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2 1.1.2 Benefits of Permeable Asphalt Mixtures Porous asphalt allows water to drain from the surface of pavement because of its gradation and aggregate make-up. Because of the high air-void content in the mix, the surface water can drain, which reduces road spray, improves skid resistance and decreases road glare (increasing vision). 1.1.3 Materials and Mix Design Irelandsâ porous asphalt has a nominal maximum aggregate size of 14mm. The aggregates are gap-graded and held together with a binder that is polymer modified. The air voids in the porous asphalt are approximately 20 percent. In the design, an inverted pavement surface texture is created which allows for acoustic absorption, which will reduce noise level. 1.1.4 Construction Practices No specifics on construction practices were given. 1.1.5 Maintenance Practices No specifics on maintenance practices were given. 1.1.6 Rehabilitation Practices No specifics on rehabilitation practices were given. 1.1.7 Performance Because of the performance of porous asphalt with respect to reducing noise levels, water drainage and friction, it is being used on many roads in Ireland. 1.1.8 Structural Design No specifics on inclusion within structural design were given. 1.1.9 Limitations No specific limitations were given. 1.2 Decoene, Y. âContribution of Cellulose Fibers to the Performance of Porous Asphalts.â Transportation Research Record No. 1265. Transportation Research Board. National Research Council. Washington, D.C. pp 82-86. 1990. 1.2.1 General The use of porous asphalt mixes has increased in Belgium due to the many benefits that they provide. Many roadways have been covered with a porous asphalt wearing layer and the performance has been good. Decoene states that the performance has been good even though unmodified binders have been used. However, in some instances problems have occurred due to the draining of the asphalt binder from the aggregate structure. As a result of this draindown problem, raveling has occurred in areas with low asphalt binder contents. In order to solve the draindown problem and to allow porous asphalt to be placed with higher asphalt binder contents (for better durability), fibers can be added to
3 the mixture. Small amounts of asbestos fibers had been used; however, because of the health issues related to asbestos, Belgium decided to investigate the use of cellulose (organic) fibers within porous asphalt. This paper summarizes research conducted to evaluate the use of cellulose fibers within porous asphalt. 1.2.2 Benefits of Permeable Asphalt Mixtures Benefits associated with porous asphalt layers cited by Decoene included improved skid resistance, reduced hydroplaning, reduced splash and spray, improved visibility, reduced tire/pavement noise and resistance to permanent deformation. 1.2.3 Materials and Mix Design Organic fibers used within the research study were characterized as gray fibers with a cellulose content of at least 75 percent and a pH of between 6 and 8.5. The length of fibers was 1.2 mm and the density was 1.5 g/cm3. Decoene stated that the fibers were resistant to temperatures up to 180ºC (356ËF). Laboratory evaluation of the cellulose fibers within porous asphalt included testing for draindown potential, the influence on compaction (in terms of air voids) and durability. Two draindown potential tests were utilized: a Basket Drainage test and the Schellenberger Drainage test. During the basket Drainage testing, a total of seven porous asphalt mixes were evaluated (Table 1). Of these seven mixes, cellulose fibers were added to six of the mixes at either a rate of 0.3 percent or 0.5 percent, by total mix mass. In order to conduct the Basket Drainage test, the mixes were first compacted in Duriez molds under a pressure of 30 bars (435 psi). The molds containing the compacted porous asphalt were then placed in a grid pattern within an oven maintained at 180ºC. The samples were held at this elevated temperature for 7.5 hours. During the course of the test, asphalt binder would drain from the compacted samples. At the conclusion of the test, the percent binder loss was calculated based upon the initial asphalt binder content. Results of this Basket Drainage testing are provided in Table 1. Table 1: Results of Basket Drainage Tests Mix Designation Composition A B C D E F G Diorite (10/14) 55.5 55.5 55.5 55.5 55.5 55.5 55.5 Diorite (6/10) 30 30 30 30 30 30 30 Sand (0/2) 13 12 12 12 12 12 12 Filler 1.5 2.2 2.2 2.2 2.0 2.0 2.0 Organic Fibers 0 0.3 0.3 0.3 0.5 0.5 0.5 Bitumen (80/100) 4.7 5.5 5.7 5.9 5.5 5.7 5.9 Binder Loss, % 13.5 1.5 2.9 3.4 0.3 1.3 1.2 Based upon the data presented in Table 1, Decoene provided several observations. First, the porous asphalt mixture without any fibers (Mix A) lost 13.5 percent of its binder
4 during the test, even at the low asphalt binder content. For the six mixes that did contain cellulose fibers, the amount of binder loss was significantly lower than the mix with no fibers. There was very little difference in the drainage characteristics between mixes containing 0.3 percent fiber and those having 0.5 percent fiber. The second test method employed by the author that measured the potential for draindown was the Schellenberger Drainage test. This test entails placing 1,000 to 1,100 grams of porous asphalt into a glass beaker and then placing the sample into an oven for 1 hour. Similar to the Basket Drainage test, results are reported as the percent binder loss as a percentage of the asphalt binder content. For this experiment, a total of 12 porous asphalt mixtures were utilized as shown in Table 2. In order to evaluate different sources of cellulose fiber, Decoene utilized four different sources of cellulose fibers. Table 2: Results of Schellenberger Drainage Tests Mix Designation Composition A B C D E F G H I J K L Durite (10/14) 55.5 55.5 55.5 55.5 55.5 55.5 55.5 55.5 55.5 55.5 55.5 55.5 Durite (6/10) 30 30 30 30 30 30 30 30 30 30 30 30 Sand (0/2) 12 12 12 12 12 12 12 12 12 12 12 12 Filler 2.5 2.5 2.5 2.5 2.2 2.2 2.2 2.0 2.2 2.2 2.5 2.2 Fiber A - - - - 0.3 0.3 - - - - - 0.3 Fiber B - - - - - - 0.3 0.5 - - - - Fiber C - - - - - - - - 0.3 - - - Fiber D - - - - - - - - - 0.3 - - Binder 60/70 4.5 4.7 4.9 4.9 5.9 6.1 5.9 5.9 5.9 - - - Binder 80/100 - - - - - - - - - 5.9 4.7 5.9 Binder Loss, % 15 16 21 17 0.1 1.1 1.1 0.3 0.6 1.1 18 0.5 Results of the Schellenberger Drainage testing yielded similar results as the Basket Drainage test. Mixtures without fibers had significantly more binder loss (draindown). This again suggested that the fibers help hold the asphalt binder to the aggregate structure of a porous asphalt. Whether 0.3 percent or 0.5 percent fiber was used in the mix did not appear to significantly influence draindown potential. The data also indicated that relatively high asphalt binder contents could be used with the porous asphalt containing cellulose fibers with little potential for draindown problems. The Belgian Road Research Centre had recommended air void contents of Marshall compacted specimens and Cantabro Abrasion tests during the design of porous asphalt. Therefore, Decoene evaluated these properties on four mixes with and without fibers. Based on the results of this testing, there was little difference in air voids of mixes with and without fibers. Also, the Cantabro loss was deemed acceptable for all four of the mixes. 1.2.4 Construction Practices Cellulose fibers are supplied to the plant site wrapped in polyethylene packaging. This packaging has a relatively low melting point that will dissolve if the cellulose fiber bag is added to the production process. Alternatively, cellulose fibers can also be pre-coated with some amount of asphalt binder in the form of a pellet. These pelletized fibers are specifically intended for drum mix plants.
