2.1 Wet-Pavement Skid Resistance
2.1.4 Factors Affecting Wet-Pavement Skid Resistance
Numerous experimental studies on wet-pavement skid resistance have been conducted globally using various measurement devices and techniques (Wu and Nagi, 1995; Henry, 2000; PIARC, 2005; Woodside and Woodward, 2002). The majority of these research studies focused on analyzing the factors affecting pavement skid resistance performance. For simplification, these factors are broadly classified into four groups: (a) pavement surface characteristics; (b) tire and vehicle properties; (c) presence of lubricant; and (d) environment factors. The following sub-sections shall discuss in detail the influences of the critical factors affecting skid resistance.
2.1.4.1 Pavement Surface Characteristics
Early experiments had emphasized the comparison between frictional performances of different pavement types, for example Moyer (1933) and Stinson and Roberts (1933). Either the towed trailer or the stopping distance method was used
25 in past experimental studies (Moyer, 1935; Martin, 1939; Michael and Grunau, 1956;
Nichols et al., 1956; Clemmer, 1958; Wambold et al., 1986; Roe et al., 1998;
Choubane et al., 2003). It was found that pavement surface characteristics have profound effects on its frictional performance. It includes both surface texture and deterioration. Pavement surface textures can be classified into microtexture, macrotexture, megatexture and unevenness based on their wavelengths (ISO, 1997b;
PIARC, 1991). Microtexture and macrotexture (see Figure 2.11) are dominant factors in wet pavement skid resistance, while megatexture and unevenness result essentially in poor riding comfort (Wu and Nagi, 1995).
Microtexture
Microtexture refers to the irregularity measured on aggregate surface at micro scale, although the definition of its wavelength range is controversial among different standards (Forster, 1989; PIARC, 2005; ASTM, 2012b). Microtexture is a function of aggregate mineralogy under the conditions of weather, traffic and pavement aging (Kokkalis and Panagouli, 1998). It plays a fundamental role in the skid resistance by enhancing the adhesion effect. A harsh microtexture provides good friction condition, while a polished microtexture commonly gives poor frictional performance even at low speeds (Leland and Taylor, 1965). The mechanisms of microtexture effect are complex because it affects the molecular and electrical interaction between tire and pavement surface (Kummer, 1966). However, it is understood that the microtexture contributes to skid resistance at all speeds and in both dry and wet conditions.
Macrotexture
Macrotexture refers to the irregularity on pavement surface measured in the scale of millimeter, which is usually visible to naked eyes. It is determined by the mixture design, attributed to aggregate size, shape, angularity, spacing and gradation.
The main function of macrotexture in skid resistance is to provide necessary surface
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drainage to help dispel the water at tire-pavement interface (Martin, 1939; Giles, 1963). It also contributes to skid resistance by inducing deformations on tire tread to produce more hysteresis losses. Rougher macrotexture can make the decline of tire- pavement friction with increasing speed less rapid (see Figure 2.12), so that good skid resistance is maintained at high speed (Sabey, 1966; Highway Research Board, 1972).
Roughness
Pavement roughness, considered to be 50 mm and larger, covers the range of both megatexture (50 to 500 mm) and unevenness (0.5 to 50 m). It affects vehicle dynamics, ride quality, dynamic loading, and drainage (Wu and Nagi, 1995). Studies on traffic safety have noted that multiple-vehicle accidents increase as pavement roughness increases (Al-Masaeid, 1997; Cenek et al., 2004). It was also found that the dynamic changes in normal load with variable pavement roughness may result in skid resistance reductions. A recent experimental study conducted by Fuentes et al. (2010) presented a significantly lower skid resistance on relatively rougher pavement section even though the microtexture and macrotexture were similar. Further analysis showed that dynamic load coefficient (the ratio between standard deviation of dynamic load variation and static normal load) is more appropriate than international roughness index to represent the effect of roughness on skid resistance.
Pavement Deterioration
Most surface distresses not only affect the structural capacity of pavement but also can affect traveling safety. Aggregate polishing directly reduces pavement microtexture and surface wear reduces its macrotexture. Therefore, skid resistance diminishes at all travel speeds as a result of traffic polishing (Gandhi et al., 1991).
Bleeding occurs at high temperatures on bituminous pavements due to the excess asphalt. It reduces both macrotexture and microtexture, resulting in a lower skid resistance as well as higher hydroplaning risk. Rutting and pothole collect rainwater
27 to form thicker water films locally. This excess water may lead to hydroplaning and low skid resistance, as well as splash and spray.
2.1.4.2 Tire Properties and Vehicle Operation Condition
Tire properties and vehicle operation condition were found to significantly affect frictional performance. Patterned tire tread is an effective measure to discharge water from the contact patch, especially on the smooth pavements with poor macrotexture (Giles and Lander, 1956; Marick, 1959; Gengenback, 1968; Lander and Williams, 1968). Sufficient tread depth is required by legal provisions in many countries, such as a minimum tread depth of 1.6 mm specified in UK, and 4 mm in Germany (Woodside and Woodward, 2002). On wet pavements, worn tires cannot provide enough drainage capacity through their treads and may pose serious hazards to drivers and passengers. All the geometric features of tread pattern, such as groove depth, groove width and rib dimension, affect the frictional performance (Marick, 1959; Maycock, 1967; Kelly, 1968). Moreover, Williams and Meades (1975) and Dijks (1976) reported that truck tires normally provide remarkably lower skid resistance than passenger car tires.
Vehicle speed is another important factor known to affect wet-pavement skid resistance. It was reported that most of the wet-skid accidents happens at high speeds (Wambold et al., 1986). As shown in Figure 2.13, skid resistance decreases with an increase in vehicle speed on wet pavements. The early Iowa study (Moyer, 1933), the study at Michigan State University (Mercer, 1958) and the NACA study (Trant, 1959) all provided consistent observations with regard to the speed dependency of skid resistance on wet pavement surfaces.
According to Kummer and Meyer (1967), vehicle tires may be in one of the following three operation modes when running on the roads: skidding, slipping or rolling. The wheel slip condition, including slip ratio and slip angle, was also found to be influential on skid resistance (Moyer, 1933). The slip ratio (S) is defined as:
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t
S 100 (2.6)
where ω is the angular velocity of a rolling tire corresponding to the travel speed and ωt is the angular velocity of a slipping tire. As shown in Figure 2.14, the coefficient of friction grows initially with an increase of slip ratio until the peak value at critical slip and decreases afterwards until a lower value at locked wheel condition, namely 100%
slip (Wu and Nagi, 1995). The critical slip ratio was reported to be between 7% and 25% slip (Trant, 1959; Holmes, 1970) and can vary with speeds. A similar trend was found with an increase in slip angle, which is defined as the angle between the tire plane and the forward direction of vehicle motion (Gillespie, 1992).
2.1.4.3 Presence of Lubricant
Dry-pavement friction is commonly sufficient and hardly speed-dependent.
Moyer (1963) suggested a skid number of 8010 for the typical dry tire-pavement interfaces. However, significant reductions in skid resistance occur once contaminant such as water or oil is present on pavement surface (Moyer, 1933; Staughton and Williams, 1970; Delanne and Gothie, 2005). It is noticed that even a very thin film of water can cause a remarkable decrease in skid resistance, especially on surfaces with poor microtexture (Leland et al., 1968). As shown in Figure 2.13, at the same travel speed, a thicker water film will cause a greater decrease in skid resistance (Trant, 1959; Benedetto, 2002). Studies (Pelloli, 1976; Balmer and Gallaway, 1983; Veith, 1983) also found that the effect of water depth is more obvious at higher speed.
Staughton and Williams (1970) indicated that the influence of water depth on the reduction rate of skid resistance was significant in the first 4 mm water film thickness.
2.1.4.4 Environment Factors
Temperature and precipitation are two major environment factors affecting short-term skid resistance performance. Generally, with all the other variables being equal, a lower skid resistance can be expected at a higher temperature. Changes in
29 water and asphalt viscosities were suggested by Moyer (1959) as the main reason of decreasing friction with the increasing temperature. Another possible cause may be the lower hysteresis loss in a rubber tire at higher temperature (Giles and Sabey, 1959). Studies on precipitation-related variations of pavement friction indicated that the wet-pavement skid resistance is a bit lower after a dry period and higher after a rainstorm (Barry and Henry, 1981; Saito and Henry, 1983). The accumulation of dust and debris within pavement surface texture is believed to be the reason for such precipitation-related variations.
Pavement skid resistance has been found to vary with season as well. Studies showed that, on the same pavement section, a higher friction level can be expected in winter and spring, while a lower level in summer and autumn (Giles and Sabey, 1959;
Burchett and Rizenbergs, 1980; Kulakowski et al., 1990). The varying range can be as large as 25% across seasons (Gargett, 1990; Jayawickrama and Thomas, 1998).
Temperature and precipitation are believed to be two important reasons for the seasonal variations in skid resistance (Burchett and Rizenbergs, 1980). It was also observed that pavement microtexture is harsher in winter, providing higher adhesion effect in tire-pavement interaction (The Highways Agency, 2004).
Other influencing factors on wet-pavement skid resistance found in the past experimental studies include tire pressure (Moyer, 1933; Gengenback, 1968), wheel load (Stinson and Roberts, 1933; Giles, 1963) and aggregate property (Nichols et al., 1956; Stutzenberger and Havens, 1958; White, 1958). Quantitative analysis of various influencing factors have provided researchers and engineers the intuitive knowledge on wet-pavement skid resistance and made further understanding of its mechanisms possible. However, most of the previously discussed experiments were performed on conventional pavements. Although similar principles are applicable on porous pavements, there will be more influencing factors affecting porous pavement skid resistance.
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