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9 Over the years, pavement marking removal methods have evolved and new methods have been developed. In general, pavement marking removal is completed using some form of blasting, grinding, burning, laser, chemical, or masking technique (Berg and Johnson 2009, Bryden and Kenyon 1986, Ellis 2003, Ellis et al. 1999, Heydon 1997, Kilgore 1980, Migletz et al. 1994, Niessner 1979). Each of these techniques is explained in Chapter 3. The effectiveness of each of these removal methods is impacted by the type of material being removed, the material thickness, the pavement surface, the allowed duration of the work activity, and the skill of the equip- ment operator(s) (Berg and Johnson 2009). Some examples of inadequate pavement marking removal can be seen in Figure 1. Past Research Over the years, there have been several different research studies with regard to pavement marking removal methods. However, there have only been a few completed recently with some of the newer technologies, and even those studies have not captured the entire matrix of available technologies, pavement marking types, and pavement surfaces. A recent study conducted in Utah by Berg and Johnson focused on evaluating five specific removal technologies (Berg and Johnson 2009). There were three blasting methods (i.e., high-pressure water, soda, and dry ice) and two grinding methods (i.e., carbide and diamond bit). The researchers con- ducted the high-pressure water blasting and the two grinding methods using large mobile truck units that provided greater productivity and ease of operation than the soda and dry ice blasting. Test removal sections consisted of waterborne paint on an asphalt chip seal pavement and waterborne paint over an existing epoxy line on a Portland cement concrete (PCC) pavement. The researchers evaluated the amount of time with respect to linear feet (lf) of line removed per minute for each method for each test section (see Table 1). The researchers also assessed other subjective factors, such as pavement surface damage and overall impact of the removal method (i.e., pavement surface was left wet, the method reduced visibility during operations, etc.). While the grind- ing removal methods were faster than the high-pressure water blasting, the high-pressure water blasting resulted in the least amount of pavement damage. The high-pressure water blast- ing also had the least amount of complications during and post application with regard to dust and noise concerns. The only noted limitation of the high-pressure water blasting method was that it is potentially limited to above-freezing conditions. The soda and dry ice blasting were both noted to be very slow compared to the vehicle-mounted removal methods, but the removal left very little pavement degradation except for some pitting of the chip seal surface. The soda blasting gener- ated a large amount of dust that could be a potential safety hazard by lowering visibility. The research recommendations/implementations indicated that the two grinding technologies are still the most effective in removing lines quickly and leaving the surface ready to be restriped. It was also suggested that the soda and dry ice tech- nologies should be investigated if space is limited or there are other special circumstances, but the speed and possible vis- ibility issues need to be considered. Finally, the water blasting technology was the most effective at marking removal with the least amount of damage to the pavement and should be investigated for future use. The Florida DOT sponsored two separate research efforts to investigate how to eradicate pavement markings, with one focused on the actual removal of the pavement markings (Ellis et al. 1999) and one focused on methods to mask or cover the pavement markings with an inexpensive surface treat- ment or black tape (Ellis 2003). In the first study, Ellis et al. investigated the removal of paint, thermoplastic, and tem- porary tape on asphalt concrete (AC) using high-pressure water blasting (full truck and hand-operated walk-behind systems), grinding (hand-operated walk-behind system), and a C H A P T E R 2 Literature Review
10 combination of those two methods. The researchers focused on AC because it is the most common pavement surface in Florida, and this pavement type had the most pavement marking removal problems. The researchers evaluated the removal methods based on the measured rate of removal, degree of removal, condition of the pavement surface after removal, and potential for scarring to confuse the motorist. The researchers based the degree of removal on subjective evaluation. They based the condition of the surface after removal on subjectively evaluated changes in color and texture from the surrounding pavement. They subjectively based the potential for the scarring to confuse a motorist on the visual appearance of any scar present after the removal. The researchers then conducted the subjective evaluations during the day as well as during the night in dry and wet conditions. The nighttime evaluations resulted in similar findings to the daytime evaluations. While the researchers did not recommend a specific removal method, they had several useful findings. They indi- cated that pavement scarring is possible with both grinding and water blasting, but grinding appears to present the largest possibility for pavement scarring. Subsequently, they reported that the high-pressure water blasting method appears to be the most effective at removing pavement markings with the least amount of surface scarring. The researchers also experi- mented with using reflectance to evaluate quantitatively the removal. The results were promising, so the researchers rec- ommended further study. In the second Florida study (Ellis 2003), Ellis investigated pavement marking eradication alternatives that masked or covered pavement markings with an inexpensive surface treatment or temporary black tape, thus negating the need for actual marking removal. Both methods resulted in com- plete eradication; hence, the measures of effectiveness were focused on the blending of the masking material with the existing pavement, the durability of the surface, the surface friction of the seal coat material, and the associated costs with each method. The surface friction measurements and the estimated costs were objective measures, and the dura- bility and blending were subjective measures. The estimated costs were $0.47 per linear foot and $1.83 per linear foot for the modified sand seal coat and temporary black pavement markings, respectively. While the study period with regard to durability was short at 30 days, each method proved effective. Friction charac- teristics of the modified sand seal coat were deemed accept- able. The blending of the black tape to the surrounding new black asphalt road surface was deemed satisfactory but would not have been satisfactory on an aged/faded asphalt surface or PCC surface. Ellis recommended both methods to be adopted as optional methods to either remove temporary markings or temporarily remove pavement markings. a) Ghost markings resulting from scarring and/or surface characteristic changes and incomplete marking removal. b) Insufficient marking removal and poor temporary markings. c) Color contrast of markings masked with black material. Figure 1. Examples of inadequate pavement marking removal. Method Average Speed (ft/min) Paint on Chip Seal Paint over Epoxy on PCC Mechanical (Carbide Grinding ) 132.2 81.1 Mechanical (Diamond Grinding ) 116.1 57.5 High-Pressure Water Blasting (40,000 psi truck unit) 109.6 45.1 Soda Blasting Three tests (4.8, 6.2, 8.6) 1.4 Dry Ice Blasting 1.0 0.7 Source: Data summarized from Berg and Johnson 2009 report. Table 1. Average Utah study removal rate.
11 Oregon DOT (ODOT) evaluated several different pavement marking removal methods and reported their results (Oregon DOT 2001). Oregon DOT had contractors remove 4-inch wide, 15-mil and 30-mil thick paint pavement markings from AC. The removal methods evaluated were soda blasting and three mechanical methods, a scarifier, a grinder, and a pla- nar. No one method was reported to be better than another; however, the mechanical methods were all faster than the soda blasting, but the soda blasting outperformed the mechanical methods with respect to minimal pavement surface scarring. The authors noted that pavement scarring is possible with any mechanical removal method and that operator skill and expe- rience can affect results. The removal rates and the percentage removed on the first pass are reported in Table 2. In 2000, Pew and Thorne completed a report describing the use of lasers to remove pavement markings (Pew and Thorne 2000). The focus of the study was the further development of a prototype laser pavement marking removal system. In this study, paint was removed from an asphalt pavement. The removal was successful, and it was completed at a rate of approximately 0.42 ft/min. While this rate would be considered slow, there have been several years of improvements in laser technology that may have improved the removal time. One company that makes laser removal equipment for commercial applications other than pavement marking removal states its system can remove paint at a rate of 7.2 ft/min (Coherent 2010), so there is the potential that faster removal rates may be possible. Pew and Thorne also noted that the removal process removed the paint and some of the surface coating from the aggregate, and that it would be beneficial to add a vacuum system to collect both the debris and any unwanted gas releases from the removal process. In 2006, Mathis and Ward completed a pavement marking removal synthesis for Washington DOT (WSDOT) (Mathis and Ward 2006). This effort did not focus on investigating new methods of pavement marking removal, but rather on the existing policies and methods that various agencies were using to provide WSDOT guidance on how to minimize ghost markings in work zone activities. The resulting recom- mendations emphasized the use of tape for masking unneces- sary pavement markings during construction, specification revisions, preplanning, solid white lane markings in transi- tion areas, and detailed review during project development. The University of Tennessee conducted a research study to evaluate the removal and placement of pavement markings in work zones (Jackson et al. 2001). The researchers carried out an extensive literature review and survey covering the most promising pavement marking materials and methods. Inter- viewed contractors reported that for effective application of temporary tape the weather needs to be warm and dry, and preferably dry for several days after placement. It is also pref- erable to limit lateral movement across temporary tape to extend its service life. The researchers found that some agen- cies do not use paint as a temporary marking if later project phases require removal of the marking. The researchers also found that temporary or removable tape is often used on PCC surfaces due to ease of removal. The Nebraska Department of Roads sponsored the Univer- sity of Nebraska-Lincoln to conduct a research project on the effectiveness of temporary pavement marking removal meth- ods (Cho et al. 2011). The project sought to identify effective removal methods and procedures on concrete and asphalt pavements. The research team conducted a five-question survey on which removal methods are used, which are most common, which are most satisfactory, what common problems exist, and what marking materials are used most. The survey was com- pleted by 50 respondents including at least one representative from 25 states. Grinding was indicated for use in all responding states, with 80 percent of states stating they use water blasting, and 60 percent using sand blasting. The most commonly used removal methods by the respondents were grinding (92 per- cent); water blasting (56 percent); and sand blasting (24 per- cent). The removal method with the most satisfactory results was grinding (48 percent), water blasting (52 percent), and sand blasting (20 percent). The research team generated a list of common problems identified for each of the removal methods. Based on the comments, each technique has the ability to dam- age the road surface or leave a scar while removing markings. Paint (85 percent) and temporary tape (20 percent) were the most common types of temporary markings used. In addition to the survey, the research team conducted a controlled field evaluation of several removal techniques: water blasting, dry ice blasting, grinder, scarifier, polycrystal- line diamond cutter grinder, chemical removal, and heat torch (Cho et al. 2011). All removal methods were hand operated Method Average Speed (ft/min) % Removed First Pass 15 mil 30 mil 15 mil 30 mil Mechanical (Scarifier) 10.2 15.0 95 95 Mechanical (Tungsten Carbide Grinding ) 23.6 16.1 99 50 Mechanical (Planar) 44.0 14.6 75 100 Soda Blasting 0.9 0.3 100 100 Source: Data summarized/modified from Oregon DOT 2001 report. Table 2. Average ODOT study removal rate.
12 including the water blasting, which was a lower pressure setup that used a wand. A total of 40 yellow paint lines 50 ft in length were applied to a concrete and an asphalt surface. Half of the lines were 12 mil thick, and the other half 20 mil. Half of the markings of each thickness were water-based paint, whereas the other half were solvent based. In addition to the paint, foil-backed tape was also evaluated, but weather conditions likely caused the tape to not properly bond, so those results will not be discussed. Evaluation criteria consisted of rate of removal, completeness of removal, and condition of the surface after removal (degree of scarring). Completeness of removal was subjectively and objectively evaluated through the use of digital image analysis to determine the percentage of material removed. The research results showed that the blasting and grind- ing techniques could remove most, if not all, of the mark- ings. The exception was that the dry ice blasting on PCC did not remove the paint very well. The shot blasting and grind- ing techniques scarred the PCC surface the most, and all removal techniques scarred the asphalt surface. The chemi- cal removal system was an off-the-shelf product that does not contain methylene chloride (MeCl). Therefore, it was determined to be environmentally safe, as the Environmen- tal Protection Agency (EPA) only has regulations for chemi- cal paint strippers that contain MeCl. The paint stripper was coated on the marking, allowed to sit for 30 min, and then power-washed off. The markings were completely removed on both surfaces, leaving no scar. The objective image analysis of the removal provided results similar to those of the sub- jective analysis as far as completeness of removal. The objec- tive image analysis could be used to determine the change in the surface color from the removed area to the surrounding area. Overall, the researchers found that the paint was most effectively removed with the chemical stripper and that the image analysis could be a useful tool in quantifying marking removal. Measures of Effectiveness There have been several research studies that have evaluated the different types of pavement marking removal methods, and the researchers in these studies used several different measures of effectiveness, or metrics, to evaluate each method (Berg and Johnson 2009, Bryden and Kenyon 1986, Cho et al. 2011, Ellis 2003, Ellis et al. 1999, Kilgore 1980, Niessner 1979, Pew and Thorne 2000). The majority of the studies focused on some form of subjective assessment with respect to pavement mark- ing removal. Factors such as surface scarring and other changes in the pavement surface characteristics that create a contrast were still subjectively rated/ranked. A few studies included objective, quantifiable data. Some studies avoided the subjective assessment and focused first on 100 percent pavement marking removal, then the depth of any surface scarring, and finally the duration and/or cost with respect to linear feet removed as the metrics. Berg and Johnson measured the depth of any resulting surface scarring and cal- culated an average removal rate in feet per minute (Berg and Johnson 2009). Ellis et al. developed a prototype device to assess the pavement diffuse reflectance and then used those values to generate a ratio of the removed region versus the surrounding pavement (Ellis et al. 1999). This ratio indicated the extent to which the surface characteristics of the removed area differed from those of the surrounding pavement. Cho et al. used digital image analysis to determine the percentage of material remain- ing and found favorable agreement with their subjective ratings (Cho et al. 2011).