Annotated Literature Review for NCHRP Report 640 (2009)

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5 1.2.5 Maintenance Practices No specific maintenance practices were given. 1.2.6 Rehabilitation Practices No specific rehabilitation practices were given. 1.2.7 Performance When describing a full scale pavement experiment within the paper, Decoene indicated that permeability testing was included at the time of construction as a performance indicator. No details on the test method or typical results were provided. 1.2.8 Structural Design No specifics on inclusion within structural design were given. 1.2.9 Limitations No limitations on use were given. 1.3 Isenring, T., H Köster and I. Scazziga. “Experiences with Porous Asphalt in Switzerland.” Transportation Research Record No. 1265. Transportation Research Board. National Research Council. Washington, D.C. pp 41-53. 1990. 1.3.1 General The first porous asphalt in Switzerland was placed in 1972 on an airport runway. For highway pavements, porous asphalt has been used since the late 1970’s and early 1980’s. This paper presents the results of a research program where 17 pavements were monitored during the life of the various porous asphalt layers. 1.3.2 Benefits of Permeable Asphalt Mixtures Benefits of porous asphalt highlighted by the authors included: • Reduction in the potential for hydroplaning. • Reduction in splash and spray. • Good frictional properties at higher speeds. • Reduction in noise. • Reduced glare at nighttime and in wet weather. • Resistance to permanent deformation. 1.3.3 Materials and Mix Design Table 3 presents typical properties of porous asphalt in Switzerland. The authors also stated that polymer modified binders are generally used with porous asphalt.

6 Table 3: General Data on Materials and Mix Property Porous Asphalt (0/10) Porous Asphalt (0/16) Max. Aggregate Size (mm) 10 16 Layer Thickness (mm) 28-42 43-50 Binder Content (%) 4.65 – 5.82 4.23-4.99 Air Voids (Marshall, %) 10.9 – 22.5 14.9 – 17.0 Air Voids (Cores, %) 14.6 – 21.1 14.6 – 19.6 1.3.4 Construction Practices No specific construction practices were given. 1.3.5 Maintenance Practices For general maintenance, the authors indicate that the cleaning of porous asphalt layers that have become filled with debris can be very difficult. High pressure water with subsequent vacuuming has been used to clean porous pavements; however, this technique has not been very successful in restoring permeability. The authors state that if cleaning techniques are begun while the layer is still permeable, instead of clogged, results should be to maintain permeability for a longer period of time. The authors used skid testing to evaluate the behavior of porous asphalt wearing layers during winter conditions. A skid trailer was used for this testing, with the wheel in a braking condition instead of a locked condition. This was done because of the possible formation of a snow or slush wedge in front of a locked wheel. The authors state that porous asphalt wearing layers are very variable during winter conditions and can change rapidly. Generally, the frictional properties of the porous asphalt layers were similar to comparison dense-graded layers. It was also noted that porous asphalt layers that had lost the ability to drain water behaved very similar to dense-graded layers. Differences in behavior between porous asphalt and dense-graded layers mainly occurred on heavily trafficked roads where traffic does not pass (i.e., between wheel paths and edges of road). Some advantages of porous asphalt during winter conditions include: ice does not generally form on wet porous asphalt surfaces because of the ability to drain water and the good macrotexture exhibited by porous asphalt; the high level of macrotexture is beneficial when snow and slush exist; and the tendency for ice formation within wheelpaths covered with snow is reduced due to the macrotexture, water absorption (draining of water) and limited thaw. Disadvantages of porous asphalt during winter conditions include: the need for deicing salts and other thawing products; the use of sand and small aggregates to improve frictional properties is not possible because they clog the void structure of the porous asphalt layer; snow and ice tend to stick to the porous asphalt layer sooner because the surface is generally cooler by about 0.5°C; snow and icing rain can form earlier on porous asphalt because deicing salts do not remain on the surface; preventative salting is not as beneficial because the salt penetrates into the void structure; if the porous asphalt layer drainage capacity is reduced, ice can build up within the layer and expand onto the pavement surface; and some icing problems can occur within the

7 initial portion of a subsequent dense-graded surface which does not receive salt through transportation by traffic. 1.3.6 Rehabilitation Practices No specific rehabilitation practices were given. 1.3.7 Performance The first performance issue discussed by the authors was frictional properties. Frictional properties were measured once or twice a year with a skid trailer. This trailer allowed the testing of the wearing layer with either a braked or locked wheel using a water film. Initial testing of each pavement was conducted within two months of opening to traffic. Within this investigation, locked-wheels were used to evaluate frictional properties. Based upon the friction testing, the authors state that vehicle speed does not influence the frictional properties of porous asphalt wearing layers as much as typical dense-graded layers due to the higher level of macrotexture. On lower vehicle speed roadways where microtexture in more important, the authors indicate that porous asphalt has lower (but acceptable) frictional resistance. Because of the aggregate structure of porous asphalt, vehicle tires are more in contact with individual coarse aggregates than a pavement surface. For this reason, the polish resistance of the coarse aggregate is important. The next performance indicator utilized by the authors was a field permeability test. Permeability was defined as a performance measure because the benefits derived from porous asphalt are related to the ability of the layer to drain water (or allow noise absorption). One of the first tasks related to the research project was to identify a method for measuring the permeability characteristics of porous asphalt. Several devices were considered, however most of the devices considered were insufficiently precise or too complicated for use. Therefore, the researchers developed a new methodology, the IVT Permeameter. This device is made of a plexiglass cylinder having an interior diameter of 190 mm and a height of 250 mm. The plexiglass cylinder contains five engraved markings that are 20 mm apart, with the “zero” marking being a height of 250 mm above the pavement surface. Special putty is used to seal the flow of water through the surface texture of the porous asphalt. Permeability is expressed as the time required for water to travel between the “zero” mark and the 80 mm marking. This will result in 2.27 liters of water flowing into the pavement. The authors state that this test of permeability is a single point measurement and that a number of tests should be conducted in order to adequately characterize the properties of the pavement. Initial testing with the developed permeameter showed a water level decrease of 80 mm ranging from 23 to 105 seconds [shorter times reflect more permeability]. Permeability of the porous asphalt wearing layers decreased over time and the rate was pavement specific. The authors listed a number of causes for this reduction in permeability. First, dust and debris can fill the void structure of the porous asphalt layer. Secondly, a slight consolidation of the porous asphalt layer will reduce permeability from the initial values. Other factors that can affect the reduction in permeability include environment (amount of rain) and type of traffic volume. Typically, permeability will be maintained longer within the wheel paths. Wheel paths will maintain permeability longer because of the

8 cleaning suction action caused by tires traveling over the layer. The authors state that some porous asphalt wearing layers will maintain permeability for more than 5 years and some will become almost impermeable within one year. When comparing the permeability of mixes having different maximum aggregate sizes (10 mm or 16 mm), permeability values were similar at the time of construction. However, porous asphalt mixtures having a maximum aggregate size of 16 mm tended to have higher permeability values for a longer period of time. The authors listed a number of favorable conditions for maintaining permeability: • Reduced amount of dirt and debris. • Good drainage (daylighted pavement edge, sufficient cross slope in underlying layer) • Layer with high percentage of larger air voids • Cleaning action of rapid and intense traffic. Another performance measure discussed within the paper was traffic noise. Traffic noise was evaluated in three ways: measurement of tire/pavement noise using a noise trailer, measurement of roadside noise, and measurement of sound absorption due to the pavement surface. Five values were used from these measurement techniques to compare the noise levels of porous asphalt wearing layers: • Degree of noise reflection by the pavement surface and the quantity of sound absorbed by the pavement. • LMA-value: Value of tire/pavement noise determined from the noise trailer. • Coasting noise: sound level of a passenger car rolling by with the engine turned off determined by a wayside measurement. • Traffic noise: sound level of a passenger car passing at a constant speed determined by a wayside measurement. • Traffic noise (Leq): the energy-equivalent continuous sound of a traffic stream determined by a wayside measurement. Measurement of sound absorption, or reflection of sound, can be done either in the laboratory using an impedance tube or in the field using special equipment. The research showed that porous asphalt layers that are in good functional condition (permeability has been maintained) are capable of absorbing sound. A maximum of about 20 percent of the sound was absorbed. The authors state that layers thicker than the 50 mm (maximum) used in Switzerland had the potential for absorbing more sound. The authors also showed a relationship between the permeability of the porous layers and the ability to absorb sound. As permeability increased, sound absorption also increased. However, the authors stated that surface texture seems to be more important than permeability. Several pavements exhibiting relatively low permeability values (clogged) still provided a reduction in noise levels. When using the noise trailer, the authors found that porous asphalt layers that were in good functional condition resulted in lower noise levels than typical dense-graded layers. At speeds less than 50 to 60 km/hr, the noise levels were similar. The difference in noise