Until comparatively recent years but little was known of the relative values of the different varieties of rock as road material, and good results were obtained more by chance and general observation than through any special knowledge of the subject. These conditions, however, do not obtain at present, for the subject has received a great deal of careful study, and a fairly accurate estimate can be made of the fitness of a rock for any conditions of climate and traffic.
In road-building the attempt should be made to get a perfectly smooth surface, not too hard, too slippery, or too noisy, and as free as possible from mud and dust, and these results are to be attained and maintained as cheaply as possible. Such results, however, can only be had by selecting the material and methods of construction best suited to the conditions.
In selecting a road material it is well to consider the agencies of destruction to roads that have to be met. Among the most important are the wearing action of wheels and horses' feet, frost, rain, and wind. To find materials that can best withstand these agencies under all conditions is the great problem that confronts the road-builder.
Before going further, it will be well to consider some of the physical properties of rock which are important in road-building, for the value of a road material is dependent in a large measure on the degree to which it possesses these properties. There are many such properties that affect road-building, but only three need be mentioned here. They are hardness, toughness, and cementing or binding power.
By hardness is meant the power possessed by a rock to resist the wearing action caused by the abrasion of wheels and horses' feet. Toughness, as understood by road-builders, is the adhesion between the crystal and fine particles of a rock, which gives it power to resist fracture when subjected to the blows of traffic. This important property, while distinct from hardness, is yet intimately associated with it, and can in a measure make up for a deficiency in hardness. Hardness, for instance, would be the resistance offered by a rock to the grinding of an emery wheel; toughness, the resistance to fracture when struck with a hammer. Cementing or binding power is the property possessed by the dust of a rock to act, after wetting, as a cement to the coarser fragments composing the road, binding them together and forming a smooth, impervious shell over the surface. Such a shell, formed by a rock of high cementing value, protects the underlying material from wear and acts as a cushion to the blows from horses' feet, and at the same time resists the waste of material caused by wind and rain, and preserves the foundation by shedding the surface water. Binding power is thus, probably, the most important property to be sought for in a road-building rock, as its presence is always necessary for the best results. The hardness and toughness of the binder surface more than of the rock itself represents the hardness and toughness of the road, for if the weight of traffic is sufficient to destroy the bond of cementation of the surface, the stones below are soon loosened and forced out of place. When there is an absence of binding material, which often occurs when the rock is too hard for the traffic to which it is subjected, the road soon loosens or ravels.