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25 TABLE 18 MICROSURFACING AGGREGATE GRADATIONS (ISSA 2010b) AND USAGE FOUND IN THE SURVEY Percentage Passing Sieve Size Type II Type III Stockpile Tolerance 3/8 (9.5 mm) 100 100 -- # 4 (4.75 mm)) 90100 7090 5% # 8 (2.36 mm) 6590 4570 5% # 16 (1.18 mm) 4570 2850 5% # 30 (600 m) 3050 1934 5% # 50 (330 m) 1830 1225 4% # 100 (150 m) 1021 718 3% # 200 (75 m) 515 515 2% Survey Usage Other Gradation U.S. 16 11 6 Canada 2 7 1 Total 18 18 7 Content Analysis Total 7 6 8 Grand Total 25 24 15 mineral fillers describes the mineral filler requirements as follows: Additives found in the study: Aluminum sulfate crystals, Mineral filler shall consist of carbonate dust, Portland cement, hydrated lime, crushed rock screenings, fly ash, or rotary lime Ammonium sulfate kiln dust, subject to approval by the Engineer . . . Crushed rock Inorganic salts screenings to be used as mineral filler shall be of such composi- Liquid aluminum sulfate, tion and quality that the bituminous mixture containing the rock Amines screenings will have stability and durability equivalent to those Anti-stripping agents of the comparable mixture containing one of the other acceptable filler materials. The rock screenings shall be free from clay and shale (Minnesota DOT 2005). The content analysis revealed that most microsurfacing specifications (87%) direct the contractor to include "addi- Mineral fillers found in the study: tives approved by the emulsion manufacturer . . . to the emul- sion mix or to any of the component materials to provide Portland cement control of the set time in the field" (New Mexico DOT 2009) or Hydrated lime a similarly worded specification clause. The survey also sought Limestone dust information on additives, and only one U.S. (Tennessee) and Crushed rock screenings one Canadian agency (Nova Scotia) indicated that they require Fly ash an anti-stripping agent. Thus, the following effective practice Kiln dust is identified: Baghouse fines Compounds added to microsurfacing job mix formulae can be selected by the emulsion manufacturer and the agency can then verify that they are compatible with the approved Additives job mix formula. Other materials are sometimes added to the microsurfacing DEVELOPING AND LABORATORY TESTING mix. These additives vary and are often specific to proprietary OF A JOB MIX FORMULA microsurfacing systems. The National Highway Institute's Pavement Preservation Manual (2007) notes that additives The development of a job mix formula fundamentally involves normally act as retardants to the reaction with emulsions. Typ- calculating the proportions of each component to the micro- ical additives include emulsifier solutions, aluminum sulfate, surfacing mix. ISSA A143 (2010b) is normally used as the aluminum chloride, and borax. Varying the concentration of guideline from which the job mix formula is completed. It an additive allows the contractor to control the breaking and starts by estimating the approximate proportions using ISSA curing times. For example, the contractor can change the con- Technical Bulletin 102. This entails creating a matrix of mix centration to account for increasing and decreasing air tem- recipes and recording the manual mixing time for each option. peratures across the work day (Hicks et al. 2000; Caltrans During this operation, the technician looks for and visually 2007, 2008). assesses changes such as foaming and coating. The standard of