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27 TABLE 20 SUGGESTED APPLICATION RATES Aggregate Type Location Suggested Application Rate per Pass Type II Urban and Residential Streets 1020 lb/SY (5.410.8 kg/SM) Airport Runways 1020 lb/SY (5.410.8 kg/SM) Scratch or Leveling Course As required Type III Primary and Interstate Routes 1530 lb/SY(8.116.3 kg/SM) Wheel Ruts As required (see below) Scratch or Leveling Course As required Rut Depth Application Rate 0.50.75 in. (12.719.1 mm) 2030 lb/SY ( 10.816.3 kg/SM) 0.751.00 in. (19.125.4 mm) 2535 lb/SY(13.619.0 kg/SM) 1.001.25 in. (25.431.75 mm) 2838 lb/SY (15.220.6 kg/SM) 1.251.50 in. (31.7538.1 mm) 3240 lb/SY(17.421.7 kg/SM) Source: ISSA (2010a). SERVICE LIFE be expected to last at least 4 years to keep this road in full ser- vice. Table 14 lists Type II and Type III microsurfacing as Microsurfacing is normally used as a pavement preservation having 2 to 4 years and 3 to 5 years of expected service life, tool to extend the service life of the existing pavement. Thus, respectively. Therefore, the engineer could select either as the service life of the microsurfacing is equal to the amount it an appropriate treatment to extend the overlay's service life extends the service life of the underlying pavement. Pittenger until its scheduled reconstruction in 4 years. (2010) adjusted the classic service life concept for pavement design to the pavement preservation and maintenance domain. She posits that the service life of a pavement preservation Service Life Determination treatment is one of two possible states: Table 21 shows the effective service life for the microsur- facing reported in the literature and the results of the survey 1. Continuous: The treatment will remain [in place] until it fails. 2. Terminal: The treatment will be removed before the end of responses regarding observed microsurfacing service life. its continuous service life by a programmed rehabilitation or The table shows broad agreement that the average service life restoration project such as milling and overlaying (Pittenger of a microsurfacing application is 6 to 7 years. It is noted that 2010). both the literature and the survey response service life esti- mates are invariably presented with the caveat that the under- lying road be in good condition. This provides further confir- Pavement Life Extension mation for the effective practice that microsurfacing not be Ideally, microsurfacing service life would be at least as long used on structurally deficient roads. as the period from the present to the given road's next sched- uled rehabilitation, upgrade, or replacement. For example, an SUMMARY asphalt pavement overlay is nearing the end of its expected 10-year service and is scheduled to be rehabilitated in 4 years This chapter focused on the design of microsurfacing. It as part of a reconstruction project. The engineer selects a furnished information on the process found in the litera- pavement preservation and maintenance treatment that can ture, the results of a microsurfacing specification content TABLE 21 SUMMARY OF MICROSURFACING SERVICE FROM THE LITERATURE AND THE SURVEY Minimum Maximum Average Source Service Life Service Life Service Life Bausano et al. (2004) 6 7 6 Lyon and Persaud (2008) 5 7 6 Smith and Beatty (1999) 7 10 7 Watson and Jared (1998) 5 7 6 Hicks et al. (1997) 5 7 6 Labi et al. (2007) 5 15 7 Temple et al. (2002) 4 10 7 Chehovits and Galehouse (20100 2 5 3.5 Average Literature 4.8 8.5 6 U.S. Survey Respondents 1 15 6 Canadian Survey Respondents 3 9 7 Average of Survey Responses 3.6 13 7

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28 analysis and the corresponding results of the survey of high- 6. Microsurfacing can be expected to provide an average way agencies. The trends seen in these independent sources service life of 6 to 7 years if the underlying road is in resulted in the identification of six conclusions and four good condition. effective practices. Effective Practices Conclusions 1. Project selection is critical to microsurfacing success 1. Microsurfacing can be procured using a performance- and those agencies that only apply microsurfacing to based contract. The content analysis found that a number structurally sound pavements are generally satisfied of agencies are already using performance specifications with its performance. in their microsurfacing contracts. 2. Microsurfacing design can be successfully assigned to 2. Microsurfacing is a pavement preservation and main- the microsurfacing contractor with the agency review- tenance tool with very few technical or operational ing and approving the final job mix formula. limitations. 3. Microsurfacing performs best when applied to correct 3. U.S. agencies are not maximizing the potential benefits surface friction and/or rutting on pavements that have of microsurfacing when they do not see it as the primary adequate structural capacity. tool to fill ruts as their Canadian counterparts do. 4. It is important that compounds added to microsurfac- 4. The majority of maintenance practitioners do not con- ing job mix formulas are specific to the requirements sider environmental impact in their project development of the emulsion manufacturer and that the agency ver- process ifies that they are compatible with the approved job 5. The majority of the respondents that use microsurfac- mix formula. ing assign the contractor the responsibility for com- 5. Microsurfacing programs can be successfully imple- pleting the job mix formula. The majority of the same mented with a single binder type with a record of sat- population rated their microsurfacing project perfor- isfactory performance in a given agency's climate and mance as satisfactory. traffic conditions.