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26 This chapter only presents items identified by the litera- ture as benefits of using PFC mixtures as wearing layers. Chapter 10 will discuss the performance of PFCs, which will include discussions on how long the benefits can be realized on the roadway. For the most part, the benefits are based upon the ability of the PFC layer to drain water from the pavement surface. Lefebvre (21) states that the benefits of PFCs can be categorized based upon three general areas: safety, driving comfort and environmental. Safety Related Benefits Benefits related to safety include reduced potential for hydroplaning, improved skid resistance (especially during wet weather), reduced splash and spray, and reduced light reflection. Hydroplaning occurs when a layer of water builds up between a tire and the pavement surface (21). This layer of water breaks the contact between the tire and road (21, 22). When this occurs, the vehicle will not respond to braking or turning by the driver. There are two aspects of PFCs that help prevent the occurrence of hydroplaning. First, because the water drains from the pavement surface into the PFC layer, the film of water is not available to break the bond between the tire and pavement surface (21). The second is the macro- texture provided at the pavement surface by PFC layers. Even when clogged, PFCs provide a significant amount of macro- texture. This macrotexture provides small channels for water to be dissipated as a tire crosses over the pavement (23). There- fore, in wet weather driving conditions, the skid resistance of PFC wearing layers is generally very good. Many references mention that OGFCs used on the pave- ment surface will improve frictional properties, especially dur- ing wet weather. Similar to how PFCs reduce the potential for hydroplaning, the ability to drain water from the pavement surface and the relatively high macrotexture of PFCs also im- prove wet weather friction. Kandhal (4) cited a number of ref- erences in his synthesis describing research conducted in the United States, Canada, and Europe that showed the improved wet pavement frictional properties of OGFCs. Much of the re- search dealt with comparing the speed gradient (or friction gra- dient) encountered on OGFC layers. A frictional speed gradient can be defined as the rate of decrease in the friction number per mile per hour increase in speed. Therefore, low frictional speed gradients are desirable. Table 21 presents data from a Pennsylvania DOT project that shows a decreased friction gra- dient for OGFC layers. Similar work in Oregon and Louisiana presented by Kandhal (4) also showed decreased friction gra- dients for OGFCs compared to dense-graded layers. Isenring et al. (15) also conducted friction testing in Switzerland on 17 different PFC test sections at different speeds including 25, 37, 50, and 62 mph. Friction measure- ments were made using the PIARC skid test and a ribbed tire. Results showed that PFC pavement surfaces had much higher coefficients of friction at higher speeds than typical dense- graded surfaces. Similar to the referenced literature by Kandhal (4), the friction speed gradients for PFC surfaces were lower than for typical dense-graded layers. Bennert et al. (12) presented the results of wet skid tests on various wearing surfaces, including PFCs, in New Jersey. The skid measurements were made in accordance with ASTM E274-97, Standard Test Method for Skid Resistance of Paved Surfaces Using a Full-Scale Tire. A test speed of 40 mph was utilized using a ribbed-tire on the skid trailer. A total of 19 dif- ferent pavement sections were tested. Included within the evaluation were asphalt rubber OGFC, PFC (termed modified OGFC), Novachip, stone matrix asphalt (SMA), microsurfac- ing, Superpave designed dense-graded HMA, and portland cement concrete. Table 22 presents the results of testing on the 19 test sections. Based upon the results, the asphalt rub- ber OGFC had the highest frictional resistance of the thin lift wearing layers followed by the microsurfacing and PFC. The PFC layers tested did provide higher wet-skid numbers than Novachip and the Stone Matrix Asphalt (SMA) surfaces. As further evidence that wet weather friction is improved with the use of PFCs, many references stated that the use of C H A P T E R 4 Benefits of Permeable Friction Courses
PFCs reduced the number of wet weather accidents. Research in Virginia, France, and Canada showed that OGFC and PFC layers reduced wet weather accidents (4). In Virginia, wet weather accidents were reduced from 39 percent of all acci- dents on State Route 23 to 17 percent of all accidents, a reduc- tion of approximately 50 percent. On the A7 Motorway in France, the number of accidents fell from 52 (1979 to 1985) to none (1985 to 1989) after a PFC replaced a dense-graded surface on the roadway. In Canada, the placement of OGFC on a section of roadway reduced the number of wet weather accidents by 54 percent and the total number of accidents by 20 percent (4). Greibe (24) discussed a study in Austria that showed no difference in traffic accidents on dry pavements but fewer wet weather accidents on PFC surfaces. Iwata et al. (20) reported an 80 percent reduction in wet weather accidents when using PFCs in Japan. Recent work in the United States by McDaniel and Thornton (13) also has shown that PFCs provide relatively more macro- texture and a higher International Friction Index (IFI) than other HMA wearing layers. Tables 23 and 24 present macro- texture and friction measurement data for three test sections in Indiana, respectively. Pavement surfaces included within the research were PFC, SMA, and dense-graded HMA. McDaniel and Thornton (13) indicated that the PFC and SMA wearing layers showed significantly more macrotexture (reported as mean profile depth) than did the dense-graded HMA (Table 23). The PFC layer provided the highest average mean profile depth measurement. The variability in measured mean profile depths also was found to be higher for the PFC and SMA layers compared to the dense-graded surface. The authors indicated that this was expected since the PFC and SMA mixes have gap- or open-graded aggregate structures. McDaniel and Thornton (13) also reported results of dy- namic friction measurements made with the Dynamic Fric- tion Tester (Table 24). Based upon the raw friction numbers, the PFC and dense-graded surfaces were comparable, whereas the SMA surface showed the lowest values. The authors also converted the mean profile depth and friction number data into the IFI. In terms of IFI, the PFC showed the highest frictional properties followed by the SMA and dense-graded surface. Another benefit related to safety from the use of PFC wear- ing layers is the reduction in splash and spray. During rain events, water will sit on the surface of a dense-graded HMA wearing layer. As vehicles pass over the pavement surface, the water will be splashed (splash) or thrown into the air in the form of a mist (spray) (7). The existence of the splashed 27 Friction Number Mix Type 30 mph 40 mph Friction Gradient 01.03747)levarg(CFGO 01.00717)etimolod(CFGO Dense-graded HMA (gravel) 68 60 0.80 Dense-graded HMA (dolomite) 65 57 0.80 Surface Type Age Wet-Skid Number (SN40) Avg. Wet-Skid Number (SN40) per Surface AR-OGFC 9 47.8 AR-OGFC 10 55.9 51.9 MOGFC 1 47.9 MOGFC 4 44.8 MOGFC 2 51.2 48.0 Novachip 3 45.4 Novachip 8 45.7 45.6 9.5 mm SMA 7 42.5 12.5 mm SMA 9 42.0 42.3 MS Type 3 1 49.6 MS Type 3 1 49.1 49.4 12.5 mm SP 10 51.8 12.5 mm SP 4 54.3 53.1 PCC (no finish) 44 38.6 PCC (no finish) 39 39.1 PCC (no finish) 48 41.4 39.7 PCC (Trans. tined) 14 57.2 PCC (Trans. tined) 14 55.8 56.5 PCC (Diamond Grind) 14 54.6 54.6 AR-OGFC = asphalt rubber open-graded friction course MOGFC = modified asphalt binder open-graded friction course SMA = stone matrix asphalt MS = microsurfacing SP = Superpave PCC = portland cement concrete Table 21. Friction data from Pennsylvania (4). Table 22. Wet-skid numbers for various pavement surface types (12).
or sprayed water reduces visibility more severely than fog because the airborne particles within the mist are larger than the particles within fog (7). PFCs reduce (and almost elimi- nate) the droplets of water caused by vehicles passing over the roadway because water infiltrates into the interconnected voids of the pavement. Greibe (24) indicated that splash and spray can be reduced as much as 95 percent when using PFC sur- face layers compared to dense-graded HMA. Flintsch (25) conducted a study to assess the performance of several roadway surface layers under rain and snow. The pave- ment surfaces included five HMA mixes and one portland cement concrete. Three of the HMA surfaces were a dense- graded mix, one was SMA, and the final surface was an open- graded mix. This work was conducted on the Virginia Smart Road and utilized artificial rain and snow. During the exper- iment dealing with rain, Flintsch (25) conducted a qualitative evaluation of splash and spray. Based upon the visual observa- tion of the various road surfaces, Flintsch (25) stated that the open-graded mixture enhanced splash and spray performance compared to the other HMA surfaces. In a similar fashion, McDaniel and Thornton (13) quali- tatively compared the splash and spray of the three test sec- tions in Indiana. Based on visual evaluations, McDaniel and Thornton indicated that sight conditions for the driver im- proved significantly (even when passing or passed by semi- trailer trucks) with the use of PFC as compared to SMA. Drivers traveling down a roadway will observe the pavement at a glancing angle of about 1 degree or less. When surfaces are very smooth (e.g., dense-graded layers), the reflection of light will look similar to a mirror in the distance. This is espe- cially true when water is on the pavement surface. PFC will diffuse the reflection of light due to the high macrotexture, even when observed from a glancing angle (21). This reduction in glare allows the driver to see pavement markings better, especially at night and/or wet weather, as well as providing overall better visibility. Driver Comfort Related Benefits Lefebvre (21) identifies increased driving speeds during wet weather as a benefit related to safety. During rain events, the de- creased potential for hydroplaning and splash and spray allows drivers increased confidence that results in increased speeds. This increased confidence leads to less traffic moving at lower speeds. The net effect of the increased speeds is a greater traffic capacity during wet weather compared to dense-graded layers. The increased confidence of drivers also leads to less stress to drivers during rain events which increases driver comfort. Greibe (24) stated that less light is reflected by wet PFC layers than dense-graded surfaces. This is due to the fact that water does not pool on the surface of PFC layers. Since less light can be reflected from oncoming vehicles, pavement mark- ings are more visible. A number of references indicate that the use of PFC wear- ing layers improves smoothness; however, very little specific research was encountered that provided relative improvements in smoothness when PFCs are utilized. Bennert et al. (12) did compare the results of ride quality measurements for a num- ber of pavement surfaces in New Jersey including: asphalt rubber OGFCs, PFCs (termed modified OGFCs within the paper), Novachip, SMA, microsurfacing and three types of rigid pavement surfaces (transverse tined, diamond grind, and no finish). Table 25 presents results of testing related to ride quality by Bennert et al. (12). Two measures of ride quality are provided within this table. The Ride Quality Index (RQI) was measured using an ARAN vehicle. Previous studies in New Jersey cited by Bennert et al. (12) developed correlations between the ARAN van and userâs perceptions to ride quality. The RQI is based upon a scale between 0 and 5, with an RQI of 5 being the âsmoothestâ ride according to userâs percep- tion. Results from the ARAN van also were used to determine the International Roughness Index (IRI) for each of the pave- ments. According to the IRI definition and scale, lower values of IRI are desirable. Bennert et al. (12) state that based upon the RQI data it was difficult to determine the âbestâ pavement surface because of so many variables (most notably age). However, for the thin lift HMA mixes included in the study (PFC, AR-OGFC, Novachip, and microsurfacing), the PFC mixes did have the highest aver- age RQI values. Similar results were obtained using the IRI measurements. 28 Mix Mean profile depth, mm (Standard Deviation) )31.0(73.1CFP )41.0(71.1AMS )50.0(03.0AMH Average Dynamic Friction Tester (DFT) Number (Standard Deviation) Mix 20 kph 40 kph 60 kph International Friction Index (F60) PFC 0.51 (0.03) 0.45 (0.03) 0.42 (0.03) 0.36 SMA 0.37 (0.01) 0.31 (0.01) 0.29 (0.01) 0.28 HMA 0.52 (0.01) 0.47 (0.01) 0.44 (0.01) 0.19 Table 23. Results of surface texture measurement (13). Table 24. Results of friction measurement (13).
Because of the increased smoothness when using PFCs, Bolzan et al. (26) indicate that fuel economy increases. Re- search at the National Center for Asphalt Technology also has indicated a relationship between smoothness and fuel economy (27); however, this research was not conducted to specifically evaluate PFC layers. Increased fuel economy also is considered a benefit related to the environment. Decoene (18) has stated that a benefit of the use of PFCs is resistance to permanent deformation. Resistance to perma- nent deformation also was cited by Isenring et al. (15). The lack of rutting, combined with other benefits discussed above, leads to improved driver comfort. Environmental Benefits Environmental benefits related to the use of PFCs in- clude reduction in tire/pavement noise, pavement smooth- ness, and use of waste materials. Two relatively recent areas where PGCs have been shown to be beneficial to the envi- ronment are filtration of stormwater runoff and a method of potentially providing a cool pavement surface. Of these environmental benefits, likely the most researched is the re- duction of tire/pavement noises that result from using PFC wearing layers. Kandhal (4) cited numerous research studies that showed PFC layers reduced tire/pavement noise approx- imately 3 dB(A) compared to dense-graded HMA. To put a 3 dB(A) reduction in tire/pavement noise into perspective, this reduction also can be achieved by reducing the traffic volume in half. Brousseaud et al. (28) reported on the tire/pavement noise reduction of PFCs compared to typical dense-graded HMA surfaces. In France, PFCs have typical noise levels of approx- imately 71 to 73 dB(A) when measured by the Statistical Pass-By Method. Comparatively, typical dense-graded HMA surfaces will have a noise level of approximately 76 dB(A) when measured by the same method. This indicates a 3 to 5 dB(A) reduction in tire/pavement noise levels when using PFC wear- ing layers. Similar reductions in tire/pavement noise also have been reported in Switzerland by Graf and Simond (29). In their study, they evaluated four different pavement locations in which a PFC layer had replaced a dense-graded wearing sur- face. Table 26 presents the results of testing conducted by Graf and Simond (though the method of measuring tire/ pavement noise was not provided, it is assumed to be Statis- tical Pass-By). The results of this testing indicated that PFCs reduced tire/pavement noise on average of about 6 dB(A). The recent study conducted by Bennert et al. (12) compared the noise levels measured using the close proximity (CPX) method of various pavement surface types. In this method, microphones are placed near the tire/pavement interface to directly measure the tire/pavement noise levels. This method was developed in Europe and is defined by ISO Standard 11819-2. Results of the CPX testing on the various pavement surfaces are presented in Table 27. When comparing the thin lift wearing surfaces, the OGFC mixes yielded the lowest aver- age noise levels. The OGFC mixture containing an asphalt- rubber binder had the lowest average noise levels, while the 29 Surface Type Age RQI value RQI Rating IRI (inch/mile) Avg. IRI per Surface Type AR-OGFC 9 3.54 Good 121 AR-OGFC 10 4.34 V. Good 82 102 MOGFC 1 4.14 V. Good 90 MOGFC 4 4.05 V. Good 68 MOGFC 2 4.08 V. Good 113 90 Novachip 3 4.47 V. Good 65 Novachip 8 3.51 Good 123 94 9.5 mm SMA 7 4.10 V. Good 84 12.5 mm SMA 9 3.72 Good 194 139 MS Type 3 1 3.79 Good 108 MS Type 3 1 4.02 V. Good 111 110 12.5 mm SP 10 4.15 V. Good 56 12.5 mm SP 4 4.31 V. Good 74 65 PCC (no finish) 44 3.39 Good 178 PCC (no finish) 39 3.13 Good 206 PCC (no finish) 48 3.42 Good 137 174 PCC (Trans. tined) 14 2.66 Fair 274 PCC (Trans. tined) 14 2.54 Fair 295 285 PCC (Diamond Grind) 14 4.21 V. Good 75 75 AR-OGFC = asphalt rubber open-graded friction course MOGFC = modified asphalt binder open-graded friction course SMA = stone matrix asphalt MS = microsurfacing SP = Superpave PCC = portland cement concrete Table 25. Results of ride quality measurements for various pavement surfaces (12).
PFC (termed MOGFC in the table) had the next lowest aver- age noise levels. Both OGFC types were slightly quieter than the Novachip, SMA, and microsurfacing layers. McDaniel and Thornton (13) showed PFC having lower tire/pavement noise than SMA or dense-graded surfaces. Results of their research are shown in Table 28. McDaniel and Thornton (13) included both CPX and Statistical Pass-By testing to evaluate tire/pavement noise. Results of CPX test- ing indicated that PFC was 4.7 dB(A) quieter than SMA and 3.5 dB(A) quieter than dense-graded HMA surfaces. Simi- lar results also were found when using the Statistical Pass-By method for evaluating tire/pavement noise. Because of the noise reducing properties of PFC (and other types of OGFC), it has been suggested that these mix types may be an alternative to the construction of sound barriers to mitigate traffic noise (4). A reduction in the noise level by 3dB(A) has the net effect of either cutting the traffic volume in half or the noise protection distance to the road can be doubled (4). Larsen and Bendtsen (30) presented the only paper that specifically compared costs between various noise abatement techniques. Techniques of noise abatement in- cluded in the research on noise barriers (walls), building insulation and PFC wearing layers. Larsen and Bendtsen (30) evaluated three different scenarios: city streets, arterials, and interstates. Based upon the assumptions and analyses con- ducted, the authors concluded that PFC layers are an effective method for noise abatement. For each of the three scenarios, PFC was the cheapest noise abatement technique. A relatively new concept with the use of PFCs to reduce noise consists of using two-layer PFC layers (31). A two-layer PFC system is composed of a bottom layer of PFC with a large aggregate size gradation and a top layer of PFC having a smaller aggregate size gradation. The top layer of PFC keeps dirt and debris from clogging the lower layer. The bottom layer utilizes a larger aggregate size gradation in order to produce larger sized air voids. According to Dutch experience (31), the two-layer system has good noise reducing characteristics compared to dense-graded layers. France also has utilized a two-layer PFC system (28). Huber (7) also indicated that the two-layer systems are used to combat clogging tendencies in slow-speed environments. The smaller aggregate size wearing layer will trap larger debris 30 Installation date Location Reduction in noise emission after the installation of PFC dB(A) 2.6...1.4titreP1991 6.8...4.5segroM3991 4.8...2.6yanoL9991 0.6...5.4xeB9991 Surface Type Age Noise Level (dB(A)) at 60 mph Avg. Noise Level (dB(A)) at 60 mph per Surface AR-OGFC 9 96.8 AR-OGFC 10 96.2 96.5 MOGFC 1 97.0 MOGFC 4 97.6 MOGFC 2 98.4 97.7 Novachip 3 98.2 Novachip 8 99.4 98.8 9.5 mm SMA 7 98.0 12.5 mm SMA 9 100.5 99.3 MS Type 3 1 98.8 MS Type 3 1 98.8 98.8 12.5 mm SP 10 97.1 12.5 mm SP 4 98.5 97.8 PCC (no finish) 44 102.9 PCC (no finish) 39 104.2 PCC (no finish) 48 103.3 103.5 PCC (Trans. tined) 14 105.6 PCC (Trans. tined) 14 106.6 106.0 PCC (Diamond Grind) 14 98.7 98.7 AR-OGFC = asphalt rubber open-graded friction course MOGFC = modified asphalt binder open-graded friction course SMA = stone matrix asphalt MS = microsurfacing SP = Superpave PCC = portland cement concrete Table 26. Reduction in noise levels when comparing PFC and dense-graded HMA in Switzerland (29). Table 27. Tire/pavement noise results for various pavement surfaces (12).
in order to maintain permeability in the lower layer. Also, the air void space in the lower layer allows a water/jet vacuum machine to restore permeability. Recent research has indicated that PFCs impact the quality of stormwater runoff. Barrett et al. (32) state that the use of PFC wearing surface might be expected to reduce the gener- ation of pollutants, retain a portion of generated pollutants within the void structure, and impede the transport of pollu- tants to the pavement edge. Barrett et al. (32) cited previous research that the constituents within runoff are related to the number of vehicles that pass during a storm event. Splash and spray generated from tires are assumed to wash pollutants from the vehicleâs engine compartment and bottoms. Because PFCs reduce the amount of splash and spray, it is assumed that fewer contaminants are washed form vehicles. Barrett et al. (32) also suggest that the void structure within a PFC layer may act to filter pollutants, especially suspended soils and other pollutants associated with solid particles. Barrett et al. (32) cited work by Berbee et al. (33) in which the researchers compared the concentrations of pollutants in runoff from porous and dense-graded surfaces in the Netherlands. The porous layer was three years old and 55 mm in thickness. Runoff water samples were obtained for a week. Berbee et al. (33) found lower concentrations of pollutants in runoff sampled from the pavement having a porous wear- ing surface than the dense-graded surface. Based upon the test results, the following was observed: 91 percent reduction in total suspended solids (TSS); 84 percent reduction in total Kjeldahl nitrogen (TKN); 88 percent lower chemical oxygen demand; and 67 to 92 percent lower cooper, lead, and zinc. Another environmental benefit that has recently been observed is the ability of PFC to act as a cool pavement to combat Urban Heat Islands (UHI). Urban heat islands are a temperature phenomenon that occurs in urban areas. The sunâs radiation is absorbed by rooftops, pavements, sidewalks, buildings, etc. Because of the close proximity of these types of structures within an urban area, the sunâs energy can be reflected or radiated from the structures resulting in increased temperatures. According to the EPA, air temperatures within urban areas can be 50 to 90°F (27 to 50°C) hotter on hot dry days than in nearby more rural areas (34). According to the Heat Island Reduction Initiative being conducted by the EPA, âcool pavementsâ are a strategy for reducing the UHI. Open-graded mixes, whether PFCs or porous pavement parking lots, are technologies that are con- sidered cool pavements. Summary The use of PFC wearing layers provides a number of ben- efits compared to dense-graded layers. Benefits related to safety include reduced potential for hydroplaning, improved wet weather frictional properties, reduced wet weather acci- dents, reduced splash and spray, reduced glare, and improved vision in seeing pavement markings. Benefits related to driving comfort include smooth wearing layers (and, thus, improved fuel economy). Environmental benefits include improved smoothness (and, thus, improved fuel economy) and reduced traffic noise. 31 Average CPX Sound Pressure Levels (Time averaged level over the length of pavement, LAEQ) Method Speed PFC SMA HMA 72 kph 89.7 dB(A) 94.2 dB(A) 93.0 dB(A) 97 kph 92.6 dB(A) 97.6 dB(A) 96.4 dB(A) Average 91.2 dB(A) 95.9 dB(A) 94.7 dB(A) CPX Difference from PFC 0.0 dB(A) 4.7 dB(A) 3.5 dB(A) elciheVdeepS Impala 68.1 dB(A) 74.8 dB(A) 72.6 dB(A) Volvo 70.1 dB(A) 75.5 dB(A) 75.2 dB(A) Silverado 71.6 dB(A) 77.0 dB(A) 74.5 dB(A) 80 kph Average 69.9 dB(A) 75.8 dB(A) 74.1 dB(A) Difference from PFC 0.0 dB(A) 5.9 dB(A) 4.2 dB(A) Impala 71.7 dB(A) 78.5 dB(A) NA* Volvo 74.3 dB(A) 80.5 dB(A) NA Silverado 74.4 dB(A) 79.4 dB(A) NA 110 kph Average 73.5 dB(A) 79.5 dB(A) NA Pass-By Difference from PFC 0.0 dB(A) 6.0 dB(A) --- * Could not be tested due to speed limits. Table 28. Results of all sound measurements (13).
