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13 a porous asphalt mixture will provide an avenue for water to penetrate into the pavement structure, thereby, increasing the potential for pavement deterioration. 1.4.9 Limitations Ruiz et al indicate that the use of porous asphalt mixtures should be carefully studied prior to being placed in the following situations: ⢠Areas of frequent snow ⢠Urban or industrial areas ⢠Areas with a high potential for reflective cracking ⢠Bridge pavements. 1.5 Van Der Zwan, J.T., T. Goeman, H.J.A.J. Gruis, J.H. Swart, and R.H. Oldenburger. "Porous Asphalt Wearing Courses in the Netherlands: State of the Art Review.â Transportation Research Record No. 1265. Transportation Research Board. National Research Council. Washington, D.C. pp 95-110. 1990. 1.5.1 General This paper presents a detailed summary on the use of porous asphalt in the Netherlands. The Netherlands is located in northwest Europe with a temperate climate. Average temperatures during January and July are 1.7ºC (35ºF) and 17ºC (63ºF), respectively. Annual precipitation is almost 800 mm (32 in) with the precipitation distributed throughout the year. Because of the amount of precipitation, the authors indicated that road surfaces tend to be wet about 13 percent of the time. Porous asphalt was first utilized in the Netherlands in 1972. Since that time, many pavement sections have been placed that included porous asphalt wearing layers. The paper discusses all aspects of porous asphalt use within the Netherlands. Information provided within the paper was used to conduct a life-cycle cost analysis to compare porous asphalt wearing layers and dense-graded wearing layers. Based upon the results of the cost-benefit analysis, the Dutch Department of Works decided that porous asphalt wearing layers would be preferable on the following types of pavements: ⢠Busy motorways with an average of more than 35,000 vehicles per day. ⢠Limited access roadways that do not allow slow moving traffic. ⢠At discontinuities such as superelevations where excess water may cause difficulties. ⢠On roadways with a recognized noise nuisance problem. 1.5.2 Benefits of Permeable Asphalt Mixtures Benefits noted by the authors centered on improvements in safety for the traveling public. These benefits were directly related to the ability of porous asphalt to remove water from the pavement surface. By eliminating water films from the pavement surface, porous asphalt layers reduce the amount of splash and spray and improve the visibility of pavement markings. Additionally, the authors state that hydroplaning is basically eliminated. Skid resistance on wet pavements is also increased.
14 The authors did note that porous asphalt layers reduce tire/pavement noise by approximately 3dB(A) when compared to typical dense-graded wearing layers. 1.5.3 Materials and Mix Design The authors state that the porous asphalt mixture being utilized in the Netherlands [as of 1990] was very similar to the OGFC mixes used in the U.S. Table 5 presents typical requirements for porous asphalt in the Netherlands. In order to improve bonding between the aggregates and asphalt binder, the authors state that a limestone filler is added during the production process. The limestone filler must have a hydrated lime content of at least 25 percent. During design, a minimum air void content of 20 percent is required. A dynamic bending test is used to evaluate the stiffness of designed porous mixtures. Wheel tracking tests are also used to evaluate resistance to rutting. The authors indicate that the results of the dynamic bending tests (in terms of modulus) are only about 20 percent of typical dense-graded mixes. However, the wheel tracking tests indicate that porous asphalt has much less potential for permanent deformation. Table 5: Typical Requirements for Porous Asphalt Mixes in the Netherlands Percent Mass Passing, % Sieve, mm Target Maximum Minimum Tolerance 16 100 96 ± 1.0 11.2 85 70 ± 8.0 8 50 35 ± 7.0 5.6 30 15 ± 7.0 2 15 ± 4.0 0.063 4.5 ± 1.0 Binder Content (80/100 binder) 4.5 ± 0.5 1.5.4 Construction Practices The authors indicate that all HMA is produced in the Netherlands using a batch plant facility. This makes the production of porous asphalt much more straightforward than the use of other plant types. During construction, the authors state that handwork is very difficult and should be avoided. Compaction of porous asphalt is best achieved using static steel-wheel rollers. The temperature of the mixture during compaction is critical. If temperatures are too high, the mortar may drain from the aggregate structure. If the temperatures are too low, the mixture will be difficult to compact. Ideal placing and compaction temperatures are between 140 and 170ºC (280 to 340ºF). The authors indicate that the costs for placing and compacting porous asphalt are comparable to placement and compaction of dense- graded mixes. 1.5.5 Maintenance Practices The authors provide issues with dealing with porous asphalt during wintry conditions. Icy roads are a recurrent problem in the Netherlands; however, snowfall is not a major
15 problem in the temperate climate of the Netherlands. Salting operations are the typical method of dealing with icy roads. Electronic monitoring systems were being installed along the main road network [at the time of this paper] to assist in selecting the appropriate time for conducting winter maintenance. Field measurements have shown that porous asphalt layers remain below 0ºC (32ËF) longer than dense-graded wearing layers. As a result, ice problems are likely to develop sooner and last longer on porous asphalt wearing layers. Due to the open nature of porous asphalt, salt applications will disappear into the void structure of the layer. This will be further exacerbated by the salt being removed from the pavement surface by melting ice. As a result, the time that salt remains on the pavement surface is relatively short compared with dense-graded wearing layers. The authors note that special attention must be paid to transitions between porous asphalt and dense-graded wearing layers. At these transitions, there is little salt transport. Experience has shown, however, if pre-wetted salt is applied that these transition points are not as big of a problem. In conclusion, the authors state that porous asphalt wearing layers are considered as safe as dense-graded wearing layers during the winter, provided that timely measures are taken into account for the different behaviors of the two wearing layers. 1.5.6 Rehabilitation Practices Minor rehabilitation strategies are similar to conventional dense-graded layers; however, the authors state that the inherent drainage characteristics of the porous asphalt should be maintained. The preferred method of rehabilitating porous asphalt layers is to mill the existing layer and replace with a new wearing layer. 1.5.7 Performance The authors state that the service life of porous asphalt layers is about 10 years. This is compared to the expected 12 year service life of dense-graded wearing layers. The most common distress encountered on porous asphalt layers is raveling, with the most potential for raveling being within the first year. 1.5.8 Structural Design Porous asphalt layers are placed at a thickness of 50 mm. This thickness was selected based upon the typical rainfall rates experienced within the Netherlands. The pavement design methodology in the Netherlands entails designing to prevent classical bottom-up fatigue cracking. When designing pavement thicknesses, dense- graded mixes are assigned a dynamic modulus value of 7,500 MN/m2 and the mixture must also meet specific fatigue properties. The authors provided a discussion on the structural contribution of a porous asphalt layer within pavements by comparing dense- graded and porous asphalt.
16 One area that dense-graded and porous asphalt mixes were compared was in terms of dynamic modulus. The authors indicated that the dynamic modulus of porous asphalt is generally 5,400 MN/m2, or about 70 to 80 percent of dense-graded mixes. This value of dynamic modulus was input into their pavement design models and the results indicated that 10 to 20 percent more thickness was required to maintain a specific fatigue strain at the bottom of the pavement layer when using porous asphalt as compared to dense- graded mixes. The authors also evaluated the effect of aging and stripping on pavement design. Due to the open nature of porous asphalt, the asphalt binder coating aggregates is susceptible to accelerated oxidative aging. Oxidative aging of the asphalt binder results in an increase in stiffness within the porous asphalt layer which would reduce pavement thickness. Alternatively, the authors state that water within the porous asphalt layer can lead to a loss of adhesion between the porous asphalt layer and the underlying layer. This loss of adhesion impairs the load transfer characteristics of the structure. The authors state that there is no evidence that the loss of adhesion between layers [delamination] has taken place in the field; they conservatively assumed a loss of adhesion to evaluate the net effect on pavement structure using the BISAR program. When delamination occurs, the effective bearing capacity of the debonded layer is reduced to between 2 and 10 percent of the original value. By applying Minerâs modified linear damage law, the authors stated that the combined effect of aging and stripping [delamination] would result in about 35 to 40 percent effective contribution of porous asphalt when compared to dense- graded layers. Because porous asphalt has different thermal properties, the authors also evaluated the effect of temperature on pavement structure when comparing dense-graded and porous asphalt wearing layers. Van der Zwan et al provided a hypothesis that the suction and pumping action of tires passing over porous asphalt surface, coupled with wind action, promotes continuous air circulation within a porous asphalt layer. As a result, the temperature of the porous asphalt layer will tend to be lower than for comparable dense- graded layers. In order to investigate this hypothesis, the authors conducted experiments on newly placed and 8-year old porous asphalt layers to compare the temperatures of pavements with porous asphalt and dense-graded wearing layers at the surface and at depth. Results from these experiments, which included 1-year of data, indicated that the weighted average temperature over a year was found to be 1ºC lower in pavements containing porous asphalt wearing layers. Due to the viscoelastic properties of asphalt, the lower effective temperature in pavements including a porous asphalt wearing layer means that the stiffness (modulus) of layers within these pavements is higher. The net result being that less thickness is required to resist fatigue cracking. The combined effect of the above mentioned factors suggests that porous asphalt can be expected to contribute about 50 percent of the equivalent structural capacity compared to a dense-graded layer. However, if adhesion between the porous asphalt layer and the underlying layer is not lost (as was conservatively assumed), then the effective contribution of porous asphalt can be 100 to 110 percent of conventional systems.
