CHAPTER 4: ANALYSIS OF THE INFLUENCING FACTORS ON SKID
4.3 Effect of Influencing Factors on Porous Pavement Skid Resistance
The effects of some critical factors on the skid resistance of porous pavements are quantitatively analyzed using an illustrative hypothetical case study.
The parameters being examined include the porosity, porous layer thickness, rainfall intensity and sliding speed. The analysis considers the interaction between different
157 influencing factors. The hypothetical problem is first defined based on a porous asphalt overlay project. The skid resistance performance on porous pavements with various design parameters are then evaluated using the developed simulation model and the numerical results are compared and analyzed for each factor. Some recommendations on porous pavement design are made based on the conclusions drawn from the parametric study.
4.3.1 Description of Hypothetical Problem
A hypothetical problem is defined for illustration purpose in this study. It is assumed that a two-way four-lane crowned tangent highway section with a total width of 15 m is considered to be rehabilitated using a porous asphalt overlay. The existing impermeable dense-graded pavement surface has a longitudinal grade of 0%
and a cross slope of 2%. The overlaid surface will preserve the same geometric features as the old pavement. An identical wet tire-road friction coefficient of 0.5 (SN0 = 50) is assumed for all the porous surface candidates for comparison basis since the asphalt binder and aggregates are assumed to be from the same sources for all porous mixtures. Alternative porous overlay designs with different porosities and thicknesses are considered in design period with focus on their frictional performance.
The skid numbers on these porous surfaces under different rainfall intensity levels at different vehicle sliding speeds are predicted and analyzed using the developed numerical simulation model.
The levels of porous layer porosity used in this study cover the common porosity range of porous pavements found in practice. The typical porosity of a newly paved porous layer usually ranges from 18% to 22% (Alvarez et al., 2011). Higher porosity values of as much as 25% have also been used to enhance the safety benefits and reduce the tire-pavement noise (Alvarez et al., 2011). The porosity of porous pavement tends to reduce with time due to the clogging effect. To cater for the deterioration of porosity with time, some road agencies adopt a design terminal value
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of 15% for porosity (Liu and Cao, 2009). These considerations form the basis for selecting five porosity levels in this study, i.e. 15%, 17.5%, 20%, 22.5% and 25%.
The levels of porous layer thickness considered in this analysis cover the regular thickness range of porous surface course found in practice as well. It is noticed from the extensive review of porous pavement projects that most of porous layers used as overlays or wearing courses vary from 50 mm to 100 mm thick (Smith, 1992; Moore et al., 2001). There are also instances where thin lift of porous layers with a thickness of 25 mm are used (Smith, 1992). Moreover, porous surface layers in the United States are usually paved in an integer multiple of 1 inch (25.4 mm).
Therefore, four porous layer thicknesses are considered in this study, namely 25 mm, 50 mm, 75 mm and 100 mm.
The levels of rainfall intensity considered in this analysis have to be sufficiently large so that a significant thickness of free standing water film can form on the porous pavement surfaces. The upper bound of examined rainfall intensity is determined according to the meteorology of project location.. From past research studies on extreme rainfall intensities (Hershfield, 1984; De Toffol et al., 2009;
Langousis and Veneziano, 2009) and computations of water film thickness on porous pavements (Anderson et al., 1998), five rainfall intensity levels are involved in this study, i.e. 60 mm/h, 100 mm/h, 150 mm/h, 225 mm/h and 300 mm/h.
The range of vehicle sliding speeds considered in this analysis cover the common operating conditions encountered by passenger cars on the roadways in a raining weather. A 20 km/h speed step is selected in order to properly develop the speed-dependency of skid number on porous pavements. Five speed levels are included in this study, namely 20 km/h, 40 km/h, 60 km/h, 80 km/h and 100 km/h.
The skid resistance performance of the various porous overlays is evaluated and compared under the above defined conditions using the proposed analysis framework. The range of values examined for each variable are believed to be adequate for most porous pavement applications and common wet-weather operating
159 conditions. The simulation results are analyzed for each individual influencing factor, taking the interactions between different factors into consideration.
4.3.2 Influence of Porosity
As recognized from past experimental studies (Page, 1993; Younger et al., 1994), the superior skid resistance performance of porous pavements mainly results from their excellent drainage capacity. Rainwater on pavement surfaces can be easily discharged through the inter-connected pores and the surface macrotexture. It not only reduces the water film thickness on pavement surface, but also releases hydrodynamic pressure developing between tire tread and pavement surface. In most applications, the porous surface layer serves only as a functional surface course without structural contribution. Therefore, porous pavement design typically focuses on its drainage capacity if the primary purpose is to enhance skid resistance. From this perspective, the porosity of a porous surface layer is the most crucial factor closely related to its drainage capacity. The influence of porosity on the frictional performance of porous pavements is quantitatively analyzed in this section.
Based on the skid numbers computed by the developed numerical simulation model, Figure 4.8 is plotted to illustrate the influence of porous layer porosity on the skid resistance for different levels of porous layer thickness, rainfall intensity and vehicle speed, respectively. In general, higher skid numbers are seen on pavements with higher porosity levels, when all the other three parameters are held constant.
This is expected because more interconnected air voids are available when the porosity of a porous pavement increases, thereby resulting in more effective water discharge underneath the vehicle tires and enhancing the skid resistance performance of the porous pavement. It is noted from Figure 4.8 that skid number increases with porosity increase in an approximately linear manner. This indicates that skid resistance is constantly improved as porosity increases. Taking a closer look at the three plots, the following additional observations can be made from Figure 4.8:
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Figure 4.8(a) illustrates the variations of skid number with porosity for the four porous layer thickness levels under the same operation condition with a 150 mm/h rainfall intensity and an 80 km/h vehicle sliding speed. This plot could be used to examine the effect of porous layer thickness on the efficiency of porosity-induced skid resistance improvement. When the porosity increases from 15% to 25%, the skid number increases by 1.6, 1.9, 2.0 and 2.1 SN units respectively for 25, 50, 75 and 100 mm porous layer thickness. These results suggest that increasing porosity in a thicker porous surface layer produces better results in improving skid resistance.
Figure 4.8(b) examines the effect of increasing porosity under different rainfall intensities, for the case of 50 mm porous layer thickness and 80 km/h vehicle sliding speed. Skid resistance improvement resulted from the increasing porosity is found to be more effective at higher rainfall intensities. When porosity value increases from 15% to 25%, the skid number increases by 1.4 SN units at 60 mm/h rainfall intensity, and increases by 2.1 SN units at 300 mm/h rainfall intensity.
Figure 4.8(c) compares the skid number variation with porosity increase for different vehicle speeds under the travel condition of 150 mm/h rainfall intensity and 50 mm porous layer thickness. When the porosity value increases from 15% to 25%, the skid resistance improvement is 2.6 SN units at a vehicle speed of 100 km/h.
The corresponding increase at vehicle speed of 20 km/h is only 0.2 SN units. These results show that the skid resistance improvement due to increasing porosity is more efficient and more significant at higher sliding speed than at lower speed.
To summarize, the above observations indicate that wet skid number on porous overlay increases with increasing porous layer porosity and the enhancing effect of porosity becomes more significant in the case of thicker porous layer, higher rainfall intensity and higher vehicle speed. Therefore, porous mixture with a larger porosity value should be used for skid resistance enhancement, as long as the large air void content does not adversely affect other performance such as durability, revelling
161 resistance and rutting resistance. In addition to skid resistance improvement, larger porosity may also give benefits in splash/spray reduction and clogging resistance.
4.3.3 Influence of Porous Layer Thickness
In addition to porosity value, the thickness of porous surface layer is also a critical factor in porous pavement design. It affects the drainage capacity of porous pavement system. A thicker porous layer is capable to store more rainwater within its pores in raining weather before a significant water film appears on pavement surface.
A thicker porous layer may also provide a larger outlet area at roadsides to make the lateral drainage of water more efficient. The influence of porous layer thickness on porous pavement skid resistance performance is analyzed in this section based on numerical simulation results.
Figure 4.9 illustrates the skid numbers computed from numerical simulation models for pavements with different porous layer thicknesses. Skid number variations with porous layer thickness are plotted with respect to pavement porosity, rainfall intensity and vehicle speed in each sub-figure, respectively. The following trends are observed from these plots:
Generally, all the plots show increasing trends of skid number as porous layer thickness increases, although the rates of increase in skid number tend to diminish when porous surface becomes thicker. These increasing trends indicate that, all other parameters being equal, a pavement with thicker porous surface layer produces a superior skid resistance performance.
Figure 4.9(a) illustrates the relationships between skid number and layer thickness for porous overlay courses with different porosities at an identical operating condition of 150 mm rainfall intensity and 80 km/h vehicle speed. This plot actually interprets the same set of data as shown in Figure 4.8(a) from a different perspective.
It is observed that the increase of skid number with porous layer thickness is slightly higher on a surface with a larger porosity. At 25% porosity, the skid number increases
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by 2.4 SN units when the porous layer thickness increases from 25 to 100 mm, while at 15% porosity, the corresponding increase is 1.9 SN units.
In Figure 4.9(b), the variation of skid number with porous layer thickness at different rainfall intensities is presented. Vehicle speed and pavement porosity are fixed at 80 km/h and 20%, respectively. The trends of curves show that, provided all other conditions being equal, higher skid resistance improvement can be obtained at higher rainfall intensity when porous layer thickness increases. In the examined thickness range (i.e. 25 to 100 mm), skid number increases by 2.8 SN units at 300 mm/h rainfall intensity, and 1.6 SN units at 60 mm/h rainfall intensity.
Figure 4.9(c) shows the effect of vehicle sliding speed on the relationship between porous pavement layer thickness and skid resistance. In this plot, surface porosity and rainfall intensity are held constant at 20% and 150 mm/h respectively.
The skid resistance improvement through thickening porous surface layer is found to be more effective at higher skidding speed. When porous layer thickness increases from 25 mm to 100 mm, the skid number increases by 3.6 SN units at a vehicle speed of 100 km/h, but only increases by 0.2 SN units at 20 km/h.
To summarize, skid resistance on porous pavement increases with increasing porous layer thickness. The influence of porous layer thickness on skid resistance is found to be more significant in the case of larger porosity, higher rainfall intensity or higher vehicle speed. A thicker porous surface layer should be used to enhance skid resistance performance if the added materials do not reduce the cost effectiveness of the whole pavement structure. In addition to skid resistance improvement, larger porous layer thickness may also provide benefits in terms of reduction of splash/spray and improvement of urban runoff quality.
4.3.4 Influence of Rainfall Intensity
The present of rainwater on pavement surface is the essential cause of friction reduction in wet weather. Since water viscosity is much lower than the frictional bond
163 between tire rubber and aggregate, the water film serves as lubricant at the tire- pavement interface. The hydrodynamic pressure developed within the water film underneath a traveling tire is closely related to the water film thickness accumulated on pavement surface, which is directly determined by the rainfall intensity. Therefore, it is necessary to examine the skid resistance performance of porous pavements under different rainfall intensities in order to further understand the frictional properties of porous pavements. The influence of rainfall intensity on the skid resistance of porous pavements is analyzed in this section.
The results of numerical simulation analysis are summarized in Figure 4.10 to illustrate the influence of rainfall intensity on porous pavement skid resistance.
Each of these three plots shows the variation of skid number with rainfall intensity for different values of pavement porosity, porous layer thickness or vehicle speed. The following characteristics of the rainfall effect are observed:
All three plots in Figure 4.10 show the general decreasing trend of skid number with rainfall intensity. This is expected because for a specific porous surface, its drainage capacity is identical at various rainfall intensities. Heavier rainfall can cause a thicker water film on pavement surface, where more water may be trapped underneath the tires when a vehicle slides over, resulting in a reduced skid resistance performance. Furthermore, it is also observed that the skid number decreases with the increase of rainfall intensity in a concave manner. It declines more rapidly in the beginning as rainfall intensity rises from 60 mm/h, and tends to level off at higher rainfall intensities.
In Figure 4.10(a), the vehicle speed and porous layer thickness are kept constant (i.e. 80 km/h and 50 mm, respectively), while varying the pavement porosity.
Skid resistance appears to be affected marginally more by rainfall intensity at a lower porosity level. At a 15% porosity, skid resistance drops by 3.5 SN units when rainfall intensity increases from 60 to 300 mm/h; while at a 25% porosity level, the reduction is 2.8 SN units.
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In Figure 4.10(b), vehicle speed and pavement porosity are kept constant (i.e. 80 km/h and 20%, respectively), while varying the thickness of porous surface layer. It is seen that the largest fall of skid number with rainfall intensity occurs when the porous surface layer is the thinnest at 25 mm. The reduction of skid resistance is 3.7 SN units for 25 mm thick porous layer when rainfall intensity increases from 60 to 300 mm/h, while it is 2.5 SN units on the 100 mm thick layer. Similar decreasing rates of skid number with increasing rainfall intensity are observed on porous surfaces thicker than 50 mm.
In Figure 4.10(c), the pavement porosity and surface layer thickness are maintained constant (i.e. 20% and 50 mm, respectively), while varying the vehicle sliding speed. It is apparent from the plot that skid resistance reduction due to increasing rainfall intensity is more significant at high sliding speeds. The skid resistance performance is reduced by 4.9 SN units at a 100 km/h vehicle speed when the rainfall intensity increases from 60 to 300 mm/h, while it is only 0.1 SN units at 20 km/h under the same rainfall intensity increment.
In summary, the simulation results demonstrate that tire-pavement skid resistance is adversely affected by increasing rainfall intensity. The influence of rainfall intensity on porous pavement frictional performance is more severe when pavement surface porosity is lower, porous layer thickness is thinner, or when vehicle speed is higher.
4.3.5 Influence of Vehicle Speed
Vehicle sliding speed is another critical factor influencing porous pavement skid resistance. It may be the most important parameter in vehicle operation that has a significant impact on the wet-pavement skid resistance performance. In the standard lock-wheel measurement (ASTM, 2011a), test speed is specified as 40 ± 1 mph (65 ± 1.5 km/h). However, the operational traveling speeds on roads vary dramatically between different locations and vehicle types. The benefits in skid resistance
165 improvement obtained from applying porous pavements may differ significantly at different sliding speeds. The influence of vehicle sliding speed on porous pavement skid resistance is numerically investigated in this section.
The illustrative numerical simulation results are presented in Figure 4.11 to analyze the effects of vehicle sliding speed on tire-pavement skid resistance. These plots provide insights on how vehicle speed influence skid resistance when surface porosity, porous layer thickness, and rainfall intensity are varied. The following observations on skid number variations with vehicle sliding speed can be achieved:
In general, the three plots show that wet-pavement skid resistance on porous pavements reduces as vehicle speed increases. This observation is similar to that on dense-graded pavements (Fwa and Ong, 2008). It is understood that when a vehicle skids on road at a higher speed, there is more water to be discharged from tire-pavement contact patch in a certain time instant. More water may be trapped underneath the tire, resulting in a reduced skid resistance at higher speed. All the curves are convex in shape, indicating that skid resistance reduces faster with unit speed incremental at higher sliding speeds.
Figure 4.11(a) shows how surface layer porosity affects the decreasing trend of skid resistance with vehicle speed. The results are plotted for the case of 150 mm/h rainfall intensity and 50 mm porous layer thickness. It is found that the fall of skid number with speed is more severe on porous pavement with lower porosity.
When vehicle speed is raised from 0 to 100 km/h, the fall in skid resistance is 8.4 SN units at 15% porosity, and 5.8 SN units at 25% porosity.
Figure 4.11(b) shows how different porous layer thicknesses affect the decreasing trend of skid resistance with vehicle speed. The illustrated data are for the case of 20% porous layer porosity and 150 mm/h rainfall intensity. The plot indicates that larger skid resistance reduction is observed on the surface with a thinner porous course for a certain vehicle speed increase. With a 25 mm thick porous surface, the
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skid number reduces by 9.4 SN units when vehicle speed increases from 0 to 100 km/h; while with a 100 mm thick porous surface, the reduction is 5.8 SN units.
Figure 4.11(c) illustrates the effect of rainfall intensity on the relationship between skid resistance and vehicle speed. The plotted data are for the case of 20%
porous layer porosity and 50 mm porous layer thickness. The influence of vehicle speed on skid resistance is found more significant at higher rainfall intensity. The skid number is reduced by 8.4 SN units at a 300 mm/h rainfall intensity when vehicle speed increases from 0 to 100 km/h. For the same speed incremental, the skid number only drops by 3.5 SN units at the rainfall intensity of 60 mm/h.
Overall, the above findings show that skid number on porous pavements deteriorates as vehicle sliding speed increases. The effects of vehicle speed on tire- pavement skid resistance is more significant at lower pavement porosities, thinner porous layer thicknesses and higher rainfall intensities. Therefore, increasing the porosity and thickness of porous surface layers can help to reduce the speed- dependency of wet skid number and provide a more consistent frictional condition in wet-weather traveling.