National Academies Press: OpenBook

Microsurfacing (2010)

Chapter: Chapter Five - Construction Practices

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Suggested Citation:"Chapter Five - Construction 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 Five - Construction 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 Five - Construction 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 Five - Construction 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 Five - Construction 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 Five - Construction Practices." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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37 INTRODUCTION Construction practices and procedures vary from region to region and are generally associated with the climatic con- ditions in which the microsurfacing will be applied. This chapter will draw information from both the survey and the specification content analysis to identify those construction practices that are associated with successful microsurfacing projects. CONSTRUCTION PROCESS The Pavement Preservation Treatment Construction Guide (National Highway Institute 2007) asserts that the primary components of the construction process are as follows: • Safety and traffic control • Equipment requirements • Stockpile and project staging area requirements • Surface preparation • Application conditions • Types of applications • Quality issues • Post-construction conditions • Post-treatments. Safety and traffic control will be discussed in chapter six along with equipment requirements and stockpile and project staging area requirements. Quality issues and post-treatment performance will be covered in chapter seven. Surface Preparation Surface preparation’s purpose is to furnish a clean and sound surface on which the new microsurfacing is installed and to which the microsurfacing will bond (New Mexico DOT 2009). As with most pavement preservation treatments, the agency needs to complete necessary crack sealing and patching. “Crack sealing provides the most cost-effective use of dollars over time compared to other pavement mainte- nance techniques” (Nebraska DOR 2002). Shortly before microsurfacing, the road is swept clean of foreign materials. Sometimes this requires the use of high power pressure washing if clay or other hard-to-remove materials, such as organic matter, are present. Failure to remove contaminants may lead to delamination of the treatment in the contaminated areas. Road markings are also removed or abraded to produce a rough surface before placing microsurfacing. Paint mark- ings require no pretreatment. Rubber from skid marks is also removed. Utility inlets can be covered with heavy paper or roofing felt to prevent the microsurfacing from interfering with their proper operation. Additionally, “all starts, stops, and handwork on turnouts should be done on roofing felt to ensure sharp, uniform joints and edges” (Caltrans 2009). The content analysis found that 100% of the specifications reviewed contained a requirement to thoroughly clean the sur- face of the road. In addition, all the specifications included a requirement to pre-wet the road’s surface before beginning microsurfacing. The pre-wetting process was described in one specification as “Pre-wet the surface by spraying water ahead of and outside of the spreader box at a rate that dampens the sur- face without allowing water to flow freely ahead of the spreader box” (Georgia DOT 2001). Another common practice was the requirement to spray a tack coat before microsurfacing, which was found in 7 of 18 specifications sampled in the content analysis. The tack coat application rates ranged from 0.05 gal- lon per square yard to 0.25 gal/y2 (0.25 to 1.5 l/m2). ISSA (2010a) recommends that the tack coat consist of CSS-1h, although some of the specifications in the content analysis required SS-1h. The Michigan DOT specifications (2005) require a “bond coat” and the New Mexico DOT requires a “paint binder” as a tack coat on concrete pavement surfaces (New Mexico DOT 2009). Both of these are a less diluted emulsion binder. For example, SS-1h diluted to 50:50 was applied at a rate of 0.10 gal/y2 (0.45 l/m2) over jointed concrete pavement in the Kansas DOT (Moulthrop et al. 1996). Application Conditions It is widely recognized that weather-related factors are often responsible for the failure of a newly constructed microsurfac- ing (WSDOT 2003; Olsen 2008). Although microsurfacing CHAPTER FIVE CONSTRUCTION PRACTICES Agencies that require tack coats: • Alabama • Georgia • Michigan • Minnesota • Ohio • Pennsylvania • Tennessee

emulsions depend on a chemical set to develop their adhe- sion characteristics, air temperature, relative humidity, wind velocity, and precipitation will impact the constructability of microsurfacing (ISSA 2010a). Ideal microsurfacing weather conditions are those with low humidity, a slight breeze, and with sustained high temperatures into the forthcoming days (National Highway Institute 2007). High humidity is a detriment to any microsurfacing owing to its acting to retard the breaking of the emulsion (Asphalt Institute 1988). ISSA recommends that microsurfacing only be placed if the humidity is 60% or lower (ISSA 2010a). The Georgia DOT specification allows microsurfacing placement up to 80% relative humidity (Georgia DOT 1998) and the New Mex- ico DOT specification limits humidity to no more than 50% (New Mexico DOT 2007). Hot temperatures accelerate the set and increase the need to use spray bars to fog the surface, which cools the surface and keeps the emulsion from breaking on contact (Caltrans 2009). Additional water may also be needed in the mix to “counter- act the higher pavement temperatures and dehydration in the spreader box” (ISSA 2010a). When temperatures are high, the operator might need to accelerate the ground speed of the microsurfacing machine and/or decrease the rate at which the materials are mixed in the pug mill. A common rule of thumb is to set a production rate that corresponds to a mix dwell time in the spreader box of 45 s or less (Wood 2007). Figure 16 summarizes the results of the content analysis. The majority fall at or above 50°F (10°C). The two that were less come from Michigan and the FLHD. The net effect of permitting a lower temperature is to functionally extend the construction season. Michigan’s northern geography makes this understandable. However, the geographic working area for the three FLHDs is the national parks and other federally owned land. Much of this land is in the mountainous areas of the country and hence logic for a slightly lower than average temperature has the same impact as in Michigan. 38 Types of Applications There are four types of microsurfacing applications: 1. Full lane width 2. Scratch coat 3. Rut filling 4. Hand-applied. Full-Lane Width Microsurfacing This is the most common type of microsurfacing application. When applying a full width seal a standard spreader box such as the one shown in Figure 17 is used. The National Highway Institute Pavement Preservation Treatment Construction Guide (2007) describes the process for installing a full lane-width microsurfacing in the following manner: The edge of each pass should align with the longitudinal joints or paint lines on the roadway. Three passes are typically used for a two-lane roadway. This allows clean edges and minimizes overlaps (usually 75 mm (3 in)). Overlapped seals should only be used when the pavement being sealed is level and in sound condition. Keeping the spreader box level and pulling it smoothly without vibration is the key to installing a full-width micro- surfacing free of surface discontinuities. The ISSA Inspector’s Manual for Slurry Systems (2010a) summarizes the issues sur- rounding spreader box operation in this manner: • Cleanliness is mandatory in a spreader box. The box must be cleaned at the end of every work period and may require clean- ing (especially the rear rubber) during the work day if excessive buildup of mixture causes streaking in the finished surface (mat). • The spreader box should not leak the mixture. Side rubbers (where appropriate) should be installed so that edges are kept neat. The rear box rubber (or steel) should leave a uniform thickness and strike off the mixture so that there are no uneven FIGURE 16 Summary of U.S. and Canadian ambient air and surface temperature specifications. FIGURE 17 Full lane-width microsurfacing (Courtesy: Intermountain Slurry Seal, Inc. 2010).

39 ridges or longitudinal ripples left in the mat. The rear rubber may be changed in thickness, width, and hardness to achieve desired results. • The spreader box should pull smoothly and evenly without vibration. Machine speed should be kept uniform. Excessive speed can cause the box to vibrate or jump, leaving transverse ripple lines in the finished surface. If using a drag, excess speed can cause it to leave a rippled and uneven mat. Spreader boxes of different designs react differently to spreading stresses. A normal speed on one type may be an excessive speed on a dif- ferent box. The most important factor in determining the allow- able speed of application is the end result and quality of the treatment. Laying speed is also affected by application rate, gradation of aggregate, viscosity of the mixture, and existing surface conditions, both texture and smoothness (ISSA 2010a). Scratch Coats “For irregular or shallow rutting less than 1⁄2 inch depth (1.26 mm), a full-width scratch coat pass may be used as directed by the project manager [for] each individual rut fill . . . [r]uts that are in excess of 1–1⁄2 inches (3.8 mm) depth may require multiple placements with the rut filling spreader box to restore the original cross section” (Labi et al. 2007). Fig- ure 18 illustrates the theory behind this. All pavements suffer some degree of rutting during their service life. Structurally sound pavements can rut owing to consolidation of the asphalt surface in the wheel paths (Hicks et al. 2000). Con- crete pavements will develop minor ruts over time owing to abrasion in the wheel paths and both will rut in areas where studded snow tires are authorized for use (Washington State DOT 2009). Both pavements can become uneven in the transverse direction as well. ISSA (2010a) describes these as “minor transverse irregularities and longitudinal ruts less than 0.5 inches (12.5 mm) deep.” The objective of the scratch coat is to create a uniformly level surface upon which to apply the surface course of full-width microsurfacing. Scratch coats are applied with a steel, rather than rubber, strike-off to ensure that the resulting surface is as level as possible (Price 2010). The survey asked respondents to indicate whether or not they used a scratch coat when the substrate conditions war- ranted one. There is a school of thought that believes that the aggregate in the scratch coat be a different size than that used on the full-width microsurfacing (Caltrans 2009). Therefore, the survey also asked that question. Table 27 shows the results from that question. Quebec and Missouri both decrease aggregate size for the scratch coat and both rated their micro- surfacing performance as “good.” Illinois (“fair” perfor- mance rating) and Michigan (“good” performance rating) increase it. Oklahoma (“fair” performance rating) checked “other” and changes the application rate of the mix on the scratch coat. Virginia’s “other” indicated that it could go either way. Virginia also rated its microsurfacing as “good.” The trend here is quite clear and leads to following effective practice: Scratch coat and full-width microsurfacing can use the same size aggregate with no apparent difference in performance. Rut Filling Applications Microsurfacing’s major advantage is its ability to fill ruts in an effective manner (Wood and Geib 2001). A rut box such as the one shown in Figure 19 is an essential piece of equip- ment because it is designed to channel the mix directly into the ruts. Its strike-off is also designed to leave a crowned fin- ish to compensate for compaction by traffic after installation. As a rule, ruts are filled and then covered with a full-width microsurfacing, but they can be opened to traffic without one Keys to Microsurfacing Success: • A clean spreader box • No leaks • Pulled smoothly and evenly • No vibration FIGURE 18 Scratch coat diagram (ISSA 2010a). Question U.S. Canada Total Do you use a scratch coat when conditions warrant? Yes 23 6 29 No 4 2 6 Do not know 0 0 0 If yes, is the aggregate size different in the scratch coat? No change in scratch coat aggregate size 17 5 22 Scratch coat aggregate is smaller 1 1 2 Scratch coat aggregate is larger 2 0 2 Do not know 1 0 1 Other, please specify 2 0 2 TABLE 27 SUMMARY OF SCRATCH COAT SURVEY RESPONSES

(Labi et al. 2007; New Mexico DOT 2009). Some agencies require newly filled ruts to be rolled to compact the mix placed in rutted surfaces (Main Roads 2008; PennDOT 2009). Several authors (Smith and Beatty 1999; Province of Ontario 2009; ISSA 2010a) recommend that newly filled ruts be traf- ficked to compact them for at least 24 h before covering them with the final microsurfacing course. Figure 20 illustrates the principles upon which rut filling is based. “Rut filling should only be used on stable ruts that have resulted from long-term traffic compaction rather than failures in the base or sub-base” (New Mexico DOT 2009; ISSA 2010a). “If rutting is the result of defects that cannot be treated (i.e., failure in the subbase or subgrade), filling the ruts with microsurfacing will not prevent development of ruts in the future. If the ruts are caused by an unstable pave- ment layer material or a structurally deficient pavement layer, the source of the original rutting problem generally will cause the rutting to return very quickly” (Smith and Beatty 1999). It is noted that failing to fill ruts as a separate step in the microsurfacing process will impact application rate. “A 40 single rut of even minor deformation will increase the total average application rate as the rut must be filled to the level of the existing pavement during the application process” (ISSA 2010a). Table 28 is a consolidation of the literature on rut filling and when it is appropriate. It furnishes a set of guidelines for incorporating rut filling into a typical agency pavement preservation and maintenance program. Figure 21 shows before and after pictures of an appropriate use of microsur- facing for rut filling on Interstate 90 in eastern Washington State. The ruts were the result of mechanical abrasion from studded tires, and were nearly 1 in. (25.4 mm) deep after only 6 years of service. The pavement was structurally sound. The ruts were flat and no fatigue cracking was evident in the wheel paths. Therefore, this was a good candidate for rut fill- ing with microsurfacing (Washington State DOT 2009). Hand Work Most projects have areas of pavement that are not accessible to the spreader box. These areas will be covered using hand- held squeegees or lutes (New Mexico DOT 2007). Although microsurfacing these areas has the same technical require- ments as the rest of the pavement, aesthetics is generally the primary problem. The goal is to match the surface texture to the machine-laid microsurfacing. Therefore, if the spreader box uses a drag mop, drag mops need to be attached to the hand tools to obtain a matching surface texture (ISSA 2010a). Because hand work is substantially different from the rest of the process, its quality is driven by the amount of time the workers have to spread the mix before it breaks (ISSA 2010a). Therefore, because emulsions break faster at higher temperatures, it is advisable to schedule hand work in the cooler hours of the day, if possible, to give the workers the maximum amount of time to not only spread the mix where it needs to go but also achieve the desired texture match. Addi- tionally, the surface of the pavement where the hand work will take place needs to be wet before starting. This reduces FIGURE 19 Rut box (Courtesy: Bergkamp Inc. 2010). FIGURE 20 Rut filling diagram (ISSA 2010a).

41 the pavement’s surface tension, making it easier to push the mix around and level it off. ISSA adds this important caveat to its manual regarding hand work: The cardinal rule for handwork is ‘least is best.’ The more the mix is worked, the more segregation takes place. As the squeegee moves the matrix back and forth the larger aggregate is worked to the surface while the fines may be lost and the mix can dehy- drate. The coarse aggregate is then inadequately embedded and may ravel. Large areas requiring handwork are necessarily applied in small sections, allowing sufficient time to place and finish the material without causing segregation or sizeable areas to break (ISSA 2010a). Post-Construction Conditions The first post-construction issue that is planned is removing traffic control and opening the newly sealed surface to traf- fic. Before trafficking is allowed, the emulsion must be allowed to break and the mix must cure. The ambient tem- perature and humidity will affect the overall curing time. Warming temperatures and low humidity reduce the time it takes for the emulsion to break and expel the water. On the other hand, cool, humid conditions increase curing times and delay opening to traffic. Currently, the chemical change can only be seen rather than measured; ISSA has collected two empirical tests that permit the observer to estimate whether or not traffic control can be removed. • The Stick Test—“The asphalt emulsion should begin to break no more than 30 to 45 s after the mixture is deposited by the spreader box. A small stick drawn across the deposited mixture will tear the surface and if the tear cannot be smoothed over by the stick the break has occurred. • The Shoe Test—The shear strength and bond can be checked subjectively by placing your full weight flatly on the sole of your shoe on the placed treatment. If the sole can be placed on the new treatment for 2 s without picking up aggregate, then the pavement can be opened to rolling traffic without significant negative effects. If you can place your weight on the heel of one shoe on the placed treatment and twist the heel (about 180°) with only minor surface marks and without the large aggregate being displaced, the mixture can probably be opened to turning traffic without significant damage. However, sharp turns, especially by heavy vehicles, can damage micro-surfacing for some time after applica- tion, particularly in hot weather” (ISSA 2010a). Surface Distress Characteristic That Favors the Use of Rut Filling ISSA (2010a) NHI (2007) NM DOT (2007) Penn DOT (2009) WS DOT (2009) Smith and Beatty (1999) Pavement is structurally sound X X X X X X Rutting is due to mechanical compaction of pavement structure X X X Ruts are flat, not sharp or showing dual wheel marks X X X Ruts do not contain fatigue cracking X X X X X X TABLE 28 LITERATURE SUMMARY OF APPROPRIATE CHARACTERISTICS FOR RUT FILLING FIGURE 21 Microsurfacing applied to rutting caused by studded tires in Washington State: before and after (Washington State DOT 2009).

The above-mentioned tests are hardly scientific in that they require an experienced person to conduct them with some degree of reproducibility. This points to the need for research to develop a suite of field tests that allow an inspector to test the microsurfacing mix after it has been laid as well as tests to identify when the mix has cured to a sufficient degree to open it to traffic without fear of damaging it. Microsurfacing can normally handle rolling traffic without damage less than one hour after placement (Price 2010). However, in areas such as intersections where stop and go traffic is prevalent, additional curing time may be needed, especially during unexpectedly hot or cold weather (National Highway Institute 2007). Microsurfacing emulsions can retain some water for several weeks. If during this period freezing temperatures are experienced, the binder film will rupture and the surface will ravel. As a result the Caltrans Maintenance Technical Advisory Guide (MTAG) recom- mends that projects “not be started” unless there is a “2-week window” without freezing weather in the forecast (Caltrans 2009). Another concern is that asphalt emulsions “cannot re- emulsify if not fully cured, but they can be tender enough to re-disperse under the effects of traffic loading and excessive water, especially ponded water. In this process, broken aggre- gates or asphalt particles that have not fully coalesced into films are dispersed in water, which disintegrates the emul- sion” (Caltrans 2009). Microsurfacing can typically withstand a light rain 3 h after application. However, a heavy rain and heavy traffic will damage the surface, especially in areas with shear resulting from turning movements (Caltrans 2009). 42 Post-Treatments The ISSA inspector’s manual (ISSA 2010a) contains infor- mation on two post-microsurfacing treatments that are cogent to the discussion. The first is sweeping to remove excess stone on heavily trafficked roads, which ISSA recom- mends be done with a suction broom if possible and com- pleted before opening the road to traffic. Sweeping is also needed if excessive stone loss is experienced after opening a heavily trafficked road. The second treatment is sanding, which can be used to furnish extra protection for special areas such as intersections. Wet spots can also be sanded to permit their opening to traffic. Sanding can commence as soon as the microsurfacing can support traffic without pick up (Caltrans 2009). SUMMARY This chapter reviewed the microsurfacing construction process from concept to traffic opening. Two types of spreader boxes were discussed in detail, but these are mere appurtenances that are attached to other pieces of construction equipment, and that is the subject of the next chapter. One effective prac- tice was developed: Scratch coat and full lane-width microsurfacing can use the same size aggregate with no apparent difference in performance.

Next: Chapter Six - Microsurfacing Equipment Practices »
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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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