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8 According to the Unified Facilities Guide Specification ing relatively smooth pavement. The upper limit is for old, (UFGS) 02744N (8), the advantage of the slow-setting grades oxidized, cracked, pocked, or milled asphalt pavement and over the rapid-setting grades is that they can be diluted. PCC pavements. The residual asphalt contents, as specified in Diluted emulsions are reported to give better results because the Hot-Mix Asphalt Paving Handbook 2000 (12), should (1) diluted emulsion provides the additional volume needed range from 0.04 to 0.06 gal/yd2. Open-textured surfaces for the distributor to function at normal speed when lower require more tack coat than surfaces that are tight or dense. application rates are used and (2) diluted emulsion flows eas- Dry, aged surfaces require more tack coat than surfaces that ily from the distributor at ambient temperatures allowing for are "fat" or flushed. A milled surface would require even more a more uniform application (9, 10). On the other hand, diluted residual asphalt because of the increased specific surface area, slow-setting emulsions may take several hours to break or even as much as 0.08 gal/yd2. Only half as much residual asphalt several days to completely set. In addition, an overlay tacked is typically required for new HMA layers, 0.02 gal/yd2 (7, with slow-setting emulsion may be vulnerable to slippage dur- 12). Recently, Ohio published typical tack coat application ing its early life (8). Such an overlay exposed to heavy traf- rates for various pavement types using slow-setting asphalt fic immediately after construction could experience excessive emulsions (SS1, SS1-h) (13). As shown in Table 2, the over- slippage in a short period of time. all residual rates vary from 0.03 to 0.08 gal/yd2 for different pavement types. 2.2 Tack Coat Application Rate 2.3Tack Coat Breaking A proper bond between pavement layers is essential in and Setting Time order to provide a monolithic pavement structure. Selection of an optimum tack coat material and application rate is cru- Before asphalt emulsion breaks, it is brown in color because cial in the development of this bond. Pavement surfaces with it contains both asphalt cement and water. After broken, the different conditions (e.g., new, old, or milled) require differ- water separates from the emulsion and the color of the emul- ent tack application rates to achieve a proper interface bond. sion changes from brown to black. Once all water is evaporated, Excessive tack coats may promote shear slippage at the inter- the emulsion is said to have "set." Under most circumstances, face. Most importantly, it is the residual amount of asphalt an emulsion will set in 1 to 2 hours (12), but the literature cement, not the application rate of diluted asphalt emulsion, generally lacks complete agreement concerning how long a that should be specified. tack coat should remain uncovered before placing the subse- From their survey, Paul and Scherocman (6) found that quent asphalt layer. The IBEF survey indicated that the lapse the residual application rates of the emulsions varied between of time required between the application of the tack coat 0.01 and 0.06 gal/yd2, depending on the type of surface for and the application of the next asphalt layer ranges from application. The IBEF survey (4) indicated that the residual 20 minutes for a broken or cold binder to several hours for a asphalt content ranged from 0.02 to 0.09 gal/yd2 for tack "dry" binder (after all water has evaporated or set) (4). Paul coats applied on conventional asphalt surfaces. The Asphalt and Scherocman (6) found that many state DOTs specified Institute (AI) specifications on tack coats reported that the a minimum time between tack coat application and place- application rates ranged from 0.05 to 0.15 gal/yd2 for an ment of HMA to provide adequate curing time for the emul- emulsion diluted with one part water to one part emul- sion to break and set. Three state DOTs had a maximum time sion (11), which is equivalent to residual application rates that a tack coat could be left before placement of the asphalt between 0.02 to 0.05 gal/yd2. The lower application rates are concrete: Alaska DOT specified a maximum setting period recommended for new or subsequent layers, while the inter- of 2 hours for CSS-1; Arkansas DOT specified a maximum mediate range is for normal surface conditions on an exist- setting period of 72 hours for SS-1; and Texas DOT specified Table 2. Typical tack coat application rates (13). Application Rate (gal/yd2) Pavement Condition Residual Undiluted Diluted (1:1) New HMA 0.03 ~ 0.04 0.05 ~ 0.07 0.10 ~ 0.13 Oxidized HMA 0.04 ~ 0.06 0.07 ~ 0.10 0.13 ~ 0.20 Milled Surface (HMA) 0.06 ~ 0.08 0.10 ~ 0.13 0.20 ~ 0.27 Milled Surface (PCC) 0.06 ~ 0.08 0.10 ~ 0.13 0.20 ~ 0.27 Portland Cement Concrete 0.04 ~0.06 0.07 ~ 0.10 0.13 ~ 0.20