Annotated Literature Review for NCHRP Report 640 (2009)

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179 buildup if traffic is concentrated into one lane. Padmos also states that there is no definitive solution for winter maintenance; however, the addition of brine is a must. He also states that the concentration of traffic within the design lane allows the traffic to “suck” the brine solution out of the void structure of the porous asphalt onto the pavement surface. 1.43.6 Rehabilitation Practices Raveling is the major cause for rehabilitation. However, Padmos does not provide any techniques for rehabbing porous asphalt. 1.43.7 Performance Padmos states one of the problems with porous asphalt is the low frictional resistance of these layers immediately after construction. After 3 to 6 months, the asphalt binder film coating the aggregates at the surface wears away and friction significantly increases. Raveling is the major distress that influences performance. Due to raveling, the service life of porous asphalt is generally 10 to 12 years. 1.43.8 Structural Design Porous asphalt is constructed to a thickness of 50 mm in the Netherlands. Padmos also states that porous asphalt provides approximately 50 percent of the structural capacity of typical dense-graded wearing layers. 1.43.9 Limitations No specific limitations were given. 1.44 Ranieri, Vittorio, Runoff control in porous pavements, Transportation Research Record No: 1789, Transportation Research Board, National Research Council, Washington, D.C. 2002. 1.44.1 General In this paper, Ranieri presents a model linking hydraulic conductivity of porous pavement with the geometrical characteristics of the road section and the rainfall intensity. The author supports the model with verification results from laboratory tests. The tests consisted of simulating rainfall on porous pavements and measuring the depth of water over the impervious layer during the seepage motion. Based on these experiments, Ranieri provides a chart for design of porous pavements. The chart provides a relationship between a function of the maximum depth of the water table over the impervious layer and the length (L) of the seepage path and a function of rainfall intensity and permeability, for different values of slope of the porous pavement. Note that since this paper contains models, design chart and example, some of these have been scanned and presented as they are in separate figures. The reviewer feels that this is important for full understanding of the material presented in this paper.

180 1.44.2 Benefits of Permeable Asphalt Mixtures No benefit of permeable asphalt mixture is mentioned. 1.44.3 Materials and Design No information on materials and design has been presented 1.44.4 Construction Practices No information on construction practices has been presented. 1.44.5 Maintenance Practices No information is provided on maintenance practices of friction course. 1.44.6 Rehabilitation Practices No information is provided on rehabilitation practices of friction course. 1.44.7 Performance No information is provided on performance of friction course 1.44.8 Structural Design Ranieri starts with a list of models developed for design of hydraulic conductivity related models for structural design– specifically thickness and slope of porous pavements. The relationships are shown in Table 94. Ranieri mentions that all of these models are for dense-graded overlays and are not applicable for porous pavements with open-graded courses, and that the attempt by Ross and Russam (1968, see Table 94) to modify an existing relation with a coefficient to take care of the type of overlay has not been successful.

181 Table 94: Models Relating Hydraulic Conductivity to Structural Design of Porous Pavements Author/Researcher/Year Topic/Model/Equation C. F. Izzard/1942 Airport runway runoff (model not provided in this reference). N.F. Ross and K. Russam/1968 Model relating the height of water film (h) over the grains to the length (L) and slope (i) of the pavement and to the rainfall intensity: H = 0.015*(√LI/5√i); H = 0.015*(√cLI/5√i); c = coefficient for the capacity of infiltration within the various kinds of new overlayers. R. Laganier/1977 Hr = a · t-b, where Hr is the height of water brought to its maximum value at equilibrium, t is time in seconds and a, b are coefficients affecting drainage. F. Giannini and A. Noli/ 1974, L. Domenichini and G. Remedia/1994 Theoretical models (not provided in this reference) Franklin Institute, 1972, T. J. Jackson and M. R. Ragan, 1974 Improved understanding of rain water discharge through porous pavements, model of full depth porous pavements Ranieri differentiates porous pavements into two main types: full depth porous pavements and pavements in which only the wearing course is porous. He mentions that both types of pavements can help in avoiding the detrimental effects of water on the road surface by storing and draining water, with drainage being the predominant mechanism in open-graded friction courses. Ranieri then presents the assumptions of his model of rainwater flow through porous pavements. He contends that rain water flow through porous pavements can be studied as an unconfined aquifer moving within a homogeneous porous medium that lies over an inclined impervious layer and is subjected to constant replenishment along the flow path. He formulates the problem as one of finding a solution given the specific boundary conditions. Citing his work on theoretical groundwater flow model specific for porous roads, Rainier mentions that this paper presents the latest studies with this model. Rainier then presents the model, solutions and charts based on the solution of the equation describing the model. For convenience, the equations, solutions and chart are shown in Figure 17. The chart relates Hmax/L to 4I/k for different slopes. Specifically, the chart gives the minimum thickness of the porous course so that rain water always discharges within it, if the design rainfall rate (I), the geometric characteristics of the road (longitudinal gradient, cross slope and width) and the permeability k of the porous asphalt are provided.

182 Figure 17. Model and chart from Ranieri’s work

183 Ranieri also provides details of a laboratory experiment carried out to evaluate his model. Ranieri conducted tests with three different materials that he had used originally - one porous asphalt and two unbound aggregates. Each material was tested for different values of slope i and rainfall intensity I; the piezometric heads along the flow path were observed and the corresponding free water surfaces were plotted. In addition each of these plots was compared to the corresponding surfaces mathematically obtained by the application of the model. The properties of the materials are shown in Table 95. Table 95: Properties of One Porous Mix and Two Unbound Aggregates Material Dmax, mm Porosity, % Permeability, cm/s A (porous mix?) 0.40 41 0.025 B Not applicable 22 0.10 C 0.15 42 5 The testing/simulating device consisted of a rain simulator positioned over a 1450 mm x 750 mm reclining basin containing the porous pavement material to be tested. The thickness of this material can be up to 70 mm. Water table depth can be assessed with seven piezometers fixed at the bottom of the basin. Ranieri indicates that the experimental results fit well with the predicted results (from the model) if a coefficient β is introduced in the theoretical model. The theoretical background of this coefficient, as postulated by Rainier, is that this coefficient is needed to compensate for the fact that the flow during the experiment is not laminar but of transitional nature. Two important points are that the coefficient is not constant, since in this case there is constant replenishment of flow from the top, and that the sole coefficient, β, is sufficient for adjusting the theoretical model. Ranieri then provide details of experiments carried out to assess the value of the coefficient β. These experiments were carried out with three unbound materials. The properties of these materials are shown in Table 96. From these tests, Rainier provides an equation relating β to i, slope and I/Kd, where I is the rate of rainfall (intensity) and KD is the Darcy’s permeability (coefficient of permeability). Rainier then provides a design example to use the chart to determine minimum thickness of a porous pavement. This example is shown in Figure 18. Table 96: Properties of One Porous Mix and Two Unbound Aggregates Material Dmax, mm Porosity, % Permeability, cm/s S 0.85 44 0.015 G 5 46 2.31 P 10 47 4.8 1.44.9 Limitations No information is provided on limitations of use.

184 Figure 18. Design example