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10 Figure 3. Proper nozzle angle setting (14). will result in some areas of the pavement having thicker or thinner coverage and possible interference between noz- zles. Differing coverage will result in streaks and gaps in the tack coat (see Figure 4). The size of the nozzles needed to apply an asphalt emul- sion material for a surface treatment, chip seal, or seal Figure 5. Small nozzle opening (19). coat is significantly larger than the size of the nozzles needed to apply a tack coat. Using a nozzle that is too small with too much pressure results in a surface that has a and 54C. Excessive heating may cause the emulsion to spider web coating of tack coat material (see Figure 5). break while still in the distributor. Spray bar height should remain constant. As tack coat is applied, the vehicle will become lighter, causing the spray bar to rise. The tack coat application vehicle should be able 2.4.2 Proper Tack Coat Application to compensate for this. Excessively low spray bars result Proper application of tack coat is a key component in high- in streaks (see Figure 4), while excessively high spray bars quality asphalt pavement rehabilitation. Proper tack coat appli- cause non-uniform transverse coverage. cation begins with properly calibrated application equipment. Pressure within the distributor must be capable of forc- If the distributor has not been used for some period of time, ing the tack coat material out of the spray nozzles at a con- the operator should place a trial tack coat application over stant rate. Inconsistent pressure will result in non-uniform some convenient, unused area to ensure that all of the nozzles application rates. are open and operating properly. In addition, the distributor Tack distributors must be capable of maintaining tem- application rate needs to be calibrated, both in the transverse perature of the asphalt cement material to ensure the direction and in the longitudinal direction, using the proce- material will adequately flow. For slow-setting asphalt dure described in ASTM Method D 2995 (19). Spray bar height emulsions such as SS-1, the spraying temperature within depends on truck speed, nozzle configuration, and application the distributor should be maintained between about 24C pressure. Operators should adjust the spray bar height through- out the day depending on the amount of emulsion in the tank. As a summary, the literature suggests the fundamental aspects of achieving tack coat success are Having a thoroughly clean roadway surface, Ensuring all the equipment functions properly and is set up correctly, Choosing the proper application rate for the tack material used and the existing surface conditions, Applying the materials uniformly, and Allowing the tack to set prior to paving to ensure the best possible bond between layers. One perpetual problem often associated with tack coat application using distributor trucks is that haul trucks normally drive on the applied tack coat, thus tracking the Figure 4. Non-uniform tack coat: streaks. tack coat material and removing it from the pavement, as