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37 CHAPTER FIVE CONSTRUCTION PRACTICES INTRODUCTION areas. Road markings are also removed or abraded to produce a rough surface before placing microsurfacing. Paint mark- Construction practices and procedures vary from region to ings require no pretreatment. Rubber from skid marks is also region and are generally associated with the climatic con- removed. Utility inlets can be covered with heavy paper or ditions in which the microsurfacing will be applied. This roofing felt to prevent the microsurfacing from interfering chapter will draw information from both the survey and the with their proper operation. Additionally, "all starts, stops, specification content analysis to identify those construction and handwork on turnouts should be done on roofing felt to practices that are associated with successful microsurfacing ensure sharp, uniform joints and edges" (Caltrans 2009). projects. The content analysis found that 100% of the specifications reviewed contained a requirement to thoroughly clean the sur- CONSTRUCTION PROCESS face of the road. In addition, all the specifications included a The Pavement Preservation Treatment Construction Guide requirement to pre-wet the road's surface before beginning (National Highway Institute 2007) asserts that the primary microsurfacing. The pre-wetting process was described in one components of the construction process are as follows: specification as "Pre-wet the surface by spraying water ahead of and outside of the spreader box at a rate that dampens the sur- · Safety and traffic control face without allowing water to flow freely ahead of the spreader · Equipment requirements box" (Georgia DOT 2001). Another common practice was the · Stockpile and project staging area requirements requirement to spray a tack coat before microsurfacing, which · Surface preparation was found in 7 of 18 specifications sampled in the content · Application conditions analysis. The tack coat application rates ranged from 0.05 gal- · Types of applications lon per square yard to 0.25 gal/y2 (0.25 to 1.5 l/m2). ISSA · Quality issues (2010a) recommends that the tack coat consist of CSS-1h, · Post-construction conditions although some of the specifications in the content analysis · Post-treatments. required SS-1h. The Michigan DOT specifications (2005) require a "bond coat" and the New Mexico DOT requires a Safety and traffic control will be discussed in chapter six "paint binder" as a tack coat on concrete pavement surfaces along with equipment requirements and stockpile and project (New Mexico DOT 2009). Both of these are a less diluted staging area requirements. Quality issues and post-treatment emulsion binder. For example, SS-1h diluted to 50:50 was performance will be covered in chapter seven. 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). Surface Preparation Application Conditions Surface preparation's purpose is to furnish a clean and It is widely recognized that weather-related factors are often sound surface on which the new microsurfacing is installed responsible for the failure of a newly constructed microsurfac- and to which the microsurfacing will bond (New Mexico ing (WSDOT 2003; Olsen 2008). Although microsurfacing 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- Agencies that require tack coats: nance techniques" (Nebraska DOR 2002). Shortly before microsurfacing, the road is swept clean of foreign materials. · Alabama · Ohio Sometimes this requires the use of high power pressure · Georgia · Pennsylvania washing if clay or other hard-to-remove materials, such as · Michigan · Tennessee organic matter, are present. Failure to remove contaminants · Minnesota may lead to delamination of the treatment in the contaminated
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38 emulsions depend on a chemical set to develop their adhe- Types of Applications sion characteristics, air temperature, relative humidity, wind velocity, and precipitation will impact the constructability of There are four types of microsurfacing applications: microsurfacing (ISSA 2010a). Ideal microsurfacing weather conditions are those with low humidity, a slight breeze, and 1. Full lane width with sustained high temperatures into the forthcoming days 2. Scratch coat (National Highway Institute 2007). High humidity is a 3. Rut filling detriment to any microsurfacing owing to its acting to retard 4. Hand-applied. 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 Full-Lane Width Microsurfacing specification allows microsurfacing placement up to 80% This is the most common type of microsurfacing application. relative humidity (Georgia DOT 1998) and the New Mex- When applying a full width seal a standard spreader box such ico DOT specification limits humidity to no more than 50% as the one shown in Figure 17 is used. The National Highway (New Mexico DOT 2007). Institute Pavement Preservation Treatment Construction Guide (2007) describes the process for installing a full lane-width Hot temperatures accelerate the set and increase the need to microsurfacing in the following manner: use spray bars to fog the surface, which cools the surface and keeps the emulsion from breaking on contact (Caltrans 2009). The edge of each pass should align with the longitudinal joints Additional water may also be needed in the mix to "counter- or paint lines on the roadway. Three passes are typically used for act the higher pavement temperatures and dehydration in the a two-lane roadway. This allows clean edges and minimizes spreader box" (ISSA 2010a). When temperatures are high, overlaps (usually 75 mm (3 in)). Overlapped seals should only be used when the pavement being sealed is level and in sound the operator might need to accelerate the ground speed of the condition. microsurfacing machine and/or decrease the rate at which the materials are mixed in the pug mill. A common rule of thumb Keeping the spreader box level and pulling it smoothly is to set a production rate that corresponds to a mix dwell time without vibration is the key to installing a full-width micro- in the spreader box of 45 s or less (Wood 2007). surfacing free of surface discontinuities. The ISSA Inspector's Manual for Slurry Systems (2010a) summarizes the issues sur- Figure 16 summarizes the results of the content analysis. rounding spreader box operation in this manner: 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 · Cleanliness is mandatory in a spreader box. The box must be permitting a lower temperature is to functionally extend the cleaned at the end of every work period and may require clean- construction season. Michigan's northern geography makes ing (especially the rear rubber) during the work day if excessive this understandable. However, the geographic working area buildup of mixture causes streaking in the finished surface (mat). · The spreader box should not leak the mixture. Side rubbers for the three FLHDs is the national parks and other federally (where appropriate) should be installed so that edges are kept owned land. Much of this land is in the mountainous areas of neat. The rear box rubber (or steel) should leave a uniform the country and hence logic for a slightly lower than average thickness and strike off the mixture so that there are no uneven temperature has the same impact as in Michigan. FIGURE 16 Summary of U.S. and Canadian ambient air and FIGURE 17 Full lane-width microsurfacing (Courtesy: surface temperature specifications. Intermountain Slurry Seal, Inc. 2010).
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39 Keys to Microsurfacing Success: · A clean spreader box · No leaks · Pulled smoothly and evenly · No vibration FIGURE 18 Scratch coat diagram (ISSA 2010a). ridges or longitudinal ripples left in the mat. The rear rubber may be changed in thickness, width, and hardness to achieve desired results. are applied with a steel, rather than rubber, strike-off to ensure · The spreader box should pull smoothly and evenly without that the resulting surface is as level as possible (Price 2010). 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 The survey asked respondents to indicate whether or not can cause it to leave a rippled and uneven mat. Spreader boxes they used a scratch coat when the substrate conditions war- of different designs react differently to spreading stresses. A ranted one. There is a school of thought that believes that the normal speed on one type may be an excessive speed on a dif- aggregate in the scratch coat be a different size than that used ferent box. The most important factor in determining the allow- able speed of application is the end result and quality of the on the full-width microsurfacing (Caltrans 2009). Therefore, treatment. Laying speed is also affected by application rate, the survey also asked that question. Table 27 shows the gradation of aggregate, viscosity of the mixture, and existing results from that question. Quebec and Missouri both decrease surface conditions, both texture and smoothness (ISSA 2010a). 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) Scratch Coats increase it. Oklahoma ("fair" performance rating) checked "For irregular or shallow rutting less than 1/2 inch depth "other" and changes the application rate of the mix on the (1.26 mm), a full-width scratch coat pass may be used as scratch coat. Virginia's "other" indicated that it could go directed by the project manager [for] each individual rut fill . . . either way. Virginia also rated its microsurfacing as "good." [r]uts that are in excess of 11/2 inches (3.8 mm) depth may The trend here is quite clear and leads to following effective require multiple placements with the rut filling spreader box practice: to restore the original cross section" (Labi et al. 2007). Fig- ure 18 illustrates the theory behind this. All pavements suffer Scratch coat and full-width microsurfacing can use the same some degree of rutting during their service life. Structurally size aggregate with no apparent difference in performance. sound pavements can rut owing to consolidation of the asphalt surface in the wheel paths (Hicks et al. 2000). Con- Rut Filling Applications crete pavements will develop minor ruts over time owing to abrasion in the wheel paths and both will rut in areas where Microsurfacing's major advantage is its ability to fill ruts in studded snow tires are authorized for use (Washington State an effective manner (Wood and Geib 2001). A rut box such DOT 2009). Both pavements can become uneven in the as the one shown in Figure 19 is an essential piece of equip- transverse direction as well. ISSA (2010a) describes these as ment because it is designed to channel the mix directly into "minor transverse irregularities and longitudinal ruts less the ruts. Its strike-off is also designed to leave a crowned fin- than 0.5 inches (12.5 mm) deep." The objective of the scratch ish to compensate for compaction by traffic after installation. coat is to create a uniformly level surface upon which to apply As a rule, ruts are filled and then covered with a full-width the surface course of full-width microsurfacing. Scratch coats microsurfacing, but they can be opened to traffic without one TABLE 27 SUMMARY OF SCRATCH COAT SURVEY RESPONSES 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
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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- FIGURE 19 Rut box (Courtesy: Bergkamp Inc. 2010). ing with microsurfacing (Washington State DOT 2009). (Labi et al. 2007; New Mexico DOT 2009). Some agencies Hand Work require newly filled ruts to be rolled to compact the mix placed in rutted surfaces (Main Roads 2008; PennDOT 2009). Most projects have areas of pavement that are not accessible Several authors (Smith and Beatty 1999; Province of Ontario to the spreader box. These areas will be covered using hand- 2009; ISSA 2010a) recommend that newly filled ruts be traf- held squeegees or lutes (New Mexico DOT 2007). Although ficked to compact them for at least 24 h before covering them microsurfacing these areas has the same technical require- with the final microsurfacing course. ments as the rest of the pavement, aesthetics is generally the primary problem. The goal is to match the surface texture to Figure 20 illustrates the principles upon which rut filling the machine-laid microsurfacing. Therefore, if the spreader is based. "Rut filling should only be used on stable ruts that box uses a drag mop, drag mops need to be attached to have resulted from long-term traffic compaction rather than the hand tools to obtain a matching surface texture (ISSA failures in the base or sub-base" (New Mexico DOT 2009; 2010a). Because hand work is substantially different from ISSA 2010a). "If rutting is the result of defects that cannot be the rest of the process, its quality is driven by the amount of treated (i.e., failure in the subbase or subgrade), filling the time the workers have to spread the mix before it breaks ruts with microsurfacing will not prevent development of (ISSA 2010a). Therefore, because emulsions break faster at ruts in the future. If the ruts are caused by an unstable pave- higher temperatures, it is advisable to schedule hand work in ment layer material or a structurally deficient pavement layer, the cooler hours of the day, if possible, to give the workers the the source of the original rutting problem generally will maximum amount of time to not only spread the mix where it cause the rutting to return very quickly" (Smith and Beatty needs to go but also achieve the desired texture match. Addi- 1999). It is noted that failing to fill ruts as a separate step in tionally, the surface of the pavement where the hand work the microsurfacing process will impact application rate. "A will take place needs to be wet before starting. This reduces FIGURE 20 Rut filling diagram (ISSA 2010a).
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41 TABLE 28 LITERATURE SUMMARY OF APPROPRIATE CHARACTERISTICS FOR RUT FILLING Smith NM Penn WS and Surface Distress Characteristic That ISSA NHI DOT DOT DOT Beatty Favors the Use of Rut Filling (2010a) (2007) (2007) (2009) (2009) (1999) Pavement is structurally sound X X X X X X Rutting is due to mechanical X X X compaction of pavement structure Ruts are flat, not sharp or showing dual X X X wheel marks Ruts do not contain fatigue cracking X X X X X X the pavement's surface tension, making it easier to push the only be seen rather than measured; ISSA has collected two mix around and level it off. ISSA adds this important caveat empirical tests that permit the observer to estimate whether to its manual regarding hand work: or not traffic control can be removed. The cardinal rule for handwork is `least is best.' The more the · The Stick Test--"The asphalt emulsion should begin mix is worked, the more segregation takes place. As the squeegee to break no more than 30 to 45 s after the mixture is 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- deposited by the spreader box. A small stick drawn drate. The coarse aggregate is then inadequately embedded and across the deposited mixture will tear the surface and if may ravel. Large areas requiring handwork are necessarily applied the tear cannot be smoothed over by the stick the break in small sections, allowing sufficient time to place and finish the has occurred. material without causing segregation or sizeable areas to break (ISSA 2010a). · 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 Post-Construction Conditions sole can be placed on the new treatment for 2 s without picking up aggregate, then the pavement can be opened The first post-construction issue that is planned is removing to rolling traffic without significant negative effects. If traffic control and opening the newly sealed surface to traf- you can place your weight on the heel of one shoe on fic. Before trafficking is allowed, the emulsion must be the placed treatment and twist the heel (about 180°) allowed to break and the mix must cure. The ambient tem- with only minor surface marks and without the large perature and humidity will affect the overall curing time. aggregate being displaced, the mixture can probably be Warming temperatures and low humidity reduce the time it opened to turning traffic without significant damage. takes for the emulsion to break and expel the water. On the However, sharp turns, especially by heavy vehicles, can other hand, cool, humid conditions increase curing times and damage micro-surfacing for some time after applica- delay opening to traffic. Currently, the chemical change can tion, particularly in hot weather" (ISSA 2010a). FIGURE 21 Microsurfacing applied to rutting caused by studded tires in Washington State: before and after (Washington State DOT 2009).