Road surface texture

The previous section started with a brief review of classical friction theory.

However, it was noted that the application of that theory, where the properties of rubber had not been taken into account, is not necessarily relevant to the study of tyre/road interaction. Similarly, research dealing with idealised surfaces created in the laboratory or surfaces existing only in theory, may not apply to road or aggregate surfaces.

In his thesis dealing with the dry adhesive friction of elastomers, Savkoor (1987) notes:

“...it may be argued from experience that when the many surfaces which may not be "smooth textured" initially, do become so after having "run-in"

for some time so that the shapes of their asperity tips are most likely to be smooth and convex. Such smooth and convex shapes can be represented generally by a curved surface having two principal radii of curvature...” Savkoor goes on to restrict his theoretical work to the case of ideal spherical asperities.

Another frequently used simplification is that of a self-affine fractal surface (Persson, 2001). In these surfaces, given the correct scale transformation, very fine texture has the same form as texture on a larger scale, which in turn has the same form as texture on a larger scale again. Fractal dimensions will be discussed in more detail when considering methods for characterisation of surface texture.

These are common, and perhaps necessary, assumptions because rubber friction is complicated enough without also dealing with surface texture.

However, since the research presented in this thesis is aimed at measurement and characterisation of surface texture (rather than a study of friction) this section skips over theoretical treatments of surface texture straight to a review of accepted definitions of road surface texture and its perceived necessity.

The geometric profile of the road in the vertical plane has traditionally been divided into different scales depending on the dynamic response of interest

1987). It should be noted that, in general, large amplitudes are desirable when wavelengths are below 10 mm and small amplitudes are desirable for wavelengths above 10 mm. The surface characteristics are divided into four general pavement surface features – unevenness, megatexture, macrotexture and microtexture – which are discussed below.

Figure 2.8 Surface texture characteristics

 The unevenness of the road surface, with wavelengths from 0.5 m to 50 m, is associated with longitudinal profiles larger than the tyre footprint. It affects vehicle dynamics, ride quality, dynamic loads, and drainage. In extreme cases, unevenness can lead to loss of contact with the surface, and it is normally caused either by poor initial construction or deformation caused by loading.

 The road’s megatexture refers to deviations with wavelengths from 50 mm to 500 mm. Examples of megatexture include ruts, potholes, major joints and cracks. It affects vibration in the tyre walls but not the vehicle suspension, and it is therefore strongly associated with noise and rolling resistance. Although megatexture generally has larger dimensions than those which affect skid resistance, it is possible that this scale of texture could influence tyre/road contact.

 Macrotexture is the amplitude of deviations with wavelengths from 0.5 mm to 50 mm, and is effected by the size, shape, spacing and arrangement of coarse aggregate particles. Macrotexture affects

mainly tyre noise and water drainage from the tyre footprint. This scale of texture is thought to be important for hysteretic friction, especially at high speed.

 Microtexture is the amplitude of deviations with wavelengths less than or equal to 0.5 mm. This scale of texture is measured on the micron scale, and is typically found on the surfaces of coarse aggregate particles (rather than being due to the gaps between them as in macrotexture) or the texture of bituminous mortar and fine material. Microtexture is frequently therefore a function of aggregate particle mineralogy and petrology and is affected by climate/weather effects and traffic action. The microtexture of the road surface is thought to affect skid resistance at all speeds for dry and wet conditions.

The scales of surface irregularities that affect skid resistance are illustrated in Table 2.1 along with terms used to describe them .

Table 2.1 Illustration of terms used to describe road surface texture

Surface Scale of texture

Macro (large) Micro (fine)

A Rough Harsh

B Rough Polished

C Smooth Harsh

Experimental results show that surfaces with harsh microtexture, but smooth macrotexture, have high skid resistance at low speed, but the rate at which friction reduces with increasing speed is greater than for surfaces with rough macrotexture – this is expanded in Section 2.5.

Considering a pavement under wet conditions, the presence of a thin film of water obscuring contact between the tyre and surface drives the requirement for roughness, and the shape of roughness required. The following is an analysis of the requirement for pavement roughness in wet conditions from Moore (1975).

Two extreme examples of roughness shape, the cone and the sphere, are illustrated in Figure 2.9. Let p* be the minimum pressure required to break through the water film on the surface. If a load is applied to the cone shape such that the maximum pressure exerted on the tyre, pmax, is just more than p*, then the same load applied to the sphere will result in a maximum pressure exerted which is less than p* and no penetration of the water film can occur. However, since the cone would be subject to severe wear at its peak, a compromise between the two shapes is required.

Figure 2.9 Examples of pressure distribution in tyre surface It is generally agreed that surface macrotexture is required in wet conditions so that the bulk of water present can be drained away and also to generate friction through tyre deformation and therefore bulk hysteresis.

A surface requires microtexture so that it can break through any remaining film of water. The graph in Figure 2.10 demonstrates the trade-off between macrotexture and microtexture. On the extreme left of the graph, if the pavement has relatively smooth macrotexture, then the surface will have insufficient drainage and there will be little or no contact with the tyre regardless of the amount of microtexture present. On the extreme right of the graph, the shape of asperities and therefore the roughness of the

macrotexture is sufficient by itself to break through any water present on the surface; again microtexture is not necessary. Most pavement surfaces will have macrotexture somewhere in between these two extremes and microtexture, therefore, will be required for adequate friction in wet conditions.

Figure 2.10 Microtexture requirements for pavement design Sabey (1958) investigated the friction between rubber and various sliding shapes under wet conditions. Her tests showed that the coefficient of friction under wet conditions is closely related to the pressure over the contact area between the sliders and the rubber. It was concluded that, to ensure good skid resistance in wet conditions, surface asperities should be such that average pressures of 1000 lb in^-2 (approximately 6.9 MPa) are set up on them, and that individual projections should have angles at their tips of 90˚ or less.

Peak pressure on asperity

Increasing macrotexture roughness

Increasing requirement for microtexture harshness Design range for microtexture

partly supported by asperities emerging through the water film and partly supported by the water film itself (see Section 2.1.3). Consideration of the theoretical deformation of the rubber slider alone could not differentiate between surfaces with and without microtexture and so it was concluded that an adhesion component of force, relating to the contact between the slider and the emerging asperities was required.