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26 a minimum mixing time of 120 s at 25C (77F) is applied and portions to be used shall be within the limits provided . . . If more the process is repeated at different temperatures to model per- than one aggregate is used, the aggregates shall be blended in designated proportions as indicated in the job mix formula, and formance in expected field conditions. The best mix is selected those proportions shall be maintained throughout the placement using aggregate coating as the prime criterion among those process. If aggregate proportions are changed, a new job mix for- alternative designs whose mixing time exceeded the minimum mula shall be submitted for approval (Missouri DOT 2004). across the expected temperature range (Moulthrop 2007). This specification relies heavily on the microsurfacing The optimum emulsion content is selected and three new contractor and its emulsion supplier to develop the job mix mixes are created. The first is at the percentage determined in formula. The Missouri DOT rated the performance of its the previous step and the other two are plus and minus 2% of microsurfacing projects as "Good" in their response to the that emulsion content to bracket the desired mix proportions. survey. In addition, the content analysis identified this agency Next, cohesion build-up is tested in accordance with ISSA as having the least prescriptive microsurfacing specification. TB 139 and performed at the expected field temperatures. Those two items of information, combined with the chap- Table 19 contains the mix properties that are tested and com- ter two finding that most agencies make the microsurfac- pared. The optimum binder content is determined by plotting ing contractor responsible for the job mix formula, indicate the output from the Wet Track Test (TB 100) and the Excess that microsurfacing is a good candidate for performance- Binder Test (TB 109) and determining where the two curves based contracting. intersect. This amount is subsequently adjusted using pro- fessional judgment to account for expected traffic volume (National Highway Institute 2007). The process demands an APPLICATION RATES experienced designer to select the optimum binder content (Austroads 2003b). Microsurfacing can be placed in relatively thick lifts. Table 20 contains the guidance found in an ISSA manual. However, it Final testing of the job mix is conducted when its compo- does have a maximum limit for Type III of 30 pounds per nents have been selected. The final mix is tested to ensure square yard (18.3 kg/m2) when placed unconfined or with a it meets the specifications listed in Table 19. The emulsion spreader box. Excessive application rates may cause the mix content and aggregate grading are reported as the "job mix to segregate and leave a flushed or excessively smooth sur- formula." Often adjustments within the allowable mix ranges face texture. If the engineer needs to exceed the stated maxi- need to be made to the job mix formula in the field to account mum application rates, the microsurfacing is then placed in for climatic variables encountered during installation. These multiple lifts. A study of microsurfacing's effectiveness as field adjustments are "limited to the amount of additives a rut filler was completed by the Alaska DOT and validated (cement and retardant) and water content needed to ensure a the application rates shown in Table 16 (McHattie and Elieff good homogeneous mix at the time of application" (National 2001). Getting application rates correct in the field is impor- Highway Institute 2007). A typical U.S. specification that tant as the production of the microsurfacing operation is con- describes the job mix formula process comes from Missouri: strained by the application rate. Additionally, the depth of the ruts to be filled also affects the application rate. ISSA states The manufacturer of the emulsion shall develop the job mix for- it as follows: mula and shall present certified test results for the engineer's approval. The job mix formula shall be designed in accordance with the ISSA recommended standards by an ISSA-recognized The correct application rate of the treatment can have a pronounced laboratory. Mix acceptance will be subject to satisfactory field effect on the success of the project. Excessive thickness can result performance. The job mix formula, all material, the methods and in rippling, displacement, and segregation. Inadequate thickness the proportions shall be submitted for approval prior to use. Pro- can cause excessive raveling and reduced life (ISSA 2010a). TABLE 19 TYPICAL MIX REQUIREMENTS Property Test (ISSA) Microsurfacing Wet-Track Abrasion Loss TB 100 (1 hour soak) 50 g/SF (538 g/SM) max. (wear loss) (6 day soak) 75 g/SF (807 g/SM) max. Wet Cohesion TB 139 (30 minutes) 12 kg-cm min. (traffic time) (60 minutes) 20 kg-cm min. Wet Stripping TB 114 Pass 90% Minimum (adhesion) Classification Compatibility TB 144 11 Grade Points Minimum (AAA, (integrity) BAA) Excess Asphalt by LWT Sand TB 109 50 g/SF (538 g/SM) max. Adhesion (excess binder) Lateral Displacement TB 147 5% max. (deformation) Source: ISSA (2010b). LWT = loaded wheel tester.