CHAPTER 4: ANALYSIS OF THE INFLUENCING FACTORS ON SKID
4.2 Effect of Porous Surface Layer on Skid Resistance Performance
Porous pavements have been extensively used worldwide to improve the skid resistance and reduce the accident occurrences in wet weather. These benefits mainly result from their capability to drain water off the pavement surface and hence achieve better frictional performance during wet weather. Most of the past research studies on the effectiveness of porous pavements in improving skid resistance and restraining hydroplaning are experimental or empirical in nature (Deuss, 1994; Huddleston et al., 1991; Isenring et al., 1990). It is essential to mechanistically compare and analyze the skid resistance performances of porous and non-porous pavements under identical wet weather condition. This section presents a study to analyze the overall effect of a porous surface layer on skid resistance and interpret how porous pavements provide friction control during wet weather. An illustrative case study is investigated to quantify and compare the skid resistance performances between porous and non- porous pavements, with considerations of the influence of pavement surface type on rain water accumulation.
4.2.1 Description of Hypothetical Problem
Numerous experimental studies in literature (Isenring et al., 1990; Kowalski et al., 2009; McGhee and Clark, 2010; Dell'Acqua et al., 2012; Ivan et al., 2012) have indicated that porous pavements tend to have better skid resistance performance compared to conventional dense-graded pavements. However, two critical questions remain unanswered from past experimental studies: (a) how does porous pavement improve skid resistance? and (b) what is the expected skid number increase when a porous layer is used? The first question is aimed at understanding the mechanisms of
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skid resistance on porous pavements and how the improvement in skid number comes about, while the second question aims to determine quantitatively the improvement in skid resistance performance brought forth when using porous pavements instead of non-porous pavement surfaces.
In order to answer the above questions, the developed analysis framework in Section 4.1 could be applied to quantify and compare the skid resistance performance between porous and non-porous pavements. The following hypothetical case study is analyzed to illustrate the differences in skid resistance behavior on porous and non- porous pavements:
Case I: A dense-graded smooth asphalt pavement on a two-way four-lane tangent section with a 2% cross slope and 15 m road width. The pavement surface course is assumed to be impermeable, with a macrotexture of 0.5 mm mean texture depth (MTD = 0.5 mm). The wet friction coefficient of the tire-pavement interface is assumed to be 0.5 (SN0 = 50, μ = 0.5).
Case II: A porous friction course overlaid on a dense-graded asphalt pavement on a two-way four-lane tangent section with a 2% cross slope and 15 m road width. The porous layer thickness is 50 mm with a porosity of 20%. In this study, a hydraulic conductivity of 5.6 mm/s at 20% porosity is assumed based on the past experimental studies (Chuai, 1998; Tan et al., 1999). The wet friction coefficient of tire-pavement interface for this case is also assumed to be 0.50 for comparison basis.
The lock-wheel skid resistance test in accordance to the ASTM E274 method (ASTM, 2011a) is simulated using the proposed numerical model. The ASTM E524 smooth tire (ASTM, 2008b) is considered to be used with a tire inflation pressure of 165.5 kPa and a wheel load of 4800 N. Skid tests are assumed to be performed under different speeds (40 km/h, 60 km/h and 80 km/h) and rainfall intensities (60 mm/h, 150 mm/h and 300 mm/h).
153 4.2.2 Results and Discussions
The numerical analysis results of water film accumulation and skid number prediction on porous and non-porous pavements are presented and compared in this section. The skid number improvement is quantified and discussed as well. The mechanisms of skid resistance enhancement on a porous pavement is analyzed based on comparisons in fluid force development, tire deformation and tire-pavement contact status.
4.2.2.1 Reduction of Water Film Thickness on Porous Pavements
A direct consequence of the porous pavement applications is the reduction in water accumulation on pavement surfaces during wet weather. Figure 4.3 compares the water film thicknesses on Case I and Case II pavements for different rainfall intensities, which were obtained from the PAVDRN software (Anderson et al., 1998).
It can be seen from the figure that water film thickness increases as the rainfall intensity increases on both porous and non-porous pavements. More importantly, water film thickness is higher for impermeable surface than for porous pavement under the same rainfall intensity and the difference is larger at higher rainfall intensity.
For Case II, rain water does not start to accumulate on porous pavement surface until the rainfall intensity is higher than a threshold level of about 30 mm/h. The reduction in water-film thickness on porous pavement is highly associated to its drainage capability and may have a significant impact on its frictional performance in wet weather. These computed water-film thicknesses serve as input parameters to the skid resistance simulation model.
4.2.2.2 Quantifying Skid Resistance Performance
Using the water film thicknesses presented in Figure 4.3 at a specific rainfall intensity, the skid numbers at different sliding speeds can be computed using the skid resistance simulation model developed in Chapter 3. Figure 4.4 compares the skid
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numbers on Case I and Case II pavements at different speeds and rainfall intensities.
The following observations can be made from this figure:
For Case I, there is a significant decrease in skid number with an increase in sliding speed or an increase in rainfall intensity. At a rainfall intensity of 150 mm/h, the loss in skid number is as high as 57% when the sliding speed increases from 40 to 80 km/h. At a speed of 60 km/h, the loss in skid number is as high as 19% when rainfall intensity increases from 60 to 300 mm/h. These findings indicate that non- porous pavements are highly susceptible to low skid resistance at high sliding speeds and high rainfall intensities.
For Case II, the decrease in skid number with an increase in sliding speed or an increase in rainfall intensity is relatively small (less than 5 SN units). At a rainfall intensity of 150 mm/h, the loss in skid number is only 6% when the speed increases from 40 to 80 km/h. At a 60 km/h sliding speed, the loss in skid number is a mere 3% when the rainfall intensity increases from 60 to 300 mm/h. Such a small variation in skid number may have given researchers such as Isenring et al. (1990) and McDaniel et al. (2004) the impression that the skid number of porous pavements is “hardly speed-dependent”.
Case II has a consistently higher skid number than Case I for an identical rainfall intensity and sliding speed. The percentage increase in skid number achieved by applying the porous pavement technology ranges between 15% and 215% for the different speeds and rainfall intensities tested in this study and the difference is more pronounced at higher sliding speeds and higher rainfall intensities. This demonstrates that porous pavements can provide superior skid resistance performance in wet weather.
Based on the above findings, it can be seen that Case II is better than Case I in providing high-speed skid resistance and hence ensuring better braking and directional control in wet-weather traffic operations. The benefit can reach up to more
155 than 200% skid number at 80 km/h sliding speed and 300 mm/h rainfall intensity, according to the presented numerical computations.
4.2.2.3 Contribution of Forces Acting on Tire to Skid Resistance
In order to achieve a fundamental understanding on why porous pavement exhibits a higher skid number than non-porous pavement under the same wet-weather condition, there is a need to examine the major force components acting on the tire in the presence (or absence) of a porous surface course. Figure 4.5 compares the normal contact forces, the fluid uplift forces and the wheel loads acting on the tire for Cases I and II at different traveling conditions. It is clearly observed that the porous surface results in a lower fluid uplift force compared to the dense-graded pavement under the same condition. This may be due to the presence of additional drainage paths within the porous layer, reducing the hydrodynamic pressure build-up underneath the sliding tire. As a consequence, a higher contact reaction force is observed for Case II as opposed to Case I.
Another interesting noteworthy point in Figure 4.5 is that for Case I, the fluid uplift force is equal to the wheel load at about 84 km/h under a rainfall intensity of 300 mm/h. At this point, hydroplaning is said to occur and the tire is separated from the pavement surface by a thin film of water. This hydroplaning speed of 84 km/h corresponds well to that computed by the well-known NASA hydroplaning equation (Horne and Dreher, 1963). For Case II, however, the tire will remain in contact with the pavement surface for the whole practicable range of highway operation speeds.
This indicates that adopting of porous surfaces can effectively prevent the occurrence of hydroplaning even at an extreme rainfall intensity of 300 mm/h.
As a result of tire-water-pavement interaction, the skid resistance develops from the traction force at the tire-pavement interface and the drag force at the tire- fluid interface. The former is proportional to tire-pavement contact force, and the latter is the fluid drag force computed from the numerical simulation model. Figure
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4.6 illustrates the percentage contributions of traction force and fluid drag force to skid resistance. It is obvious that, for Case I, fluid drag force becomes the dominant component of skid resistance beyond 73 km/h sliding speed at a 300 mm/h rainfall intensity, and at the hydroplaning speed of 84 km/h, a vehicle will only be experiencing the fluid drag (since the wheels are “lifted” off the pavement surface).
For Case II, it was noted that even at high sliding speeds, traction force is still the main component of skid resistance. This stark contrast indicates that a major reason why porous pavements can maintain the excellent skid numbers at high speed is to keep fluid uplift and drag forces low while maintaining the contact between tire tread and pavement surface and relying on the traction forces resulting from the tire- pavement contact. This is further demonstrated in Figure 4.7 where it can be seen that at high speed, there is a significant tire deformation causing loss in tire-pavement contact for Case I as compared to Case II at the same rainfall intensity.
From the above case study, it can be concluded that porous pavement is an effective engineering measure to improve wet pavement skid resistance. The superior skid resistance performance of porous pavements is mainly due to its higher porosity and inner drainage capacity. Such characteristics result in a significant reduction in fluid uplift force, providing better contact between tire tread and pavement surface even at extremely high speeds. This develops higher traction force, which dominates the skid resistance on porous pavements. Resulting from the same mechanisms, porous pavements can also prevent the occurrence of hydroplaning at highway operational speed.