Road surfacing

Fillers are also used in the preparation of paper to improve the properties of the material and are added to bituminous materials such as plastics and road surfacing materials. 

The situation is complicated, however, because some of the drag on a skidding tire is due to the elastic hysteresis effect discussed in Section XII-2E. That is, asperities in the road surface produce a traveling depression in the tire with energy loss due to imperfect elasticity of the tire material. In fact, tires made of high-elastic hysteresis material will tend to show superior skid resistance and coefficient of friction. 

As might be expected, this simple picture does not hold perfectly. The coefficient of friction tends to increase with increasing velocity and also is smaller if the pavement is wet [14]. On a wet road, /x may be as small as 0.2, and, in fact, one of the principal reasons for patterning the tread and sides of the tire is to prevent the confinement of a water layer between the tire and the road surface. Similarly, the texture of the road surface is important to the wet friction behavior. Properly applied, however, measurements of skid length provide a conservative estimate of the speed of the vehicle when the brakes are first applied, and it has become a routine matter for data of this kind to be obtained at the scene of a serious accident. 

A further apphcation of the concept may be found in the patterning of automobile tires to channel residue incident to contact with the road surface away from the automobile tire. Any of these apphcations of the central idea of providing a solution to increase road traction also may be patentable. 

For dry traction more contact is desired and the stopping distance is directiy related, ie, the more contact area the shorter the stopping distance. A softer, more pHable compound conforms to the road surface topography. Too soft a compound (low mechanical strength) abrades more easily and can therefore acts as a roUer and not allow sufficient contact area to be maintained. This is not readily encountered in nominal tires and conditions but has been encountered in cases of extremely high torque conditions for very fast acceleration and sudden stops. 

Until the end of World War II, coal tar was the main source of these aromatic chemicals. However, the enormously increased demands by the rapidly expanding plastics and synthetic-fiber industries have greatly outstripped the potential supply from coal carbonization. This situation was exacerbated by the cessation of the manufacture in Europe of town gas from coal in the eady 1970s, a process carried out preponderantly in the continuous vertical retorts (CVRs), which has led to production from petroleum. Over 90% of the world production of aromatic chemicals in the 1990s is derived from the petrochemical industry, whereas coal tar is chiefly a source of anticorrosion coatings, wood preservatives, feedstocks for carbon-black manufacture, and binders for road surfacings and electrodes. 

Roadbed Stabilization/Dust Control. One of the earliest uses of calcium chloride was for dust control and roadbed stabilization of unpaved gravel roads. Calcium chloride ia both dry and solution forms are used both topically and mixed with the aggregate. When a calcium chloride solution is sprayed on a dusty road surface, it absorbs moisture from the atmosphere binding the dust particles and keeping the surface damp. Calcium chloride does not evaporate, thus this dust-free condition is retained over along period of time. 

Asphalt Asphalt is used as a flexible protective coating, as a bricklining membrane, and as a chemical-resisting floor covering and road surface. Resistant to acids and bases, alphalt is soluble in organic solvents such as ketones, most chlorinated hydrocarbons, and aromatic hydrocarbons. 

Fig. 26.7. Skidding on a rough road surface deforms the tyre material elastically.

The resins have also found use in a number of other directions. The use of the resins in floorings and road surfacings is somewhat spectacular. In spite of the high initial cost, such floorings have excellent chemical resistance and resistance to wear. The resins are claimed to be of particular value at road junctions and roundabouts, where severe wear is experienced, but where repairs and maintenance operations need to be kept to a minimum because of the resultant disruption in the flow of traffic. 

Rolling resistance is almost directly proportional to the total weight on the tires. It is the sum of the deformation of the wheel, tire, and road surface at the point of contact. Energy loss occurs when the three do not return all of the energy to the cycle. 

Over rough surfaces, an opposite effect, not easily measured in the laboratoiy, becomes apparent to the rider. A tire inflated to veiy high pressures (for example, 120 pounds) bounces off the peaks of the road surface, making the bike harder to control, and negating any theoretical decrease in rolling resistance. For... 

Sulfur can replace 30-50% of the asphalt in the hlends used for road construction. Road surfaces made from asphalt-sulfur hlends have nearly double the strength of conventional pavement, and it has been claimed that such roads are more resistant to climatic conditions. The impregnation of concrete with molten sulfur is another potential large sulfur use. Concretes impregnated with sulfur have better tensile strength and corrosion resistance than conventional concretes. Sulfur is also used to produce phosphorous pentasulfide, a precursor for zinc dithiophosphates used as corrosion inhibitors. 

The main value of salt-spray tests is in the evaluation of the effectiveness of phosphate coatings in restricting the spread of rust from scratches or other points of damage in a paint film. This feature is of particular interest to the motorcar industry, as vehicles are often exposed to marine atmospheres and to moisture and salt when the latter is used to disperse ice and frost from road surfaces. Great care is needed in the interpretation of a salt-spray test, as it has been found to favour thin iron phosphate coatings more than is justified by experience with natural weathering. In the motorcar industry the present custom is to use zinc phosphate coatings on the car bodies and all other parts exposed to the outside atmosphere. 

FIGURE 26.28 Side force coefficient and self-aligning torque of a radial ply tire 175 R 14 on two wet road surfaces of different friction coefficient, at three slip angles and loads as function of the quantity c (Equation 26.17c) aU on log scales. The sohd hnes correspond to the brush model. (From Schallamach, A. and Grosch, K.A., Mechanics of Pneumatic Tires, S.K. Clark (ed.). The US Department of Transportation, National Highway Safety Administration, Washington DV.)... 

But even the structures of a chosen test route vary with the season. It is well known that road surfaces in areas of moderate climate are much sharper in winter than in summer. Even in much shorter periods the sharpness changes often drastically with the weather conditions. 

Table 26.7 gives a list of the boundary conditions which define a tire wear test simulation and in fact also an acmal road test. The road surface is the laboratory surface on which the abrasion data for the simulation were obtained. There is as yet no definition of a road surface and even if there were one, it would be of httle use since road surface structures change frequently along the road surface as pointed out earlier. 

This basic mbber friction process is present on all surfaces, dry, wet or icy, being modified only by the external conditions. On wet surfaces this is primarily water lubrication which itself is influenced by the water depth, roughness of the road surface, and the state of the tire tread pattern. The low friction on ice near its melting point is mainly due to the properties of the ice. 

---