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160 In order to evaluate the concept, 500 mm square specimens of porous asphalt were fabricated in the laboratory. The lower layer of the specimen was comprised of 120 mm of dense-graded HMA. On top of this lower layer, the resistive filaments were placed. A polymer modified emulsion was then placed on top of the filaments. Finally, 50 mm of a porous asphalt was placed as the upper layer. The resistive filaments were shaped in a sinusoidal form of 24 mm width and 8 mm wavelength. As an initial attempt at the distance between the filaments, a distance of 150 mm was utilized. Thermal images were taken over time to evaluate the ability of the filaments to increase the temperature of the porous asphalt. Giuliani concluded that the heating elements would increase the road surface temperature above the external ambient temperature. The sinusoidal shaped filaments created a bigger heating surface inside the pavement and could withstand construction better. Placing a system of resistive elements within a pavement is not complicated. 1.37.6 Rehabilitation Practices No specific rehabilitation practices were given. 1.37.7 Performance No specific performance measures were given. 1.37.8 Structural Design No specifics on inclusion within structural design were given. 1.37.9 Limitations No specific limitations were given. 1.38 Greibe, A. P. âPorous Asphalt and Safety.â Proceedings of the Ninth International Conference on Asphalt Pavements. Copenhagen, Denmark. August 2002. 1.38.1 General This paper provides an overview of a research study being conducted in Denmark. The objective of the project was to evaluate the noise reducing effects, durability, performance and safety of porous asphalt. For the study, a section of two-layer porous asphalt was constructed 300m in length and compared to a 100m section of dense-graded HMA. The majority of the paper provides the results of a literature review that covers a wide range of topics. The first topic is traffic accidents. Greibe cites work in Holland that suggests that there are no differences in safety, whether wet or dry pavements, between porous asphalt and dense-graded HMA. Work in France showed that placement of porous asphalt resulted in 17 percent more accidents but a 25 to 30 percent decrease in wet weather accidents. Greibe noted, however, that this French study omitted pavements where no accidents occurred which may have skewed the results. A similar study in Austria showed a decrease in wet weather accidents with little change on dry pavements.
161 Greibe indicated that some of these studies suggested that the benefits of porous asphalt may be somewhat offset by the higher speeds and shorter distance between vehicles resulting from the increased confidence exhibited by drivers on porous asphalt. The next topic discussed by Greibe was splash and spray. Studies in Europe have indicated that splash and spray may be reduced by as much as 95 percent for porous pavements when compared to dense-graded layers. Greibe states that new porous asphalt layers have relatively low frictional resistance. This is primarily due to the asphalt binder film coating the aggregates. When a vehicle brakes hard, the friction developed between the tire and the thin film of asphalt binder causes heat which melts the asphalt binder film. This can result in a slippery surface and breaking distances that are 20 to 40 percent longer than on dense-graded layers. The thin asphalt binder layer generally takes 3 to 6 months to wear away. Less light is reflected by wet porous pavements than dense-graded pavements. This is due to the fact that porous asphalt does not allow water to pool on the pavement surface. Since less light from oncoming vehicles is reflected, pavement markings are more visible. 1.38.2 Benefits of Permeable Asphalt Mixtures As detailed above, a number of benefits were identified which include: reduced splash and spray, less light reflection and improved wet weather friction. 1.38.3 Materials and Mix Design No specifics on materials and mix design were given. 1.38.4 Construction Practices No specifics on construction practices were given. 1.38.5 Maintenance Practices Greibe states that the winter maintenance of porous asphalt is different than for dense- graded layers. On dense-graded pavement layers, salts mix with moisture on the surface. Porous pavements contain more interconnected voids and the salt disappears in the void structure so more salt is required. An increase of 25 to 100 percent should be expected in salt consumption. The pumping action caused by traffic passing over a porous asphalt layer will continually circulate the salt solution within the void structure of the layer. Therefore, as long as the traffic volume remains high, the driver should not notice any differences between porous asphalt and dense-graded HMA surfaces. Greibe states that since the behavior of salt on porous asphalt surfaces is so different, special locally adjusted salt spreading strategies are required. Due to the difference in thermal conductivity of porous asphalt compared to dense-graded HMA, the temperature of porous asphalt will fall below freezing before dense-graded layers. Porous asphalt will also stay below freezing longer than dense-grade layers.
