Abstract:
A method of applying building a pavement structure using a polymer modified hot-applied tack coat is disclosed. This polymer modified tack coat is non-adhesive at ambient temperatures, and, thus, also non tracking. The tack is applied while hot, but cools quickly. The subsequent application of hot mix asphalt results in a superior bond between the asphalt layer and the tack layer. It is particularly well suited to bonded friction course applications since it removes the necessity of specialized spray paving machinery and allows the use of conventional asphalt distributors and pavers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/307,749 which was filed on Feb. 24, 2010. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is generally directed toward a non-tracking, hot applied tack coat for bonding two layers of hot mix asphalt together and its methods of application. 
       BACKGROUND OF THE INVENTION 
       [0003]    Thin Mix HMA, Bonded Friction Course, and Open Graded Friction Courses (OGFC) (also known as Porous friction Courses, Gap Graded Asphalts, or Stone Matrix Asphalts) have grown in popularity in recent years due to their important advantages over the roadways paved with dense graded hot mix asphalt (HMA) and over concrete pavements. These advantages include a reduced risk of hydroplaning, improved drainage, improved visibility, coarse surface for improved friction values, and reduced noise. However, the disadvantages of OGFC have been well documented and include reduced durability, debonding of the OGFC layer, and stripping in the OGFC and underlying areas. 
         [0004]    The durability issues have been largely addressed by the use of a Bonded Friction Course (BFC) system that employs the use of improved polymerized tack coat materials and processes. One such Bonded Friction Course system, known as the NOVACHIP process and described in U.S. Pat. No. 5,069,578, uses a specialized “Spray Paver” machine to apply a thick layer of polymer modified tack coat immediately before a thin gap-graded HMA layer is applied. This polymer modified tack coat wicks into the new gap graded mix by displacement and water vaporization. The tack coat provides a degree of adhesion or bonding between the layers and also acts to reduce slippage and sliding of the layers relative to other layers in the pavement structure during use or due to wear and weathering of the pavement structure. The thick application of the tack coat further seals minor cracks in the existing surface layer and forms a strong bond between the new HMA layer and the existing pavement. 
         [0005]    However, the NOVACHIP bonded friction course system can be prohibitively expensive due to the requirement that the specialized “Spray Paver” machine be used. Currently, each spray paver machine costs almost $500,000.00, and many contractors and state agencies cannot justify the expense. However, without the use of the NOVACHIP Spray Paver, the thick layer of emulsified polymer modified tack coat used in a bonded friction course system would be very difficult to work with. The thick layer of emulsion tack coat would have a very slow cure rate, resulting in unacceptable delays and also tracking of the tack coat layer. Tracking occurs when the tack or bonding coat is picked up on the tires or tracks of vehicles traveling over the coated surface. Where this occurs, the asphalt compositions often are tracked onto other pavement surfaces causing disruption to the surrounding area. This tracking also reduces the effectiveness of the tack coat by displacing a portion of the intended volume from the area awaiting a new pavement layer. 
         [0006]    Insufficient adhesion between a new layer of pavement and an existing base course, a previously laid pavement layer, or a prepared pavement surface can cause pavement separation and cracking during construction of the structure, as well as subsequent failures and premature deterioration of the pavement structure and/or surface. Such conditions often require costly repairs, can cause damage to vehicles traveling on the surface and may cause dangerous traffic conditions threatening damage to property and injury to vehicles and passengers. 
       SUMMARY OF THE INVENTION 
       [0007]    We disclose a new system for creating a bonded friction course pavement structure that does not require the use of specialized machinery for its application. The system employs conventional asphalt distributors to place a hot-applied, polymer modified tack coat having the properties after cooling of being non-adhesive at ambient temperatures. The polymer modified tack coat layer is applied while hot, at temperatures greater than 212° F. in a liquid form, and may be allowed to cool to ambient temperatures. At ambient temperatures, the polymer modified tack coat is non-tracking and non-adhesive. However, when it comes into contact with a new hot mix asphalt layer, the polymer modified tack coat becomes adhesive, again. The resulting pavement structure made through use of the claimed methods has improved strength compared to other known paving systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Further advantages of the invention will become apparent by reference to the detailed description of preferred embodiments when considered in conjunction with the drawings: 
           [0009]      FIG. 1  is a graph comparing the interface strengths of different tack materials. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The following detailed description is presented to enable any person skilled in the art to make and use the invention. For purposes of explanation, specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that these specific details are not required to practice the invention. Descriptions of specific applications are provided only as representative examples. Various modifications to the preferred embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the scope of the invention. The present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest possible scope consistent with the principles and features disclosed herein. 
         [0011]    By using a non-tracking, polymer modified tack coat that is non-adhesive at ambient temperatures, a specialized “Spray Paver,” such as that used in the NOVACHIP bonded friction course system, is no longer required. Instead, conventional distributors and paving equipment can be used. The hot-applied polymer modified tack coat is heated until it is liquid and sprayable and, then, sprayed on the pavement to create the thick layer. Typically this tack coat is applied at the rate of 0.04 to 0.8 gals/yd 2  for a conventional HMA overlay, or 0.09 to 0.18 gals/yd 2  for a Bonded Friction Course. Once applied, the layer of tack coat cures hard to the touch in seconds to form a non-tracking surface. Cracks that may exist in existing pavement are filled by this thick layer, thus sealing the surface. A hot-mix asphalt layer can, then, be placed over the tack coat layer almost instantly after the tack coat layer has cooled. 
         [0012]    As the tack coat cools, it becomes non-adhesive, and, therefore, non-tracking. Vehicles can drive over this layer without fear of the tack coat sticking to the tires of the vehicles. When the hot-mix asphalt layer is applied on top of the tack coat, the heat of the HMA layer causes the tack coat to liquefy, and this liquefied polymer modified membrane is wicked into the HMA layer by displacement. At the higher temperatures of the hot mix layer, the tack coat is extremely adhesive, allowing it to form a strong structural bond with the OGFC or other hot mix asphalt layer. As the polymer modified tack coat cools, the bond with the hot mix asphalt layer becomes stronger. However, the polymer modified tack coat retains its flexibility. 
         [0013]    It should be appreciated that this polymer modified trackless tack coat is particularly useful in Open Graded Friction Course, Bonded Friction Course, and thin overlay mixes where the material was previously applied with specialized distributors, such as “Spray Pavers.” However, using the claimed method only a conventional distributor and paver are required. As a result, the methods disclosed are available to all contractors and government agencies that do not want to purchase a proprietary or specialized machine. 
         [0014]    The disclosed method may use any tack coat formulation that has the desired properties of being adhesive only at higher temperatures, but not at ambient temperatures. The tack coat can be made by blending a low pen asphalt and/or with a polymer. Harder asphalts with low pen values have higher softening points. By reducing the amount of hard pen asphalt in an asphalt composition, and increasing the amount of other ingredients such as polymers, an asphalt composition can be made that has a softening point that is lower than the temperature of hot mix asphalt, but high enough that it is not adhesive when it cools to ambient temperatures. In one embodiment, the non-tracking polymer modified tack coat can be made by producing an asphalt cement having a penetration of 0 to 50 dmm and combining its polymers and additives to achieve a softening point of 135° C. or higher. 
         [0015]    The polymers and additives in the tack coat add strength and flexibility to the tack coat. The polymers and additives are added at various concentrations to an asphalt cement in order to achieve the desired physical properties of the trackless tack. The polymers that can be used in modifying the tack coat include, but are not limited to, SBS, SB, SEBS, XSB, EVA, polypropylene, acrylic polymers, Ground Tire Rubber, natural and synthetic waxes, Gilsonite, Trinidad Lake Asphalt, plastomers, elastomers, hardeners and softeners, or any combination, including oxidizing, thereof known in the art that allows the tack coat to achieve the properties of being non-adhesive at ambient temperatures. For the purposes of this application, ambient temperature is any temperature that is typically used in paving applications. Currently, paving is typically only performed at temperatures greater than 40° F. 
       EXAMPLES 
       [0016]    The following standard procedures of the American Association of State Highway and Transportation Officials (AASHTO) were used in preparing and testing the pavement compositions. Softening Point (using Ring and Ball Apparatus) was tested as per AASHTO T53. Penetration was tested as per AASHTO T49. Rotational Viscosity was determined as per AASHTO T316. Rheological properties were tested using a Dynamic Shear Rheometer pursuant to AASHTO T315 or PAV(Pressure Aging Vessel) DSR as per AASHTO T315. Flexural Creep Stiffness was tested using Bending Beam Rheometer as per AASHTO T313. Separation of Polymer from Polymer Modified Asphalt was tested as per AASHTO T53 and ASTM D7173. 
         [0017]    In one embodiment, the polymer modified trackless tack has the following properties: 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 PHYSICAL PROPERTIES 
               
             
          
           
               
                 PARAMETER 
                 TEST METHOD 
                 MIN 
                 MAX 
               
               
                   
               
             
          
           
               
                 Rotational Viscosity @ 135° C., Cp 
                 AASHTO T316 
                 — 
                 3000 
               
               
                 Penetration @ 25° C. 
                 ASTM D5 
                 — 
                 50 
               
               
                 Softening Point (° C.) 
                 ASTM D36 
                 70 
                 — 
               
               
                 Original DSR @ 82° C. (G*/SIN 
                 AASHTO T315 
                 1.0 
                 — 
               
               
                 δ, 10 rad/sec) 
               
               
                   
               
             
          
         
       
     
         [0018]    In another embodiment, the polymer modified trackless tack is produced by creating a blend consisting of Marathon Hard Pen asphalt with 47% asphalt concentrate of SBS using paddles agitation at 350° F. and 0.5% anti-strip agent, such as Adhere LOF 65-00. The resulting polymer modified tack coat had the following properties: 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 PARAMETER 
                 TEST METHOD 
                 Result 
               
               
                   
                   
               
             
             
               
                   
                 Penetration @ 25° C. 
                 ASTM D5 
                 18 dmm 
               
               
                   
                 Softening Point (° F.) 
                 ASTM D36 
                 149° F.       
               
               
                   
                   
               
             
          
         
       
     
         [0019]    The softening point is well above ambient temperatures for pavement applications, and, therefore, the resultant polymer modified tack is non-adhesive at ambient temperatures. The tack may then be heated to above 149° F. and applied to a substrate pavement layer, such as a pre-existing pavement at the conventional spray rate of 0.04 to 0.08 gals/yd 2  for a conventional HMA overlay, or 0.09 to 0.18 gals/yd 2  for a Bonded Friction Course. The hot tack coat will bond with the substrate layer and cool to a hard surface that is non-adhesive. A hot mix asphalt layer is then applied to the pavement layer. The heat from the hot mix asphalt is significantly higher than the softening point of the tack coat, causing it to liquefy and be wicked into the hot mix asphalt where it can form a strong bond as the asphalt cools below the softening point of the tack. 
         [0020]    Tests show that the use of the disclosed polymer modified hot tack results in a significantly increased interfacial bond strength between the pavement layers, compared to regular tack methods and materials. As previously discussed, separation of layers will cause premature failure of the roadway, possibly resulting in damage to vehicles or even death. In one test by the National Center for Asphalt Technology, nine slabs consisting of two inches of 12.5 mm open-graded friction course overlaid on two inches of 12.5 mm dense-graded asphalt were prepared and tested. The three tacks tested included the hot applied polymer modified tack as disclosed herein (also referred to as Ultrabond at the time of testing); CQS-1HP, a generic form of NOVABOND tack used by the Alabama Department of Transportation; and NTSS-1HM, a trackless tack made by 
         [0021]    Blacklidge Emulsions, Inc. (Gulfport, Miss.) which is also the subject of U.S. Pat. No. 7,503,724. Six 6-inch specimens were cored from each slab. Two specimens from each slab were cut in half to evaluate the extent of tack coat migration into the OGFC layer. The remaining four specimens were evaluated for bond strength. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Testing specifications: 
               
             
          
           
               
                   
                 Spray 
                 Residual 
                   
                   
                   
                   
               
               
                   
                 Application 
                 Application 
                 Number 
                 Number 
                 Cores for 
                 Cores for 
               
               
                   
                 Rate 
                 Rate 
                 of Slabs 
                 of 
                 Migration 
                 Bond Strength 
               
               
                 Tack 
                 (gal/yd 2 ) 
                 (gal/yd 2 ) 
                 Prepared 
                 Cores 
                 Investigation 
                 Testing 
               
               
                   
               
             
          
           
               
                 Hot Applied 
                 0.080 
                 0.080 
                 1 
                 6 
                 2 
                 4 
               
               
                 Polymer 
               
               
                 Modified Tack 
               
               
                 Hot Applied 
                 0.130 
                 0.130 
                 1 
                 6 
                 2 
                 4 
               
               
                 Polymer 
               
               
                 Modified Tack 
               
               
                 Hot Applied 
                 0.180 
                 0.180 
                 1 
                 6 
                 2 
                 4 
               
               
                 Polymer 
               
               
                 Modified Tack 
               
               
                 CQS-1HP 
                 0.130 
                 0.080 
                 1 
                 6 
                 2 
                 4 
               
               
                 CQS-1HP 
                 0.215 
                 0.130 
                 1 
                 6 
                 2 
                 4 
               
               
                 CQS-1HP 
                 0.300 
                 0.180 
                 1 
                 6 
                 2 
                 4 
               
               
                 NTSS-1HM 
                 0.160 
                 0.080 
                 1 
                 6 
                 2 
                 4 
               
               
                 NTSS-1HM 
                 0.260 
                 0.130 
                 1 
                 6 
                 2 
                 4 
               
               
                 NTSS-1HM 
                 0.360 
                 0.180 
                 1 
                 6 
                 2 
                 4 
               
               
                 Total 
                   
                   
                 9 
                 54 
                 18 
                 36 
               
               
                   
               
             
          
         
       
     
         [0022]    Test specimens were conditioned in an environmental chamber at 77° F. (25° C.) for a minimum of two hours prior to testing. The specimens were, then, loaded into a bond strength device, with the marked layer interface centered between the edge of the shearing block and the edge of the reaction head. Only the shearing block was allowed to move, and the reaction block was stationary. The specimen and the bond strength device were placed in the Geotest S5840 test apparatus with the loading head on top of the bonded interface. The loading apparatus applied a vertical shear load in a controlled displacement mode (0.1 inches/minute) to determine the maximum shear load and maximum displacement of the interface. For each test specimen, the interface bond strength was calculated by dividing the maximum shear load by the cross-sectional area of the specimen. 
         [0023]      FIG. 1  compares the average interfacial bond strengths of the three tack coat materials at three application rates. It should be appreciated that interfacial bond of the presently disclosed tack coat was significantly higher than either of the two prior art tacks. Furthermore, the cores in which the CQS-1HP and NTSS-1HM tack coat materials were used broke cleanly at the interface, indicating that it was the weakest junction between the pavement layers. However, in the cores in which the presently disclosed hot-applied tack material was used, they sheared through the OGFC layer instead of the interface, presumably due to the interface shear strength exceeding the shear strength of the OGFC mix. 
         [0024]    The interfacial strength that results from using the disclosed invention is impressive. It suggests, as it indicates, that a road made according to the methods herein would result in a lower chance of failure. Not only will this increase the life of the paved surface, but it could result in decreased damage to vehicles or their passengers due to OGFC that separates from the substrate layers. Therefore, not only does the invention provide significant benefit due to its non-tracking properties at ambient temperatures and the removal of the need for specialized paving machinery, it also creates stronger and safer pavement structures. 
         [0025]    The terms “comprising,” “including,” and “having,” as used in the claims and specification herein, shall be considered as indicating an open group that may include other elements not specified. The terms “a,” “an,” and the singular forms of words shall be taken to include the plural form of the same words, such that the terms mean that one or more of something is provided. The term “one” or “single” may be used to indicate that one and only one of something is intended. Similarly, other specific integer values, such as “two,” may be used when a specific number of things is intended. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention. 
         [0026]    The invention has been described with reference to various specific and preferred embodiments and techniques. However, it should be understood that many variations and modifications may be made while remaining within the spirit and scope of the invention. It will be apparent to one of ordinary skill in the art that methods, devices, device elements, materials, procedures and techniques other than those specifically described herein can be applied to the practice of the invention as broadly disclosed herein without resort to undue experimentation. All art-known functional equivalents of methods, devices, device elements, materials, procedures and techniques described herein are intended to be encompassed by this invention. Accordingly, the scope of the invention should be limited only by the attached claims. Whenever a range is disclosed, all subranges and individual values are intended to be encompassed. This invention is not to be limited by the embodiments disclosed, including any shown in the drawings or exemplified in the specification, which are given by way of example and not of limitation. 
         [0027]    All references throughout this application, for example patent documents including issued or granted patents or equivalents, patent application publications, and non-patent literature documents or other source material, are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in the present application (for example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference).