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CHAPIER 5 SI~MARY, FINDINGS, AND RECOMMENDATIONS SUGARY The primary objective of this research was to identify unproved methods for draining rainwater from the surface of multi-lane pavements and to develop guidelines for their use. The guidelines, along with details on the rationale for their development, are presented in a separate document' "Proposed Design Guidelines for Improving Pavement Surface Drainage" (2J. The guidelines support an interactive computer program, PAVDRN, that can be used by practicing engineers In the process of designing new pavements or rehabilitating old pavements' is outlined In figure 39. The intended audience for the guidelines is practicing highway design engineers that work for transportation agencies or consulting firms. Improved pavement surface drainage is needed for two reasons: (~) to minimize splash and spray and (2) to control the tendency for hydroplaning. Both issues are primary safety concerns. At the request of the advisory panel for the project, the main focus of this study was on ~mprov~g surface drainage to mammae the tendency for hydroplaning. In terms of reducing the tendency for hydroplaTuT g, the needed level of drainage is defined in terms of the thickness of the film of water on the pavement. Therefore, the guidelines were developed within the context of reducing the thickness of the water film on pavement surfaces to the extent that hydroplaning is unlikely at highway design speeds. Since hydroplaning is ~7

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DESIGN CRITERIA Pavement Geometry Number of lanes Section type - Tangent - Horizontal curve - Transition - Vertical crest curve - Vertical sag curve Enviromnental oramaters Rainfall intensity ~ Temperature Pavement Tvpe Dense-graded asphalt Porous asphalt Portland cement concrete ~ Grooved Portland cement concrete Desion Soeed Allowable speed for onset of hydroplaning Recommend Desion Changes Alter geometry Alter pavement surface Add appurtenances Groove (Portland cement concrete) CALCULATIONS Lenoth of flow path Calculate on basis of pavement geometry IT Hydraulic Analvses . No? Water film thickness Equation No. 10 Equation No.'s. 16-19 1 Hvdroolanino Analvsis Hydroplaning speed Equation No.'s 21-24 Rainfall Intensity Equation No. 25 -A I / Meet Design ~ \ Cntena? / \ OCR for page 117
controlled primarily by the thickness of the water film on the pavement surface, the design guidelines focus on the prediction and control of ache depth of water flowing across the pavement surface as a result of rainfall, often referred to as sheet flow. Water film thickness on highway pavements can be controlled In three fundamental ways, by: I. Minimizing the length of the longest flow path of the water over We pavement and thereby the distance over which the flow can develop; 2. Increasing the texture of the pavement surface; and 3. Removing water from the pavement's surface. In the process of using PAVDRN to implement the design guidelines, the designer is guided to (~) minimize the longest drainage path length of the section under design by altering the pavement geometry and (2) reduce the resultant water film thickness that will develop along that drainage path length by increasing the mean texture depth, choosing a surface that maximizes texture, or using permeable pavements, grooving, and appurtenances to remove water from the surface. Through the course of a typical design project, four key areas need to be considered in order to analyze and eventually reduce the potential for hydroplaning. These areas are: ~9

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I. Environmental conditions: 2. Geometry of the roadway surface; 3. Pavement surface (texture) properties; and 4. Appurtenances. Each of these areas and their influence on the resulting hydroplaning speed of the designed section are discussed In detail In the guidelines (21. The environmental conditions considered are rainfall ~ntensibr and water temperature, which determines the kinematic viscosity of the water. The designer has no real control over these environmental factors but needs to select appropriate values when analyzing the effect of flow over the pavement surface and hydroplaning potential. Five section types, one for each of the basic geometric configurations used In highway design, are examined. These section are: 1. TaIlgent; 2. Superelevated curve; 3. Transition; 4. Vertical crest curve; and 5. Vertical sag curve. 120

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Pavement properties that affect the water fihn thickness mclude surface characteristics, such as mean texture depth and grooving of Portland cement concrete surfaces, are considered In the process of applying PAVDRN. Porous asphalt pavement surfaces can also reduce He water film thickness and thereby contribute to the reduction of hydroplaning tendency and their presence can also be accounted for when using PAVDRN. Finally, PAVDRN also allows the design engineer to consider the effect of drainage appurtenances, such as slotted drain inlets. A complete description of the various elements that are considered In the PAVDRN program is illustrated In figure 40. A more complete description of the design process, the parameters used in the design process, and typical values for the parameters is presented In the "Proposed Design Guidelines for Improving Pavement Surface Drainage" (2) alla in Appendix A. fIN1)INGS The following findings are based on the research accomplished during the project, a survey of the literature, and a state-of-the-art survey of current practice. I. Model. The one~unensional mode} is adequate as a design tool. The simplicity and stability of the one~imensional mode} offsets any increased accuracy afforded by a two-d~mensional model. The one~mensional model as a predictor of water fiDn thickness and How path length was verified by using data from a previous study (11). 121

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No. of Planes Length of Plane Grade Step Increment Wdth of Plane Cross Slope Section T,rne 1) Tangent 2) Honzontal Curare 3) Transition 4) Vertical Crest 5) Vertical Sag U=tS 1)U.S. 2) S. I. Rainfall Intenstity ~ , \ |Kinematic Viscosity |Design Speed Note: PC = Point of Curvature PI. = Point of Tangency PCC = Portland cement concrete WAC = Dense graded asphalt concrete 0GAC = 0pcn~raded asphalt concrete where OGAC includes all types of intentally draining asphalt surfaces GPCC = Grooved Ponland cement concrete Taneent Pavement Type Mean Texture Depth 1) PCC 2) DGAC 3) OGAC 4) GPCC Horizontal Cun~c Grade Cross Slope Radius of Cunran~re Wdth Pavement Type _ 2) DGAC 3) OGAC 4) GPCC Mean Texture Depth Step Increment _ Transition Length of Plane Super Elevation Tangent Cross Slope Tangent Grade width of Curve Transition Width Pavement Type_ 1) PCC 3) OGAC 4) GPCC Mean Texture Depth Step Increment Horizontal Length Cross slope width PC Grade PI' Grade Elevation: Pr-PC Vertical Crest Flow Direction Step Increment Pavement Type 1) PC Side I 2) PI. Side | 1)PCC 2) DGAC 3) OGAC 4) GPCC Mean Tex~rc Depth _ _ ~ Figure 40. Factors considered in PAVDRN program. 122 ~1 r - . , Vertical Sad | Horizontal Length | Cross slope Wldth PC Grade PI Grade Elevation: PIE Flow Direction Step Increment / Stored :_ ~ cats ~ 1) PC Side | 2) PI Side | . Pavement Typed 1) PCC 3) OGAC 14) GPCC Mean Texture Depth I I

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~ Stored data V ~ 3 L IN1T For use with a second nut using data from the first run.) , 1 EPRINT (Echos input to output ) 1 CONVERT (Converts units to and from SI and English.) ~ , ADVP (Advances Page of output.) KINW (Calculates Minning's n, Water Film Thickness (WEIR), and Hydroplaning Speed UPS).) , EDGE (Determines if flow has reached the edge of the pavement.) out roar Figure 40. Factors considered in PAVDRN program (continued). 123

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2. Occurrence of Hydropl~r g. In general, based on the PAVDRN mode! and the assumptions inherent in its development, hydroplaning can be expected at speeds below roadway design speeds if the length of the flow path exceeds two lane widths. 3. Water Film Thickness. Hydroplaning is initiated primarily by the depth of the water film thickness. Therefore, the primary design objective when controlling hydroplaning must be to limit the depth of the water film. 4. Reducing Water Film Thickness. There are no simple means for controlling water John thickness, but a number of methods can effectively reduce water film thickness and consequently hydroplaning potential. These include: Optimizing pavement geometry, especially cross-slope. Providing some means of additional drainage, such as use of grooved surfaces (PCC) or porous mixtures (HMA). Including slotted drains within the roadway. 5. Tests Needed for Design. The design guidelines require an estimate of the surface texture (MTD) and the coefficient of permeability Porous asphalt only). The sand patch is an acceptable test method for measuring surface texture, except for the more open (20-percent air voids) porous asphalt mixes. In these cases, an estimate of the surface texture, based on tabulated data, is sufficient. As an alternative, 124

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sand patch measurements can be made on cast replicas of the surface. For the open mixes, the glass beads flow into the voids within the mixture, giving an inaccurate measure of surface texture. Based on the measurements obtained In the laboratory, the coefficient of permeability for the open-graded asphalt concrete does not exhibit a wide range of values, and values of k may be selected for design purposes from tabulated design data (k versus air voids). Given the uncertainty of this property resulting from compaction under traffic and clogging from contaminants and anti-skid material, a direct measurement (e.g., drainage lag permeameter) of k is not warranted. Based on the previous discussion, no new test procedures are needed to adopt the design guidelines developed during this project. 6. Grooving. Grooving of PCC pavements provides a reservoir for surface water and can facilitate the removal of water if the grooves are placed parallel to the flow oath. Parallel orientation is generally not practical because the flow on highway pavements is typically not transverse to the pavement. Thus, the primary contribution offered by grooving is to provide a surface reservoir unless the grooves comlect with drainage at the edge of the pavement. Once the grooves are filled with water, the tops of the grooves are the datum for the Why and do not contribute to the reduction in the hydroplaning potential. 125

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7. Porous Pavements. These mixtures can enhance the water removal and Hereby reduce water film tHch~ess. They merit more consideration by highway agencies In the United States, but they are not a panacea for eliminating hydroplaning. As with grooved PCC pavements, the internal voids do not contribute to the reduction of hydroplaning; based on the field tests done In this study. hv~ronImiina can be if, , , ~ expected on these mixtures given sufficient water fiLn thickness. Other than their ability to conduct water through internal flow, the large MTD offered by porous asphalt is the main contribution offered by the mixtures to the reduction of hydroplaning potential. The high-void ~ > 20 percent), modified binder mixes used In Europe merit further evaluation in the United States. They should be used In areas where damage from freezing water and the problems of black ice are not likely. 8. Slotted Drains. These fixtures, when installed between travel lanes, offer perhaps the most effective means of controlling water film thickness from a hydraulics standpoint. They have not been used extensively In the traveled lanes and questions remain unanswered with respect to their installation (especially in rehabilitation situations) and maintenance. The ability to support traffic loads and still maintain surface smoothness has not been demonstrated and they may be susceptible to clogging from roadway debris, ice, or snow. 126

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RECOMMENDATIONS AND CONCLUSIONS The following recommendations are offered based on the work accomplished during this project and on the conclusions given previously: I. Implementation. The PAVDRN program and associated guidelines need to be field tested and revised as needed. The program and the guidelines are sufficiently complete so that they can be used in a design office. Some of the parameters and algorithms will I~ely need to be modified as experience is gained with the program. 2. Database of Material Properties. A database of material properties should be gathered to supplement the information contained in PAVDRN. This information should Include typical values for the permeability of porous asphalt and topical values for the surface texture (MTD) for different pavement surfaces to include toned Portland cement concrete surfaces. A series of photographs of typical pavement sections and their associated texture depths should be considered as an addition to the design guide (21. 3. Pavement Geometry. The AASHTO design guidelines (~) should be re-evaluated In terms of current design criteria to determine if they can be modified to enhance drainage without adversely affecting vehicle handling or safety. ~27

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4. Use of appurtenances. Slotted drams should be evaluated In the field to determine if they are practical when Installed In the traveled way. Manufacturers should reconsider the design of slotted drains and their Installation recommendations currently In force to maximize them for use In multi-lane pavements and to determine if slotted drains are suitable for installations In the traveled right of way. 5. Porous Asphalt Mixtures. More use should be made of these mixtures, especially the modified high a~r-void mixtures as used In France. Field trials should be conducted to monitor HPS and the long-term effectiveness of these mixtures and to validate the MPS and WDT predicted by PAVDRN. 6. Two-D~mensional Model. Further work should be done with two~mensional models to determine if they improve accuracy of PAVDRN and to determine if they are practical from a computational standpoint. ADDITIONAL STUDIES On the basis of the work done during this study, a number of additional items warrant furler study. These Include: 1. Full-scale skid resistance studies to validate PAVDRN in general and the relationship between water film thickness and hydroplaning potential in particular are needed in light of the unexpectedly low hvdronlanin~ speeds predicted during 128 , . ~. , ~

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this study. The effect of water infiltration into pavement cracks and loss of water by splash and spray need to be accounted for In the prediction of water fihn Sickness. Surface Irregularities, especially rutting, need to be considered in the prediction models. 2. Field trials are needed to confirm the effectiveness of alternative asphalt and Portland cement concrete surfaces. These include porous Portland cement concrete surfaces, porous asphalt concrete, and various asphalt m~cro-surfaces. 3. The permeability of porous surface mixtures needs to be confirmed with samples removed from the field, and the practicality of a simplified method for measuring in-situ permeability must be investigated and compared to alternative measurements, such as the outflow meter. 4. For measuring pavement texture, alternatives to the sand patch method should be investigated, especially for use with porous asphalt mixtures. 129

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