Annotated Literature Review for NCHRP Report 640 (2009)

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17 1.5.9 Limitations No limitations on the use were given. 1.6 Van Heystraeten, G. and C. Moraux. “Ten Years’ Experience of Porous Asphalt in Belgium.” Transportation Research Record No. 1265. Transportation Research Board. National Research Council. Washington, D.C. pp 34-40. 1990. 1.6.1 General Porous asphalt has been used in Belgium since 1979. Based upon the success of initial trials, more sections were placed starting in 1981. As of 1990, a total of about 70 projects encompassing approximately 2 million square meters of porous asphalt had been placed. The authors define porous asphalt as a bituminous road mixture designed so that after construction, the mixture will form a surface having an air void content of about 22 percent. These mixes must be designed so that water is drained through the layer laterally; therefore, the underlying layer must have some cross-slope to promote drainage through the layer laterally. Also, the underlying layer must be impervious so that water cannot infiltrate into the layer. Porous asphalt is most commonly used in Belgium where water will tend to stagnate, like superelevations, wide pavements (motorways and airfield runways) and sags in horizontal curves. Another location that the authors indicate porous asphalt can be used is within tunnels. Any water that may seep up through the pavement would be carried by the porous laterally to a drainage system to remove the water. Other areas where porous asphalt is commonly utilized would be areas where traffic noise is an issue. The authors state that porous asphalt will provide a reduction in noise of 6 to 10 dB(A) compared with concrete pavements. In some instances, porous asphalt is used on noisy urban arterials, even curbed sections. When used on curbed sections, adequately designed drainage systems at the pavement edge are a necessity. 1.6.2 Benefits of Permeable Asphalt Mixtures Porous asphalt makes it possible for the tires on vehicles to stay in contact with the pavement surface during wet weather, thus avoiding hydroplaning. Porous asphalt also eliminates splash and spray behind vehicles and avoids reflections from the pavement surface during the day and night. The authors state that porous asphalt accounts for a great part of the success considered in reducing traffic noise both inside and outside vehicles. This reduction in traffic noise is due to the absorption of sound that occurs in the voids of the porous asphalt layer. Also, the void structure eliminates the air pumping at the tire/pavement interface. 1.6.3 Materials and Mix Design The philosophy of designing porous asphalt in Belgium is to provide sufficient coarse aggregate in order to provide the gap-grading required of porous asphalt. Typically, 80 percent of the aggregate would be larger than 2 mm. The gap-grading is generally provided by omitting the 2/7mm or 2/10mm fraction from a 0/14 mm gradation size. Mix designers also try to minimize the amount of asphalt binder so that the voids are not filled

18 with the asphalt binder. Table 6 presents typical requirements for porous asphalt in Belgium. Table 6: Belgian Specification for the Composition of Porous Asphalt Property Specification Grading 0/14 mm gap Stones (≥ 2mm) 83 % Crushed Sand (0.080 mm – 2 mm) 12 % Filler (< 0.080 mm) 5 % Binder - 80/100 bitumen 4 to 5 % - Modified bitumen 4 to 5 % - Rubber bitumen 5.5 to 6.5 % Air Void Content - Average 19 to 25 % - Individual 16 to 28 % The mix design method used in Belgium includes to first determine the voids in coarse aggregate for the plus 2 mm aggregate fraction [though no test method was given]. The next step entails compacting mixture with the Marshall hammer [no blow count given] and evaluating air voids and the percent wear using the Cantabro Abrasion test method. Optimum asphalt binder content is selected such that the granular materials are coated correctly but not excessively while meeting all other criteria. 1.6.4 Construction Practices Conventional batch plants are used to produce porous asphalt in Belgium. The authors note that care must be taken to ensure that the temperature of the mix does not exceed 170°C (338°F) in order to avoid draindown problems. Haul distances must also be minimized. As haul distance increases, the potential for draindown also increases. Paver operations during the placement of porous asphalt are very similar to that of typical dense-graded layers. Static steel-wheel rollers are the recommended equipment for compacting porous asphalt. Vibrator rollers are not recommended because of the potential for degrading the aggregate within the mix. Pneumatic tire rollers are generally not used as there is generally a pick-up problem when compacting porous asphalt with these rollers. Care must be taken at longitudinal joints. The joints should not be tacked as this will obstruct the drainage of water across the layer. Finished porous asphalt tends to stick to the tires of vehicles. Therefore, it is common practice in Belgium to place fines (<0.080 mm) onto the porous asphalt surface at a rate of 50 g/m2. 1.6.5 Maintenance Practices The authors state that porous asphalt and dense-graded HMA wearing layers do not behave differently in snowy weather if deicing salts are “intensively” spread on the pavement surface. If the deicing salts are not “intensively” spread, snow may remain on