Annotated Literature Review for NCHRP Report 640 (2009)

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111 1.24.9 Limitations No limitations on the use were given. 1.25 Pasetto, M. “Porous Asphalt Concretes with Added Microfibres.” 2nd Eurasphalt & Eurobitumen Congress. Barcelona, Spain. pp. 438-447. 2000. 1.25.1 General This paper describes a research effort designed to evaluate different fibers within porous asphalt. Three different fiber types were investigated: polypropylene, polyacrylonitrile and cellulose. Additionally, the use of a Superpave gyratory compactor to provide compactive effort during mix design was evaluated. 1.25.2 Benefits of Permeable Asphalt Mixtures No specific benefits were given. 1.25.3 Materials and Mix Design Materials utilized for the porous asphalt during the research included aggregates, asphalt binder and the three types of fiber. Three aggregate stockpiles were included within the mixture: porphyric aggregate, basaltic aggregate and sand. Pasetto indicates that the two coarse aggregate sources had Los Angeles Abrasion loss values of 14 percent. Also, the Polishing Stone Value was higher than 0.59 for both aggregate sources. Table 63 provides the specified gradation band and the design gradation utilizing the selected aggregates. A polymer-modified penetration-graded 50/60 asphalt binder was used. Table 63: Gradation Requirements and Design Gradation Sieve, mm Gradation Band Design Gradation 16.0 100 100 12.5 22 – 100 63.2 9.50 20 – 35 26.4 6.30 17 – 30 17.2 4.75 15 – 27 17.1 2.00 10 – 18 14.6 0.425 7 – 12 9.6 0.180 6 – 10 7.4 0.075 5 – 8 5.1 As stated above, three different types of fibers were included within the research. Table 64 provides properties of the three fibers. The polypropylene fiber was added at 0.35 percent, the polyacrylonitrile fiber was added at 0.126 percent and the cellulose was added at 0.3 percent.

112 Table 64: Properties of Fibers Property Units Polyacrylonitrile Polypropylene Cellulose Length mm 12 6 1.1 Thickness mm 0.016 0.020 0.045 Specific Weight g/cm3 1.180 1.32-1.40 --- Elastic Modulus GPa 23 --- --- Tensile Strength MPa 800 550 --- Ultimate Elongation % 9.8 33 --- Samples of the porous asphalt were compacted using both the Marshall hammer and the Superpave gyratory compactor. [No specifics on the number of blows per face used to compact samples with the Marshall hammer were provided; however, it is assumed that 50 blows per face were used.] Samples having a diameter of 150 mm were compacted to a maximum of 130 gyrations in the Superpave gyratory compactor. Results of testing indicated an increase in the strength properties of porous mixes containing fibers when compared to a control mix with no fibers. This was true for mixes containing all three fiber types. Strength tests within the study included Marshall stability and indirect tensile strength. Also included within the study was the Cantabro Abrasion test. Again, inclusion of fibers reduced the amount of abrasion loss when fibers were added to the porous asphalt. Figure 13 illustrates the results for the comparison between the Marshall hammer and Superpave gyratory compactor. This figure shows that 50 gyrations of the Superpave gyratory compactor is comparable to the standard Marshall hammer compactive effort (assumed as 50 blows per face).