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7 7 Section 2 State of Practice A review of the existing state of practice was conducted to Original emulsion--an emulsion of paving-grade asphalt and identify factors related to the use of tack coats for both new water that contains a small amount of emulsifying agent. HMA pavements and overlays on new, old, milled HMA and Original slow-setting grade emulsions contain up to 43% for PCC pavements. This review involved an extensive search water, and original rapid-setting grade emulsions contain of all published materials and ongoing research projects to up to 35% water. obtain the latest information on the research of the bonding Diluted emulsion--an original emulsion that has been mechanisms of tack coat in pavement structure. A worldwide diluted by adding an amount of water equal to or less than survey on current tack coat practices was conducted to bet- the total volume of original emulsion. ter understand the current state of tack coat practices and Residual asphalt content--the amount of paving asphalt assist in designing an ensuing research experiment. Results remaining on a tacked pavement surface after the emulsion of the survey provided the basis for the experimental factorial has broken and set (i.e., after all water has evaporated). design that was used in Phase II of the NCHRP Project 9-40 Tack coat break--water separates from the emulsion and research project. the color of the tack coat changes from brown to black. A worldwide survey on tack coat application was con- 2.1 Tack Coat Materials ducted by the International Bitumen Emulsion Federation According to ASTM D8, Standard Terminology Relating to (IBEF) (4, 5). Seven countries--Spain, France, Italy, Japan, the Materials for Roads and Pavements, "Tack coat (bond coat) is an Netherlands, the United Kingdom, and the United States-- application of bituminous material to an existing relatively non responded through their professional associations. The sur- absorptive surface to provide a thorough bond between old and vey results indicated that the most frequently used tack coat new surfacing" (1). Generally, hot paving asphalt cement, cut- material is cationic emulsion. Paul and Scherocman (6) con- back asphalt, and emulsified asphalt have all been used as tack ducted a survey of tack coat practices in the United States. coat materials, but cutback asphalts (asphalts dissolved in sol- This survey received responses from 42 state DOTs and the vents such as kerosene or diesel) are not typically used for tack District of Columbia. They found that almost all the state coat applications today due to environmental concerns. The DOTs use slow-setting emulsions for tack coats. The emul- most widely used tack coat material in the world is emulsified sions mostly used are SS-1, SS-1h, CSS-1, and CSS-1h. Only asphalt. Emulsified asphalt, or asphalt emulsion, is a nonflam- one responding state (Georgia) routinely used hot asphalts mable liquid substance that is produced by combining asphalt (AC-20 and AC-30) as tack coats. A recent phone survey and water with an emulsifying agent such as soap, dust, or cer- conducted by Cross and Shrestha (7) in 13 mid-western and tain colloidal clays (2). The most common types of emulsions western U.S. states indicated that slow-setting emulsions are used for tack coats include slow-setting grades of emulsion the primary materials for tack coat, except for California, such as SS-1, SS-1h, CSS-1, and CSS-1h and the rapid-setting where the AR-4000 was the most common tack coat material grades of emulsion such as RS-1, RS-2, CRS-1, CRS-2, CRS-2P followed by either SS-1 or CSS-1. The Kansas DOT was the (polymer-modified), and CRS-2L (latex-modified). According only agency that reported occasionally using cutback asphalts to the Construction Procedure Bulletin (CPB) of the California as tack coat. New Mexico DOT and Texas DOT reported DOT, several basic terms used in an asphalt emulsion tack coat that performance-grade (PG) binders (asphalt cement) were application are as follows (3): occasionally used as tack coat materials.