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17 was rated. However, it was not recommended by the major- It can also be used to correct a loss of pavement friction ity of the authors in the literature for addressing cracking (net 9). However, based on the net, it shows that micro- issues. This leads to the idea that microsurfacing will only surfacing has limited ability to address most cracking issues perform properly when applied to structurally sound pave- as it was given a negative rating more times than it was rated ments. Additionally, its operational benefits support its use in positive. Microsurfacing is also shown to be viable for all pavement preservation programs. Microsurfacing's ability to levels of traffic as well as useful in both urban and rural set- accept traffic within 1 h of installation (Johnson et al. 2007) tings. Finally, it appears to be robust enough to be effectively not only reduces life-cycle costs by minimizing user-delay used in locations where the work has to be done at night or costs but it also enhances work zone safety by minimizing in cool weather, as well as where stresses resulting from stop- the time workers are exposed to active traffic. That the liter- ping and snow plowing are present. This analysis is validated ature shows its use on all types of roads with no limitation on by a study of microsurfacing performance. "Microsurfacing traffic volume accentuates its value for high-volume urban generally will not be effective if it is applied to pavements freeway preservation projects. This is reinforced by research that have working cracks, that are structurally inadequate, or that proved it performs well on both asphalt and concrete pave- that have unstable pavement layer materials. Microsurfacing ments (Moulthrop et al. 1996; Wood and Geib 2001). Thus, applied to pavements in the appropriate condition provides this analysis leads to the conclusion that microsurfacing is a seven or more years of service" (Smith and Beatty 1999, ital- pavement preservation and maintenance tool with very few ics added). Thus, the mantra cited in the opening sentence technical or operational limitations. of this section is found to be very correct for using micro- surfacing as a pavement preservation tool and leads to the Table 10 consolidates the recommendations in each of the following effective practice: categories. "Sum 1" is the number of observations where it was rated either "good" or "fair." "Sum 2" is the number of Project selection is critical to microsurfacing success and observations where it was rated either "poor" or "not recom- those agencies that only apply microsurfacing to struc- mended." "Net" is Sum 1 minus Sum 2. Thus, a positive turally sound pavements are generally satisfied with its number would indicate that microsurfacing would generally performance. be recommended for those categories and a negative number would argue against its use. The table shows that microsur- CANDIDATE ROAD CHARACTERIZATION facing is expected to perform well to treat pavement distresses in the following in order: Each public highway agency has its own method of assem- bling a list of roads that need some form of preservation or 1. Rutting--16 for shallow ruts; 14 for deeper ruts maintenance to either restore their serviceability or to extend 2. Raveling--12 their service life. Thus, the selection of appropriate candidates 3. Oxidation--12 for microsurfacing cannot be done in a vacuum. The process 4. Bleeding--7 necessitates that those candidates be evaluated as a group and TABLE 10 QUANTIFIED OUTPUT FOR MICROSURFACING ONLY FROM TABLE 9 Traffic Maintenance Pavement Condition Rutting Cracking Type Desired Benefits Volumes Issues Minimize User Delay Costs Early Opening to Traffic Recommendation Multiple Lifts Needed Night-time Work Stopping Points ADT = 3K to 5K Snow plow use Cool Temps Longitudinal Transverse Oxidation ADT >5K Raveling Bleeding ADT<3K Alligator Friction Urban < 1/2" Rural >1/2" Good 9 9 12 8 15 14 0 1 1 12 13 11 4 6 2 7 7 6 6 4 5 Fair 0 3 0 1 1 1 5 4 4 0 0 1 2 0 2 1 1 0 0 2 1 Sum1 9 12 12 9 16 15 5 5 5 12 13 12 6 6 4 8 8 6 6 6 6 Poor 0 0 0 0 0 0 4 3 3 0 0 0 0 0 1 0 0 0 0 0 0 Not 0 0 0 2 0 1 5 6 6 1 0 0 0 0 0 0 0 1 1 0 0 Sum2 0 0 0 2 0 1 9 9 9 1 0 0 0 0 1 0 0 1 1 0 0 Net 9 12 12 7 16 14 -4 -4 -4 11 13 12 6 6 3 8 8 5 5 6 6

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18 TABLE 11 SURVEY RESPONSES FOR MICROSURFACING SELECTION LOGIC Reason for Selecting Number of Reason for Selecting Number of Microsurfacing Responses Microsurfacing 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 each road assigned the appropriate treatment based on its divergence of practice in the two countries. The U.S. agen- condition and the physical and financial environment in which cies favor microsurfacing to furnish a surface wearing course the treatment will be applied. Many factors are considered. and to seal the surface from water infiltration, whereas the First, the following parameters are normally used to pair roads Canadians use microsurfacing primarily as a rut filler. The with appropriate treatments: U.S. results contradict the analysis shown in Table 10. This indicates that the practice is not reflective of the literature Existing pavement type and age; and might show a misunderstanding with regard to the tech- Traffic volume; nique's effectiveness in filling ruts. The literature reviewed Type, severity, and extent of distress; in Table 9 cites the ability to be installed in multiple layers Surface friction; as one of microsurfacing's desired qualities. Rut filling will Expected service life of the treatment; and. generally consist of a rut filling course followed by a full lane- Program for the next major rehabilitation or recon- width surface course. Therefore, it is one of the few pavement struction. preservation and maintenance treatments that can restore the transverse geometry of a rutted road. This leads to the con- Once an appropriate treatment is selected, the environ- clusion that U.S. agencies are not maximizing the potential ment in which it will be constructed is then accounted for and benefits of microsurfacing when they do not see it as the pri- treatment decisions adjusted accordingly (Hicks et al. 2000). mary tool to fill ruts as their Canadian counterparts do. These factors include: Table 12 reports the responses of the agencies when asked Time of year in which the treatment will be placed. to indicate what factors were used to characterize the exist- Climatic conditions, such as humidity, wind, tempera- ing substrate as part of their design process. Texas was the tures, etc. U.S. "other" response and it uses a combination of the distress Cost of the treatment and availability of funds. score and ride score developed in its pavement management Availability of qualified contractors. information system to provide input to the Texas Transpor- Availability of quality materials. tation Institute design method. Ontario was the Canadian Pavement noise requirements after application. "other" response and it uses the pavement condition index in Impact on traffic flow and disruption during construction. its pavement management information system in design. One can see in the table that qualitative characterization is the The survey asked respondents to name the reasons they dominant design factor followed by roughness. chose microsurfacing in their programs. Table 11 shows the possible responses and the number of times each was cited Table 12 also contains the responses regarding the differ- by U.S. and Canadian respondents. It shows an interesting ences in the design process for urban versus rural roads. TABLE 12 SURVEY RESPONSES FOR FACTORS USED IN DESIGN 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.

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

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

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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. FIGURE 8 Pavement friction model (Hall 2006). 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 restore pavement friction if an unsafe level of polishing occurs respect to the greenhouse gas emissions, as well as information in the pavement surface. on raw material consumption. Both studies merely constructed a simplified snapshot of the comparative environmental impact Environmental Aspects of Microsurfacing of microsurfacing. Neither included the impact of work zone delays nor the life safety benefits accrued from microsurfac- Considering environmental impact when choosing pave- ing owing to its ability to minimize the duration of work zone ment preservation and maintenance treatments appears to delays and increased congestion during pavement maintenance be rare based on the results shown in Table 13, with only 5 operations. of 36 respondents indicating that it was part of their process. On this issue the literature was markedly divided into two The previous discussion and the survey show that the envi- camps. One side advocated the use of bituminous surface treat- ronmental impact of pavement maintenance or preservation ments as pavement preservation tools and opined that because projects has yet to be adequately evaluated (Takamura et al. the consumption of raw materials and energy by micro- 2001). The survey results (Table 13) that show that the major- surfacing was decidedly less than that of a hot-mix asphalt ity of knowledgeable maintenance practitioners do not con- overlay that the treatment inherently had less impact on the sider environmental impact in their project development environment (Takamura et al. 2001; Uhlman 2010). The process validates this conclusion and points to the need for other side argued that the use of any kind of petroleum prod- future research in the area of sustainable maintenance practices. 55.0 1.4 1.3 50.0 Microtexture (Skid Number) 1.2 45.0 Macrotexture (mm) 1.1 1 40.0 0.9 35.0 0.8 30.0 0.7 0.6 25.0 0.5 20.0 0.4 e 1 2 3 4 5 6 7 8 9 lin B# B# B# B# B# B# B# B# B# se O O O O O O O O O Ba Microtexture Macrotexture FIGURE 9 Microsurfacing change in skid number and macrotexture over time (Riemer et al. 2010).