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65 A substantial amount of research has been conducted and published in the United States and Europe concerning general and winter maintenance of PFC highway pavements. Follow- ing are discussion on general and winter maintenance of PFC pavements. General Maintenance General maintenance consists of cleaning clogged PFC, preventive surface maintenance, and corrective surface maintenance. Cleaning Clogged PFC Over time PFCs may gradually be choked and partially lose permeability (48) due to dirt and debris entering the void structure. Frequent cleaning may be necessary. Three methods of cleaning PFC: (a) cleaning with a fire hose, (b) cleaning with a high-pressure cleaner, and (c) cleaning with a specially manufactured cleaning vehicle were tested for effectiveness in Switzerland (64). The special cleaning vehicle manufactured in Switzerland can wash and vacuum clean the surface in one pass. Deposited dirt in the PFC is washed out by a high- pressure water stream with a working pressure of about 500 psi (3,450 kPa) from a front washing beam, mounted on the vehicle. The water-dirt mixture on the pavement is then sucked into a container by a heavy-duty vacuum cleaner. During the investigation, cleaning with the high-pressure cleaner was found to be most effective based on permeability tests after cleaning. A similar piece of self-contained equipment (Figure 40) from Japan was reported on for cleaning PFC layers at the meet- ing of the International Conference on Asphalt Pavements held in Copenhagen, Denmark (65). A high-pressure water blast (125 psi or 860 kPa) followed by a vacuum to remove the solids and water was used. The primary difference between the Japanese equipment and Swiss equipment is that the Japanese system causes cavitation of the water through the use of spe- cial nozzles. Field trials with the equipment were successful at restoring permeability to PFC layers. Isenring et al. (15) stated that cleaning clogged PFC layers can be difficult. They suggest that cleaning techniques should begin while the layer is still permeable. By starting while the layer is still permeable, regular maintenance should maintain permeability of the layer for a longer time period. The summary of agencies indicated only one agency, Austria, conducts maintenance to unclog PFC layers. Austria stated that they use a special equipment to unclog PFC layers, but no specifics were provided. It is assumed to be similar to the equipment from Switzerland or Japan described above. Preventive Surface Maintenance It is expected that the asphalt binder in the PFC pavement will get oxidized and become brittle after many years service. This may precipitate surface raveling. Many highway agencies such as those in New Mexico, Wyoming, South Carolina, and Oregon have used fog seals to perform preventive maintenance of PFC pavements. Fog seals provide a thin film of neat asphalt binder at the surface and, therefore, are believed to extend the life of PFC pavements (66). The FHWA recommends fog seal application in two passes (at the rate of 0.05 gal per sq. yd. in each pass) using a 50:50 mixture of asphalt emulsion and water without any rejuvenating agent (9). Research in Oregon (66) indicated that the application of fog seals reduces the permeability of PFC layers. Also, application of fog seals to PFC layers will reduce the frictional properties of PFC layers. However, friction increases significantly in the first month after application as the fog seal is worn away by traffic. Fog seals did not affect the macrotexture of PFC layers; there- fore, the reduced potential for hydroplaning was maintained. Rogge (66) concluded that the expected benefits of fog seals to prolong the lift of PFC layers were not substantiated with quantitative studies. Additionally, he recommended that when it was acceptable to abandon the free draining characteris- C H A P T E R 8 Maintenance of Permeable Friction Courses
tics of PFC layers and the pavement structure was sound, chip seals may be applied. Chip seals are, however, more expensive but they more completely seal the surface than fog seals (66). However, Oregon responded in the survey that they had con- cerns with the use of chip seals. These concerns were related to increased potential for moisture damage in underlying layers. None of the agencies indicated that they currently uti- lize fog seals. Ruiz et al. (16) also stated that the primary problem with PFC in Spain has been raveling. The raveling generally occurs shortly after traffic is applied to the pavement layer. They indi- cated that this problem generally originates from placing PFC too cold, not enough compaction, or draindown problems. In British Columbia, light raveling is addressed by applying a light application of asphalt sealant. Wimsatt and Scullion (67) stated that it was standard practice by Texas DOT to use seal coats over distresses open-graded surfaces. Corrective Surface Maintenance Occasionally, the PFC layers will require repair of delami- nated areas and potholes. Milling and inlay using PFC mix has been recommended by the Oregon DOT to repair PFC when the quantities of material are enough to justify this activity. If only a small quantity is needed, a dense-graded conven- tional asphalt mix is suggested for such patch repairs (66). The FHWA advises to consider the drainage continuity of the PFC when undertaking patch repairs (9). When the patched area is small and the flow of water around the patch can be ensured, use of dense-graded asphalt mix can be considered. Rotation of the patch to 45 degrees to provide a diamond shape is rec- ommended because it will facilitate the flow of water along the dense mix patch and also will diminish wheel impact on the patch joint (3). In Britain, patch repairs are recommended with PFC material only both for small and large potholes. Patching with dense-graded mix is limited to sizes 1.64 ft by 1.64 ft (35). If a dense mix is used in urgency it must be replaced with PFC mix later (68). When patch repairs are made with PFC material, only a light tack coat (preferably emulsion) should be applied to the verti- cal faces of the existing pavement. Heavy tack coats will impede the flow of water through the patch. The PFC pavement also can develop transverse and longitu- dinal cracks while in service. Narrow cracks are usually not vis- ible on the PFC surface because of its very open texture. When cracks appear on the PFC surface they need to be sealed. There is no problem in sealing the transverse cracks because the crack sealer will not impede the flow of water within the PFC, which takes place in a transverse direction (69). Sealing longitudinal cracks in PFC is problematic because the crack sealer could impede the transverse flow of water within the PFC. One potential solution, although expensive, is to mill off the PFC in a narrow strip right over the longitu- dinal crack and place an inlay with PFC material. If the longi- tudinal crack also is present in the underlying course, it must be sealed properly. Only a light tack coat should be applied to the vertical faces of the existing pavement. The other option is to rehabilitate the pavement if the severity of the crack becomes too high (69). Winter Maintenance The intent of this chapter was to provide the state of practice on maintenance, with this section specifically dealing with winter maintenance. However, after evaluating all of the infor- mation from the literature review and survey, it became obvi- ous that each agency appears to have different thoughts on proper winter maintenance of PFC surfaces. This statement is backed up by statements from three different authors from three different countries. Padmos (51) stated there is no defin- itive solution for winter maintenance of PFCs. Greib (24) stated that since the behavior of the road salts on different PFC sur- face is so different, special locally adjusted strategies are needed. Brousseaud et al. (28) stated experience is the only true method of developing a winter maintenance program. Therefore, this section presents the experiences with winter maintenance of PFCs found in the literature. Winter maintenance (snow and ice control) often has been cited and assumed to be a serious problem with PFCs (4). Isenring et al. (15) listed some advantages and disadvan- tages of PFCs during winter conditions. Some advantages of PFC during winter conditions include: 1) ice does not gen- erally form on wet PFC surfaces mixes; 2) the high level of macrotexture is beneficial when snow and slush exist; and 3) the tendency for ice formation within wheelpaths covered 66 Figure 40. Truck mounted PFC cleaning system (65).
in snow is reduced due to the macrotexture, permeability, and limited thaw. Disadvantages of PFC in winter conditions listed by Isenring et al. (15) include: 1) the need for deicing salts and other products; 2) the use of sand and small aggre- gates to improve frictional properties is not a viable option because these materials clog the void structure of PFCs; 3) snow and ice tend to stick to PFC layers sooner because the surface is generally cooler by about 1°F (0.5°C) than dense-graded mixes; 4) snow and icing rain can form earlier on PFC because deicing salts do not remain on the pavement surface; 5) preventative salting is not as beneficial because the salt penetrates into the void structure; 6) if the PFCâs per- meability is reduced, ice can build up within the layer and expand onto the pavement surface; and 7) some icing prob- lems can occur within the initial portion of a subsequent dense-graded surface because transportation of salt by traf- fic is reduced with the use of PFCs. Lefebrve (21) describes three winter conditions that cause concern for PFC layers. The first is a freezing fog/hoar frost. This condition occurs at certain humidity temperature com- binations and results in a very thin layer of ice on the pavement surface due to condensation and near freezing temperatures. Ketcham (70) defines the cause of hoar frost as dew or water vapor forming ice crystals in the form of scales, needles, feathers, or fans on surfaces when the temperature of the pave- ment is at or below freezing. Lefebrve (21) stated that both France and the Netherlands have shown PFC layers are gener- ally 3.6 to 5.4°F (1 to 2°C) lower than dense-graded layers. He also stated that research in Austria indicated that PFC behaves differently at temperatures in the range of 23 to 32°F (-5 to 0°C) than dense-graded layers. Below this temperature range, PFCs act similar to dense-graded layers. The second condition listed by Lefebrve (21) was frozen wet surfaces. This condition con- sists of ice building up on the pavement surface due to rain falling on a frozen PFC layer. Finally, snow or sleet falling onto PFC layers is a concern. There is evidence that PFC layers have different thermal properties than typical dense-graded layers. Huber (7) states that the heat conductivity of PFC layers is 40 to 70 percent that of dense-graded layers. As stated above, PFC layers during cold climates are generally cooler than nearby dense-graded layers. Research has indicated that PFC layers will be 3.6 to 5.4°F cooler than dense-graded layers (7). This means that the PFC layer can drop below freezing sooner, resulting in the formation of ice/frost when nearby dense-graded sur- faces do not freeze. Also, PFC layers will stay frozen longer than dense-graded layers. Iwata et al. (20) conducted an experiment in Japan to compare the temperature of PFCs and dense-graded surfaces during cold weather. During the daytime, the pavement sur- face of dense-graded layers was higher than nearby PFC by about 3.6°F (2°C). At nighttime, the road surface tempera- ture was higher on PFC layers by about 1°F (0.5°C). During snowfalls, Iwata et al. (20) indicated that the temperature of PFCs was about 0.4°F (0.2°C) lower. Iwata et al. (20) also conducted a qualitative evaluation of road surface conditions comparing PFC and dense-grade sur- faces during winter events. Tables 34 and 35 present the results of these evaluations. The upper right part of each table repre- sents the number of cases where the visual surface condition of PFC layers was considered to be worse than nearby dense- graded surfaces. The lower left portion of the table represent when the visual surface conditions were considered worse on comparison dense-graded surfaces. Iwata et al. (20) highlighted two conditions from the tables. First was when the dense-graded layer was wet. In this situa- tion, the snow falling onto the dense-graded surface was melt- ing while on the PFC surface it was becoming slush. The second condition noted was when the dense-graded surfaces were cov- ered with ice and the surface of the PFC was either wet, slush, or snow. This suggested that it was difficult for freezing of PFC surfaces. Another experiment conducted by Iwata et al. (20) included monitoring the salinity concentration on the pavement surface using three different spreading methods: solution, solid, and wet salt. Data presented by the authors suggested that the rate of decrease in salinity concentration was generally less on PFC surfaces than for dense-graded. Layers of PFC contain more interconnected voids than dense-graded mixes; therefore, the salt disappears into the void 67 Porous Asphalt Pavement Rutted Sections dry wet slush snow ice total dry 33 33 wet 1 297 23 321 slush 14 96 2 112 snow 10 2 88 100 ice 10 3 3 23 39 Dense- Graded Pavement total 34 331 124 93 23 605 Table 34. Road surface conditions during a snowfall (rutted sections) (20).
structure. This leads to an increased need for salt. Greibe (24) reports a 25 to 100 percent increase in salt consumption. The pumping action caused by traffic passing over PFC will contin- ually circulate the salt solution within the void structure of the layer. This may explain the observations of Iwata et al. (20) when they observed a lower rate of decrease in salinity for PFC surfaces when compared to dense-graded. As long as the traf- fic volume remains high, drivers should not notice any differ- ence between PFC and dense-graded surfaces. The influence of traffic volume on winter performance also was noted by Bennert and Cooley (71). They showed a clear influence of traf- fic volume on surface friction in two separate winter storm events. When evaluating surface friction on both the passing lane and travel lane during snow events, skid numbers for the travel (design) lane were maintained at a higher level. Also, Padmos (51) stated that in the Netherlands, all lanes are closed except for the design lane during severe winter events. Similar to Bennert and Cooley (71), Iwata et al. (20) showed higher skid numbers for PFC than dense-graded surfaces using a locked wheel skid tester. One exception to this observation was when PFC layers were covered with compacted snow. In this instance, skid numbers were similar for both wearing sur- faces. This indicated that the PFC maintained equal or higher friction coefficients than dense-graded surfaces during snow events. Litzka (49) provided an overview of the evolution in win- ter maintenance of PFCs in Austria as well as a summary of a summit held in 1999 on European winter maintenance practices. Because of their open nature, PFC surfaces are about 1.8°F (1°C) cooler when compared to dense-graded surfaces. Therefore, PFCs remain at a colder temperature longer and reach freezing temperatures earlier than dense- graded surfaces. Because of the extended time at freez- ing temperatures, the consumption of deicing materials is higher. During slushy conditions, Litzka (49) indicates that the performance of PFCs is slightly poorer than dense- graded surfaces. Snowplows tend to push the slushy material into the void structure of PFC layers. Freezing temperatures cause the slushy material to swell, such that the slushy materials become a road hazard. To prevent the slushy material from swelling, salting of the roadway must be conducted immediately after the snowplows pass. In Austria, this is in contrast to dense-graded layers. The extra placement of salt results in the increased usage of deicing materials. An Austrian survey described by Litzka (49) indicated that preventative application of anti-icing materials may delay or prevent icing. However, on PFCs, immediate and continuing applications of anti-icing materials are required. When road salt is applied to PFC too late or the anti-icing materials are ineffective, the removal of the resulting ice layer is much more difficult on PFC layers than dense-graded. Litzka (49) summarized that PFC layers will require 25 to 50 percent more deicing agents. Winter maintenance crews must be able to respond quickly and flexibly to different weather and road conditions. Weather forecasting and elec- tronic monitoring systems are very helpful in this quest. Litzka (49) also documented a 1999 International Exchange of Experiences held in Austria that brought together experts from throughout Europe to discuss issues with PFC layers. In Germany, winter maintenance of PFC is generally consid- ered more expensive and slightly more difficult than dense- graded surfaces. The standard quantity of salt applied to PFC surfaces is 0.02 lb per sq yd (10 gr per sq m); however, within problem areas the required quantity of salt may reach 0.07 lb per sq yd (40 gr per sq m). The availability of weather fore- casting systems helps facilitate timely response to winter main- tenance activities. In Italy, salt is generally applied to wet pavements in quantities of 0.02 to 0.04 lb per sq yd as a pre- ventative maintenance technique. During snowfalls, the dry rodded salt is again applied at the same rate. After snowplows have removed the snow, another 0.02 to 0.06 lb per sq yd of road salt is applied, depending upon the road conditions. Litzka (49) noted that Italy stated that a change from a 20 mm maximum aggregate size PFC to a 16 mm maximum aggre- gate size PFC had led to a significant improvement in road conditions during winter conditions. In the Netherlands, road salt consumption increased by about 25 percent when 68 Porous Asphalt Pavement Non-Rutted Sections dry wet slush snow ice total dry 34 34 wet 2 247 29 4 282 slush 13 114 8 3 138 snow 1 11 109 2 123 ice 4 5 1 18 28 Dense- Graded Pavement total 36 265 159 122 23 605 Table 35. Road surface conditions during a snowfall (non-rutted sections) (20).
69 Austria Belgium Denmark France Germany Italy Japan Netherlands Sweden Switzerland United Kingdom Solid NaCl VC VC VC VC VC VC VC VC VC VC VC NaCl brine RE RE RE VC RE CaCl2 flakes VC RE RE VC VC LC CaCl2 brine RE VC VC Solid mixture NaCl/CaCl2 LC LC RE VC Wet salt method NaCl + CaCl2 solution VC RE LC VC RE VC LC RE Wet salt method NaCl + NaCl solution LC VC LC VC RE RE VC = very commonly used LC = less commonly used RE = rather exceptionally used Blank = never used Table 36. Forms in which salts are used in Europe and Japan (21). Country Normal Treatment Preventive Treatment 02-01ynamreG 20-30 - 02-703-02muigleB 01-501>kramneD 51-0103-02ecnarF 51-0103-51ylatI 01>001<napaJ -02-5sdnalrehteN United Kingdom 20-40 10-20 01-502nedewS 51-0102-51dnalreztiwS Table 37. Average spreading rates for solid NaCl (g/m2) (21). Country Normal Treatment Preventive Treatment Belgium 20-30 7-20 -03-02ecnarF Italy 10-20 5-10 Japan 10-50 10-50 Switzerland 15-40 15-30 Table 38. Average spreading rates for CaCl2 flakes (g/m2) (21). using PFCs. In very severe winter conditions, speed limits or road closures had been employed. In Slovenia, salt consump- tion is up to 100 percent higher for PFCs. Finally, Litzka (49) reported that salt usage in Austria was about 25 percent higher for PFCs. Best results occur when using a wet salt application (salt/brine ratio of 2:1). Lefebvre (21) provided typical chemicals used as deicing materials in Europe and Japan. Table 36 presented the typical materials and Tables 37 through 39 provide typical dosage rates. Van Doorn (72) did warn that too much dry deicing salt placed on a PFC surface during dry conditions can lead to slip- periness. Also, when the temperature is below -26°F (-15°C), the rock salt can freeze. In these instances, calcium chloride can be sprayed onto the pavement surface. Giuliani (73) reported on the use of anti-icing techniques on PFCs in Italy. Anti-icing techniques were defined as those operations necessary to avoid the formation and development of bonded snow or ice on the pavement surface. Anti-icing differs from deicing as deicing is the process of weakening or destroying the bond between snow or ice and the pavement. Giuliani (73) states that PFC makes anti-icing operations diffi- cult and expensive. The porous structure of PFCs allows the saline substances to drain from the surface. Therefore, approx- imately 30 percent more anti-icing materials are needed com- pared to dense-graded layers. With respect to traffic safety, late application of anti-icing techniques can lead to safety issues. When winter precipitation occurs without anti-icing opera- tions, tires cannot achieve sufficient traction because the void structure becomes clogged with fresh snow. The snow is com- pacted by passing vehicles forming an ice layer within the pavement layer. Because of the potential problems, Giuliani developed a system of heating elements to be placed under a PFC layer that would maintain the temperature of a PFC layer at about freezing. Bennert et al. (12) reported on experiences with winter main- tenance on PFC mixes in New Jersey. They mention that the New Jersey DOT uses rock salt for deicing, whereas the New Jersey Garden State Parkway (NJGSP) uses liquid magnesium chloride. The New Jersey DOT has found that PFC layers are more difficult to maintain ice-free than nearby dense-grade
layers. The NJGSP, on the other hand, has had good success with liquid magnesium chloride although PFC requires twice the amount. Also, the NJGSP continually monitors forecasts of temperature and measures pavement surface temperatures. The NJGSP pre-treats PFC surfaces with liquid magnesium chloride to avoid icing. By pre-treating, the NJGSP has found the PFC surfaces manageable and can be lowed the same as dense-graded mixes. 70 Country Normal Treatment Preventive Treatment 51-0103-01ynamreG 01-airtsuA 02-703-02muigleB 01-501>kramneD 51-01-ecnarF 7-502-5sdnalrehteN 551nedewS 51-5-dnalreztiwS Table 39. Average spreading rates for wet salt (g/m2) (21).
