Annotated Literature Review for NCHRP Report 640 (2009)

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175 1.41.3 Materials and Mix Design The only information provided on materials and mix design was that single layer porous asphalt mixes have an 8 mm maximum aggregate size. When two layer porous asphalt layers are used, the top layer has an 8 mm maximum aggregate size and the lower layer of porous asphalt has a 16 mm maximum aggregate size. 1.41.4 Construction Practices No specifics on construction practices were given. 1.41.5 Maintenance Practices The authors indicated that two layer porous asphalt as studied in this paper could maintain permeability if cleaned by high pressure water twice a year. 1.41.6 Rehabilitation Practices No specifics on rehabilitation practices were given. 1.41.7 Performance No specifics on performance were given except for that the noise reduction capabilities of porous asphalt do diminish over time. 1.41.8 Structural Design The authors state that two layer porous asphalt pavements include 25 mm of the 8 mm maximum aggregate size porous asphalt and 45 mm of the 16 mm maximum aggregate size porous asphalt. Additionally, the authors state that the total of 70 mm of porous asphalt provides more structural capacity than 30 mm of typical dense-graded HMA. 1.41.9 Limitations No specific limitations were given. 1.42 Litzka, J. “Austrian Experiences with Winter Maintenance on Porous Asphalt.” Proceedings of the Ninth International Conference on Asphalt Pavements. Copenhagen, Denmark. August 2002. 1.42.1 General This paper describes the evolution of winter maintenance for porous asphalt within Austria. Also included within this paper are the results of a summit, held in Austria during 1999, that describes the typical practices of a number of European countries for the winter maintenance of porous asphalt. Porous asphalt test sections were first placed in Austria during 1984. From 1984 until approximately 1988, porous asphalt was sparingly used within Austria; however, from 1988 until 1991, approximately half of the pavements using porous asphalt were placed. Since 1991, the use of porous asphalt has only been minimal. The paper states that the use of porous asphalt has declined due to two problems: structural life and maintenance. Additionally, the development of dense surface layers that reduce tire-pavement noise, like small nominal maximum aggregate size SMA mixes, has led to the decrease in use of porous asphalt.

176 1.42.2 Benefits of Permeable Asphalt Mixtures The authors did not specifically conduct testing to evaluate the benefits of permeable friction courses; however, benefits of PFCs discussed within the paper include noise reduction and reduced hydroplaning. 1.42.3 Materials and Design This paper does not specifically deal with the design of friction courses; however, the authors state that only modified binders are used for porous friction courses. Fibers may also be included. Air void contents are specified at greater than or equal to 17 percent. 1.42.4 Construction Practices This paper does not specifically deal with the construction practices of friction courses; however, the authors state that porous asphalt is placed onto an impervious layer (SAMI). Acceptance testing for porous asphalt pavements include: layer thickness, smoothness, percent compaction, drainage (permeability) and noise emission. 1.42.5 Maintenance Practices According to the paper, Austria’s climate is humid, cool and characterized by cloudy conditions, wind and precipitation throughout the year. In winter (December to February), average temperatures generally range from -0.2˚C to -2.2˚C (32˚F to 28˚F, respectively). The paper also states that there are a large number of freeze-thaw cycles. Three summaries were provided on the maintenance of porous asphalt: the results of a survey conducted in Austria during 1993/1994, an international exchange of experiences in 1999, and another Austrian survey conducted in 2002. 1993/1994 Austrian Survey Because of their open structure, porous asphalt surfaces are about 1˚C colder when compared to dense-graded surfaces. Therefore, porous asphalt surfaces remain at a colder temperature longer and reach freezing temperatures earlier than dense-graded surfaces. Because of the extended time for cold temperatures, the consumption of de- icing materials is higher. During slushy conditions, the performance of porous asphalt surfaces is slightly poorer than dense-graded surfaces. Snowplows tend to push the slushy material into the void structure of the porous pavement. The cold temperatures cause the slushy material to swell such that the slushy materials are again a road hazard. To prevent the slushy material remaining in the void structure of the porous asphalt from swelling, salting of the roadway must be conducted immediately after the snowplows pass. This is in contrast to dense-graded surfaces. The extra salting also leads to increased usage of de- icing materials for porous asphalt surfaces compared to dense-graded surfaces. The survey states that on dense-graded surfaces, the preventative application of anti-icing materials may delay or even prevent icing. However, on porous asphalt surfaces, immediate and continuing applications of anti-icing materials are required. When road salt is applied to porous asphalt surfaces too late or the anti-icing agents are ineffective,

177 the removal of the resulting ice layer is much more difficult on porous asphalt surfaces than dense-graded surfaces. The authors state that porous asphalt will require 25 to 50 percent more deicing agents than dense-graded surfaces. Winter maintenance crews must be able to respond quickly and flexibly to different weather and road conditions. Weather forecasting systems and electronic road condition surveillance systems are very helpful in this quest. 1999 International Exchange of Experiences In 1999, an International Exchange of Experiences was hosted in Austria that brought experts from throughout Europe together to discuss issues with porous asphalt. The following presents a summary of the experiences of various countries with respect to the maintenance of porous asphalt. Germany Winter maintenance of porous asphalt is generally considered more expensive and slightly more difficult than dense-graded surfaces. The standard quantity of salt applied to porous asphalt surfaces is 10 g/m2; however, within problem areas the required quantity of salt may reach 40 g/m2. The availability of weather forecasting systems helps facilitate timely response to winter maintenance activities. Italy Dry road salt is generally applied to wet pavements in quantities of 10 to 20 g/m2 as a preventative maintenance. During snowfalls, the dry road salt is again applied at the same rate. After snowplows have removed the snow from the pavement surface another 10 to 30 g/m2 of road salt is applied depending upon the road conditions. It was stated that Italy’s change from a coarse maximum aggregate particle size porous asphalt (20 mm) to a smaller maximum particle size (16 mm) has led to significant improvement in road conditions during winter months. Netherlands Road salt consumption increased by about 25 percent when using porous asphalt. In very severe winter weather, speed limits or road closures have been employed. Slovenia It was stated that in Slovenia, salt consumption is up to 100 percent higher for porous asphalt mixtures than for dense-graded surfaces. Austria The increase in salt usage for porous asphalt surfaces is about 10 to 15 percent over dense-grades surfaces. Best results occur when using a wet salt application (salt/brine ratio of 2:1). 2002 Survey Within Austria Statements by the survey respondents indicated that preventative anti-icing measures are difficult and more expensive. Tests with dry road salt did not produce positive results [no