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29 Arizona, California, Colorado, Idaho, Louisiana, Rhode Island, High Float 70% Texas, and Wisconsin. All respondents from Australia, New Emulsions 19% Zealand, and South Africa indicated the use of precoated aggre- Polymer 50% gate with asphalt cement binders. Alaska, Pennsylvania, Texas, Modifiers 69% and Wisconsin indicated that they also use precoated aggregate 100% with emulsion binders. Emulsions 100% Hot Asphalt Cement 9% BINDER SELECTION % of Agencies The Asphalt Institute's Asphalt Surface Treatments-- Construction Techniques (1988) outlines the following require- United States Canada ments for chip seal binders: FIGURE 26 Distribution of North American chip seal · The binder should not bleed when applied at the appro- binder selection. priate rate. · At the time of application, the binder needs to be fluid opened for traffic early after chip seal application and broom- enough to uniformly cover the surface, yet viscous ing. However, the disadvantages include high application enough to not puddle or run off the pavement. · The binder should develop adhesion quickly and hold temperatures, sensitivity to moisture in rock particles, and a requirement for more rolling energy. High working tempera- the aggregate tightly to the roadway surface. tures can also create safety concerns that may limit the appli- cation season to hot summer months. Harder asphalt cements There are two main binder types used for chip seal opera- hold cover stone more tightly, but initial retention is more dif- tions: asphalt cements and emulsified asphalts. Climate and ficult to obtain (Benson and Gallaway 1966). Table 7 shows weather play an extremely important role in chip seal binder the typical hot asphalt binders being used in the United States, selection. The selection of the binder should be influenced by as found in the survey responses. surface temperature, aggregate, and climate of region during construction operations (McLeod 1969). One of the most important environmental factors to account for when using Emulsion Binders any bituminous binder is the ambient air temperature. It is accepted that ambient temperatures at the time of construc- Emulsified asphalts have three primary constituents: asphalt tion closely affect the quality of chip seal (Gransberg et al. cement, emulsifying agent, and water (A Basic Emulsion 1998). In hot weather, bleeding can be prevented with binder Manual 1997). The asphalt cement is suspended in the water selection directed toward the use of "harder" hot applied with the help of an emulsifier. At the time of the application asphalts and emulsions. During construction with low ambi- of the binder, the water evaporates, leaving behind the resid- ent air temperatures, high humidity, or damp aggregate ual asphalt that bonds with the aggregate. One of the major and pavement surfaces, emulsions are generally believed to concerns with using emulsions is the spreading time of the be more successful than hot asphalts (Sprayed Sealing aggregate after the emulsion is applied. The phenomenon Guide 2004). that occurs when the water evaporates is called "breaking," evidenced when the binder's color changes from brown to As a result of differences in nomenclature between North black. The aggregate chips must be applied and rolled before America and overseas, international responses to questions the emulsion has broken (Jackson 1990). This emulsion- about binder were not effective. Figure 26 is a graphical rep- specific issue indicates that if there is too long a wait, the resentation of binder selection practices in Canada and the ability for the rollers to properly seat the aggregate is greatly United States. One specific practice that is apparent is that reduced. high float emulsions are more widely used in Canada than in the United States. Emulsions can be either anionic or cationic depending on the chemistry created by the emulsifying agent. Generally, Asphalt Cement Binders TABLE 7 HOT ASPHALT CEMENT BINDER USE IN Some agencies use hot-applied asphalt cement as the binder THE UNITED STATES for chip seals. Soft asphalt cement grades are recommended for use in chip seal applications (Asphalt Surface Treatments-- Binder Type State DOT AC-10 Georgia Specifications undated). Adhesion agents may be added to AC15-P Texas these asphalt cements to enhance chip retention. Asphalt AC15-5TR Arizona, Texas cements are advantageous because the roadway can be AC-20 Georgia
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30 cationic emulsions outperform anionic emulsions on a chip ing as much as 5% passing the No. 200 sieve (Janisch and seal project because they are less sensitive to weather, inher- Gaillard 1998). ently have antistripping qualities, and are electrostatically compatible with more types of aggregate (McHattie 2001). Cationic emulsions have a positive charge, and because oppo- Modified Binders site charges attract, they are drawn toward most aggregate particles. Thus, a direct and very rapid bonding between the The survey results show that modified binders are used by most emulsion and an aggregate or pavement is possible. In add- agencies, with the only limit to their use being the additional ition, emulsions are not as sensitive as asphalt cements to the cost. The most common type of modification is through the use moisture in aggregate and in the atmosphere. Also, because of polymers. Research has shown that polymer modification excessive presence of water reduces the viscosity of the binder, reduces temperature susceptibility, provides increased adhe- emulsions require much lower material application temper- sion to the existing surface, increases aggregate retention and atures than asphalt cements. Asphalt emulsions are graded flexibility, and allows the roadway to be opened to traffic based on setting speed and the relative viscosity of the emul- earlier (Zaniewski and Mamlouk 1996). Polymers are con- sion. Table 8 lists emulsion use as found from the survey sidered to be beneficial in minimizing bleeding, aiding chip responses. retention, and enhancing the durability of the chip seal, and they are recommended for high traffic volume roads and late season work (Shuler 1990; Wegman 1991). High Float Emulsions Integrating crumb rubber into chip seal binders has proven High float emulsions are those emulsions that pass the float successful at mitigating reflective cracking, improving aggre- test (AASHTO T50, ASTM D139). High float emulsions gate retention, and reducing noise emissions. When blended allow for a thicker residual asphalt film on the aggregate, with bitumen, the binder behaves as an elastomer (Sprayed and this prevents runoff of the asphalt from the surface of Sealing Guide 2004). In Australia, crumb rubber is added at a the road (Seal Coat . . . 2003). The wetting agents used in rate of 16% to 20% by volume (Beatty et al. 2002). this type of binder penetrate the dust coating and provide a good bond with the aggregate particles. Agencies that use Proprietary additives, known as adhesion agents, are used high float emulsions commonly state that they used them in to improve the degree of wetting of the aggregate by the situations where local aggregate is excessively dirty or binder, thus enhancing the adhesion between the binder and dusty and the cost to wash them to meet a specification of aggregate. Adhesion agents are generally proprietary products. less than 1% passing the No. 200 sieve would be too expen- Therefore, their application rates are usually specified by their sive. This type of binder can be used with aggregates hav- manufacturers. Also known as antistripping agents, these addi- TABLE 8 ASPHALT EMULSION BINDER USE IN THE UNITED STATES Binder Type U.S. Locations Non-U.S. Locations CRS-1 Nevada None CRS-1H Kansas, Nevada None CRS-2 Connecticut, Iowa, Maryland, Ontario Michigan, Montana, Nevada, New York, North Carolina, Oklahoma, Utah, Virginia, Washington, Wisconsin CRS-2H Arizona, California, Texas None CRS-2P Arizona, Arkansas, Alaska, New Zealand, Nova Scotia Idaho, Iowa, Louisiana, Michigan, Minnesota, Mississippi, Montana, Nebraska, North Carolina, New York, North Dakota, Oklahoma, Texas, Washington, Wisconsin, Wyoming HFRS Alaska, Colorado, New York, British Columbia, Manitoba, Wisconsin Ontario, Saskatchewan, Quebec, Yukon HFRS-2P Colorado, New York, North Saskatchewan, Quebec Dakota, Oregon, Texas, Wisconsin, Wyoming Note: Includes city and county responses in state/province designation.