National Academies Press: OpenBook

Microsurfacing (2010)

Chapter: Chapter Three - Design Practices

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Suggested Citation:"Chapter Three - Design Practices." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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Suggested Citation:"Chapter Three - Design Practices." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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Suggested Citation:"Chapter Three - Design Practices." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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INTRODUCTION First, the survey asked respondents to indicate if they used a formal design and, if so, which one. The U.S. sample contained 31 responses, of which 12 reported using a single formal design method and 3 indicated that they had used 2 different methods in the past 5 years. The results regarding the survey are as follows: 1. ISSA Design Method for Microsurfacing (2010b): ISSA A143—12 responses. 2. ASTM Design Method for Slurry Seals (ASTM 2007a): ASTM D 3910-98—3 responses. 3. ASTM Design Method for Microsurfacing (ASTM 2007b): ASTM D 6372-99a—2 responses. 4. Texas Transportation Institute Design Method for Microsurfacing (West et al. 1996): TTI 1289—1 response. Additionally, 11 responses noted the use of empirical methods 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 (2010b) to design their microsurfacing and 4 use an empir- ical method. Microsurfacing design is essentially a six-step process: 1. Identifying and characterizing the roads where a micro- surfacing treatment is appropriate. 2. Selecting materials: emulsion, aggregate, mineral filler, additives, and water. 3. Developing a job mix formula. 4. Laboratory testing of the job mix formula [also referred to as the “mix design” by some authors such as ISSA (2010b)]. 5. Developing application rates. 6. Preparing construction documents based steps 1–5. ISSA stresses the importance of the design process in its most recent technical publication: Mix designs must be completed by a competent laboratory that is experienced with state of the art asphalt emulsion/aggregate mixing technology as it applies to slurry systems. The laboratory must possess the necessary specialized equipment and knowl- edgeable staff to perform the required tests. Knowing the specific system and the relationships of all the components is critical to the development of a good mix design. Each of the material components, aggregate, asphalt emulsion, water, and additives, must meet all job specifications and test requirements. Indi- vidual materials must be qualified through testing before the laboratory will perform further tests to determine the mix com- patibility and performance under simulated wear conditions (ISSA 2010b). The job mix formula development process seeks to deter- mine the quality of the materials and evaluate how they will interact with each other during and after treatment curing. The job mix formula procedure includes the various phases of the microsurfacing process in which the following questions are answered for each phase: • Mixing: Will the components mix together and form true, free- flowing microsurfacing? • Breaking and Curing: Will the emulsion break in a controlled way on the aggregate, coat the aggregate, and form good films on the aggregate? Will the emulsion build up cohesion to a level that will resist abrasion owing to traffic? • Performance: Will the microsurfacing resist traffic-induced stresses? (National Highway Institute 2007). The process involves prescreening the possible alternatives for materials, the job mix formula itself, and final testing. At every step, the laboratory addresses mixing, breaking, curing, and performance issues to ensure that the final design is opti- mized both for the actual materials and for the environment in which the microsurfacing will be installed. Table 6 is a summary of survey responses to the question: Which entity is responsible for performing the microsurfacing design? The table shows that only 7 agencies do their own job mix formula, whereas 22 U.S. and 6 Canadian agencies delegate this respon- sibility to the microsurfacing contractor through the construc- tion contract. Additionally, of the agencies that out-source the job mix formula, only two U.S. and two Canadian agencies do not require that the final job mix formula be reviewed by an agency representative. That the majority of the respondents that use microsurfac- ing (77%) chose to give the contractor the responsibility for the job mix formula leads to the conclusion that microsur- facing projects are being delivered as de facto performance contracts. The Canadian respondents indicated that all Cana- dian road agencies require a warranty that ranges from 1 to 2 years. In the United States, seven agencies reported requir- ing a warranty on their microsurfacing projects. The details of the warranties are shown in Table 7. CHAPTER THREE DESIGN PRACTICES 14

15 Finally, when asked to rate how they perceived the per- formance of their microsurfacing program (Table 8), the majority of the agencies rated their microsurfacing per- formance as “Good” or “Excellent” and none rated it as one of the unsatisfactory ratings (“Poor” or “Very Poor”). Taking the results of Tables 6, 7, and 8 leads to the fol- lowing effective practice: Microsurfacing design can be successfully assigned to the microsurfacing contractor with the agency reviewing and/or approving the final job mix formula. However, applying the perfectly designed job mix to a road that will not benefit from microsurfacing is a formula for failure. Therefore, “project selection play[s] a key factor in overall success of microsurfacing” (Wood and Geib 2001). PROJECT PLANNING AND ROAD SELECTION The mantra of the pavement preservation movement in the United States is “the right treatment, on the right road, at the right time” (Galehouse et al. 2003). As such, project selec- tion becomes the key to a successful microsurfacing program. Moulthrop (2007) emphasizes Wood and Geib’s (2001) find- ing when he states: “Failures are generally a result of poor proj- ect selection—there is a need to educate users on the proper use of slurry and microsurfacing.” Table 9 is a summary of the project selection criteria found in the literature. Its format came from the most detailed source (Caltrans 2009). As noted in chapter one, it shows the cited project selection criteria for both treatments in those sources where both were given. Analysis of Table 9 shows that microsurfacing is the appropriate option in most of the categories where its utility Number of Responses Entity That Develops the Job Mix Formula U.S. (of 28) Canada (of 8) Agency in-house design section 2 1 Agency in-house maintenance group 2 0 Agency in-house materials lab section 1 1 Microsurfacing contractor under the construction contract 21 6 Independent lab for the microsurfacing contractor under the construction contract 1 0 Do not know 1 0 TABLE 6 JOB MIX FORMULA DEVELOPMENT RESPONSIBILITY SUMMARY Agency Microsurfacing Warranty Length Warranty Criteria Louisiana 1 year Standard construction warranty for surface defects New Hampshire 1 year Surface defects New York 1 year Raveling, flushing, delamination, snowplow damage Oklahoma 1 year Standard construction warranty for surface defects Alberta 1 year Raveling British Columbia 1 year Standard construction warranty for surface defects Quebec 1 year Standard construction warranty for surface defects Saskatchewan 1 year Standard construction warranty for surface defects Nevada 2 years Standard construction warranty for surface defects Texas 2 years Raveling, flushing Manitoba 2 years Raveling, friction New Brunswick 2 years Standard construction warranty for surface defects Nova Scotia 2 years Standard construction warranty for surface defects Ontario 2 years Raveling, flushing Indiana 3 years Raveling, friction, rutting TABLE 7 MICROSURFACING WARRANTY SUMMARY Performance Rating Excellent Good Fair Poor Very Poor U.S. 1 20 4 0 0 Canada 1 7 0 0 0 Total 2 27 4 0 0 TABLE 8 MICROSURFACING PERFORMANCE RATINGS

16 Source Reference Type F r i c t i o n R a v e l i n g O x i d a t i o n B l e e d i n g < 1 / 2 " > 1 / 2 " A l l i g a t o r L o n g i t u d i n a l T r a n s v e r s e A D T < 3 K A D T = 3 K t o 5 K A D T > 5 K C o o l T e m p s S t o p p i n g P o i n t s S n o w p l o w u s e U r b a n R u r a l Ohio DOT Hicks et al 2000 Oregon DOT Hunt 1991 Asphalt Inst AI 1983 Wisconsin DOT Shober 1997 Indiana DOT Labi et al 2007 Texas DOT Smith and Beatty 1999 Iowa DOT Jahren & Behling 2004 New York State DOT NYSDOT 1999 Parameters Desired Benefits E a r l y O p e n i n g t o T r a f f i c M u l t i p l e L i f t s N e e d e d N i g h t - t i m e W o r k M i n i m i z e U s e r D e l a y C o s t s FLHD 2009FLHD Caltrans Olsen 2008 Austroads Austroads 2003 USACE ASTM 1998 Asphalt Contractor Mouthrop et al 1999 FHWA FHWA 2007 Caltrans Hicks et al 2000 Iowa DOT Jahren et al 1999 G =Good; F = Fair; P = Poor; N = Not M = Microsurfacing (general); MII = Microsurfacing (Type II); MIII = Microsurfacing (Type III); S = Slurry Seal Surface Condition Maintenance issues Type Pavement Condition Rutting Traffic VolumesCracking E x p e c t e d s e r v i c e l i f e MII F G N G N N N N G G G F G F G G G G F G MIII - G - G N G G P N N G G G F G F G G N N F F M G G G - G G N N N G G - - - - - - - - - - S - G G - N N P N N - - - - - - - - - - - - - S - G - - - - F P P - - - - - - - - - - - - - M G G - G G G N N N G G G - - G - - - - S F G - G G G N N N G P N - - F - - - - M G G G G G G - - - - - - - - - - - - - - - M - F - - G G P P P G G G - - - - - - - - - - S - P - - P P P P P G N N - - - - - - - - - - M G G G - F F P F F - - - - - - - - - - - - - S G F F - - - P P P - - - - - - - - - - - - - M G G G G G G F G G G G G - - G G G G G - G - S G G N G F N N P F G G G - - G G N N N - N - MII - G G G G G N N N G G G G G - - - - - G - MIII - G G G G G N N N N G G G G - - - - - G - M - - - - G G - - - G G G G - - G G G G G G - S - - - - N N - - - G N N - - - N G N N - - - M G - - - G G P P P G G G - - - F G G G - G - S F - - - F N N N N F N N - - - G N - - - - - M - G G G G G F F F G G F - - - G F - M G - G F G G F F F G G G - G - G G G G - - M G - G G G G F F F G G G G G - G G G G G G S G - F F - - P P P G G G N - - G - N N N P 3 TO 4 3 TO 4 3 TO 8 2 TO 6 7 TO 10 7 TO 10 7 TO 10 5 TO 7 5 TO 6 5 TO 7 M G F G G G G F P P - - - - - P - - - - - - - AI = Asphalt Institute. TABLE 9 PROJECT PLANNING AND SELECTION CRITERIA SUMMARY

17 was rated. However, it was not recommended by the major- ity of the authors in the literature for addressing cracking issues. This leads to the idea that microsurfacing will only perform properly when applied to structurally sound pave- ments. Additionally, its operational benefits support its use in pavement preservation programs. Microsurfacing’s ability to accept traffic within 1 h of installation (Johnson et al. 2007) not only reduces life-cycle costs by minimizing user-delay costs but it also enhances work zone safety by minimizing the time workers are exposed to active traffic. That the liter- ature shows its use on all types of roads with no limitation on traffic volume accentuates its value for high-volume urban freeway preservation projects. This is reinforced by research that proved it performs well on both asphalt and concrete pave- ments (Moulthrop et al. 1996; Wood and Geib 2001). Thus, this analysis leads to the conclusion that microsurfacing is a pavement preservation and maintenance tool with very few technical or operational limitations. Table 10 consolidates the recommendations in each of the categories. “Sum 1” is the number of observations where it was rated either “good” or “fair.” “Sum 2” is the number of observations where it was rated either “poor” or “not recom- mended.” “Net” is Sum 1 minus Sum 2. Thus, a positive number would indicate that microsurfacing would generally be recommended for those categories and a negative number would argue against its use. The table shows that microsur- facing is expected to perform well to treat pavement distresses in the following in order: 1. Rutting—16 for shallow ruts; 14 for deeper ruts 2. Raveling—12 3. Oxidation—12 4. Bleeding—7 It can also be used to correct a loss of pavement friction (net 9). However, based on the net, it shows that micro- surfacing has limited ability to address most cracking issues as it was given a negative rating more times than it was rated positive. Microsurfacing is also shown to be viable for all levels of traffic as well as useful in both urban and rural set- tings. Finally, it appears to be robust enough to be effectively used in locations where the work has to be done at night or in cool weather, as well as where stresses resulting from stop- ping and snow plowing are present. This analysis is validated by a study of microsurfacing performance. “Microsurfacing generally will not be effective if it is applied to pavements that have working cracks, that are structurally inadequate, or that have unstable pavement layer materials. Microsurfacing applied to pavements in the appropriate condition provides seven or more years of service” (Smith and Beatty 1999, ital- ics added). Thus, the mantra cited in the opening sentence of this section is found to be very correct for using micro- surfacing as a pavement preservation tool and leads to the following effective practice: Project selection is critical to microsurfacing success and those agencies that only apply microsurfacing to struc- turally sound pavements are generally satisfied with its performance. CANDIDATE ROAD CHARACTERIZATION Each public highway agency has its own method of assem- bling a list of roads that need some form of preservation or maintenance to either restore their serviceability or to extend their service life. Thus, the selection of appropriate candidates for microsurfacing cannot be done in a vacuum. The process necessitates that those candidates be evaluated as a group and R ec om m en da tio n Fr ict io n Ra ve lin g Ox ida tio n Bl ee din g < 1 /2 " > 1/ 2" Al lig at or Lo ng itu din al Tr an sv er se AD T< 3K AD T = 3K to 5 K AD T >5 K Co ol Te m ps St op pin g Po int s Sn ow p low u se Ur ba n Ru ra l Ea rly O pe nin g to T ra ffic M ul tip le Lif ts Ne ed ed Ni gh t-t im e W or k M in im ize U se r D ela y C os ts Good Fair Sum1 Poor Not Sum2 Net 9 8 1 12 13 11 4 0 2 9 6 0 0 0 9 9 3 12 0 0 0 12 12 0 12 0 0 0 12 1 9 0 2 2 15 1 16 0 0 0 7 16 14 1 15 0 1 1 0 5 5 4 5 9 4 5 3 6 9 14 -4 -4 1 4 5 3 6 9 0 12 0 1 1 0 13 0 0 0 1 12 0 0 0 -4 11 13 12 6 0 0 0 6 6 0 0 0 0 6 4 2 2 1 0 1 3 8 7 1 0 0 0 8 8 7 1 0 0 0 8 6 6 0 0 1 1 5 6 6 0 0 1 1 5 6 4 2 0 0 0 6 6 5 1 0 0 0 6 Cracking TypeMaintenanceIssues Traffic VolumesPavement Condition Rutting Desired Benefits TABLE 10 QUANTIFIED OUTPUT FOR MICROSURFACING ONLY FROM TABLE 9

each road assigned the appropriate treatment based on its condition and the physical and financial environment in which the treatment will be applied. Many factors are considered. First, the following parameters are normally used to pair roads with appropriate treatments: • Existing pavement type and age; • Traffic volume; • Type, severity, and extent of distress; • Surface friction; • Expected service life of the treatment; and. • Program for the next major rehabilitation or recon- struction. Once an appropriate treatment is selected, the environ- ment in which it will be constructed is then accounted for and treatment decisions adjusted accordingly (Hicks et al. 2000). These factors include: • Time of year in which the treatment will be placed. • Climatic conditions, such as humidity, wind, tempera- tures, etc. • Cost of the treatment and availability of funds. • Availability of qualified contractors. • Availability of quality materials. • Pavement noise requirements after application. • Impact on traffic flow and disruption during construction. The survey asked respondents to name the reasons they chose microsurfacing in their programs. Table 11 shows the possible responses and the number of times each was cited by U.S. and Canadian respondents. It shows an interesting 18 divergence of practice in the two countries. The U.S. agen- cies favor microsurfacing to furnish a surface wearing course and to seal the surface from water infiltration, whereas the Canadians use microsurfacing primarily as a rut filler. The U.S. results contradict the analysis shown in Table 10. This indicates that the practice is not reflective of the literature and might show a misunderstanding with regard to the tech- nique’s effectiveness in filling ruts. The literature reviewed in Table 9 cites the ability to be installed in multiple layers as one of microsurfacing’s desired qualities. Rut filling will generally consist of a rut filling course followed by a full lane- width surface course. Therefore, it is one of the few pavement preservation and maintenance treatments that can restore the transverse geometry of a rutted road. This leads to the con- clusion that U.S. agencies are not maximizing the potential benefits of microsurfacing when they do not see it as the pri- mary tool to fill ruts as their Canadian counterparts do. Table 12 reports the responses of the agencies when asked to indicate what factors were used to characterize the exist- ing substrate as part of their design process. Texas was the U.S. “other” response and it uses a combination of the distress score and ride score developed in its pavement management information system to provide input to the Texas Transpor- tation Institute design method. Ontario was the Canadian “other” response and it uses the pavement condition index in its pavement management information system in design. One can see in the table that qualitative characterization is the dominant design factor followed by roughness. Table 12 also contains the responses regarding the differ- ences in the design process for urban versus rural roads. Reason for Selecting Microsurfacing Number of Responses Reason for Selecting Microsurfacing Number of Responses U.S. Canada U.S. Canada Provide a Surface Wearing Course 9 0 Fill Surface Rutting 2 4 Prevent Water Infiltration 6 1 Improve Striping Visibility 0 0 Oxidation 3 1 Distress (cracking) 1 0 Raveling 3 2 Improve Friction (skid) Resistance 1 0 TABLE 11 SURVEY RESPONSES FOR MICROSURFACING SELECTION LOGIC Design Characterization Factor U.S. Canada Qualitative (visual) factors 17 6 Roughness 9 3 Level of oxidation 5 0 Rutting 2 3 Other 1 1 Do not characterize existing conditions in the design process 4 0 Do you vary design based on urban vs. rural? Yes 7 1 No 15 7 Do not know 6 0 If yes, what factors are used? AADT 4 1 Number of ESALs 2 0 Proximity to urban areas 2 0 Proximity to rural areas 1 0 ESAL = equivalent single axle load. TABLE 12 SURVEY RESPONSES FOR FACTORS USED IN DESIGN

19 Because the majority of respondents do not differentiate in their design and because many reported using microsurfacing to extend the life of the underlying pavement, there appears to be no need to differentiate between urban and rural micro- surfacing design. Decision Tools A Foundation for Pavement Preservation study developed a decision tree-based framework with which to select an appropriate surface treatment (Hicks et al. 2000). The project covered most pavement preservation and maintenance treat- ments used on asphalt pavements including microsurfacing. The study furnishes an example that illustrates a rigorous methodology to ensure that microsurfacing is indeed the appropriate treatment for a given road. A number of U.S. and Canadian agencies have followed suit and developed decision tools to assist maintenance engineers in pairing a pavement distress condition with an appropriate pavement preserva- tion and maintenance treatment (Helali et al. 1996; Nebraska Department of Roads 2004; Li et al. 2006; Berg et al. 2009). Hicks et al. demonstrated how the pavement engineer can opti- mize the treatments with the distresses with which they are most effective. Figures 6 and 7 are conceptual decision trees for asphalt and concrete pavement preservation and mainte- nance treatment selection. They were created by synthesizing microsurfacing decision trees found in the literature and they are not meant to be comprehensive but rather are illustra- tive of the process of selecting a road whose distresses can be adequately addressed by microsurfacing. Figure 6 shows the details of the microsurfacing decision process for raveling as found in Helali et al. (1996), and rutting, a combination of Hicks et al. (1997) and Caltrans (2009). The decision tree for raveling comes from a Canadian study that was implemented by the Ministry of Transporta- tion (MTO) in Ontario. It provides a clear example that micro- surfacing is not an appropriate treatment for asphalt pave- ments with structural distresses (Helali et al. 1996). The same can be said for the rutting decision tree that springs from research conducted for Caltrans (Hicks et al. 1997; Caltrans 2009). The concrete pavement friction loss tree in Figure 7 is used by the Utah DOT. It shows that when friction loss is localized that mechanical retexturing using shotblasting or diamond grinding will be more cost-effective than micro- surfacing. However, as the magnitude of the unsafe area increases, microsurfacing becomes the preferred option based on a lower production cost and reduced user-delay costs (Berg et al. 2009). Finally, the rutting decision tree shows that microsurfacing can be used to correct rutting problems in both pavement types. Comparing Figures 6 and 7 with the output shown in Tables 9 and 10 confirms the conclusions drawn from that analysis. It also confirms a trend in two different sources of information and leads to identification of the following effective practice: Microsurfacing performs best when applied to correct sur- face friction, oxidation, raveling, and/or rutting on pave- ments that have adequate structural capacity. Friction Restoration The review of the literature found two other factors that could influence the decision to use microsurfacing on a given road. The first involved the use of microsurfacing to restore surface friction on an interim basis to quickly react to potential safety hazards found in the course of routine skid testing (Yager 2000). The second was the relative impact of microsurfacing on the environment (Uhlman 2010). Table 13 summarizes the survey results for both issues. The survey question that FIGURE 6 Conceptual decision tree for asphalt pavement preservation and maintenance treatment selection (adapted from Helali et al. 1996; Hicks et al. 2000; Caltrans 2009).

resulted from the first issue sought to determine the use of skid resistance as a metric to characterize candidate roads. The relative ambivalence of the responses in the survey (i.e., 87% either do not consider skid numbers or they vary) clearly demonstrates that microsurfacing projects are rarely selected on a basis of skid numbers alone. The skid resistance of a pavement is the result of a “complex interplay between two principal frictional force components— adhesion and hysteresis” (Hall 2006). There are other compo- nents such as tire shear, but they are not nearly as significant as the adhesion and hysteresis force components. Figure 8 shows these forces and one can see that the force of friction (F) can be modeled as the sum of the friction forces owing to adhesion (FA) and hysteresis (FH) per Equation 1 here: From Figure 8 one can see that the frictional force of adhe- sion is “proportional to the real area of adhesion between the tire and surface asperities,” which makes it a function of pave- ment microtexture. The hysteresis force is “generated within the deflecting and visco-elastic tire tread material, and is a func- tion of speed,” making it primarily related to pavement macro- texture (Hall 2006). Thus, if an engineer wants to improve pavement skid resistance through increasing the inherent fric- tion of the physical properties of the pavement that engineer F F FA H= + ( )1 20 needs to improve both surface microtexture and macrotexture (Davis 1999). Microsurfacing does both. Figure 9 was developed from data collected for an on-going pavement preservation research project that includes a micro- surfacing field test section (Riemer et al. 2010). It shows the pre-microsurfacing baseline measurement for microtex- ture, measured by a treaded tire skid number, and macro- texture, measured using the sand circle test. Once the micro- surfacing was applied, both values show a marked increase and as time goes on they start to deteriorate. The observations in the graph correlate to quarterly measurements and both val- ues appear to be leveling off after two years of service. This research demonstrates the potential for microsurfacing to quickly and effectively enhance the surface friction of a struc- turally sound pavement that has become unsafe owing to pol- ishing of its aggregate or the loss of macrotexture resulting from flushing or bleeding. The FHWA issued a technical advisory on pavement fric- tion management (FHWA 2010b) that requires agencies to use the measurements shown in Figure 9 that states: “Because all friction test methods can be insensitive to macrotexture under specific circumstances, it is recommended that friction testing be complemented by macrotexture measurement.” Therefore, microsurfacing is shown in Figure 9 to satisfy both com- ponents of the FHWA technical advisory and can be used to FIGURE 7 Conceptual decision tree for concrete pavement preservation and maintenance treatment selection (adapted from Berg et al. 2009; Anderson et al. 2007; Bennett 2007). Nation SN > Agency Minimums SN< Agency Minimums SN at or Close to Agency Minimums SN Vary SN Not Used in This Context Consider Environmental Impact Do Not Consider Environmental Impact U.S. 2 2 1 10 13 3 25 Canada 0 0 0 2 6 2 6 Total 2 2 1 13 19 5 31 SN = skid number. TABLE 13 SKID NUMBER AND ENVIRONMENTAL CONSIDERATION SUMMARY

21 uct had a deleterious impact on the environment (Gillies 2006). Table 14 was extracted from a 2010 study by Chehovits and Galehouse that quantified energy use and greenhouse gas emissions of the full suite of FHWA-approved pavement preservation treatments. It shows that there is a huge differ- ence in microsurfacing’s environmental impact when com- pared with a 2-in. hot-mix asphalt overlay. The last column shows that when the savings are annualized to account for microsurfacing’s shorter service life that the savings remain large. These numbers do not account for the greenhouse gas emissions from vehicles being delayed in work zones. If those were added, microsurfacing’s ability to return the road to full- speed traffic in one hour would overwhelm the typical 8 to 12 h it takes to mill and install an overlay. The information in Table 14 agrees with findings from an earlier study by Takamura et al. (2001). Figure 10 illustrates the output from that study and provides greater detail with respect to the greenhouse gas emissions, as well as information on raw material consumption. Both studies merely constructed a simplified snapshot of the comparative environmental impact of microsurfacing. Neither included the impact of work zone delays nor the life safety benefits accrued from microsurfac- ing owing to its ability to minimize the duration of work zone delays and increased congestion during pavement maintenance operations. The previous discussion and the survey show that the envi- ronmental impact of pavement maintenance or preservation projects has yet to be adequately evaluated (Takamura et al. 2001). The survey results (Table 13) that show that the major- ity of knowledgeable maintenance practitioners do not con- sider environmental impact in their project development process validates this conclusion and points to the need for future research in the area of sustainable maintenance practices. FIGURE 8 Pavement friction model (Hall 2006). 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 20.0 Ba se line OB #1 OB #2 OB #3 OB #4 OB #5 OB #6 OB #7 OB #8 OB #9 25.0 30.0 35.0 40.0 45.0 50.0 55.0 M ac ro te xt ur e (m m) M ic ro te xt ur e (S ki d Nu m be r) Microtexture Macrotexture FIGURE 9 Microsurfacing change in skid number and macrotexture over time (Riemer et al. 2010). restore pavement friction if an unsafe level of polishing occurs in the pavement surface. Environmental Aspects of Microsurfacing Considering environmental impact when choosing pave- ment preservation and maintenance treatments appears to be rare based on the results shown in Table 13, with only 5 of 36 respondents indicating that it was part of their process. On this issue the literature was markedly divided into two camps. One side advocated the use of bituminous surface treat- ments as pavement preservation tools and opined that because the consumption of raw materials and energy by micro- surfacing was decidedly less than that of a hot-mix asphalt overlay that the treatment inherently had less impact on the environment (Takamura et al. 2001; Uhlman 2010). The other side argued that the use of any kind of petroleum prod-

22 Energy Use Greenhouse Gas Emissions Treatment Composition BTU/CY MJ/CM Lbs/SY Kg/SM Life Extension Annualized Percent Savings vs. 2-in. Hot-Mix Overlay 1.5 in. (3.8 cm) 46,300 59 9.0 4.9 5–10 yr Hot-Mix Asphalt Overlay 2.0 in. (5.0 cm) 61,500 77 12.3 6.7 5–10 yr Energy Use Savings Greenhouse Gas Savings Type III 5,130 6.5 0.6 0.3 3–5 yr 83%–86% 90%–92% Micro- surfacing Type II 3,870 4.9 0.4 0.2 2–4 yr 83%–84% 91%–92% Adapted from Chehovits and Galehouse (2010). TABLE 14 ENVIRONMENTAL IMPACT COMPARISON OF MICROSURFACING VERSUS HOT MIX ASPHALT OVERLAY 0 10 20 30 40 50 60 70 Energy CO2 NO2 Ozone Raw Material Microsurfacing Hot-mix Overlay Polymer-modified Hot-mix overlay FIGURE 10 Comparative environmental impacts of three pavement preservation and maintenance treatments (adapted using data taken from Takamura et al. 2001). This conclusion is especially valid given the current wide- spread focus on sustainable design and construction practices (Takamura et al. 2001; Uhlman 2010). The crux of sustainable engineering practices revolves around judicious selection of materials, which is the subject of the next section. SELECTION OF MATERIALS Once a road has been identified as a microsurfacing project, the next step is to select the appropriate materials. The compo- nents of a microsurfacing job mix consist of emulsion, aggre- gate, mineral filler, additives, and water. It is important that each of these ingredients be compatible with each other for the microsurfacing to work as designed. Therefore, the mix design process is necessarily based on laboratory results, which are in turn used to optimize the job mix formula. Emulsion “Asphalt emulsions are dispersions of asphalt in water sta- bilized by a chemical system” (National Highway Institute 2007). They are manufactured by blending emulsifying agents with the base asphalt to permit it to disperse uniformly, creat- ing a temporary mixture. This mixture “breaks” upon place- ment and releases the water leaving behind the asphalt (called residual asphalt) (Caltrans 2009). Cationic emulsions are typ- ically used in microsurfacing. The literature cites CSS-1h (cationic–slow setting–low viscosity–hard; see the Glossary for abbreviations of emulsion types) as the most common (Moulthrop et al. 1999). Some agencies prefer quick setting emulsions such as CQS-1h to reduce the amount of traffic dis- ruption and delay (ISSA 2010a). Emulsions can also be specially formulated to ensure com- patibility with local aggregates and to meet appropriate mix design parameters. The survey asked respondents to indicate which type of emulsions were commonly used in their micro- surfacing program and the content analysis of the microsur- facing specifications also sampled for that information. The results are shown in Table 15 (note the table only lists binders that were cited by survey respondents). A trend in the data was found that showed that only 3 of 28 U.S. agencies used more than a single emulsion type. All of the specifications contained only a single emulsion, which was also true for the Canadian agencies. The use of a single microsurfacing binder coupled with the reported microsurfacing performance results in Table 8 (i.e., 100% of the agencies rated their microsur- facing performance as “Fair,” “Good,” or “Excellent”) indi- cates that an agency can select a single binder that works best for its specific climatic and traffic environment and use it exclusively in their microsurfacing program. Agency satis-

23 density. To correct this, the latex is remixed by circulating it and the emulsion in the tanker before it is transferred to the microsurfacing machine for installation (ISSA 2010a). The survey had 23 responses citing polymer use, 7 citing natural latex, and 2 styrene butadiene styrene. Typical emulsion specifications are shown in Table 16. These include binder content and residual asphalt properties. Both viscosity and storage stability are vital to ensure effective emulsion performance on the jobsite. Aggregates According to the National Highway Institute’s Pavement Pres- ervation Treatment Construction Guide (2007), the key char- acteristics of aggregates used in microsurfacing are as follows: • Geology: This determines the aggregate’s compatibility with the emulsion along with its adhesive and cohesive properties. • Shape: The aggregates must have fractured faces in order to form the necessary interlocking matrix. Rounded aggregates will result in poor mix strength. • Texture: Rough surfaces (crushed aggregate) form bonds more easily with emulsions. • Age and Reactivity: Freshly crushed aggregates have a higher surface charge than aged (weathered) aggregates. Surface charge plays a primary role in reaction rates. • Cleanliness: Deleterious materials such as clay, dust, or silt can cause poor cohesion and adversely affect reaction rates. • Soundness and Abrasion Resistance: These features play a particularly important role in areas that experience freeze-thaw cycles or are very wet. The physical properties of the aggregate are important to it achieving its design service life (Fugro-BRE/Fugro South 2004). The quality of microsurfacing aggregate is typically measured by the four properties shown in Table 17. The con- tent analysis found that some agencies also specified the fun- damental material from which the aggregate was made. A typical example from the Missouri DOT is as follows: The mineral aggregate shall be flint chat from the Joplin area, an approved crushed porphyry or an approved crushed steel slag. Blast furnace slag may be used from sources with a documented history of satisfactory use and that have been previously approved by MoDOT for use in micro-surfacing. For non-traffic areas such as shoulders, the mineral aggregate may be crushed limestone or crushed gravel (Missouri DOT 2004). Nation CSS- 1P CSS- 1h* CSS- 1hP CQS- 1h* CQS- 1hP Ralumac™ Quick Set Mixing Grade CRS- 1P CRS- 2P U.S. 1 6 7 5 5 2 1 1 1 Canada 3 2 1 0 2 0 0 0 0 Total 4 8 8 5 7 2 1 1 1 Content Analysis Totals 2 9 4 1 4 0 0 0 0 *Note all the specifications that called for CSS-1h and QS-1h also specified that it be polymer or latex modified. See Glossary and Abbreviations for emulsion grade abbreviations. TABLE 15 MICROSURFACING BINDER USAGE SUMMARY FROM SURVEY AND CONTENT ANALYSIS FIGURE 11 Emulsion with latex in dispersion and after breaking and curing (Watson 2005). faction with the performance of a single binder can be iden- tified as the following effective practice: Microsurfacing programs implemented with a single binder type can yield satisfactory performance in a given agency’s climate and traffic conditions. Microsurfacing emulsion specifications are similar stan- dard emulsion specifications and contain requirements for physical characteristics such as stability, binder content, and viscosity. Polymers are added to microsurfacing emulsions to reduce thermal susceptibility and promote aggregate retention after curing and opening to traffic (ISSA 2010a). Addition- ally, polymers improve the binder’s softening point and its flexibility, which translates to better thermal crack resistance. However, there is a low effective limit to the amount of reflec- tive cracking microsurfacing will resist. Finally, the polymers permit microsurfacing to be placed in thicker sections of two to three stones thick, which enables its use in rut filling. Emulsions are usually modified with latex in an emulsion of polymer particles. The asphalt and latex do not combine. The latex and the asphalt particles intermingle to form an integrated structure as shown in Figure 11 (note the aggre- gate particles contained in a microsurfacing mixture are not shown in this figure. Figure 11 depicts only the interaction of the latex with the asphalt in the microsurfacing mixture). Microsurfacing emulsion is modified with either natural latex or styrene butadiene styrene latex. It is possible for the latex to separate from the emulsion owing to the differences in

Gradations Table 18 shows the gradation for standard microsurfacing aggregates. ISSA (2010b) is the proponent of the A143 micro- surfacing design method and is the source for the various type classifications of aggregates for both microsurfacing and slurry seals. The primary difference between the two gradations is top size, with Type III furnishing a coarser aggregate than Type II. The gradation determines the appropriate amount of residual asphalt in the mix design as well as the particular applications, such as rut filling, for which a given mix design is effective. Type II is recommended for “raveling and oxidation on road- ways with moderate to heavy traffic volumes. Type III . . . is appropriate for filling minor surface irregularities, correcting raveling and oxidation, and restoring surface friction . . . on arterial streets and highways” (National Highway Institute 2007). The fines (i.e., aggregate particles 75 µm and finer) in the mix create “a mortar with the residual asphalt to cement the larger stones in place” (National Highway Institute 2007). Fines are essential for creating a cohesive hard-wearing mix. The National Highway Institute Pavement Preservation Man- ual (2007) recommends that the fines content be at the mid- point of the grading envelope. Additionally, a 2002 research study suggests that the distribution of the fraction that passes the #200 sieve (75 µm and finer) is critical in effectively con- trolling reaction rates in microsurfacing emulsions (Shilling 2002). This finding makes the amount of material that passes the #200 sieve an important gradation factor with regard to microsurfacing performance. Surface Texture Eight of the responses indicated that the agency used more than one gradation in their microsurfacing programs. Addi- 24 tionally, many of those that cited specifying a nonstandard gra- dation added an explanatory note that theirs was a slight mod- ification of the standard gradations. One can see the split between Type II and Type III aggregates. When asked to iden- tify the gradation that was most often specified, Type II was indicated more frequently than Type III and other gradations. The seeming preference for the smaller aggregate may have been caused by the concerns about road noise, which are dis- cussed in detail in chapters seven and eight. Type III aggregate will also produce a surface with deeper macrotexture, which will result in a surface that drains faster than Type II. Conversely, it will also produce more road noise. Road noise was the complaint most often reported in the survey. A study completed at the National Center for Asphalt Technology (NCAT) found that a “surface with a smooth texture using small maximum size aggregate” (de Fortier Smit 2008) reduced macrotexture and minimized road noise. Because Type II aggregate has a lower top size, it appears to conform to the NCAT finding. Therefore, the preference for Type II may also be to mitigate road noise complaints. Mineral Filler Portland cement or other fine materials are used as a “mixing aid allowing the mixing time to be extended and creating a creamy consistency that is easy to spread . . . hydroxyl ions counteract the emulsifier ions, resulting in a mix that breaks faster with a shorter curing time” (National Highway Insti- tute 2007). Portland cement also has a fine consistency, which absorbs water from the emulsion and causes it to break faster. As previously discussed, fine materials in the min- eral filler also promote cohesion by forming a mortar with the residual asphalt. The Minnesota DOT specification for Test Typical Specification Method Residue 62% min. AASHTO T59 Sieve Content 0.3% max. AASHTO T59 Viscosity at 77°F (25°C) 15–90 AASHTO T59 Stability (1 day) 1% max. ASTM D244 Storage Stability (5 days) 5% max. ASTM D244 Residue Penetration at 77°F (25°C) 40–90 ASTM D244 Elastic Recovery 5%–60% AASHTO T301 Softening Point 135°F (57°C) min. ASTM D5 Distillation at 350°F (177°C) 62% min. ASTM D6997 Polymer Content 3.0% min. ASTM D6372 Sources: National Highway Institute (2007) and ISSA (2010b). TABLE 16 TYPICAL PROPERTIES OF MICROSURFACING EMULSIONS Test Microsurfacing Test Number and Purpose Sand Equivalent (min.) 65 ASTM D2419 Clay Content Soundness (max.) 15% ASTM C88 (using NaSO4) Abrasion Resistance 30% max. AASHTO T96 Resistance to traffic Crushed Particles 100% ASTM D5821 Source: ISSA (2010b). TABLE 17 GENERAL AGGREGATE PROPERTIES AND AGGREGATE REQUIREMENTS

25 mineral fillers describes the mineral filler requirements as follows: Mineral filler shall consist of carbonate dust, Portland cement, hydrated lime, crushed rock screenings, fly ash, or rotary lime kiln dust, subject to approval by the Engineer . . . Crushed rock screenings to be used as mineral filler shall be of such composi- tion and quality that the bituminous mixture containing the rock screenings will have stability and durability equivalent to those 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). Additives Other materials are sometimes added to the microsurfacing mix. These additives vary and are often specific to proprietary microsurfacing systems. The National Highway Institute’s Pavement Preservation Manual (2007) notes that additives normally act as retardants to the reaction with emulsions. Typ- ical additives include emulsifier solutions, aluminum sulfate, aluminum chloride, and borax. Varying the concentration of an additive allows the contractor to control the breaking and curing times. For example, the contractor can change the con- centration to account for increasing and decreasing air tem- peratures across the work day (Hicks et al. 2000; Caltrans 2007, 2008). The content analysis revealed that most microsurfacing specifications (87%) direct the contractor to include “addi- tives approved by the emulsion manufacturer . . . to the emul- sion mix or to any of the component materials to provide control of the set time in the field” (New Mexico DOT 2009) or a similarly worded specification clause. The survey also sought information on additives, and only one U.S. (Tennessee) and one Canadian agency (Nova Scotia) indicated that they require an anti-stripping agent. Thus, the following effective practice is identified: 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 job mix formula. DEVELOPING AND LABORATORY TESTING OF A JOB MIX FORMULA The development of a job mix formula fundamentally involves calculating the proportions of each component to the micro- surfacing mix. ISSA A143 (2010b) is normally used as the guideline from which the job mix formula is completed. It starts by estimating the approximate proportions using ISSA Technical Bulletin 102. This entails creating a matrix of mix recipes and recording the manual mixing time for each option. During this operation, the technician looks for and visually assesses changes such as foaming and coating. The standard of Percentage Passing Sieve Size Type II Type III Stockpile Tolerance 3/8 (9.5 mm) 100 100 — # 4 (4.75 mm) 90–100 70–90 ±5% # 8 (2.36 mm) 65–90 45–70 ±5% # 16 (1.18 mm) 45–70 28–50 ±5% # 30 (600 µm) 30–50 19–34 ±5% # 50 (330 µm) 18–30 12–25 ±4% # 100 (150 µm) 10–21 7–18 ±3% # 200 (75 µm) 5–15 5–15 ±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 TABLE 18 MICROSURFACING AGGREGATE GRADATIONS (ISSA 2010b) AND USAGE FOUND IN THE SURVEY Mineral fillers found in the study: • Portland cement • Hydrated lime • Limestone dust • Crushed rock screenings • Fly ash • Kiln dust • Baghouse fines Additives found in the study: • Aluminum sulfate crystals, • Ammonium sulfate • Inorganic salts • Liquid aluminum sulfate, • Amines • Anti-stripping agents

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

27 SERVICE LIFE Microsurfacing is normally used as a pavement preservation tool to extend the service life of the existing pavement. Thus, the service life of the microsurfacing is equal to the amount it extends the service life of the underlying pavement. Pittenger (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 treatment is one of two possible states: 1. Continuous: The treatment will remain [in place] until it fails. 2. Terminal: The treatment will be removed before the end of its continuous service life by a programmed rehabilitation or restoration project such as milling and overlaying (Pittenger 2010). Pavement Life Extension Ideally, microsurfacing service life would be at least as long as the period from the present to the given road’s next sched- uled rehabilitation, upgrade, or replacement. For example, an asphalt pavement overlay is nearing the end of its expected 10-year service and is scheduled to be rehabilitated in 4 years as part of a reconstruction project. The engineer selects a pavement preservation and maintenance treatment that can 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 having 2 to 4 years and 3 to 5 years of expected service life, respectively. Therefore, the engineer could select either as an appropriate treatment to extend the overlay’s service life until its scheduled reconstruction in 4 years. Service Life Determination Table 21 shows the effective service life for the microsur- facing reported in the literature and the results of the survey responses regarding observed microsurfacing service life. The table shows broad agreement that the average service life of a microsurfacing application is 6 to 7 years. It is noted that 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- mation for the effective practice that microsurfacing not be used on structurally deficient roads. SUMMARY This chapter focused on the design of microsurfacing. It furnished information on the process found in the litera- ture, the results of a microsurfacing specification content Aggregate Type Location Suggested Application Rate per Pass Urban and Residential Streets 10–20 lb/SY (5.4–10.8 kg/SM) Airport Runways 10–20 lb/SY (5.4–10.8 kg/SM) Type II Scratch or Leveling Course As required Primary and Interstate Routes 15–30 lb/SY(8.1–16.3 kg/SM) Wheel Ruts As required (see below) Type III Scratch or Leveling Course As required Rut Depth Application Rate 0.5–0.75 in. (12.7–19.1 mm) 20–30 lb/SY ( 10.8–16.3 kg/SM) 0.75–1.00 in. (19.1–25.4 mm) 25–35 lb/SY(13.6–19.0 kg/SM) 1.00–1.25 in. (25.4–31.75 mm) 28–38 lb/SY (15.2–20.6 kg/SM) 1.25–1.50 in. (31.75–38.1 mm) 32–40 lb/SY(17.4–21.7 kg/SM) Source: ISSA (2010a). TABLE 20 SUGGESTED APPLICATION RATES Source Minimum Service Life Maximum Service Life Average 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 TABLE 21 SUMMARY OF MICROSURFACING SERVICE FROM THE LITERATURE AND THE SURVEY

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

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 411: Microsurfacing explores highway microsurfacing project selection, design, contracting, equipment, construction, and performance measurement processes used by transportation agencies in the United States and Canada.

Microsurfacing is a polymer-modified cold-mix surface treatment that has the potential to address a broad range of problems on today’s highways.

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