Optimization of Tack Coat for HMA Placement (2012)

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F-1 A p p e n d i x F Tack Coat Training Manual

F-3 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 C o n t e n t s

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

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

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

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

F-8 potential sliding failure problem. The dust coating will create a slip plane and any added, excess residual tack coat will fur- ther weaken the bond between the layers and thus make the problem worse. The only remedy for a dusty or dirty pavement surface is to clean that surface and remove all loose dust or dirt. A small amount of moisture on 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 moisture is minimal, this moisture should be flashed off by the subsequent hot asphalt mixture overlay. However, if the pavement surface layer is saturated with water and the exist- ing pavement surface is damp, the ability of the tack coat material to provide adequate bond between the existing and the new pavement layers may be significantly compromised. New Pavement Surface A common perception is that a tack coat may not be needed between two new asphalt concrete pavement layers. If one 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 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 heavy as to create a slip plane or bleeding at those locations. If the existing asphalt concrete pavement surface is highly oxidized and brittle, a slightly higher residual tack coat rate may be needed. Texture of the Pavement Surface As demonstrated by the results of NCHRP Project 9-40, the surface texture of the existing pavement has a significant effect on the residual amount of tack coat needed. If that sur- face has a relatively fine texture, less residual tack coat will be required. If that surface has a relatively coarse texture, more residual tack coat will be needed. 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 cases, it will be necessary to increase the residual tack coat application rate in order to account for the rougher surface texture. Figure 1. Delamination type of sliding failure.

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. 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- ing followed by flushing of the milled surface with water or use of compressed air is needed to assure that all of the dust has been removed prior to tack coat application. If the dust is not removed from the grooves in a milled surface and a tack coat is applied, the tack coat material can cause the dust to become sticky and adhere to the tires of the construction equipment. 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 provide the bond between existing and new pavement layers (Figure 3). If a tack coat is applied to a milled asphalt concrete sur- face, the residual tack coat rate should be reduced in order to prevent the tack coat material from draining into the milled grooves and collecting in the bottom of the grooves. If this situation occurs, the degree of bond achieved will obviously vary from the top of the grooves to the bottom of the grooves and, therefore, decrease the strength of the bond instead of increasing the strength of the bond with the new asphalt con- crete layer. Use of diluted emulsion as tack coat will exacer- bate this problem. Bleeding Surface Care must be taken when a tack coat is applied to an exist- ing pavement surface that is flushed or bleeding. In this situ- ation, the tack coat application rate must be reduced in order to take into account the amount of asphalt material already Figure 2. Clean milled surface. Figure 3. Tack coat and debris picked up by construction traffic from a dirty milled/tacked surface.

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

F-11 greater residual tack coat rate will be needed to compensate for the increase in the surface area due to the rough texture. In addition, if the existing pavement surface is extensively cracked, a greater amount of residual tack coat will be needed. Because the variation of surface texture of an existing asphalt pavement surface can be significant due to a wide range of sur- face issues, the range of residual tack coat rates is also greater. In general, the residual tack coat range would be from 0.04 to 0.08 g/sy. Open-Graded Asphalt Pavement Surfaces A new open-graded asphalt pavement (OGAP) surface is generally much more open than the surface of an OGAP sur- face that has been used by traffic for a number of years. In addi- tion, some OGAP surfaces have been clogged with dust and dirt with time and traffic, and the air void content of the mix has been reduced significantly due to the amount of dust and dirt that may have accumulated in the pores of the mix. It is, there- fore, very difficult to predict the amount of residual tack coat material that is needed to create the bond between the existing OGAP surface and the new asphalt concrete overlay. Thus, no residual tack coat material rate is suggested in this manual. More importantly, it has been found that overlaying an OGAP surface with a dense-graded asphalt pavement layer has led to early failure of the new overlay due to the amount of water that may accumulate in the now underlying open- graded layer. Water can enter the open-graded layer both from above and below. Many overlaid OGAP layers have experi- enced significant stripping when overlaid with a dense-graded asphalt concrete mixture. It is generally recommended, there- fore, that the OGAP surface be removed prior to the placement of another asphalt concrete layer. Milled Asphalt 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 concrete pavement and the new asphalt concrete overlay and that the roughness and exposed new asphalt surface created by the milling operation provides the necessary bond between the old and new layers. However, results from NCHRP Proj- ect 9-40 indicated that the roughness of the milled surface was not sufficient to provide the required shear strength at the interface. Thus, a tack coat material will normally be needed. If a tack coat is applied to a milled surface, the residual amount of the tack coat should be reduced compared to that amount used for an old, aged pavement surface. A typical resid- 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 tack coat might actually reduce the bond achieved between the milled surface and the new asphalt concrete overlay. Bleeding Surface Rarely is the amount of bleeding or flushing that occurs on the surface of an asphalt concrete pavement uniform either in the transverse direction or the longitudinal direction. In most cases, the amount of bleeding is much greater in the wheel paths of the roadway as compared to the pavement areas between the wheel paths or outside the wheel paths. This sig- nificant difference in the condition of the pavement surface at different locations makes it extremely difficult to determine the amount of residual tack coat material that is needed to pro- vide the proper bond between the new overlay and the exist- ing bleeding pavement surface across the width and down the length of the roadway. If the existing pavement surface is bleeding, the best approach is to mill that surface and remove the excess binder material. If milling is not needed to correct the grade or cross slope of the existing pavement structure, the depth of milling can be minimal—the depth of the milling can be limited to ½ inch or even less. If the asphalt concrete mix is unstable, however, and is the cause of the bleeding, the deficient layer should be entirely removed. It is possible to use different size nozzles on the asphalt distributor spray bar to apply different amounts of tack coat material at different transverse locations across the width of the pavement lane being overlaid. This is feasible, however, only if the bleeding areas are consistent in width and length. In the vast majority of the cases related to the overlay of an existing asphalt pavement that is bleeding, the proper solution to the problem is to mill off the bleeding surface rather than attempt to apply the “correct” amount of residual asphalt tack material to all of the surface locations. Due to significant variation in the amount of bleeding that can occur on a pavement surface, it is basically impossible to provide a typical range for the residual asphalt binder in this guide. Portland Cement Concrete Pavement Surface In most cases, the residual amount of asphalt binder needed for a tack coat applied to a PCC surface is essentially the same as that for an old, aged asphalt concrete pavement surface. In general, the residual tack coat rate will be at the lower end of the range, usually between 0.04 and 0.05 g/sy. If the PCC surface has been diamond ground and has relatively high tex- ture, the residual asphalt tack coat rate should be in the range of 0.05 to 0.06 g/sy. Application Rate Versus Residual Asphalt Binder As discussed above, the residual asphalt binder is the amount of material that actually provides the bond between two differ- ent pavement layers. Thus, the application rate of the asphalt

F-12 material must be back-calculated from the desired residual rate of the asphalt binder. In Table 2, the application rates for the four types of materials listed are determined based on the following relationships: For the PG asphalt, the application rate is the same as the residual rate. Based on the residual rate for a new asphalt concrete pavement surface of 0.03 to 0.04 gallons per square yard (Table 1), the application rate is the same, as shown in Table 2. For the undiluted asphalt emulsion material, the applica- tion rate is approximately 1-½ times more than the residual rate (based on a ratio of the emulsion being 2⁄3 asphalt binder and 1⁄3 water, which is a useful approximation). Thus, for an undiluted asphalt emulsion applied to a new asphalt pavement surface, for a residual application rate of 0.03 to 0.04 g/sy, the required application rate for the undi- luted asphalt emulsion is in the range of 0.04 to 0.06 g/sy, or approximately 1-½ 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 residual rate (based on the diluted asphalt emulsion being approximately 1⁄3 asphalt binder and 2⁄3 water). Thus, for a 1:1 diluted asphalt emulsion applied to a new asphalt pavement surface, for a residual application rate of 0.03 to 0.04 g/sy, the required application rate for the diluted asphalt emulsion is in the range of 0.09 to 0.12 gallons per square yard. For RC-70 cutback, using an estimated asphalt content of 60 percent, the application rate would need to be approxi- mately 1.6 times greater than the residual binder rate. Thus, for a RC-70 cutback asphalt that is applied to a new asphalt pavement surface, for a residual application rate of 0.03 to 0.04 g/sy, the required application rate for the cutback asphalt is in the range of 0.05 to 0.07 gallons per square yard. It is very important to realize that the tack coat application rate MUST be determined by starting at the desired residual application rate for the type of asphalt material being used for tack coat and working backward to calculate the actual appli- cation rate. Table 3 provides a summary of the multiplication factors that need to be used to determine the application rate for the four common types of tack coat materials—PG type asphalt binder, undiluted asphalt emulsion, 1:1 diluted asphalt emulsion, and RC-70 cutback asphalt. Type of Tack Coat Material Multiplication Factor PG Type Asphalt Binder 1.0 Undiluted Asphalt Emulsion 1.5 1:1 Diluted Asphalt Emulsion 3.0 RC-70 Cutback Asphalt 1.6 Table 3. Typical residual rate—application rate multiplication factors. 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 Table 2. Typical application rates using PG asphalt.

F-13 Asphalt Distributors Parts of an Asphalt Distributor An asphalt distributor is normally employed to apply a tack coat to an existing pavement surface. The distributor (Figure 4) consists of a number of primary parts. Those parts include the truck frame, asphalt tank, liquid heating system, variable or constant speed pump, spray bar with spray noz- zles, and computer system to control the rate of the tack coat application. Tack coat material, however, can also be applied manually, using a hand wand or single spray nozzle system. Asphalt Tank An asphalt tank holds tack coat material until it is ready to be applied to the pavement surface. The tank is insulated and typi- cally has a capacity of 500 to 5,500 gallons of tack coat material. Tanks normally contain a series of baffle plates to keep the tack coat material from sloshing around when the truck is moving. Tack Coat Material Temperatures Distributors are equipped with burners, which are used to maintain the temperature of tack coat material to assure the cor- rect viscosity in order to be sprayed properly. Proper tempera- ture for the tack coat material depends on the type of product. The temperature at which an asphalt emulsion is maintained in the distributor tank depends on the grade of the emulsion. Most rapid set (RS) emulsions are applied at a temperature in the range of 70°F to 140°F. The spraying temperature for a high float rapid set (HFRS) material, however, is typically in the range of 125°F to 185°F. Most medium set (MS) and slow set (SS) emulsions are maintained at a temperature in the range of 70°F to 160°F, including high float medium set (HFMS) materials. When polymer-modified asphalt emulsions are used, the spray temperature is typically in the range of 180°F to 200°F. Table 4 presents a summary of guidelines of storage and application temperatures for tack coat materials. Details of storage, handling, and sampling are presented elsewhere (1). Application temperature for cutback asphalt, such as RC-70, is normally in the range of 120°F to 150°F. For PG asphalt, the temperature in the distributor tank is much higher, in the range of 280°F to 325°F. For a polymer-modified PG asphalt binder, the temperature in the distributor tank is typically in the range of 320°F to 340°F. Table 4 presents a summary of guidelines of storage and application temperatures for cutback tack coat materials. However, recommendations for the appropriate spraying temperatures for polymer-modified asphalt should be obtained from the supplier. Cleaning the Distributor Tank It is very important that the interior of the tank on the asphalt distributor be cleaned when changing from one asphalt materials to another. For example, PG asphalt added on top of an asphalt emulsion remaining in the distributor tank may cause severe foaming depending on the amount emulsion in the tank. In addition, mixing materials will significantly change both the properties of the desired tack coat material and the appropriate application rate. It is extremely important that the tack coat material be main- tained in the distributor tank at the appropriate temperature for the material being used in order to assure uniform flow of the material through the nozzles on the spray bar. If the tack coat material is too cold when it is sprayed onto the existing pave- ment surface, the material will come out in strings instead of a uniform spray. The distributor is equipped with a thermometer that displays the temperature of the tack coat materials in the tank. If the tack coat material is not within the proper tem- perature range for the type of product being used, application of the tack coat should be delayed until the material is brought to the correct application temperature. Distributor Pump Asphalt distributors typically are equipped with one of two types of pumps, a variable speed pump or a constant speed pump. Different distributors use different methods to main- tain the necessary pressure to pump asphalt materials at dif- ferent temperatures and different application rates. It is also important for the pump on the distributor to operate at the proper speed or pressure in order to assure the desired spray pattern for the tack coat material. On older model asphalt distributors, a tachometer is usu- ally employed to maintain a constant travel speed during the spraying process. A chart is used by the distributor operator to determine the correct combination of pump speed or pump pressure and distributor travel speed. In order to achieve a consistent tack coat application rate, it is very important on Figure 4. A typical asphalt distributor (courtesy of Etnyre).

F-14 the older distributors for the operator to maintain a constant speed when spraying the tack coat material. Newer model distributors are equipped with an onboard computer system that determines the relationship between the distributor travel speed and the pump speed or pressure. When the speed of the distributor changes, a consistent applica- tion rate is maintained by the computer, which automatically changes the pump pressure to compensate for the change in travel speed. Tack coat material is circulated from the tank on the distributor to the spray bar. In addition, when the tack coat application is complete, the pump is used to pull the material from the spray bar back into the tank. It is impossible to describe all types of distributor functions here. Therefore, the operator should refer to the owner’s man- ual or manufacturer instructions for the specific distributor. Spray Bar Nozzle Angle The spray bar is located at the rear of the distributor, behind the rear wheels of the truck. Tack coat material is applied to the Type and Grade Spraying Temperature Storage Temperature °C °F °C °F Asphalt Cements AC -2.5 149+ 300+ 160 320 AC-5 149+ 300+ 166 330 AC-10 163+ 325+ 174 345 AC-20 163+ 325+ 177 350 AC-40 177+ 350+ 177 350 AR-1000 149+ 300+ 163 325 AR-2000 149+ 300+ 168 325 AR-4000 177+ 350+ 177 350 AR-8000 177+ 350+ 177 350 PEN 40-50 177+ 350+ 177 350 PEN 60-70 177+ 350+ 177 350 PEN 85-100 163+ 325+ 177 350 PEN 120-150 163+ 325+ 177 350 PEN 200-300 149+ 300+ 168 335 Emulsified Asphalts Trackless 71-85 160-180 71-85 160-180 RS-1 21-71 70-160 20-60 70-140 RS-2 60-85 140-185 50-85 125-185 HFRS-2 60-85 140-185 50-85 125-185 MS-1 21-71 70-160 10-60 50-140 MS-2 60-85 140-180 50-85 125-185 MS-2h 60-85 140-180 50-85 125-185 HFMS-1 21-71 70-160 10-60 50-140 HFMS-2 60-85 140-180 50-85 125-185 HFMS-2h 60-85 140-180 50-85 125-185 HFMS-2s 60-85 140-180 50-85 125-185 SS-1 21-71 70-160 10-60 50-140 SS-1h 21-71 70-160 10-60 50-140 CRS-1 21-71 70-160 50-85 125-185 CRS-2 60-85 140-180 50-85 125-185 CMS-2 60-85 140-180 50-85 125-185 CMS-2h 60-85 140-180 50-85 125-185 CSS-1 21-71 70-160 10-60 50-140 CSS-1h 21-71 70-160 10-60 50-140 Cutback Asphalts (NOTE: Use Caution on upper limits due to flash point) MC-30 30+ 80+ 54 130 MC-70 50+ 120+ 71 160 MC-250 75+ 165+ 91 195 MC-800 95+ 200+ 99 210 MC-3000 110+ 230+ 99 210 RC-70 50+ 120+ 71 160 RC-250 75+ 165+ 91 195 RC-800 95+ 200+ 99 210 RC-3000 110+ 230+ 99 210 SC-70 50+ 120+ 71 160 SC-250 75+ 165+ 91 195 SC-800 95+ 200+ 99 210 SC-3000 110+ 230+ 99 210 Table 4. Guideline temperatures for tack coat materials. (courtesy of Asphalt Products Unlimited)

F-15 pavement surface using the nozzles on the spray bar. Optional extensions on the spray bar can be used to increase the width of tack application. The extensions simply fold upward when the distributor is being relocated or when they are not needed. Figure 5 illustrates the location of the spray bar. Figure 6 shows a folded spray bar extension. Alignment of the nozzles on the spray bar is of extreme importance in achieving a uniform application of tack coat. Further, use of the proper size nozzles and the correct spray bar height is very important. All nozzles used on the spray bar are 4 inches apart. Thus, there are three nozzles per foot of width of the spray bar. The angle of the opening of each nozzle must be set precisely the same in order to achieve the proper amount of overlap of the spray from each nozzle with the adjacent nozzle(s). As shown in Figure 7, the proper nozzle angle setting is between 15 and 30 degrees to the axis of the spray bar. In normal prac- tice, the angle is set at 30 degrees to the axis of the spray bar. If the nozzles are not all set at the same angle, the spray pat- tern from one nozzle will interfere with the spray pattern from the adjacent nozzles. This will result in a very non-uniform application of the tack coat material onto the pavement surface. Interference of the spray pattern from nozzle to nozzle will mean that some portions of the existing pavement surface will receive excessive tack coat material while adjacent portions will receive insufficient material. For example, if all of the nozzles are set in Figure 5. Rear and side component identification for distributor. Figure 6. Double-fold wing configuration. Figure 7. Spray bar nozzle alignment.

F-16 a direction parallel to the axis of the spray bar, there will be an extremely heavy amount of tack coat applied where the spray from the adjacent nozzles strike each other and very little tack coat applied directly under the center of the nozzles. In the other extreme, if the nozzles on the spray bar are set at an angle of 90 degrees to the axis of the spray bar, the resulting spray pattern will be strings of tack coat material on the pavement surface (Figure 8). Those strings will be 4 inches apart. In this case, less than 15 percent of the existing pavement surface will be covered with the tack coat material. Obviously, sufficient and uniform bond with the new overlay will not be achieved. Many distributor operators use a nozzle alignment wrench to set the correct angle of the spray bar nozzles. Use of the wrench simplifies setting all of the nozzles to the same angle. Spray Bar Height Normally, the height of the spray bar is set to achieve a triple overlap between the adjacent nozzles (Figure 9). This height, which depends, in part, on the make and model of the distributor, is typically in the range of 9 to 12 inches above the pavement surface. Single lap coverage is rarely employed because of the difficulty in achieving the exact meeting of the spray pattern from the adjacent nozzles. Some contractors choose to employ a double lap coverage, which is acceptable, if the resulting spray pattern uniformly covers the entire pavement surface. As the amount of tack coat material in the distributor tank decreases during application, the height of the spray may increase. On some older distributors, it may be necessary to adjust (lower) the height of the spray bar as the amount of tack coat material in the tank is reduced. On most new distribu- tors, the height of the spray bar is automatically adjusted as the weight of the tack coat material in the distributor tank is reduced. In either case, it is very important to maintain the cor- rect height of the spray bar during application of the tack coat in order to achieve a consistent double or triple coverage overlap. Spray Bar Nozzle Size Proper spray bar nozzle size depends on three primary factors: tack coat application rate, speed of the distributor, and type of material being sprayed. In addition, different asphalt distributor manufacturers may use different nozzle sizes for different application rates. It is very important to remember that the amount of asphalt material needed for tack coat application is significantly less than the amount of material needed for a chip seal or surface treatment application. Thus, the size of the nozzles on the spray bar must be checked to ensure that they are the correct size for uniform application of tack coat materials. Application rate of the asphalt material is directly depen- dent on the size of the nozzles. Table 5 provides information on the application rate (not residual rate) for various nozzle sizes for a Rosco distributor. This particular distributor man- ufacturer provides six different nozzle sizes. Those six sizes allow application rates from 0.03 to 1.00 gallons/square yard. As shown in the table, for tack coat usage, two nozzle sizes are available, 00 and 0. Figure 8. Poor spray pattern of tack coat due to improper nozzle alignment, nozzle size, and/or pump pressure. Figure 9. Spray bar height and tack coat coverage. Nozzle Size Recommended Flow Rate - GPM Application Rate Gal/Sq. Yd. 00 1.2 0.03 - 0.08 0 3.0 0.05 - 0.20 1 4.0 0.10 - 0.30 1.5 6.0 0.15 - 0.40 2 8.5 0.25 - 0.55 3 13.5 0.35 – 1.00 Table 5. Rosco nozzle sizes and flow rate ranges.

F-17 It is important to note that, for this particular model of distributor, a nozzle size change may be required if an emulsion is undiluted versus an emulsion that is diluted 1 to 1 with water. For example, if an undiluted emulsion is to have a residual rate of 0.04 gallons per square yard, the application rate would be 0.06 gallons per square yard. Thus, for the Rosco distributor, either a nozzle size of 00 or 0 could be used. If a diluted emulsion were to be used, however, for the same residual application rate of 0.04 gal- lons per square yard, the application rate for the 1:1 diluted emulsion would calculate to be 0.12 gallons per square yard. In this latter case, nozzle size 00 could not be used. Nozzle size 0 would be required to achieve the proper application rate and spray pattern. Another distributor manufacturer, Etnyre1, uses an entirely different system for the selection of the nozzle size to achieve the desired application rate. As shown in Table 6, for the vast majority of tack coat application rates, either undiluted or diluted 1 to 1 with water, two different types of nozzles are recommended, a coin slot nozzle and a V slot nozzle. For the V slot tack nozzle, the applicable application rate is shown to be 0.05 to 0.20 gallons per square yard. For the S36-4 V slot nozzle, the applicable application rate is significantly higher, 0.10 to 0.35 gallons per square yard. Using the example above, for a residual rate of 0.04 gallons per square yard, for an undiluted emulsion, the application rate would be 0.06 gallons per square yard. Only the V slot tack nozzle could be used to achieve the proper application rate. If a 1 to 1 diluted emulsion tack coat material is used, however, for the same residual rate, either the V slot tack noz- zle or the S36-4 V slot nozzle could be used for the resulting application rate of 0.12 gallons per square yard. If PG asphalt were to be used as tack coat material, for a residual tack coat rate of 0.04 gallons per square yard, the application rate would be exactly the same as the residual rate (0.04 gallons per square yard). According to the Etnyre infor- mation, no nozzle size is available for this particular situation. For a BearCat2 model distributor, again, different nozzle sizes are needed for different application rates (for the same residual tack coat rate). The choice of the proper nozzle size, however, is based on two factors: the rate of travel of the distributor in feet per minute and the required application rate of the tack coat in gallons per square yard. Four different nozzle sizes are available. A BearCat Road Oil Spreading Calculator (Figure 10) is used to determine the correct nozzle size for a distributor pump pres- sure range of 5 to 25 psi. BearCat recommends, however, that the nozzle size selected should yield a required application rate at a pump pressure of 6 to 12 psi. Ref. Part No. Description Application (per square yard) Application (Metric) Liters per square meter Flow Gallons per minute per foot 1 3353788 V Slot Tack Nozzle 0.05 - 0.20 0.19 - 0.75 3.0 to 4.5 2 3351008 S36-4 V Slot 0.10 - 0.35 0.38 - 1.30 4.0 to 7.5 3 3351009 S36-5 V Slot 0.18 - 0.45 7.0 to 10.0 4 3352368 Multi-Material V Slot 0.15 - 0.40 0.57 - 1.50 6.0 to 9.0 5 3351015 3/32-inch Coin Slot 0.15 - 0.40 0.57 - 1.50 6.0 to 9.0 6 3352204* Multi-Material V Slot 0.35 - 0.95 1.30 - 3.60 12.0 to 21.0 7 3352205* Multi-Material V Slot 0.20 - 0.55 0.75 - 2.08 7.5 to 12.0 8 3352210 End Nozzle 0.20 - 0.55 0.75 - 2.08 7.5 to 12.0 9 3351014 3/16-inch Coin Slot 0.35 - 0.95 1.30 - 3.60 12.0 to 21.0 10 3351010 ¼-inch” Coin Slot 0.40 - 1.10 1.50 - 4.16 15.0 to 24.0 * Recommended nozzles for chip seal when using emulsified asphalt Table 6. Etnyre spray bar nozzles and associated application rate ranges. 1E. D. Etnyre & Co., Oregon, Illinois 2BearCat Mfg., Wickenburg, Arizona

F-18 For a BearCat distributor, the combination of the applica- tion rate of the tack coat and travel speed is used to determine the spray bar pressure. In general, the greater the application rate, the larger is the nozzle size. But, any of the four nozzle sizes can be used to apply the same amount of tack coat, depending on the travel speed of the distributor and the spray bar pres- sure. BearCat recommends that a lower bar pressure be used, when possible, to produce a more uniform application of the tack coat material. Some contractors use the same distributor to apply asphalt material for chip seals, surface treatments, and tack coats. Although the same distributor can be used for both applica- tions, the same nozzle size can NOT be used. If, for example, the application rate for an undiluted asphalt emulsion for a chip seal is 0.36 gallons per square yard, the residual rate would be approximately 2⁄3 of the application rate or 0.24 gallons per square yard. No make of distributor can apply less than 0.08 g/sy and greater than 0.30 g/sy when using the same size nozzle. Regardless of the manufacturer of the distributor, a different nozzle size would be required for the two different application rates used in this example. This same comment is applicable if the asphalt distribu- tor was previously used to apply a prime coat material, either cutback asphalt or asphalt emulsion. The normal residual rate for prime coat is significantly greater than that for tack coat. Thus, the nozzles on the distributor may have to be changed in order to reduce the application rate, and maintain a proper spray pattern to achieve uniform coverage. Distributor Truck Inspection, Calibration, and Certification Correct tack coat application begins with proper inspec- tion and calibration of application equipment. Periodically, the operator should place a trial tack coat application over some convenient, unused area to assure that all of the noz- zles are open and operating properly. Further, the distributor application rate needs to be calibrated, both in the transverse direction and in the longitudinal direction, using the proce- dure described in ASTM Method D 2995, “Standard Practice for Estimating Application Rate of Bituminous Distribu- tors.” Furthermore, many owner agencies require a valid cer- tification to ensure the proper functioning of the distributor and its components. This is recommended practice. Calibra- tion should address, as a minimum, spray bar height, nozzle angle, spray bar pressure, thermometers, and strapping stick. Blocked Nozzles If an asphalt distributor is not properly maintained, it is very possible for some of the nozzles to become plugged. Fig- ures 11a and 11b illustrate tack coat material that was applied using a distributor spray bar with blocked nozzles (and thus a very poor spray pattern). The operator of the distributor should be able to use his rear view mirrors to observe the uniformity or non-uniformity of the tack coat application. In addition, the foreman of the paving crew should observe tack coat application regularly to ensure uniformity of the application and to stop the process if any nozzles are blocked. Hand Wand Application There are often areas on an asphalt paving project where it is not feasible to use the distributor to apply the tack coat material. Such locations are intersections, driveways, and around drainage structures. In these cases, the tack coat material is typically applied using a hand wand with the tack coat material fed from the asphalt distributor. Occasionally, a crack sealing bucket or “pot” is used to apply the tack coat material. Whichever method is employed, it is extremely important that the tack coat material be applied uniformly and completely cover the pavement surface. Figure 10. BearCat road oil spreading calculator.

F-19 Figures 12 and 13 are examples of improper application of the tack coat material using hand methods. Figures 12 (a) and (b) are examples of tack coat applied using a crack sealing bucket. Figure 13 is an example of tack coat applied using a hand wand. There is no measurable way to assure the correct applica- tion rate of the tack coat material when using a hand wand. The application rate is solely dependent on the experience and talent of the person using the wand. It is important, however, for the application rate to be, as much as possible, the same as that for the tack coat material applied using the asphalt dis- tributor. That is, hand wand application should cover essen- tially 100 percent of the existing pavement surface and should be as uniform as possible. A crack sealing bucket should NEVER be used to apply the tack coat materials, since it is practically impossible to achieve complete or uniform cover- age of the tack coat material. Summary In order for a tack coat to provide the necessary bond between the existing pavement surface and the new asphalt concrete overlay, it is extremely important that the following factors be considered: 1. Tack coat material must be maintained at the proper tem- perature in the distributor tank for the type of material being applied. 2. Required residual amount of the tack coat material must be known before starting application. 3. Application rate of the tack coat material must be calcu- lated based on the residual amount of tack coat needed. 4. For an undiluted emulsion, the application rate should typ- ically be about 1.5 times more than the residual rate (based on an assumption that the emulsion consists of 2⁄3 asphalt binder and 1⁄3 water, which is not always correct). 5. For an emulsion diluted 1:1 with water, the application rate should be approximately 3.0 times more than the residual rate. Spraying a diluted asphalt emulsion increases the total volume of material and thus can help achieve more uni- form application. 6. For MC-30 or MC-70 cutback asphalt tack coat, the application rate should be about 1.7 times more than the required residual rate. The factor for RC-70 will also (a) (b) Figure 11. Tack coat spray applied with partially blocked nozzles.

F-20 be about 1.7 and, for SC-70, about 2.0. Due to the low flash point (thus fire danger) and environmental con- cerns (emissions of volatile organic compounds), cut- backs are not recommended. For these reasons, many state DOTs have prohibited the use of cutback asphalts. 7. For a PG asphalt tack coat material, the application rate should be the same as the residual rate. 8. Height of the spray bar must be adjusted to obtain a double or triple lap of the spray. This is recommended to achieve uniform application and 100 percent coverage of the residual tack coat material. 9. All nozzles on the spray bar must be set to the same angle, typically 30 degrees to the axis of the bar. 10. All nozzles on the spray bar must be the same size, unless bleeding exists in the wheel paths and a different applica- tion rate is needed at those locations. 11. All nozzles must be clean, not blocked, and functioning properly. 12. Size of the nozzles must be selected based on recommen- dations of the distributor manufacturer for the desired application rate of the particular 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 a 1:1 diluted form, the emulsion will contain approximately 1⁄3 asphalt binder and 2⁄3 water. In addition, the emulsion will (b)(a) Figure 12. Improper application of tack coat using a crack sealing bucket. Figure 13. Improper application of the tack coat using a hand wand.

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

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- ment surface and the new asphalt concrete pavement layer. Obviously, if the tack coat is applied in one area but not in another area, or in a greater quantity in one area as compared to an adjacent area, there will be a difference in the degree of bond attained (Figure 14). Poor uniformity can be due to one or a combination of several factors. One or more of the nozzles may be blocked. One or more of the nozzles may be set at an improper angle to the axis of the spray bar. One or more of the nozzles may be of a different size compared to the other nozzles. Truck speed and/or pump pressure may be inadequate. Figures 15 (a) and (b) depict proper application of a tack coat. All of the nozzles on the spray bar are open and func- (b)(a) Figure 15. Uniform tack coat spray application. Figure 14. Non uniform tack coat spray application.

F-23 tioning correctly. All of the nozzles are set at the same angle to the axis of the bar. Height of the spray bar is adjusted to provide a triple lap of spray from the adjacent nozzles. This figure illustrates a uniform application of tack coat material. Figure 16 illustrates blocked nozzles on the spray bar. This figure shows that several of the nozzles on the spray bar are 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 not all set at the same angle to the axis of the spray bar. In this case, the spray fan from one nozzle comes in contact with the spray fan from the adjacent nozzle, resulting in an increase in the amount of tack coat applied where the two spray fans interfere with each other. Figure 18 shows the opposite prob- lem; the angles of the adjacent spray bars are so different that no overlap is achieved between the nozzles. This type of application yields excessive tack coat in some areas and little or no tack coat in adjacent areas. Figure 16. Non uniform tack coat spray application— blocked and oversize nozzles. (a) (b) Figure 17 a, b. Non uniform tack coat spray application—improper nozzle setting.

F-24 Figure 19 shows excessive tack coat applied to a pavement surface. Although the tack coat application is uniform, this amount of tack coat is extreme. Figure 20 shows a spray pattern where some of the nozzles are not functioning, some are set at improper angles, and/or some are just dribbling tack coat material onto the pavement surface. The correct solution is to remove the distributor from the project until the spray bar nozzle problems are corrected. Pick Up of Tack Coat Material on Truck Tires Until an emulsion tack coat is fully cured and all of the water has evaporated, the material is sticky. It will adhere to the tires of the haul trucks and be carried off of the pavement surface (Figure 21). If the tack coat is carried off of the roadway on the haul truck tires, it obviously is not available to provide any bond 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. The safe time to allow traffic on PG asphalt is dependent on Figure 19. Excessive tack coat spray application. Figure 20. Non uniform tack coat spray application— nozzles are not functioning and not at the correct angle. Figure 18. Non uniform tack coat spray application— improper nozzle setting.

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

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 the haul truck tires. One way to delay the break of the emulsion 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 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 water remains in the emulsion. The heat of the asphalt mix- ture causes the emulsion to break. The water in the emulsion thus escapes in the form of steam, and stripping of the new asphalt mixture does not occur. Placing asphalt mixture over the tack coat when the emulsion is still brown (unbroken), instead of black, greatly reduces the tendency of the tires on the haul trucks to pick up and carry off the tack coat material. Spray Pavers European contractors have used spray pavers for a number of years. These pavers, which have been recently introduced into the United States, carry a tank of asphalt emulsion on the frame of the paver (Figure 23). A spray bar is installed on the paver immediately in front of the asphalt mixture on the augers. Asphalt emulsion tack coat is applied to the existing pavement surface typically less than two feet in front of the placement of the mix (Figure 24). Asphalt emulsions usually used in Europe are essentially the same as those specified in the U.S. Using a spray paver eliminates the possibility of any construc- tion traffic driving through the tack coat. The fact that the spray paver has been successfully used for more than twenty years and continues to be used today in Europe is an indication that it is possible to apply emulsion to the pavement surface, place the new asphalt mixture on top of the unbroken emulsion, and still create a suitable bond between the pavement layers. Types of Tack Coat Failures Three primary types of pavement failures are related to the application of the tack coat material: • Inadequate bond between the old and the new layers. • Delamination, with time and traffic, of the new asphalt concrete overlay from the underlying pavement course. Figure 23. Asphalt paver with asphalt emulsion tank for tack coat. Figure 23a. Roadtec Spray Paver-Spray bar is located at white square between track and screed.

F-27 • Slippage failure, where the new overlay slides horizontally, usually producing crescent shaped cracks. Inadequate Bond 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 size nozzles. 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. 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 mixture to delaminate. This can occur even though the origi- nal bond between the layers was adequate. Thus, delamina- tion of the surface course of the asphalt mixture may, or may not, be related to the uniformity of the application of the tack coat material. Figure 24. Tack coat application using spray paver. Figure 25. Delamination failure in asphaltic concrete pavement.

F-28 Figure 26. Sliding failure in asphaltic concrete pavement. Sliding Failures 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 pavement structure, but this cause is relatively rare. If the existing pavement surface is dusty or dirty, as dis- cussed in details in this report, a lack of bond will occur regard- less of how uniformly and adequately the tack coat material is applied. If the tack coat is applied non-uniformly, however, or if the tack coat in the wheel paths is picked up and carried off by the tires on the haul trucks, then the sliding failure will be directly related to the application of the tack coat. In most cases, sliding failures are directly related to the lack of uniformity of the tack coat. Measuring Tack Coat Material Tack coat material is normally paid for by the gallon (or liter). The quantity of tack coat material applied to the pavement surface is determined by making measurements prior to and after spray applications. The quantity of tack 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: L T WR = 9 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 L T WR = Where: L = Linear distance of spray, meters T = Quantity of tack coat in distributor, liters W = Sprayed width of pavement, meters R = Application rate, liters per square meter Characterization of the Interface Shear Strength Tack coat materials are applied onto a pavement surface before overlay construction to ensure adequate interface bond strength between the two layers. If the interface can- not provide enough strength to resist stresses due to traffic and environmental loading, shear failure may occur at the interface. Poor interface bond strength may also acceler- ate the appearance of other distresses, such as slippage and surface cracks. A direct shear device was developed as a part of NCHRP Project 9-40, “Optimization of Tack Coat for HMA Placement,” for the characterization of interface shear strength of cylindrical specimens (Figure 27). This device is referred to as the Louisiana Interlayer Shear Strength Tester (LISST) and can be used for the determination of the inter- face shear strength of two bonded asphalt mixture layers (2). A draft standard test method was developed as a part of NCHRP Project 9-40 and is presented in Appendix E.

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

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