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31 United States with mixed success. Montana and Nevada Antistripping agents 32 responded that their trials were unsuccessful; however, Rubber crumb 19 Oklahoma and two counties in California reported that 28 theirs were a success. Latex Polymers 57 International respondents unanimously believe that 0 20 40 60 80 100 geotextile-reinforced seals are effective for treating badly No. of Agencies cracked, oxidized, or structurally distressed pavements. The construction process basically involves placing a tack coat on FIGURE 27 Use of binder modifiers. the distressed pavement, spreading the geotextile on the tack coat, spraying the geotextile with binder, and then applying the aggregate. Figure 28 shows the mixed success rate of tives may be added to either the binder or precoating asphalt geotextile chip seals in the United States, yet overwhelming (Sprayed Sealing Guide 2004). In addition, hydrated lime can success in countries overseas. None of the Canadian also be used to enhance adhesion and improve a binder's provinces responded that they had performed trials with resistance to oxidation (Dickinson 1984). Figure 27 shows geotextile-reinforced seals. the polymer modified binders to be the most popular among respondents, with 57 agencies reporting that they regularly A fiber-reinforced seal usually involves blowing glass fibers use them. onto an application of a polymer-modified binder, with the aggregate being spread quickly after this application. Fiber- reinforced chip seals require special purposebuilt equipment AGGREGATEBINDER COMPATIBILITY to spray and apply the treatment. In general, these seals are not as effective as geotextile seals, but they are less costly (Sprayed Adhesion between the aggregate and binder is governed by a Sealing Guide 2004). number of variables, but most important is the type of aggre- gate. The adhesion between aggregate and binder is a func- tion of mechanical, chemical, and electrostatic properties MATERIAL SELECTION CONCLUSIONS (Yazgan and Senadheera 2003). Possible mechanical- and AND BEST PRACTICES chemical-related factors include aggregate dust, moisture The conclusions in this area are quite evident. First, the selec- content, and binder temperature. Different types of aggregate tion of chip seal materials is project dependent, and the engi- are better suited to certain binders as a result of electrostatic neer in charge of design must fully understand not only the charges (Sprayed Sealing Guide 2004). Basically, the binder pavement and traffic conditions in which the chip seal will and aggregate must have opposite charges. If this is not the operate but also the climatic conditions under which the chip case, the binder will not form a strong bond with the aggregate seal will be applied. It appears that the widespread use of emul- and it will ravel. Therefore, local aggregate is critical to deter- sion binder chip seals results from the notion that emulsions are mining which type of chip seal to use, which type of binder to less sensitive to environmental conditions during construction. design for, and which type of construction procedures to spec- Additionally, as emulsions are installed at a lower binder tem- ify. In addition, porosity and the presence of water on the perature, they are probably less hazardous to the construction surface of the aggregate affect binderaggregate compatibil- crew. Binder performance can be improved through the use of ity. Aggregate, which is quite porous, will actually lead to modifiers such as polymers and crumb rubber. excessive absorption of the binder. Loss of aggregate shortly after construction is indicative of poor adhesion between the binder and aggregate. Before construction, it is essential to con- duct laboratory testing to determine the adhesion capability 7 between the aggregate and the binder. An antistrip test, such 8 as ASTM D1664 (AASHTO T182), will assist in determin- 6 3 ing the compatibility between the aggregate and binder. This 2 test may also highlight the need for an antistrip additive 4 (Asphalt Seal Coats 2003). 2 0 North America AU, NZ, UK, SA GEOTEXTILE- AND FIBER-REINFORCED SEALS Yes No The use of geotextiles and sprayed fibers is common prac- tice in Australia and New Zealand. A small number of FIGURE 28 Success rate of geotextile-reinforced geotextile-reinforced seals have been constructed in the chip seals.

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32 Next, the selection of the binder is dependent on the type 1. Conduct electrostatic testing of chip seal aggregate of aggregate that is economically available for the chip seal source before chip design to ensure that the binder project in the United States and Canada. That Australia and selected for the project is compatible with the potential New Zealand are willing to bear additional aggregate costs sources of aggregate. to ensure the quality of their chip seals is something that 2. Specify a uniformly graded, high-quality aggregate. should be seriously considered in North America. 3. Consider using lightweight synthetic aggregate in areas where post-construction vehicle damage is a major The aggregate should be checked to ensure that electrosta- concern. tic compatibility is met with the type of binder specified. Also, 4. Use life-cycle cost analysis to determine the benefit of precoating of the aggregate appears to be required for use with importing either synthetic aggregate or high-quality hot asphalt cement binders to ensure good adhesion after appli- natural aggregates to areas where availability of high- cation. Finally, it appears that the use of geotextile-reinforced quality aggregate is limited. chip seal is promising and should be considered for those roads 5. Use polymer-modified binders to enhance chip seal that have more than normal surface distress and for which an performance. overlay is not warranted. Therefore, several best practices can be extracted from the foregoing discussion: