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9 (a) Tack coat distributor truck (b) Paver with tack coat tank and spray bar Figure 1. Application equipment of tack coat. a maximum setting period of 45 minutes for SS-1 or MS-2. 2.4 Tack Coat Application Methods Four states indicated that paving was required the same day the tack coat was applied. 2.4.1Equipment It is generally recognized that an emulsion should be Two types of tack coat application methods are shown in completely set before new mix is placed on top of the tack Figure 1: (a) a conventional tack coat distribution truck and coat material. Laboratory studies (14, 15) agreed with this (b) a special paver with tack coat tank and spray bar. assumption showing that greater interface shear strengths are Generally, the best tack coat application results from a achieved with longer curing times for the tack coat prior to "double lap" or "triple lap" coverage. As shown in Figure 2, testing. This was true for both laboratory-fabricated samples good "double/triple lap" means that the nozzle spray patterns (14) and field cores (15). However, experience has also shown overlap one another such that every portion of the pavement that new HMA can usually be placed on top of unset tack surface receives spray from two or three nozzles. coat and even over an unbroken tack coat emulsion with no Several vehicle-related adjustments and settings are crucial detrimental effect on pavement performance (12). Indeed, in to achieving uniform tack coat placement. Essentially, the Europe, emulsified tack coat is often applied to the pavement nozzle patterns, spray bar height, and distribution pressure surface underneath the paver just before the HMA in front of must work together to produce uniform tack coat application the paver screed. Some European firms have used this tacking (14, 19). Specific guidance is summarized as follows: process with conventional dense-graded HMA mixtures and normal emulsified asphalt tack rates without negative conse- Nozzle spray patterns should be identical to one another quences, but there may be concerns with water vapor passing along a distributor spray bar. To prevent the spray of liq- through a dense-graded mat. In the United States, this emul- uid asphalt from interfering with adjacent spray nozzles, sion spray method is used in the NovachipTM construction all nozzles should be set at the same angle (about 30) to the process, as reported by Estakhri and Button (16, 17). axis of the spray bar (see Figure 3). Lack of a uniform angle Figure 2. Uniform tack coat application with double and triple overlapping (18).