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Microsurfacing (2010)

Chapter: Chapter Eight - Case Studies

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Suggested Citation:"Chapter Eight - Case Studies." 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 Eight - Case Studies." 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 Eight - Case Studies." 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 Eight - Case Studies." 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 Eight - Case Studies." 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 Eight - Case Studies." 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 Eight - Case Studies." 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 Eight - Case Studies." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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56 INTRODUCTION As with most pavement preservation tools, microsurfacing has many uses and those uses differ from agency to agency based on individual experiences, climatic conditions, and traffic vol- umes. The previous chapters have chronicled the very aspects of microsurfacing from a general practice level. This chapter will review six case studies. Each was selected because it detailed a specific aspect of microsurfacing’s state of the prac- tice. Kohn (1997) posits that case study research can be used for the following reasons: • To explore new areas and issues where little theory is available or measurement is unclear; • To describe a process or the effects of an event or an intervention, especially when such events affect many different parties; and • To explain a complex phenomenon. Microsurfacing is the domain of specialty contractors who not only install the product but, as shown in chapter three, also furnish the technical design for their product. This puts the agency at a disadvantage in terms of theoretical knowledge and experience. The survey found that fully one-fourth of the agency respondents did not even use microsurfacing, making it a new area for those entities. Pavement maintenance and preservation is by definition an intervention whose process needs to be clearly described to ensure that it is properly uti- lized and its inherent benefits are accrued by the owner agency. Finally, chapter three shows it to be a complex phenomenon that needs to be explained so that its desirable qualities can be properly exploited for pavement maintenance and preserva- tion programs. Hence, all three of these reasons apply to this synthesis report, making the case studies particularly valuable to this type of study. CASE STUDY DESCRIPTIONS Table 41 summarizes the case study programs that will follow this section. Scanning the table will show that the cases were drawn from agencies across the United States and in Canada. They also encompass both warm southern climates and cold northern climates where snowplowing impacts microsurfac- ing performance. Each was selected to demonstrate a spe- cific aspect of microsurfacing practice. The case studies were drawn from the literature and fleshed out with telephonic or face-to-face interviews where necessary to ensure an accurate interpretation of the information contained in the literature. Each case will be briefly described and then the analysis’ focus will shift to specific lessons learned. Finally, the results of the case study will be compared with the information derived from the other study instruments to generate conclusions and effec- tive practices. Microsurfacing as Pavement Preservation— Maine Department of Transportation This case study consisted of a 5-year field test of microsurfac- ing placed on asphalt pavements on two highways in northern Maine. The test compared microsurfacing with thin (1.7 in. or 43 mm) hot-mix asphalt (HMA) overlays. The objective of the trials was to evaluate microsurfacing’s ability to “extend the service life of two projects” (Marquis 2009). Table 42 con- tains the salient facts about the project. Results of the Maine Case Study This 5-year field evaluation found that microsurfaced sections showed considerably more wear than the HMA overlay sec- tions. The details are as follows: • Most high spots of the microsurfaced roadway have been abraded by winter snow removal equipment. In some areas the microsurfaced treatment has been worn away completely. • Microsurfacing has higher IRI [International Roughness Index] values and Frictional Resistance is slightly higher than the HMA sections. • Microsurfacing appeared to slow the progression of reflective cracking up [for 2 years] . . . [then] cracks reflected through the microsurfaced sections at a higher rate than the HMA sections. • Microsurface treatments claim to add between five and seven years life to existing pavements. This appears to be the case on the Limestone project where the material is performing as expected. The only apparent issue is snow plow abrasion (Marquis 2009). Lessons Learned A full analysis of the three reports available in this case study (Marquis 2002, 2004, 2009) provides two lessons learned. The CHAPTER EIGHT CASE STUDIES

57 TABLE 41 CASE STUDY PROGRAM SUMMARY TABLE 42 MAINE DOT CASE STUDY FACTS Case Study Agency/Location Reason for Inclusion Remarks Microsurfacing as a pavement preservation treatment Maine DOT Caribou, Maine Specific use for pavement preservation; long-term performance in an area with heavy snowplowing Demonstrates microsurfacing performance in cold, snowy climate; answers concerns that it is not appropriate on roads with heavy snowplowing Use of microsurfacing as a preventive maintenance treatment to improve safety York Region Ontario, Canada Focus on safety; specific use for preventive maintenance Demonstrates a use for microsurfacing that does not focus on pavement distress Long-term comparative performance of microsurfacing on asphalt and concrete pavements Oklahoma DOT Tulsa and Oklahoma City, Oklahoma Used for filling deep ruts and treating alligator cracking on high-volume interstate; 9-year record Very comprehensive look at the treatment in a variety of situations Microsurfacing on a high traffic interstate highway Georgia DOT Atlanta, Georgia Heavy urban traffic volume; road noise evaluation Agency survey response indicated they do not use microsurfacing Microsurfacing on Jointed plain concrete pavement Kansas DOT Cowley County, Kansas Evaluation of ride quality improvement; use of microsurfacing on a concrete surface Ride quality is of prime importance on concrete pavements; comparison is with a hot-mix overlay Microsurfacing using a softer binder Minnesota DOT Albertville, Minnesota Evaluation of cracking and rut filling performance Provides an alternative for situations where cracking is the primary issue Item Data Binder CSS-1H Aggregate Type III Mineral Filler Non-air entrained portland cement Job Mix Design Aggregate 100% Portland cement 1.0% Water 10.0% Binder 12.0% + 1% Test Specification Residual asphalt 8.3 Wet track abrasion—1 h 470.0 Wet track abrasion—6 day 680.0 Excess asphalt loaded wheel 453.2 Wet stripping 96.0 Compatibility 11 pts Location Route 1 between Presque Isle and Caribou, Maine Route 1A between Limestone and Caswell, Maine ADT 8,600 and 1,100, respectively Distress Level Before Microsurfacing IRI ( m/km ) 0 .97 to 2.14 (62 to 136 in./mile) Rut depth ( mm ) 9 .95 to 12.75 (0.39 to 0.50 in.) Friction number Average 53.2 Length of Test Period 5 years Snowplowin g? Yes ADT = average daily traffic. first lesson regards the impact of snowplowing on microsur- facing. Both microsurfacing sections were visibly abraded by snow removal equipment. The report states: “Most high spots of the microsurfaced roadway have been abraded by winter snow removal equipment.” Given this premise, the three per- formance measures studied (International Roughness Index, rut depth, and friction number) were all within acceptable lim- its for the Limestone road [1,100 average annual daily traffic (AADT)] and only rut depth was unacceptable on the Presque Isle (8,600 AADT). Thus, the following lesson learned can be noted: Snowplowing will abrade microsurfacing and eventually wear it away. Although this is an issue if the treatment is used to act as a seal to water intrusion it does not signifi- cantly impact the use of microsurfacing to enhance ride

quality and skid resistance or as a rut filling technique on a structurally sound pavement. The second lesson in this case deals with skid resistance. Both the Presque Isle and Limestone microsurfaced test sec- tions were found to furnish higher skid numbers throughout the 5-year test period than the HMA test sections on the same roads. The higher volume road lost 3.4% and the lower volume road lost 1.1% over the period. Therefore, the lesson learned is: Using microsurfacing to correct the loss of frictional resis- tance on a structurally sound pavement works well in a Northern climate. Effective Practices One effective practice can be derived from this case study. The analysis of the literature review shown in Table 7 (chapter three) found that among those authors that specifically men- tioned snowplowing that microsurfacing was found to be suit- able for use in those areas more often than it was cited as a concern (four positive versus one negative citations). Next, the survey received responses from 23 states where winter snow removal is an issue and only 6 did not include microsurfacing in their pavement maintenance programs. Additionally, all of the Canadian provinces except one use microsurfacing. There- fore, the intersection of those two lines of information with the one contained in this case study yields the following effective practice: Microsurfacing can be effectively employed on roads where routine winter snow removal is a factor if the under- lying pavement is structurally sound. 58 Microsurfacing as Preventive Maintenance— York Region This case study consisted of the analysis of microsurfacing’s impact on safety when used as a preventive maintenance tool to restore skid resistance. The study evaluated accident rates over a 4-year period of microsurfacing placed on pavements on two highways in the Region of York in Ontario, Canada. The test compared 28 microsurfacing sites with 12 HMA over- lays. The objective of the study was to evaluate the inclusion of safety issues in an agency’s preventive maintenance pro- gram (Erwin and Tighe 2008). It used the two types of pave- ment surface treatments and an accident rate before and after to test the study’s hypothesis. Table 43 contains the salient facts about the project. Results of the York Case Study Figure 26 is a map that shows the location of the York Region in the Canadian Province of Ontario. The results of the study are summarized as follows: • Microsurfacing has a positive safety effect when applied at loca- tions with an AADT greater than 3,000 vehicles per lane. • Microsurfacing has been demonstrated to have a positive safety effect on locations with higher traffic volumes susceptible to any one or combination of these conditions: – occurrence of wet or slick (not dry) road surface conditions; – trend in severe crashes; – frequent intersection-related crashes; and – high occurrence of rear-end crashes. • Another point to consider is that contractors [furnish lower] treatment prices for larger jobs. To capitalize on that opportunity and keeping in mind that microsurfacing was demonstrated to be very effective at reducing intersection-related crashes, when TABLE 43 YORK REGION CASE STUDY FACTS Item Data Binder CSS-1h Aggregate Type III Mineral Filler Non-air entrained portland cement Job Mix Design—Typical Aggregate 100% Portland cement 2.0% Water 10.0% Binder 11.5% + 1% Test Specification—Typical Residual asphalt 6.0% to 11.5% Wet track abrasion—1 h 538.0 Wet track abrasion—6 day 807.0 Excess asphalt loaded wheel 538.0 Wet stripping 90.0 Compatibility 11 pts Location 28 sites through the Reg io n of York AADT 1,000 to 7,000+ Distress Level Before Microsurfacing IRI ( m/km) Not available Rut depth ( mm) Not available Friction number Not available Length of Test Period 4 years Snowplowin g? Yes AADT = average annual daily traffic.

59 prioritizing treatment sites; one could group intersection to and tender them out as single job. Such foresight in the planning process can help agencies stretch their budgets farther while making the roads safer. Lessons Learned Two lessons can be drawn from this case study. First, the idea of including safety in a public highway agency’s pavement maintenance/preservation program is appropriate. A struc- turally sound pavement could be rendered unsafe merely to loss of skid resistance resulting from the polishing of the pave- ment’s aggregate (Gransberg 2009). The literature shows that countries with a tradition of performance-based pavement maintenance contracting such as Australia and New Zealand include accident rates as a key performance indicator (Grans- berg et al. 2010). Therefore, adding an analysis of accident rates to the pavement maintenance/preservation project selection process makes sense. The lesson learned here is as follows: Because microsurfacing has shown itself to be particu- larly effective in reducing intersection accidents, adding safety issues to the project-specific treatment selection process may furnish added value to an agency’s pavement maintenance/preservation program. The second lesson learned is that it would be beneficial to use microsurfacing in localized areas that are expected to experience frequent stopping. For instance, freeway ramps would benefit from a higher friction surface to enhance emer- gency stopping during unexpected situations. Thus, the lesson learned is: Microsurfacing can be effectively used to enhance skid resistance in areas where a reduction in stopping distance is critical to safe operation of a given highway feature. This approach has not been developed enough to yield an effective practice. It does intersect with the literature, but the survey responses reflected that using it to improve friction is not currently a primary reason for selecting microsurfacing for a given project. Long-Term Evaluation of Microsurfacing Performance—Oklahoma Department of Transportation This case study reports on an early large-scale field test of microsurfacing used to fill deep ruts and alligator cracking on high-volume four-lane divided highways in Oklahoma. The three sites studies were located in Oklahoma City and Tulsa. The study lasted 9 years and yielded valuable informa- tion that the Oklahoma DOT used to modify its microsurfacing program. That the Oklahoma DOT still uses microsurfacing as a major tool in its pavement maintenance and preservation pro- gram amply demonstrates the value of including the case study in this synthesis. Table 44 contains the important data about this case. Table 45 shows the aggregate gradations used by the DOT specifically for deep rut filling and alligator cracking compared with the standard microsurfacing gradation. Results of the Oklahoma DOT Case Study The major findings of this robust study are as follows: • Microsurfacing reduces the level of rutting and retards the rate of rutting for up to four years of service. • Microsurfacing provides good friction characteristics for up to nine years of service. • Microsurfacing can be used effectively to fill ruts up to 38 mm (1.5 inch) deep. • Microsurfacing works well for filling depression cracks and alligator cracks. • Microsurfacing worked successfully with mine chat (cherty limestone) and dolomite/granite aggregate mixture (Hixon and Ooten 1993). Lessons Learned Two lessons learned can be derived from this case. The first regards the sustainability of microsurfacing and the ability to increase its “greenness” by utilizing recycled waste materials such as mine chat. Not only does microsurfacing require less bituminous material, because it is cold-laid it con- sumes less energy than other treatment alternatives. There- fore, the lesson learned in this case is as follows: Microsurfacing is a “green” alternative and can be used to promote sustainable maintenance practices by using recycled waste products such as mine chat for aggregate and products such as fly ash and cement kiln dust as min- eral filler. The second lesson learned comes from the special aggre- gate gradations used by the Oklahoma DOT. The literature contradicts the Oklahoma experience with regard to alligator cracking as can be seen in Table 9. However, because the Oklahoma DOT used a special gradation designed specifically FIGURE 26 Map of the York region case study.

60 TABLE 45 OKLAHOMA DOT CASE STUDY AGGREGATE GRADATIONS Percentage Passing Sieve Size ODOT-Type I (alligator cracking) ODOT-Type II (normal) ODOT-Type III (deep ruts) 3/8 (9.5 mm) 100 99–100 98–100 #4 (4.75 mm) 98–100 80–94 75–85 #10 (2.36 mm) 68–86 44–60 45–55 #40 (420 µm) 22–41 12–30 15–25 #80 (177 µm) 10–25 8–20 8–25 #200 (75 µm) 5–15 5–15 2–8 Source: Hixon and Ooten (1993). to treat alligator cracking, the two lines of information are not directly comparable. Thus, it appears that a one-size-fits-all approach to microsurfacing design may optimize microsurfac- ing’s rut filling ability at the expense of its crack filling ability. This idea is validated because the Oklahoma DOT used a spe- cial gradation for filling deep ruts rather than the “normal” gra- dation. Therefore, the lesson learned is: The aggregate gradation in the job mix design is to be cus- tomized to match the primary purpose of utilizing micro- surfacing on a given road with specific gradations being developed for cracking versus rut filling. Effective Practices Taking the findings in this case study regarding microsurfac- ing’s ability to furnish long-term surface friction characteris- tics and intersecting it with the information found in the York case study and the literature, the following effective practice is proposed: Microsurfacing is the proper alternative to enhance skid resistance in areas where the frictional characteristics of the road’s surface are to be restored to safe operating limits. Microsurfacing on High-volume Roads— Georgia Department of Transportation This case study (Tables 46 and 47) consisted of an experimen- tal trial of microsurfacing to address wheel path raveling and cracking on 92 lane-kilometers of Interstate 285 in Atlanta. Because the motivation for the project was part of the prepara- tion for the 1996 Summer Olympics, aesthetics was also a con- sideration. The project used both a tack coat and a scratch course. The Georgia DOT evaluated friction, crack propaga- tion, and road noise. TABLE 44 OKLAHOMA DOT CASE STUDY FACTS Item Data Binder CSS-1h Aggregate See Table 45 Mineral filler Non-air entrained portland cement Job Mix Design—Typical Aggregate 90% Portland cement 2.0% Water 9.0% Binder 9% + 1% Test Specification—Typical Residual asphalt 8.0% to 13.0% Wet track abrasion—1 h Not available Wet track abrasion—6 day Not available Excess asphalt loaded wheel Not available Wet stripping Not available Compatibility Not available Location I-40 in Oklahoma City: 2 sites US-64 in Tulsa: 1 site AADT AADT = average annual daily traffic. 11,000 to 40,000 Distress Level Before Microsurfacing IRI (m/km) Not available Rut depth (mm) 72 to 81 mm (2.8 to 3.2 in.) Friction number 32–44 Length of Test Period 9 years Snowplowing? No

61 Results of the Georgia DOT Case Study The Georgia DOT case study is summarized as follows: • The microsurfacing used on I-285 has performed quite well. • No additional problems with raveling or load cracking have been encountered. • The mix has provided excellent smoothness and good friction, with a minimal increase in pavement noise levels. • It is aesthetically superior to slurry seal because of its resem- blance to hot-mix asphalt (Watson and Jared 1998). The unique feature of this study was the comparison of noise levels with other surface courses. Table 48 provides the comparison with several locations in the Atlanta metro area. It shows that the change is virtually negligible. When this is compared with the survey results where the respon- dents cited road noise as the most frequent public complaint about microsurfacing a dichotomy exists. One possible expla- nation is that public road noise complaints are the result of the differential change from the original surface, which may have seemed quieter owing to low friction characteristics, and the microsurfacing that increased the texture of the wheel paths. Lessons Learned The major lesson from this case study deals with the qualita- tive aspects of microsurfacing and its use as a “quick fix” to enhance the appearance of a road at a low cost while extend- ing its life and enhancing the safety of the traveling public by TABLE 46 GEORGIA DOT CASE STUDY FACTS TABLE 47 GEORGIA DOT CASE STUDY AGGREGATE GRADATION TABLE 48 COMPARISON OF MICROSURFACING ROAD NOISE TO OTHER COMMON SURFACES Item Data Binder CSS-1HLM (Ralumac with 3% natural latex) Aggregate See Table 47 Mineral filler Type I portland cement Job Mix Design—Typical Aggregate 100% Portland cement 1.0% Water 10.0% Binder 7.4% Test Specification—Typical Residual asphalt 6.8% Wet track abrasion—1 h 807 Wet track abrasion–6 day 538 Excess asphalt loaded wheel 538 Wet stripping 90% Compatibility Pass Location I-285 in Atlanta, Georgia AADT 55,650 Distress Level Before Microsurfacing IRI (m/km) 0.573 (36.1 in./mile) Rut depth (mm) 19 mm (0.75 in.) Friction number 46–50 Length of Test Period 2 years Snowplowing? no AADT = average annual daily traffic. Sieve Size Percentage Passing GDOT 3/8 (9.5 mm) 100 #4 (4.75 mm) 80 #8 (2.36 mm) 68–86 #50 (300 µm) 22–41 #200 (75 µm) 5–15 Surface Course Average Decibels Microsurfacing Difference Microsurfacing 74.9 — Conventional OGFC 73.9 +1.0 Modified OGFC 72.8 +2.1 Porous European Mix 72.7 +2.2 Dense Graded Surface Mix 73.1 +1.8 Portland Cement Concrete 73.1 +1.8 Source: Watson and Jared (1998). OGFC = open-graded friction course.

increasing friction numbers. This speaks to the public relations aspects that impact public highway agencies. It also demon- strated that road noise complaints are largely perceptional and that the public can be educated by showing them the numbers such as the Georgia DOT did. Thus, this lesson can be stated as follows: Microsurfacing can be used as a cost-effective means to enhance the visual quality of a high-volume road while simultaneously enhancing skid resistance, smoothness, and addressing raveling and cracking issues on a high-volume highway. This case study did not yield any effective practices. Microsurfacing Performance on Concrete Pavement—Kansas Department of Transportation The objective of this case study was to test microsurfacing’s ability to improve ride quality on a jointed, plain concrete pavement on US-77 in Cowley County, Kansas. Kansas DOT engineers investigated a number of alternatives (diamond grinding, HMA overlay, and cracking and sealing) and selected microsurfacing based on cost and time required for installa- tion. The concrete pavement was structurally sound, although the ride was rough owing to joints that were faulted approx- imately 6 mm (0.25 in.). Before installation, the joints and cracks in the substrate were sealed and a tack coat consisting of SS-1h emulsion was applied. Tables 49 and 50 contain the details of this case. 62 Results of the Kansas DOT Case Study The results of the Kansas DOT case study project in Cowley County, Kansas, can be summarized as follows: • A relatively thin application of microsurfacing (20.6 kg/m2) placed in two lifts improved the ride quality of JPCP [jointed plain concrete pavement]. • The contractor was able to complete the 1.6 km (1 mile) test section including the sealing of joints and cracks in 10 working days. • The ride quality improvement when a short-span, 2.4-m ski was attached to the paving box indicated a minor increase (16.7% on average) in smoothness from the original pavement. • The use of a 4.9-m ski produced a marked improvement (49% on average) in smoothness from the original pavement. • The average final profile index on the project for the 14.1-km (8.8-mi) section where the 4.9-m ski was used was 436 mm (27.48 in.), well within the limits established by Kansas DOT of 254 to 762 mm for a 100-mm-thick bituminous pavement (Moulthrop et al. 1996). Lessons Learned This case study project documents the successful enhancement of ride quality on jointed plain concrete pavement using micro- surfacing. It was included because much of the nation’s Inter- Item Data Binder CSS-1HLM (Ralum ac) Aggregate See Table 50 Mineral filler Type I portland cement Job Mix Design—Typical Aggregate 100% Portland cement 1.75% + 0.25% Water As required Binder 7.6% + 0.4% Test Specification—Typical Residual asphalt 6.8% Wet track abrasion—1 h Not available Wet track abrasion—6 day Not available Excess asphalt loaded wheel Not available Wet stripping Not available Compatibility Pass Location US-77 in Cowley County, Kansas AADT AADT = average annual daily traffic. 4,000 Distress Level Before Microsurfacing IRI ( m/km ) 0 .848 to 0.929 (52.6 to 57.6 in./mile) Rut depth ( mm ) N ot applicable Friction number Not applicable Length of Test Period 2 years Snowplowin g? Yes TABLE 49 KANSAS DOT CASE STUDY FACTS

63 state Highway System was constructed using this pavement type, and ride roughness is a major issue on roads with this type of pavement. Often concrete pavements are found in urban areas where the high traffic volume initially warranted the higher construction cost and lower life-cycle costs that concrete furnishes. The cardinal outcome of the case was the finding that microsurfacing delivered a smoothness that was comparable to hot mix (Moulthrop et al. 1996). Thus, this case provides a valuable tool for pavement managers dealing with this issue. Two lessons learned can be derived from this case study project. • Microsurfacing furnishes a cost-effective means to improve ride quality on jointed concrete pavements; and • Microsurfacing provides an expeditious means to improve ride quality while minimizing disruption to traffic. Effective Practices In this case study, the standard microsurfacing equipment needed to be modified for use on jointed concrete pavement. A detailed explanation of the modifications is as follows: The standard load-bearing support for the laydown box consists of three steel skis, on which the box rides as it is pulled along the pave- ment. These skis are normally 1.8 m (6 ft) long at the outside sup- port location and shorter at the middle of the box. For this project, the skis were initially changed to 2.4 m in length to help support the laydown box when it passed over the faulted joints. This did not produce the desired smoothness and, after consultation with the project engineers, it was decided that the box needed to have better support to stop it from tipping when the skis passed over the joints. The contractor fabricated a 4.9-m-long supported-beam leveling arm that attached solidly to the laydown box at the outside edge. This beam had attached at each end of it small metal skis that piv- oted when the ski passed over the joints. The beam was attached to the laydown box so that the standard 1.8 m skis were left attached to the bottom of the box. The beam supported the box, which elim- inated the tipping. The other equipment adjustment made to the standard laydown box configuration was the use of a steel (rigid) strikeoff plate instead of the rubber (flexible) strikeoff that is nor- mally used. Usually a microsurfacing laydown box uses a rubber strikeoff to finish the surface. When a flexible strikeoff is used, downward pressure is applied to the fresh mix, which causes a small amount of deformation in the surface. Rubber tends to follow the natural surface contours, thus restricting the leveling. When a rigid strikeoff is used, it does not flex. This rigidity allows for bet- ter reprofiling of the pavement section (Moulthrop et al. 1996). Therefore the following effective practice is found: When using microsurfacing to improve ride quality on jointed plain concrete pavements, the spreader box can be modified to furnish better support across the joints and the flexible rubber strike-off can be replaced with a rigid strike- off to improve smoothness. Microsurfacing Performance with a Softer Binder—Minnesota Department of Transportation The objective of this case study was to test the impact of a softer binder on microsurfacing’s ability to resist reflective cracking and act as a surface preparation measure for subsequent level- ing or rut-filling courses. Minnesota DOT engineers investi- gated this treatment on four test sections originally paved in 1993. Before installation, cracks in the substrate were sealed in only one test section and a tack coat consisting of diluted CSS-1h emulsion was applied. Table 51 contains the details of this case. Results of the Minnesota DOT Case Study The results of the Minnesota DOT case study project at the Minnesota Test Road Facility can be summarized as follows: • The construction phase demonstrated the viability of producing and placing microsurfacing slurry mixtures at 12.5% and 16.5% emulsion levels. Mixture consolida- tion did not appear problematic when very-low-volume traffic was involved. • Following a 6-month service period that included a northern climate winter, the project was evaluated for reflective cracking, smoothness, and rutting. Approxi- mately 71% of transverse cracks and 5% of longitudinal cracks had reflected through the microsurface. – Transverse cracks in lanes constructed with scratch and wear course mixtures had reflected through the microsurface to 88% of preconstruction numbers. – Transverse cracks in lanes constructed with rut-fill and wear course mixtures had reflected through the micro- surface to 60% of preconstruction numbers. – Patched locations were not reflecting through the microsurface. • Pavement IRI measurements showed little change from the post-construction condition. The 6-month IRI was found to have decreased by 22% for lanes constructed with scratch and wear course mixtures, and by 58% for lanes constructed with rut-fill and wear course mixtures. • Rut conditions as measured after construction showed the following results: – A 4% to 6% decrease for lanes constructed with scratch and wear course mixtures, TABLE 50 KANSAS DOT CASE STUDY AGGREGATE GRADATION Sieve Size Percentage Passing KDOT 3/8 (9.5 mm) 99–100 #4 (4.75 mm) 86–94 #8 (2.36 mm) 45–65 #16 (1.19 mm) 25–46 #30 (300 µm) 15–35 #50 (297 µm) 10–25 #200 (75 µm) 5–15

– A 11% to 40% decrease for lanes constructed with rut- fill and wear course mixtures, – A 7% decrease for 102-kip load-configuration lanes, and – A 32% decrease for 80-kip load-configuration lanes. • Early results from this research show that the soft asphalt concrete microsurface design has a moderate effect in decreasing transverse reflected cracks. • Data . . . also suggest that the soft asphalt concrete microsurfacing is effective at reducing rutting (Johnson et al. 2007). Lessons Learned This case study project documents the results of using a softer asphalt binder in the microsurfacing JMF. Two lessons learned can be derived from this case study project. • Microsurfacing furnishes a promising means to reduce the amount of transverse reflective cracking; and • The amount of binder can be successfully varied in the field to enhance microsurfacing ability to fill ruts. Effective Practices This case study yielded the following effective practice. The microsurfacing binder amount can be reduced by 1% to 2% in rut filling and scratch courses upon which a wear- ing course will be applied. 64 SUMMARY AND EFFECTIVE PRACTICES This chapter presented six case studies that each demon- strated a particular aspect of microsurfacing practice. The case studies covered projects in both northern and southern climates, in the United States and Canada, on rural and urban highways, and on both asphalt and concrete pavements. In summary, the case studies highlighted the robust ability of microsurfacing to effectively address many common pave- ment distresses while enhancing skid resistance, ride quality, aesthetics, and extending the service lives of the pavements upon which they are placed. This chapter produced 16 lessons learned and 4 effective practices. The effective prac- tices are as follows: 1. Microsurfacing can be effectively employed on roads where routine winter snow removal is a factor if the underlying pavement is structurally sound. 2. Microsurfacing is the proper alternative to enhance skid resistance in areas where the frictional characteristics of the road’s surface are to be restored to safe operating limits. 3. When using microsurfacing to improve ride quality on jointed plain concrete pavements the spreader box can be modified to furnish better support across the joints and the flexible rubber strike-off would be replaced with a rigid strike-off. 4. The microsurfacing binder amount can be reduced by 1% to 2% in rut filling and scratch courses upon which a wearing course will be applied. Item Data Binder CSS-1 using PG 49-34 asphalt binder Aggregate Type II Mineral filler Type I portland cement Job Mix Formula—Typical Aggregate 100% Portland cement 1.75% + 0.25% Water As required Binder 12.5% and 16.5% + 0.4% Test Specification—Typical Residual asphalt 8% to 8.5% Softeni ng po in t 12 8 o F Penetration 163 Excess asphalt loaded wheel Not available Wet stripping Not available Compatibility Pass Location Minnesota Road Test Facility Albertville, MN AADT—Test road 80 truck passes per day Distress Level Before Microsurfacing IRI ( m/km ) 1 .24 to 3.25 (52.6 to 57.6 in./mile) Rut depth ( mm ) 8 to 46 (0.33 to 1.81 in.) Friction number Not applicable Length of Test Period 2 years Snowplowin g? Yes AADT = average annual daily traffic. TABLE 51 MINNESOTA DOT CASE STUDY FACTS

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