Annotated Literature Review for NCHRP Report 640 (2009)

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19 the surface of porous asphalt longer because the brine that is created due to the melting snow will penetrate into the void structure of the layer. Accidents have been reported on porous asphalt due to icy conditions when nearby dense-graded surfaces were not icy [no explanation of conditions were given]. 1.6.6 Rehabilitation Practices No specific rehabilitation practices were given. 1.6.7 Performance During construction, a permeability test is used to evaluate the drainage characteristics of porous asphalt. A criterion of 1.4 liters of water flowing into the pavement in less than 60 seconds is used. The authors also state that porous asphalt will slowly clog with silts in places where traffic is not intense. 1.6.8 Structural Design The authors state that there are two thicknesses used with porous asphalt: 25 and 40 mm. In order to maintain the drainage characteristics and noise reducing attributes for a longer period of time, the authors indicate that the 40 mm layer thickness is best. The authors also stated that based upon modulus testing, porous asphalt constructed with an 80/100 penetration graded asphalt binder will contribute 73 to 79 percent of the structural capacity of typical dense-graded mixes. 1.6.9 Limitations The authors listed several circumstances where porous asphalt should not be used. Roads that have a high potential for debris, such as near farming operations, should not include porous asphalt because of the high potential for rapid clogging. Another location where porous asphalt should not be used is on low volume, low speed pavements. When traffic volumes or traffic speed is low, the self cleaning attributes of porous asphalt are negated. Self cleaning occurs because of the pumping and suction of the tires of numerous fast moving vehicles. The final location the authors recommended not using porous asphalt included areas subjected to very high shearing forces at the tire/pavement interface. 1.7 Lefebvre, G. “Porous Asphalt.” Permanent International Association of Road Congresses. 1993. 1.7.1 General This comprehensive document provides an overview of the state-of-art for the use of porous friction courses in Europe [as of 1993]. Complete chapters are dedicated to: 1) criteria for the selection of porous asphalt for a specific project; 2) desirable material properties; 3) mix design; 4) production and construction; 5) field performance; and 6) maintenance and repair. Experiences from many countries are combined to discuss each of the above topics.

20 1.7.2 Benefits Lefebvre mentions numerous benefits when using porous asphalt. These benefits were categorized based upon safety, driving comfort and environmental impact. Benefits related to safety include hydroplaning, skid resistance, splash and spray, light reflection, driving speed, stability of construction and the effects on accidents. Hydroplaning occurs when a layer of water builds up between a tire and the pavement surface. This layer of water breaks the contact between the tire and the road. There are two aspects of porous asphalt that help prevent the occurrence of hydroplaning. First, because water drains into porous asphalt, the film of water is not available to break the bond between the tire and pavement surface. The second aspect is the macrotexture provided by porous asphalt. Even when clogged, porous asphalt provides a significant amount of macrotexture. This macrotexture provides small channels for water to be dissipated as a tire crosses over the pavement. Therefore, in wet driving conditions, the skid resistance of porous asphalt wearing layers is generally very good. Lefebvre does mention; however, that in dry driving conditions the skid resistance of porous asphalt pavements is generally about the same as traditional dense-graded wearing surfaces. Another benefit derived from the use of porous asphalt is the reduction in splash and spray. Rolling wheels will throw water into the air from pools on the pavement surface (splash) as well as mist the surface water (spray). These water droplets reduce driver visibility. Water that comes from the roadway will also generally be contaminated with dirt and debris which can be smeared by windshield wiper blades, further reducing visibility. Through the infiltration of water into a porous asphalt layer, the pools of water will not be available to create splash and spray under rolling wheels. Lefebvre indicates that in many European countries, counteracting splash and spray was the primary reason for applying porous asphalt layers. Another benefit related to safety is the light reflectivity of pavement markings. Drivers observe a pavement at a glancing angle of about 1 degree or less. When surfaces are very smooth, the reflection of light at this angle will look similar to a mirror in the distance. This is especially true when water is on a pavement. Porous asphalt will diffuse the reflection of light due to the high macrotexture even when observed from a glancing angle. Lefebvre identifies increased driving speed during wet weather as a benefit related to safety. During rain events, the decreased potential for hydroplaning and splash/spray allows drivers increased confidence that results in increased speeds. This results in less traffic moving at slower speeds. Environmental benefits listed in this document include noise reduction, pavement smoothness and use of waste rubber. Lefebvre indicates that the average noise level resulting from traffic on porous asphalt layers is about 3dB(A) lower than on dense- graded pavement layers. Another benefit is related to pavement smoothness. Lefebvre indicates that porous asphalt layers are typically constructed smoother than dense-graded

21 layers. Under certain conditions, this has resulted in a 1 to 2 percent reduction in fuel consumption. 1.7.3 Materials and Mix Design Lefebvre provided a review of general desirable characteristics of materials needed for porous asphalt. For coarse aggregates, particles should be crushed having the proper shape and texture. Coarse aggregates should be resistant to polishing. Coarse aggregates should also be hard and resistant to the effects of freeze/thaw cycles. Fine aggregates should also be the result of crushing. Both neat and modified binders have been used in PFCs. If neat asphalt binders are utilized, stabilizing additives are needed. If modified binders are used, stabilizing fibers are used at times. Typical asphalt binder modifiers have included polymers and recycled materials. Both elastomers and plastomers have been used with success. Recycled rubber is the most common recycled material used. The document provides general overviews/concepts of various mix design methodologies used in Europe. Spain developed a mix design method based upon the Cantabro Abrasion Test. The method entails specifying a minimum air void content of 21 percent and a maximum Cantabro Abrasion loss of 30 percent (tested at 18˚C). Figure 1 illustrates the concept. The minimum asphalt binder content is selected as the binder content that meets both the air void and Cantabro loss requirements. The document indicates that Belgium utilizes a similar concept. Another concept used to design porous asphalt in Europe involves a draindown test. The maximum asphalt binder content is selected based upon draindown testing. C antabro Loss, % Asphalt Binder Content A ir V oi ds , % Air Voids Loss Minimum Binder Content C antabro Loss, %A ir V oi ds , % Figure 1: Use of Cantabro Abrasion and Air Voids to Select Minimum Binder Content

22 Each of the benefits cited earlier are related to the drainage capacity of PFCs. PFCs are designed for high air void contents in order to enhance the ability of the layer to remove water from the pavement surface. However, only interconnected voids open from the pavement surface are of benefit. Figure 2 and Table 7 illustrate the types of air voids with PFC and their relative importance in effectively removing water and enhancing noise absorption. d b1 a b3 a c a b2 Average level of surface Figure 2: Air Void Classifications Table 7: Influence of Air Void Classification on Permeability and Noise Effectiveness with regard to Description of voids Type water permeability noise absorption continuously connected a + + accessible from the surface b1 - + entering the section b2 - ? “open” (accessible) not continuous “blind canal” b3 - ? not accessible to water under atmospheric pressure c - - “closed” fully closed d - -

23 1.7.4 Construction Practices Construction practices are provided for production, transportation, laydown and specifications. During production, the plant operator must pay closer attention to mix temperature than for dense-graded mixes. Higher mix temperatures will increase the potential for draindown. When using modified asphalt binders, the supplier’s recommendations for temperature should be used. If the binder modifier is to be formulated on-site, preliminary testing should be conducted to ensure the blended material meets requirements. During transport, the potential for draindown increases as haul time increases. Also, vibrations transmitted to the porous asphalt through the truck bed can also increase the potential for draindown. Every effort should be made to maintain the temperature of the mix during transportation. This is especially true when modified asphalt binders are used. Laydown of porous asphalt mixes is no more difficult than for dense-graded HMA. Handwork can be especially hard, however. Once placed, vibratory rollers should not be used to compact porous asphalt. The energy produced by vibratory compactors can fracture aggregates. The preferred method for compacting porous asphalt in Europe includes the use of a 10 to 12 ton steel-wheel roller making 2 to 3 passes. Pneumatic tire rollers are generally not used and prohibited in some countries. When placing the porous asphalt layer, it must be daylighted at the edge. Figure 3 provides three examples of how pavement edges can be constructed. Obstructions at the pavement edge will prevent water from flowing through the porous asphalt layer. Also, longitudinal joints should not be sprayed with a tack coat. Again, this prevents water from flowing through the porous asphalt layer. Related to laydown, porous asphalt should never end in the middle of a vertical or horizontal curve. Porous asphalt layers should end in a flat region of the roadway. With respect to construction specifications, porous asphalt should only be placed on impervious layers. In order to promote drainage of water, the underlying layer should have a sufficient cross-slope and daylighted on the pavement edge. A tapered edge can be used if the PFC is not carried out to the pavement edge (Figure 3). Pavement marking materials should be such that they don’t penetrate into the pavement layer.

24 4 cm channel 1 2 1 4 cm 10 cm 2 soft shoulder 4 cm shoulder 1 2 hard shoulder slow lane 30 to 50 cm 2 to 2.5 cm 1 - Impervious Layer 2 - Porous asphalt Figure 3: Examples of Daylighting Porous Asphalt Mixtures 1.7.5 Maintenance Practices General maintenance of porous asphalt layers has many similarities to dense-graded pavement layers in that skid resistance, smoothness, cracking, rutting and/or deterioration must all be monitored. However, when dealing with porous asphalt, permeability and noise reducing capabilities must also be considered. During the life of a porous asphalt layer, dirt and debris can clog the layer. This clogging decreases the ability to drain water through the layer. Common methods for de-clogging porous asphalt layers include the use of highly pressurized water or the use of a “suck-sweep” cleaning truck. The role of winter maintenance is to clear roads from ice and snow at an acceptable cost, so that drivers can use the major roads almost normally under all but exceptional winter conditions. Lefebvre indicated that each country appears to handle winter maintenance in a different manner. The effects of three winter conditions on a porous asphalt layer were discussed. The first is a freezing fog/hoar frost. This condition occurs at certain temperature-humidity combinations and results in a very thin layer of ice on the pavement surface due to condensation and near freezing temperatures. Both France and the Netherlands have

25 shown that porous asphalt layers are generally 1 to 2˚ C lower than dense-graded layers. Research in Austria indicated that porous asphalt behaves differently at temperatures in the range of 0 to -5˚C. Below this temperature range, the porous asphalt acts similar to dense-graded layers. At these lower temperatures, preventative salt spreading is not as effective. An increased frequency is often needed. The second winter condition cited in the document was frozen wet surfaces. This condition is ice building up on the pavement surface due to rain on the frozen pavement surface. In this condition, preventative salting is not very effective. Also, during the precipitation, an increased frequency in salting is required. The final winter condition is snow fall or hail. Preventative salting in this situation is also less effective. More frequent salting is needed to melt the snow; however, too much salt can lead to formation of ice within the porous asphalt layer due to the melted snow re-freezing within the layer. Lefebvre provided typical chemicals used as deicing salts in Europe. Table 8 presents typical materials used in different countries and Tables 9 through 11 provide typical dosage rates. 1.7.6 Rehabilitation Practices Lefebvre indicates that a distinction needs to be made between minor and major rehabilitation of porous asphalt layers. Minor rehabilitation entails small local repairs necessary because of small damage or distress with the rest of the pavement layer in good condition. Major rehabilitation is conducted when the entire layer is in need of repair. Local distresses can be repaired by replacing the area with dense-graded mix. A discontinuity in the drainage path will occur; however, the discontinuity should not be considered severe when dealing with small localized areas. Major rehabilitation techniques include replacement of the entire layer or refurbishment of the entire layer. Replacement of the porous asphalt would include completely removing the layer and replacing with a new layer. Refurbishment of the layer would include in-situ recycling. It was noted that the Netherlands have been using hot in-place recycling to rehabilitate porous asphalt. Largely, this experiment has been unsuccessful. 1.7.7 Performance Several performance measures were discussed. Lefebvre indicated that rutting has not been experienced within porous asphalt layers in Europe. This is likely because of the relatively thin lifts and the stone-on-stone contact in the aggregate structure of the mix. Porosity is another important performance characteristic. Porosity can be measured using either air voids or in-place permeability. Porosity should be evaluated at the time of construction to ensure the porous asphalt is properly constructed and during the life of the pavement to evaluate the level of clogging. It was pointed out that permeability will vary across the lane transversely. This variation is mainly due to the pressure-suction action

26 of tires over the pavement during a rain. Loss of permeability is related to the amount of traffic. More heavily trafficked pavements will maintain permeability longer. In this respect, design lanes are generally more permeable than passing lanes. Raveling is another performance characteristic of porous asphalt. In the Netherlands, they expect raveling to begin in 10 years compared to 12 years for dense-graded layers. When raveling occurs, the acoustical benefits of porous asphalt are diminished. As stated previously, porous asphalt layers generally provide good wet weather friction. However, skid resistance is generally low just after construction because of the asphalt binder film coating the aggregates at the pavement surface. Once the asphalt binder film has worn away, skid resistance will improve.

27 Table 8: Forms in Which Salts Are Used in Europe 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

28 Table 9: Average Spreading Rates For Solid NaCl (g/m2) Country Normal Treatment Preventive Treatment Germany 10-20 20-30 - Belgium 20-30 7-20 Denmark >10 5-10 France 20-30 10-15 Italy 15-30 10-15 Japan <100 >10 Netherlands 5-20 - United Kingdom 20-40 10-20 Sweden 20 5-10 Switzerland 15-20 10-15 Table 10: Average Spreading Rates for CaCl2 Flakes (g/m2) County Normal Treatment Preventive Treatment Belgium 20-30 7-20 France 20-30 - Italy 10-20 5-10 Japan 10-50 10-50 Switzerland 15-40 15-30 Table 11: Average Spreading Rates for Wet Salt (g/m2) County Normal Treatment Preventive Treatment Germany 10-30 10-15 Austria - 10 Belgium 20-30 7-20 Denmark >10 5-10 France - 10-15 Netherlands 5-20 5-7 Sweden 15 5 Switzerland - 5-15 1.7.8 Structural Design No specifics were given on inclusion within structural design. 1.7.9 Limitations No specific limitations were provided; however, three disadvantages were given. First, porous asphalt generally costs more than dense-graded layers. This is a result of requiring high quality, polish resistant aggregates and polymer modified asphalt binders. Also, pavement markings have to be adapted for porous asphalt. Special impervious layers specifically placed below porous asphalt also increase construction costs. Another disadvantage of using porous asphalt is the relatively shorter economic life. Finally, maintenance is generally more expensive, especially winter maintenance.