The skid resistance of a pavement can be affected by many factors. They can be broadly classified into four categories:
(a) those related to pavement surface characteristics, such as pavement material type, and pavement surface texture in the form of microtexture and macrotexture;
inflation pressure;
(c) those related to the presence of contaminants that interfere with the tire-pavement interaction, such as presence of water, water film thickness, presence of loose particles like grit, sand and silt, presence of oils; and
(d) those related to the operating conditions, such as pavement surface temperature, and vehicle speed.
The four groups as stated above constitute the major components of the tire-fluid-pavement interaction in a very general sense. A thorough understanding of the interaction of these components would allow researchers to better understand the process of skid resistance development and the occurrence of hydroplaning. The next few sub-sections shall discuss how these parameters affect skid resistance.
2.2.1 Pavement Surface Characteristics
Pavement surface texture is the “roughness” that, in a bituminous surface, is most significantly influenced by the sizes and gradation of the aggregate and in Portland cement surface by the finishing method (e.g. burlap drag, brush finish etc). Texture not only affects the development of the necessary frictional forces under both dry and wet pavement conditions, but also influences the nature and area of contacts with the tire by projecting through water films. The tire-pavement interaction (under dry condition) and the tire-fluid-pavement interaction (under wet condition) are heavily dependent on pavement surface texture.
Pavement surface texture can be broadly classified into microtexture, macrotexture, megatexture and unevenness (ISO/CD13473, 1994). Microtexture and macrotexture are considered important for skid resistance and tire-pavement friction while unevenness is associated with road roughness and rider comfort. Megatexture generally results in vibration in tire walls but not in vehicle suspension (Wu and Nagi, 1995). Although it is a continuum between macrotexture and unevenness, it has not been generally separated or measured (Wu
pavement interaction.
2.2.1.1 Microtexture
Microtexture is a surface texture irregularity which is measured at the micro-scale of harshness and the scale of irregularities from 0.005 to 0.3mm. The lower limit of this range represents the smaller size of surface irregularities that affects wet friction (Forster, 1990). The definition of the range of microtexture is often controversial (Forster, 1990; PIARC, 1995;
ASTM, 2005g). For example, ASTM 867-02a (ASTM, 2005g) states that pavement microtexture is deviations of a pavement surface from the true planar surface with characteristic dimensions of wavelength and amplitude less than 0.5 mm. This definition is the same as that stated in the ISO/CD 13473 where microtexture refers to the peak-to-peak amplitudes varying in the range of 0.001 to 0.5 mm (ISO/CD, 1994). This research adopts the definition of microtexture as stated in the ASTM E 867-02a and the ISO/CD 13473.
Microtexture plays a fundamental role in the skid resistance behavior by locally deforming or even penetrating into the soft rubber material of the tire. A harsh pavement surface has an average microtexture depth of 0.05 mm. It is known to be a function of aggregate particles mineralogy for given conditions of weather effect, traffic action and pavement age (Kokkalis and Panagouli, 1998). On a wet pavement surface, microtexture governs the adhesion component because it controls the intimacy of contact between the rubber and the pavement surface by breaking through the thin water film that remains even after the bulk of the water is displaced. The manner in which microtexture is effective is complex because it affects the molecular and electrical interaction between the contacting surfaces (Kummer, 1966).
2.2.1.2 Macrotexture
Macrotexture is a surface texture irregularity which is measured in millimeters and is
define macrotexture as irregularities between 0.3 mm and 5.0 mm. ASTM 867-02a (ASTM, 2005g) states that pavement macrotexture is deviations of a pavement surface from the true planar surface with characteristic dimensions of wavelength and amplitude from 0.5 mm to those that can no longer affect tire-pavement interaction. ISO/CD 13473 (1994) adopts a slightly different definition which states that pavement macrotexture is the deviations of a pavement surface with characteristic dimensions of 0.5 mm to 50 mm.
A pavement surface can be considered rough if the average depth of macrotexture is more than 1.0 mm. The harsh asperities of the aggregate are able to penetrate a thin film of water on pavement surface and offer irregularities that help dispel the water between the pavement and tire tread. Inadequate macrotexture can be caused by poor construction, worn aggregates, embedded aggregates or surface bleeding. It leads to dramatically decreased skid resistance, thus increasing accident risk (Kokkalis and Panagouli, 1998).
The macrotexture of asphalt pavement surfaces is mainly attributed to aggregate size, shape, angularity, spacing, and distribution of coarse aggregates (bigger than 2.0 mm). The principle function of pavement macrotexture is to provide, together with tire tread, escape channels for rainwater, which would otherwise be trapped in the tire-pavement contact patch.
Deep macrotexture means that the pavement surface has a large void area, which is capable of draining excess water from the tire-pavement contact region. Friction between tire and wet pavement decreases with increasing speed, but deep macrotexture is helpful to lessen the gradient of such decline (Highway Research Board, 1972).
2.2.2 Presence of Contaminants
Under normal operating circumstances, dry friction between the tire and pavement never poses a serious safety problem. However, a serious loss in friction can occur once contaminants such as water from rainfall or oils from fuel leakage are present on a pavement surface. These contaminants act as lubricating agents which cause a loss in friction and the braking ability of the automobiles and aircraft. The presence of such contaminants under
hydroplaning.
2.2.3 Vehicle Speed
The influence of vehicle or aircraft speed on skid resistance is highly dependent upon the properties of the tire and the pavement surface. Figure 2.2 shows that an increase in vehicle speed causes a decrease in the dry skid resistance for dry pavement. This decrease is gradual as compared to the wet skid resistance which decreases dramatically with increasing speed. The wet skid resistance is also related to other factors such as water film thickness, tire tread pattern and depth, and pavement surface properties. Figure 2.3 highlights the effect of vehicle speed on friction factor for different tires using locked wheel trailer method as stated in ASTM E 274-97 (ASTM, 2005a). This highlights the variability of the skid resistance measured under the influence of different rubber materials for the tires, and the trend of decreasing friction with increasing speed for wetted pavements.