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8 FIGURE 3 Difference between slurry seals and microsurfacing (adapted from Bickford 2008). Microsurfacing--A mixture of cationic polymer- agrees with the approach used in Australia and New Zealand, modified asphalt emulsion, 100% crushed aggregate, where "Slurry Surfacing [is] a general term for Slurry Seal water, and other additives properly proportioned and and Microsurfacing" (Austroads 2003b). spread over a prepared surface. The special purpose polymers and additives used in microsurfacing allow With one exception, this synthesis will concentrate the higher than normal rates of application and multi-layer remainder of its discussion strictly on microsurfacing. In chap- applications for projects such as rut filling and high- ter three, the topic of project selection will include an analysis way leveling and resurfacing. A multi-layer application of those situations in which microsurfacing is uniquely appro- allows material depths to exceed the normal one stone priate and those where a slurry seal is a better option. thickness rule. PROTOCOL TO DEVELOP CONCLUSIONS, Figure 3 shows the difference between microsurfacing and EFFECTIVE PRACTICES, AND FUTURE the two slurry seals and Table 2 consolidates the ISSA defin- RESEARCH NEEDS itions and other technical definitions found in the literature. Microsurfacing appears to have three features that differentiate The major factor in developing a conclusion was the occur- it from slurry seals: rence of similar trends found in two or more research instru- ments. Additionally, greater weight was given to information 1. It always contains polymers. developed from the survey of highway agencies. The litera- 2. It cures rapidly through chemical reaction, which per- ture review and specification content analysis were consid- mits traffic to be returned in a shorter time. ered to be supporting sources. Finally, the case studies were 3. It can be placed in layers thicker than one stone deep. used to validate the conclusions as appropriate because they are examples of how U.S. and Canadian highway agencies The California DOT (Caltrans) Maintenance Technical have actually implemented microsurfacing. Advisory Guide (Caltrans 2009) goes further in its discussion of the differences in the two technologies. Table 3 was taken Effective practices were identified in the same manner from that guide and adds additional information to assist the as conclusions, with one exception. An effective practice is reader in understanding the fundamental differences between specific to a single factor in microsurfacing practice and microsurfacing and slurry seal. The Asphalt Emulsion Man- may only apply to a specific set of circumstances, such as ufacturers Association (AEMA) states that microsurfacing is agencies in northern climates; whereas, conclusions can be an alternative to hot mix for rut filling (AEMA n.d.). Finally, generalized. Future research needs were developed based a pooled fund study devoted to updating microsurfacing and on practices that were described in the literature and con- slurry seal design procedures also faced this terminology issue firmed as effective by one of the research instruments but and concluded that because "constructability issues are the generally not widely used. Gaps in the body of knowledge same for both," the two might be categorized as "Slurry found in this study were also used to define the areas where Surfacing Systems" (Fugro-BRE/Fugro South 2004). This more research would be valuable.

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9 TABLE 2 CURRENT DEFINITIONS FOR MICROSURFACING AND SLURRY SEAL Source Microsurfacing Slurry Seal ISSA A mixt ure of cationic polymer modified "A mixture of aggregate, emulsified (2010a) asphalt emulsion, 100% crushed aggregate, asphalt, water, and additives ... applied in a water and other additives...multi-layer stone thickness ..." applications for projects such as rut filling..." AEMA "...a polymer modified quick traffic slurry seal "...a blend of emulsion, aggregate, water, (undated) system...can be placed in much greater and additive." thicknesses than conventional slurry seal..." Hicks et "... a polymer-modified cold slurry "...a slow- or quick-set emulsion and al. (1999) system...uses aggregate, which normally aggregates that typically pass the 6-mm passes the 9-mm (3/8-in.) sieve. [It will] cure (1/4-in.) sieve... used to seal minor surface and develop strength faster than conventional cracks and voids, retard surface raveling, slurry seals and can be placed in thicker layers fill minor ruts, and improve surface ..." friction." Hicks et "A mixture of polymer modified asphalt "A mixture of slow setting emulsified al. (2000) emulsion, mineral aggregate, mineral filler, asphalt, well graded fine aggregate, mineral water, and other additives, properly filler, and water. It is used to fill cracks and proportioned, mixed, and spread on a paved seal areas of old pavements, to restore a surface." uniform surface texture, to seal the surface to prevent moisture and air intrusion into the pavement, and to provide skid resistance." FLHD "Microsurfacing emulsions break and cure "...slurry seals may or may not contain (2003) much more quickly than unmodified slurry polymers. Slurry seals are generally laid at seals, allowing faster return to traffic and less thicknesses of 0.40 to 0.60 in. (1 to 1.5 traffic damage ... can be placed in thicker lifts cm), whereas slower breaking slurry seals for rut-filling. By definition, microsurfacing cure on the surface, "skinning over" and contains polymers... microsurfacing can be preventing thorough breaking and curing thickly applied in multiple layers... [it] breaks when they are applied at greater chemically...[which] permits the thicknesses." microsurfacing to gain cohesive strength rapidly." Austroads "A Bituminous Slurry Surfacing, usually "A thin layer of Bitu minous Slurry (2003b) containing polymer, which is capable of being Surfacing, usually without a polymer spread in variably thick layers for rut-filling modifier." and correction courses, and for wearing course applications requiring good surface texture." AEMA = Asphalt Emulsion Manufacturer's Association. TABLE 3 CALIFORNIA DOT MAINTENANCE TECHNICAL ADVISORY GUIDE ON MICROSURFACING VERSUS SLURRY SEAL Differences In: Microsurfacing Slurry Seal Asphalt Emulsion Always polymer modified, quick set Could be polymer modified Aggregate Stricter specifications for sand Can use Type I, II, or III Quality/Gradation equivalent; use only Type II and Type III Additives/Break Chemical break largely independent of Breaking and curing dependent on weather conditions weather conditions Mix Stiffer mix, use augers in the spreader Softer mix, use drag box Stiffness/Equipment box and secondary strike-off Applications Same as slurry seal + rut filling, night Correct raveling, seal oxidized work, correction of minor surface profile pavements, restore skid resistance irregularities Source: Caltrans (2009).