2.1 Wet-Pavement Skid Resistance
2.1.5 Skid Resistance on Porous Pavements
From past research on wet-pavement skid resistance, pavement surface macrotexture has consistently been identified to be critical to the frictional performance at high speed (Highway Research Board, 1972). It governs the drainage capacity of pavement surfaces under a skidding tire. Various engineering measures have been developed to enhance the macrotexture and drainage capacity of pavement surface. This includes tining, grooving, chip seal and porous friction course. Porous pavement, whose high porosity facilitates the drainage of water from tire-pavement interface, is found to be effective in improving pavement skid resistance performance in wet weather (FHWA, 1998).
Porous pavement applications in the U.S. primarily attempt to reduce road accidents in wet weather. Numerous comparison studies have shown the superiority of porous pavements in skid resistance. Moyer (1959) conducted experiments on over 300 pavement sections in California and compared the friction levels of open-graded asphalt surfaces to those of dense-graded asphalt surfaces. It was found that open- graded mixtures had a skid resistance level 10% to 20% higher than that of dense- graded asphalt. In another experimental study sponsored by the Federal Highway Administration, 45 in-service pavement sections with various surface types were tested (Page, 1977). It was found that open-graded asphalt pavements performed better in skid resistance than other pavement types under heavy or medium traffic conditions. Dynamic friction tester was used in a field evaluation in the State of Indiana (McDaniel et al., 2004) to compare the wet frictional performances among porous friction course (PFC), stone matrix asphalt (SMA) and conventional hot mix asphalt (HMA). It was observed that the average DFT value of SMA was lower than that of PFC or HMA, and the higher friction of PFC might have resulted from its higher macrotexture. Furthermore, the long-term frictional performance of Indiana PFC pavements was monitored as well (Kowalski et al., 2009). Both DFT and lock-
31 wheel trailer were used in the measurements. It was found that the long-term skid resistance performance of PFC was much better than that of conventional HMA.
Superior skid resistance on porous pavements was also widely observed in other research studies, such as the FHWA demonstration project “Improved Skid Resistant Pavements” (Pelletier, 1976), the performance evaluation of open-graded asphalt pavements in Oregon (Huddleston et al., 1991), the experiments in Arizona using mu- meters (Hossain et al., 1992), the studies on pavement surface characteristics at Virginia Smart Road (Davis, 2001) , the development of functionally optimized PFC (McGhee et al., 2009) and the application of thin-lift asphalt surface courses in New Jersey (Bennert et al., 2005).
Besides the higher skid resistance at specific speeds, another benefit porous pavements can provide to the road users is the relatively speed-independent frictional performance. Porous pavements were involved in the study of skid number-speed gradients in Colorado (Steere, 1976). The test results provided solid evidences of a significantly less degree of skid resistance deterioration with increasing travel speed on Colorado Type-A open graded plant mix seal coats. Gallaway et al. (1979) found that skid resistance performance of open-graded friction courses was less sensitive to vehicle speed when compared to lightweight aggregate slurry seal and longitudinal grooved concrete pavements. Lock-wheel trailer was used in Florida to measure skid resistance on various types of pavements at different speeds (Page, 1993). The results showed that frictional characteristics remained relatively stable with speed variation on open graded friction courses (OGFC). Friction tests performed by Younger et al.
(1994) evaluated the performance of porous pavements used in Oregon and found that the wet friction numbers of porous pavements were retained better at higher speeds than those on conventional dense-graded pavements.
Isenring et al. (1990) also studied the skid resistance variation with vehicle speed on porous pavements in Switzerland, measuring pavement friction properties by Skiddometer BV8. Both lock-wheel and brake-wheel (with a 14% slip ratio) tests
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were conducted using a ribbed PIARC skid test tire. Several useful conclusions were drawn from this study: (a) friction values of porous asphalt pavements are hardly speed dependent; (b) skidding properties of porous asphalt is poorer than that of conventional mixtures at lower speeds, where microtexture is more relevant and the function of macrotexture is insignificant; (c) at higher speed, skid resistance of porous pavement is generally higher compared to that of dense-graded surface, due to its higher macrotexture. Although lesser speed dependency was observed in the porous pavement skid resistance when compared to that on conventional pavement, the friction values do decrease with the increase of vehicle speed (McDaniel et al., 2004).
With the consensus that porous pavements are capable to maintain a superior wet skid resistance at high speeds, research efforts were made to relate the frictional performance to various influencing factors. Correlation between British pendulum number and skid trailer measurement on open-graded course was proposed by Mullen (1972) in his research to predict field skid numbers using laboratory test BPN results.
It was found that the relationship between SN and BPN of porous pavement was different from that of dense-graded mixture. Seasonal variation of skid resistance on open-graded asphalt pavements was studied using the lock-wheel method in Pennsylvania (Dahir et al., 1979) and a model involving season and weather effects was proposed. Variables considered in this model include rainfall during the previous week, tire and pavement temperature, time in a year and polishing susceptibility and polishing rate of aggregates. McDaniel et al. (2004) indicated that the superior skid resistance of porous friction course may be a result from its higher mean profile depth (MPD), which is an indicator of surface macrotexture level. However, Jackson et al.
(2005) showed that there was no clear relationship between MPD and friction number, and macrotexture alone was not a good predictor for overall friction property. It was also found that aggregate gradation has a significant influence on wet skid resistance performance of porous pavements (Xing et al., 2010).
33 A major adverse effect of porous surface on skid resistance identified from past studies is that a potential skidding hazard may exist on a porous pavement when it is opened to traffic immediately after construction (Younger et al., 1994; Liu et al., 2010; McGhee and Clark, 2010). The lock-wheel test results from the experiments conducted by Isenring et al. (1990) clearly showed that the skid resistance on porous asphalt was relatively lower in the initial stage after construction and increased after the first winter. The generally poorer microtexture and thicker bitumen film on newly laid porous asphalt surfaces were believed to be the causes of such a phenomenon. A Dutch study by Deuss (1994) showed a 25% to 30% lower skid resistance on new porous pavements as compared to new dense-graded pavements . However, friction properties would improve after a certain time range (which can be from weeks to months, depending on the traffic condition) when the binder coating on pavement surface has worn off. These studies also suggested that aggregates with high anti- polish properties and good angular shapes should be used in porous asphalt to provide satisfactory microtexture and macrotexture.
Besides the above mentioned research studies, numerous studies on skid resistance on porous pavements were perfomed in other countries, such as Canada (Ryell et al., 1979), Japan (Yoshiki et al., 2005) and Spain (Miró et al., 2009).
Moreover, the application of porous surfaces to enhance skid resistance has been studied on runways (Brownie, 1977) and bridge deck pavements (Brewer, 1971).
Although the aspect of porous pavement skid resistance had been studied in past research, applications in the field are still experience-based. There is still a lack of understanding in porous pavement skid resistance mechanisms and how factors can influence skid resistance on porous pavement. This posts difficulties in the efficient design, construction and maintenance of porous pavements and hence limits their applications.
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