-20 (a) (b) Figure 12. Improper application of tack coat using a crack sealing bucket. be about 1.7 and, for SC-70, about 2.0. Due to the low 9. All nozzles on the spray bar must be set to the same angle, flash point (thus fire danger) and environmental con- typically 30 degrees to the axis of the bar. cerns (emissions of volatile organic compounds), cut- 10. All nozzles on the spray bar must be the same size, unless backs are not recommended. For these reasons, many bleeding exists in the wheel paths and a different applica- state DOTs have prohibited the use of cutback asphalts. tion rate is needed at those locations. 7. For a PG asphalt tack coat material, the application rate 11. All nozzles must be clean, not blocked, and functioning should be the same as the residual rate. properly. 8. Height of the spray bar must be adjusted to obtain a 12. Size of the nozzles must be selected based on recommen- double or triple lap of the spray. This is recommended to dations of the distributor manufacturer for the desired achieve uniform application and 100 percent coverage of application rate of the particular tack coat material. the residual tack coat material. 13. Speed of the distributor and pump pressure need to be based on recommendations of the distributor manufac- turer and the application rate of the tack coat material. 14. If a hand wand is used, care should be taken to assure that the application rate is as accurate and uniform as possible. A crack sealing bucket is never appropriate for applying tack. 15. Tack coat material should uniformly cover 100 percent of the existing pavement surface. Tack Coat Break Time and Set Time Type of Tack Coat Material Asphalt Emulsion Tack Coat Material As discussed above, asphalt emulsion contains approxi- mately 2/3 asphalt binder and 1/3 water, in an undiluted form. In Figure 13. Improper application of the tack coat a 1:1 diluted form, the emulsion will contain approximately using a hand wand. 1/3 asphalt binder and 2/3 water. In addition, the emulsion will

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F-21 contain a small amount of emulsifying agent, typically, less dilution rate of an asphalt emulsion, and than one percent by weight of the emulsion. type of emulsifying agent used in an emulsion. Immediately after application by a distributor to a pave- ment, the emulsion is brown in color. This color indicates that Asphalt Emulsion Tack Coat Material the material is still in emulsified form, that is, the micron-sized asphalt particles are still suspended in the water. When the One primary factor that affects the break and set times of color of the emulsion changes from brown to black, it is typi- emulsions is the application rate. The higher the application cally stated that the emulsion has "broken." This means that rate, everything else being equal, the longer it will take for the the asphalt particles have separated from the water and two emulsion to both break and set. In addition, use of a diluted distinct phases now exist. When all of the water has evapo- asphalt emulsion will require more time to both break and set rated, it is stated that the emulsion has "set." When the emul- compared to an undiluted emulsion, simply because of the sion has set, all that remains on the pavement surface is the increased amount of water in the diluted emulsion. If a rapid asphalt binder--the water essentially is gone. set (RS) emulsion is used, the break and set times will be shorter The comments in this section related to emulsions apply to than if a slow set (SS) emulsion is used. trackless tack, as it is a polymer-modified asphalt emulsion. In general, the higher the application temperature of the asphalt emulsion, the more quickly the material will break and set. Further, if the ambient air temperature and/or the Cutback Asphalt Tack Coat Material temperature of the existing pavement surface is relatively For RC-70, the cutter stock used is typically naphtha (simi- high, both the break and set times will be shorter. Further, lar to gasoline). When cutback asphalt is applied to the exist- emulsified asphalt will set more quickly on a windy day when ing pavement surface, approximately 60 percent asphalt and compared to a calm day. 40 percent naphtha is in that tack coat material. Different than In most cases, an asphalt emulsion applied as a tack coat, an emulsion based tack coat material, no break time is involved depending on its application rate and dilution rate, will break with a cutback material. There is, however, a set time. in 10 to 20 minutes. This means, as discussed above, that the The set time for the cutback material is the time required color of the tack coat will change from brown to black. Com- for the diluent to evaporate. Once the naphtha is gone, the plete setting of the emulsion typically requires from 30 min- remaining asphalt binder material is said to be "set." utes to more than 2 hours. Unless the tack coat is set, there will be a strong tendency for the tack coat to be picked up on the tires of the trucks delivering the asphalt concrete mix to PG Asphalt Tack Coat Materials the material transfer vehicle or to the paver hopper. As discussed above, if PG asphalt is used as the tack coat material, the residual rate and application rate of the material Cutback Asphalt Tack Coat Material are exactly the same. The PG material is typically applied at a temperature in the range of 280F to 325F. Because the PG One primary factor that affects the break time and set time material does not contain any water (as in an emulsion) or of a cutback material is the application rate. Higher applica- any cutter stock (as in a cutback material), no break or set tion rates require more time for the cutter stock to evaporate times are involved. and thus for the material to set. Typically, the safe time for allowing traffic on a PG tack coat The higher the application temperature of the cutback is the time required for the asphalt to cool to the same tem- asphalt material, the more quickly the material will set. Fur- perature as the pavement surface on which it has been sprayed. ther, if the ambient air temperature and/or the temperature of the pavement surface are high, the cutback asphalt set time will be relatively shorter. Further, a cutback asphalt tack coat will Factors Affecting the Break and Set Times set more quickly on a windy day as compared to a calm day. Many factors affect the break and set times, particularly for In most cases, a cutback asphalt tack coat, depending on an asphalt emulsion. Among the factors are: its application rate and amount of diluent, will set in 10 to 20 minutes. Unless the cutback is set, there will be a tendency ambient air temperature, for the tack coat material to be picked up on the tires of the relative humidity, trucks delivering the asphalt concrete mix to the paving site. wind speed, temperature of the pavement surface on which the tack PG Asphalt Binder coat material is placed, temperature of the tack coat material when sprayed, For a PG asphalt tack coat, the residual rate and the applica- application rate of the tack coat material, tion rate are exactly the same. Because the PG material does not

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F-22 contain any water or any cutter stock, no break time is involved. Safe trafficking time is the time required for the PG asphalt to reach the temperature of the pavement surface on which it has been sprayed. Typical safe trafficking times are in the range of 2 to 5 minutes, depending on environmental conditions. If the application (or residual) rate of the PG asphalt is relatively low (e.g., in the range of 0.04 gallons per square yard), and the material is uniformly applied to the existing pavement surface, there should be very little pick up of the tack coat on the haul truck tires. Construction Problems There are a number of potential problems with placement of a tack coat on an existing pavement surface. The three most common problems are (1) lack of uniformity of the tack coat application, (2) pick up of the tack coat on the haul truck tires and the paving equipment before the tack coat material is set, and (3) the need to pave over an emulsion tack coat before it is broken and/or set. Uniformity of Tack Coat Application It is extremely important that the tack coat material be uniformly applied to the pavement surface, both in a longitu- dinal direction and in a transverse direction. This is to assure that a consistent bond is achieved between the existing pave- Figure 14. Non uniform tack coat spray application. ment surface and the new asphalt concrete pavement layer. Obviously, if the tack coat is applied in one area but not in One or more of the nozzles may be set at an improper angle another area, or in a greater quantity in one area as compared to the axis of the spray bar. One or more of the nozzles may be to an adjacent area, there will be a difference in the degree of of a different size compared to the other nozzles. Truck speed bond attained (Figure 14). and/or pump pressure may be inadequate. Poor uniformity can be due to one or a combination of Figures 15 (a) and (b) depict proper application of a tack several factors. One or more of the nozzles may be blocked. coat. All of the nozzles on the spray bar are open and func- (a) (b) Figure 15. Uniform tack coat spray application.

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F-23 not functioning. No tack coat is being applied to the pavement surface at those locations. In this case, the distributor needs to be stopped, the blocked nozzles removed and cleaned, the nozzles replaced onto the spray bar, and, only then, the appli- cation of the tack coat continued. In most cases, it is easier and faster to simply remove and replace the blocked nozzles with spare nozzles that should be kept on the distributor in the event of such a problem. The blocked nozzles can be cleaned at a later time. In addition to the blocked nozzles, the overlap of the tack coat spray from one nozzle to the adjacent nozzle is not correct. The proper amount of overlap should be achieved by either adjusting the angle of the nozzles, the distributor pump pressure, and/or the speed of the distributor. Figures 17 (a) and (b) illustrate a series of nozzles that are Figure 16. Non uniform tack coat spray application-- not all set at the same angle to the axis of the spray bar. In this blocked and oversize nozzles. case, the spray fan from one nozzle comes in contact with the spray fan from the adjacent nozzle, resulting in an increase tioning correctly. All of the nozzles are set at the same angle in the amount of tack coat applied where the two spray fans to the axis of the bar. Height of the spray bar is adjusted to interfere with each other. Figure 18 shows the opposite prob- provide a triple lap of spray from the adjacent nozzles. This lem; the angles of the adjacent spray bars are so different figure illustrates a uniform application of tack coat material. that no overlap is achieved between the nozzles. This type of Figure 16 illustrates blocked nozzles on the spray bar. This application yields excessive tack coat in some areas and little figure shows that several of the nozzles on the spray bar are or no tack coat in adjacent areas. (a) (b) Figure 17 a, b. Non uniform tack coat spray application--improper nozzle setting.

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F-24 Figure 20. Non uniform tack coat spray application-- nozzles are not functioning and not at the correct angle. surface. The correct solution is to remove the distributor from the project until the spray bar nozzle problems are corrected. Figure 18. Non uniform tack coat spray application-- improper nozzle setting. Pick Up of Tack Coat Material on Truck Tires Figure 19 shows excessive tack coat applied to a pavement Until an emulsion tack coat is fully cured and all of the water surface. Although the tack coat application is uniform, this has evaporated, the material is sticky. It will adhere to the tires amount of tack coat is extreme. of the haul trucks and be carried off of the pavement surface Figure 20 shows a spray pattern where some of the nozzles (Figure 21). If the tack coat is carried off of the roadway on the are not functioning, some are set at improper angles, and/or haul truck tires, it obviously is not available to provide any bond some are just dribbling tack coat material onto the pavement between the new and the old pavement layers. The important issue in this instance is that the typical location where the tack coat is picked up on the truck tires is exactly where the bond between the layers is most needed--in the wheel paths of traf- fic to later travel over the completed pavement structure. In addition to the loss of the tack coat material, much of the tack coat that is picked up on the haul truck tires will be deposited on the adjacent pavement surface (Figure 22). Such an occurrence is unsightly. In addition, depending on how much tack coat material is deposited on the adjacent pavement, a reduction in friction, particularly during wet weather, can occur and create a hazard. Pick up of PG tack coat material can be minimized if the tack coat is permitted to reach ambient temperature before construction vehicles are allowed to drive on the material. Figure 19. Excessive tack coat spray application. The safe time to allow traffic on PG asphalt is dependent on

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F-25 Figure 22. Pick up of tack coat material by construction traffic. Figure 21. Pickup of tack coat material in wheel paths by construction traffic. A typical RC cutback asphalt tack coat will set more quickly than a typical asphalt emulsion. As discussed previ- the application rate and environmental conditions. In gen- ously, a number of factors affect the time required for the eral, the suitable time for trafficking PG asphalt is 2 to 5 min- cutter stock material in the cutback asphalt to evaporate. In utes after application. most cases, depending on the dilution rate and the applica- An asphalt emulsion will first break and then set. When tion rate of the material, a cutback asphalt tack coat will set the emulsion breaks, the microscopic asphalt particles sus- in approximately 10 to 20 minutes. If trafficked before it pended in the water separate from the water and two distinct is set, pick up of the cutback asphalt tack coat by the haul phases are present. When all of the water has evaporated, the truck tires will occur. emulsion is set. An emulsion that has broken but not set will Thus, to avoid pick up of the tack coat material, it is neces- typically be extremely susceptible to removal by the tires of sary for a tack coat to completely set so that it is not sticky the haul truck as well as by the tires or tracks of the paver. and will not adhere to the tires of the construction vehicles. Set time for an asphalt emulsion tack coat will be longer for Depending on the type of the tack coat material and many a diluted emulsion compared to an undiluted emulsion. Fur- other factors discussed previously, up to two hours may be ther, the break and set times will depend on the application required before the tack coat material is set and will not be rate as well as environmental conditions. Therefore, the set picked up. time for an asphalt emulsion tack coat is usually in the range One additional method that can be used to avoid pick up of of 30 minutes to two hours. the tack coat material on the tires of haul trucks is to employ Set time for trackless tack coat material, a type of asphalt some type of material transfer device to convey the asphalt emulsion containing much harder base asphalt, is signifi- concrete mixture from the haul truck to the paver hopper. cantly less than that for a normal asphalt emulsion. In most This can be accomplished by offsetting the material transfer cases, the set time for trackless tack is in the range of 5 to device so that it is located in an adjacent lane to the one being 15 minutes, depending on application rate and environmen- paved. Using this method of delivery, neither the haul trucks tal conditions. nor the transfer vehicle will travel over the tacked surface.

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F-26 Paving Over an Unbroken Emulsion Many believe that it is not proper to place an asphalt mix- ture over an asphalt emulsion that is not yet broken. One of the reasons most often cited is that the water in the emulsion will affect the temperature of the asphalt mixture material placed on top of it and that a good bond will not be created. Two things are important to consider. First, an emulsion, which is not yet broken, is typically not sticky. That is because the microscopic asphalt particles are still suspended in the water. If the asphalt mixture can be placed on the asphalt emul- sion before it breaks, the tack coat material will usually not be significantly picked up on the tires of the haul trucks. As dis- cussed above, the time delay for the emulsion to break depends on a number of factors. Paving over the emulsion before it breaks usually results in much less pick up of the tack coat on Figure 23. Asphalt paver with asphalt emulsion tank the haul truck tires. One way to delay the break of the emulsion for tack coat. is to dilute it with water. A 1:1 dilution rate is often used. The second factor to consider is the amount of water that is actually in the emulsion and whether the amount of water is a problem with the ability of the emulsion to create a bond between the old and the new pavement layers. The amount of water in an undiluted emulsion tack coat is actually very small. For example, for an undiluted tack coat application rate of 0.06 gallons per square yard, the amount of water is approximately 0.02 gallons per square yard. Although it is not good practice to place an asphalt mix- ture in even a light the rain, it is sometimes done. In general, the amount of water that is present on the pavement surface when it is raining, or has recently stopped raining, is signifi- cantly greater than the amount of water in undiluted emul- sion. In the vast majority of the cases, the asphalt mix that Figure 23a. Roadtec Spray Paver-Spray bar is located at white square between track and screed. has been placed in a light rain remains in place and performs properly over time and traffic. The bond between the old and the new pavement layers is formed even though some of the the mix (Figure 24). Asphalt emulsions usually used in Europe water remains in the emulsion. The heat of the asphalt mix- are essentially the same as those specified in the U.S. ture causes the emulsion to break. The water in the emulsion Using a spray paver eliminates the possibility of any construc- thus escapes in the form of steam, and stripping of the new tion traffic driving through the tack coat. The fact that the spray asphalt mixture does not occur. Placing asphalt mixture over paver has been successfully used for more than twenty years and the tack coat when the emulsion is still brown (unbroken), continues to be used today in Europe is an indication that it is instead of black, greatly reduces the tendency of the tires on possible to apply emulsion to the pavement surface, place the the haul trucks to pick up and carry off the tack coat material. new asphalt mixture on top of the unbroken emulsion, and still create a suitable bond between the pavement layers. Spray Pavers Types of Tack Coat Failures European contractors have used spray pavers for a number of years. These pavers, which have been recently introduced Three primary types of pavement failures are related to the into the United States, carry a tank of asphalt emulsion on the application of the tack coat material: frame of the paver (Figure 23). A spray bar is installed on the paver immediately in front of the asphalt mixture on the augers. Inadequate bond between the old and the new layers. Asphalt emulsion tack coat is applied to the existing pavement Delamination, with time and traffic, of the new asphalt surface typically less than two feet in front of the placement of concrete overlay from the underlying pavement course.

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F-27 Delamination of the Pavement Layers Delamination (Figure 25) is generally caused by insuf- ficient bond between the layers. In most cases, the surface course layer separates from the lower pavement course. Lit- tle, if any, tack coat can typically be observed on top of the underlying layer. In some cases, delamination is due to excessive deflec- tion of the pavement structure under load. Deflection of the pavement structure is so great that it causes the lower layer of the pavement structure to bend excessively under load and crack. With time and traffic, this deflection results in fatigue cracking of the pavement layers, from the bot- tom to the top. In most cases, the cracking appears on the asphalt concrete pavement surface in the form of fatigue or "alligator" cracking. Figure 24. Tack coat application using spray paver. In some instances, however, the bending of the pavement structure is great enough to cause the lower courses of the pavement structure to fatigue crack and the surface course Slippage failure, where the new overlay slides horizontally, mixture to delaminate. This can occur even though the origi- usually producing crescent shaped cracks. nal bond between the layers was adequate. Thus, delamina- tion of the surface course of the asphalt mixture may, or may Inadequate Bond not, be related to the uniformity of the application of the tack coat material. Many times, when a core is cut from a new asphalt con- crete pavement structure, there appears to be a lack of bond between the new and the old pavement layers or between two new pavement courses. Pavement layers often separate at the interface as the cores are extracted from the core hole. Indeed, even if the coring operation takes place a week or two after the pavement has been constructed, it is not unusual for the creation of the bond to not yet be completed. The presence or absence of a bond between the layers depends on a number of factors. Among those are residual rate of the tack coat, uniformity of the tack coat application, cleanliness of the underlying pavement surface, and expo- sure of the pavement surface to traffic at the core location. Usually, with time and traffic, a sufficient bond will develop between the old and the new layers. Periodically, when a pavement overlay fails and is removed for some reason, such as with a sliding failure or delami- nation, no tack coat is visible on the underlying pavement surface. The location of the sliding failure or the delamina- tion might have occurred in an area where the tack coat was removed due to pick up by haul truck tires during construc- tion. Or, it may be due to excessive dilution of emulsion with water. In the vast majority of the cases, the lack of bond is due to non-uniformity of the original tack coat application. This lack of uniformity can be due to blocked spray bar nozzles, nozzles set at the wrong angle to the axis of the spray bar, dribbling of the tack coat from the spray bar, and/or use of the wrong Figure 25. Delamination failure in asphaltic concrete size nozzles. pavement.

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F-28 coat material in a distributor truck is measured using either a volume gauge or a measuring stick provided by the manu- facturer of the truck. For trucks with a flow gauge, the gauge should be set to zero prior to spraying and recorded imme- diately after the spray application is completed. Whereas, when a measuring stick is used, the amount sprayed is the difference between the stick readings prior to and after spray application. It is important to measure the asphalt temperature in the distributor truck. This temperature will be used in temperature-volume corrections for spray application and payment. The linear distance of pavement that can be covered by a tack coat material in a distributor truck can be determined as followed: 9T L= WR Where: L = Linear distance of spray, feet T = Quantity of tack coat in distributor, gallons W = Sprayed width of pavement, feet R= Application rate, gallons per square yard T L= WR Figure 26. Sliding failure in asphaltic concrete pavement. Where: L = Linear distance of spray, meters T = Quantity of tack coat in distributor, liters Sliding Failures W = Sprayed width of pavement, meters R= Application rate, liters per square meter Sliding or slippage type failures (Figure 26) are usually caused by tack coat related problems. In some instances, the sliding failure might be related to excessive deflection in the Characterization of the Interface pavement structure, but this cause is relatively rare. Shear Strength If the existing pavement surface is dusty or dirty, as dis- Tack coat materials are applied onto a pavement surface cussed in details in this report, a lack of bond will occur regard- before overlay construction to ensure adequate interface less of how uniformly and adequately the tack coat material is bond strength between the two layers. If the interface can- applied. If the tack coat is applied non-uniformly, however, not provide enough strength to resist stresses due to traffic or if the tack coat in the wheel paths is picked up and carried and environmental loading, shear failure may occur at the off by the tires on the haul trucks, then the sliding failure interface. Poor interface bond strength may also acceler- will be directly related to the application of the tack coat. In ate the appearance of other distresses, such as slippage and most cases, sliding failures are directly related to the lack of surface cracks. A direct shear device was developed as a part uniformity of the tack coat. of NCHRP Project 9-40, "Optimization of Tack Coat for HMA Placement," for the characterization of interface shear Measuring Tack Coat Material strength of cylindrical specimens (Figure 27). This device is referred to as the Louisiana Interlayer Shear Strength Tester Tack coat material is normally paid for by the gallon (LISST) and can be used for the determination of the inter- (or liter). The quantity of tack coat material applied to the face shear strength of two bonded asphalt mixture layers pavement surface is determined by making measurements (2). A draft standard test method was developed as a part of prior to and after spray applications. The quantity of tack NCHRP Project 9-40 and is presented in Appendix E.

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F-29 Horizontal Sensor Vertical Sensors Normal Load Actuator Shearing Frame Reaction Frame Figure 27. The LISST device. Summary is the residual binder that is important in creating the bond between the old and the new pavement layers. For each exist- Long-term performance of an asphalt concrete pavement ing pavement condition, and for each type of applied tack structure or an asphalt concrete overlay of an existing Port- coat material, the amount of tack coat applied must be back- land cement concrete pavement is, in significant part, related calculated from the residual binder content needed to create to the bond that is developed between successive layers of an adequate bond. pavement in the roadway structure. The bond between the Tack coat is typically applied using an asphalt distributor. layers is related to the uniformity of the application of the Factors that are important in the proper operation of the dis- tack coat. tributor are temperature of the tack coat material, operation Three basic types of asphalt materials are used as a tack of the nozzles on the spray bar, angle of the nozzles compared coat material: asphalt emulsions (the most used), cutback asphalt (rarely used), and asphalt cement. Each of these three to the axis of the spray bar, height of the nozzles above the materials is capable of creating the necessary bond between pavement surface, and size of the nozzles used on the spray the pavement layers. Results of NCHRP Project 9-40, "Opti- bar. Blocked nozzles and/or nozzles set at incorrect angles are mization of Tack Coat for HMA Placement," showed that the main causes of non-uniform application of the tack coat. the type or grade of the tack coat material has a significant Different types of tack coat materials have different break influence on the resulting bond between the old and the new and/or set times. It is important to fully understand the sig- pavement courses. nificance of those times in order to prevent the tack coat from Condition of the existing pavement surface is a primary being picked up on the tires of the construction vehicles. factor that affects the performance of the tack coat material. Construction problems related to the use of tack coats Many different surface conditions can be present, includ- include non-uniformity of the tack coat application, pick up ing dusty or dirty, old or aged, rough or smooth texture, of the tack coat on the haul truck tires, and the time frame bleeding/flushing, wet, or a milled. Each of these situations needed for the tack coat to break and/or set. This is particu- requires different considerations and surface preparation larly important when using an asphalt emulsion. Paving over processes. an unbroken emulsion tack coat (while it is still brown) may It is very important to realize that there can be a signifi- be a means to reduce the pickup problem. In addition, the cant difference in the amount of tack coat applied to a pave- use of a spray paver, which applies the emulsion tack coat ment surface and the residual amount of asphalt binder immediately in front of the asphalt mixture on the augers that remains after the tack coat material has set. For asphalt of the paver, can eliminate the potential pick up problem. emulsion tack coats, in particular, whether the emulsion It is noted, however, that the uniformity of the application is diluted with additional water or not makes a major dif- of the tack coat material cannot be observed when using a ference in the quantity of residual binder remaining on the spray paver, since the majority of the length of the spray bar existing pavement surface after the water has evaporated. It is located underneath the paver.

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F-30 Three types of failures are usually related to improper appli- rate related to the specified residual rate for the tack coat, cation of the tack coat material: lack of bond between layers, and pick up of the tack coat by the construction vehicles, delamination of the layers, and sliding type failures. In each will result in an asphalt concrete pavement structure that case, the uniformity of the application of the tack coat material performs as expected under traffic. However, failure of a can be a significant contributing factor to the occurrence of pavement due to insufficient interfacial bond is extremely the failure. costly. It costs nothing to do it right, and to do it right the first time. Closure References It basically costs nothing extra to properly apply a tack coat to a pavement surface in a uniform manner. Atten- 1. "A Basic Asphalt Emulsion Manual," Asphalt Institute, Manual tion by the contractor to a few basic issues, such as clean Series No. 19, Second Edition, Lexington, Kentucky. 2. Mohammad, L.N, Elseifi, M., Button, J., and Scherocman, J. liness of the existing pavement surface, proper temperature "Optimization of Tack Coat for HMA Placement," Final Report, of the tack coat material before application, condition and National Cooperative Highway Research Program Project 9-40, position of the nozzles on the spray bar, correct application Washington, DC, 2011.