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58 Table 34: Average Permeability Results Over Time Average Permeability, ft/day Test Section 1992 1993 1996 Permeability in 1996 as % of Permeability in 1992 Std. OGFC (d) 146 21.9 7.8 5 Coarse OGFC (D) 142 31.0 28.9 20 D + Mineral Fibers (DM) 254 66.7 12.9 5 D + Cellulose Fibers (DC) 222 61.3 28.4 13 DC + SB Polymer (DCP) 220 67.4 18.6 8 D + SB Polymer (DP) 262 95.6 11.0 4 D + 16% Crum Rubber (D16R) 210 55.9 25.6 12 Another interesting observation about the permeability data was that the permeability values between the wheel paths was generally lower than the results of testing within the wheelpaths. Santha indicated that this was an indication of the action of traffic cleaning the OGFC layer. 1.12.8 Structural Design The test sections were placed at a thickness of 19 mm. 1.12.9 Limitations No specific limitations were given. 1.13 Tolman, F. and F. van Gorkum. âA Model for the Mechanical Durability of Porous Asphalt.â European Conference on Porous Asphalt. Madrid. 1997. 1.13.1 General The objective of the study reported in this paper was to demonstrate the use of cyclic tensile tests in explaining the difference in behaviors of different porous asphalt mixes. Starting with the premise that failure in porous asphalt occurs mostly at the interface between binder and aggregates, Tolman and Gorkum provided results from tensile tests conducted on cores from six different porous asphalt pavements in Netherlands. Using the minimum creep rate model, the authors derived parameters required for explanation of fracture and ductile failure mechanisms. They were able to derive fundamental parameters to explain the differences in behaviors between cores obtained from different pavements (of different ages). 1.13.2 Benefits of Permeable Asphalt Mixtures No specific benefits were given. 1.13.3 Materials and Design Tolman and Gorkum provide definition and description of porous asphalt. They mention that porous asphalt is a skeleton of stones bound by âvisco-elasticâ deteriorating mortar, which consists of asphalt, added filler, aggregate filler and particles embedded in the asphalt film. Tolman and Gorkum indicate that the principal factors affecting the strength of porous asphalt are binder stiffness, film thickness and minerals in binder film. They mention that failure in porous asphalt has been seen to occur primarily due to the fracture of binder films and not due to
59 separation from aggregates or fracture of aggregate particles. They mention that the separation or detachment (or stripping) has been assumed to be the result of physicochemical actions. The authors make reference to three papers and mention that there is a critical stiffness level for porous asphalts, below which the failure occurs by flow, termed as ductile failure, and above which failure occurs by fracture, termed as brittle failure. 1.13.4 Construction Practices No information has been provided on construction practices. 1.13.5 Maintenance Practices No information has been provided on maintenance practices. 1.13.6 Rehabilitation Practices No information has been provided on rehabilitation practices 1.13.7 Performance In discussing the objectives of the study, Tolman and Gorkum mention that loss of aggregates occur in porous asphalts. They indicate that this phenomenon, which is a combination of raveling, fretting and disintegration, occurs probably because of stretching of binder film due to traffic, particularly during warm weather. They contend that this stretching causes intrusion of dust and exposure to materials such as oxygen, UV and water, which in turn, leads to hardening of the porous asphalts. The authors indicate that most likely, this increased hardening, with a drop in temperature, leads to fracture, during the winter periods. Once the porous asphalt cracks, the ingress of water causes separation of asphalt binder from the aggregate. Tolman and Gorkum mention that it is the fracture at the interface of the aggregate and asphalt binder, and the stiffness of the binder film that are important for understanding the failure mechanism in porous asphalt. They provide estimated values of tensile stress and resulting strains in binder films under different conditions, as shown in Table 35. Table 35: Stresses and Strains in Binder Films in Porous Asphalt Condition Horizontal tensile stress, maximum, MPa / Stress gradient Strains, % Aggregates not embedded in binder 3 MPa/- 300 % Aggregates embedded in binder -/0.2 MPa/mm 100 % Note: Assumptions: 1. Asphalt binders stiffness: 1 MPa, typical length of aggregate particle = 6 mm, thickness of binder film = 1mm Tolman and Gorkum mention that high stresses (for example, 3 MPa) can overcome relatively low binder stiffness (1 MPa) and cause a high amount of strain and ultimately failure. They contend that failure can occur in one of three ways: 1. brittle failure, caused by fracture of binder layer, 2. ductile failure caused by shear failure, due to flow and contraction of the binder layer, and 3. a combination of two processes, first a flow and contraction of the binder layer and then a brittle fracture.
