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14 CHAPTER THREE DESIGN PRACTICES INTRODUCTION to the development of a good mix design. Each of the material components, aggregate, asphalt emulsion, water, and additives, First, the survey asked respondents to indicate if they used a must meet all job specifications and test requirements. Indi- vidual materials must be qualified through testing before the formal design and, if so, which one. The U.S. sample contained laboratory will perform further tests to determine the mix com- 31 responses, of which 12 reported using a single formal design patibility and performance under simulated wear conditions method and 3 indicated that they had used 2 different methods (ISSA 2010b). in the past 5 years. The results regarding the survey are as follows: The job mix formula development process seeks to deter- mine the quality of the materials and evaluate how they will 1. ISSA Design Method for Microsurfacing (2010b): ISSA interact with each other during and after treatment curing. The A143--12 responses. job mix formula procedure includes the various phases of 2. ASTM Design Method for Slurry Seals (ASTM 2007a): the microsurfacing process in which the following questions ASTM D 3910-98--3 responses. are answered for each phase: 3. ASTM Design Method for Microsurfacing (ASTM 2007b): ASTM D 6372-99a--2 responses. Mixing: Will the components mix together and form true, free- 4. Texas Transportation Institute Design Method for flowing microsurfacing? Breaking and Curing: Will the emulsion break in a controlled Microsurfacing (West et al. 1996): TTI 1289--1 way on the aggregate, coat the aggregate, and form good films response. on the aggregate? Will the emulsion build up cohesion to a level that will resist abrasion owing to traffic? Additionally, 11 responses noted the use of empirical methods Performance: Will the microsurfacing resist traffic-induced stresses? (National Highway Institute 2007). based on an agency's past experience, and 4 agencies claimed to use no formal method. The Canadian sample was similar; 4 of 8 respondents reported using the ISSA A143 method The process involves prescreening the possible alternatives (2010b) to design their microsurfacing and 4 use an empir- for materials, the job mix formula itself, and final testing. At ical method. every step, the laboratory addresses mixing, breaking, curing, and performance issues to ensure that the final design is opti- Microsurfacing design is essentially a six-step process: mized both for the actual materials and for the environment in which the microsurfacing will be installed. Table 6 is a 1. Identifying and characterizing the roads where a micro- summary of survey responses to the question: Which entity surfacing treatment is appropriate. is responsible for performing the microsurfacing design? The 2. Selecting materials: emulsion, aggregate, mineral filler, table shows that only 7 agencies do their own job mix formula, additives, and water. whereas 22 U.S. and 6 Canadian agencies delegate this respon- 3. Developing a job mix formula. sibility to the microsurfacing contractor through the construc- 4. Laboratory testing of the job mix formula [also referred tion contract. Additionally, of the agencies that out-source the to as the "mix design" by some authors such as ISSA job mix formula, only two U.S. and two Canadian agencies do (2010b)]. not require that the final job mix formula be reviewed by an 5. Developing application rates. agency representative. 6. Preparing construction documents based steps 15. That the majority of the respondents that use microsurfac- ISSA stresses the importance of the design process in its ing (77%) chose to give the contractor the responsibility for most recent technical publication: the job mix formula leads to the conclusion that microsur- facing projects are being delivered as de facto performance Mix designs must be completed by a competent laboratory that contracts. The Canadian respondents indicated that all Cana- is experienced with state of the art asphalt emulsion/aggregate dian road agencies require a warranty that ranges from 1 to mixing technology as it applies to slurry systems. The laboratory 2 years. In the United States, seven agencies reported requir- must possess the necessary specialized equipment and knowl- edgeable staff to perform the required tests. Knowing the specific ing a warranty on their microsurfacing projects. The details system and the relationships of all the components is critical of the warranties are shown in Table 7.