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.
Experience shows that a rock possessing all three of the properties mentioned in a high degree does not under all conditions make a good road material; on the contrary, under certain conditions it may be altogether unsuitable. As an illustration of this, if a country road or city park way, where only a light traffic prevails, were built of a very hard and tough rock with a high cementing value, neither the best, nor, if a softer rock were available, would the cheapest results be obtained. Such a rock would so effectively resist the wear of a light traffic that the amount of fine dust worn off would be carried away by wind and rain faster than it would be supplied by wear. Consequently the binder supplied by wear would be insufficient, and if not supplied from some other source the road would soon go to pieces. The first cost of such a rock would in most instances be greater than that of a softer one and the necessary repairs resulting from its use would also be very expensive.
A very good illustration of this point is the first road built by the Massachusetts Highway Commission. This road is on the island of Nantucket and was subjected to a very light traffic. The commission desired to build the best possible road, and consequently ordered a very hard and tough trap rock from Salem, considered then to be the best macadam rock in the state. Delivered on the road this rock cost $3.50 per ton, the excessive price being due to the cost of transportation. The road was in every way properly constructed, and thoroughly rolled with a steam roller; but in spite of every precaution it soon began to ravel, and repeated rolling was only of temporary benefit, for the rock was too hard and tough for the traffic. Subsequently, when the road was resurfaced with limestone, which was much softer than the trap, it became excellent. Since then all roads built on the island have been constructed of native granite bowlders with good results, and at a much lower cost.
If, however, this hard and tough rock, which gave such poor results at Nantucket, were used on a road where the traffic was sufficient to wear off an ample supply of binder, very much better results would be obtained than if a rock lacking both hardness and toughness were used; for, in the latter case, the wear would be so great that ruts would be formed which would prevent rain water draining from the surface. The water thus collecting on the surface would soon make its way to the foundation and destroy the road. The dust in dry weather would also be excessive.
Only two examples of the misuse of a road material have been given, but, as they represent extreme conditions, it is easy to see the large number of intermediate mistakes that can be made, for there are few rocks even of the same variety that possess the same physical properties in a like degree. The climatic and physical conditions to which roads are subjected are equally varied. The excellence of a road material may, therefore, be said to depend entirely on the conditions which it is intended to meet.
It may be well to mention a few other properties of rock that bear on road-building, though they will not be discussed here. There are some rocks, such as limestones, that are hygroscopic, or possess the power of absorbing moisture from the air, and in dry climates such rocks are distinctly valuable, as the cementation of rock dust is in a large measure dependent for its full development on the presence of water. The degree to which a rock absorbs water may also be important, for in cold climates this to some extent determines the liability of a rock to fracture by freezing. It is not so important, however, as the absorptive power of the road itself, for if a road holds much water the destruction wrought by frost is very great. This trouble is generally due to faulty construction rather than to the material. The density or weight of a rock is also considered of importance, as the heavier the rock the better it stays in place and the better it resists the action of wind and rain.
Only a few of the properties of rock important to road builders have been considered, but if these are borne in mind when a material is to be selected better results are sure to be obtained. In selecting a road material the conditions to which it is to be subjected should first be considered. These are principally the annual rainfall, the average winter temperature, the character of prevailing winds, the grades, and the volume and character of the traffic that is to pass over the road. The climatic conditions are readily obtained from the Weather Bureau, and a satisfactory record of the volume and character of the traffic can be made by any competent person living in view of the road.
In France the measuring of traffic has received a great deal of attention, and a census is kept for all the national highways. The traffic there is rated and reduced to units in the following manner: A horse hauling a public vehicle or cart loaded with produce or merchandise is considered as the unit of traffic. Each horse hauling an empty cart or private carriage counts as one-half unit; each horse, cow, or ox, unharnessed, and each saddle horse, one-fifth unit; each small animal (sheep, goat, or hog), one-thirtieth unit.
A record is made of the traffic every thirteenth day throughout the year, and an average taken to determine its mean amount. Some such general method of classifying traffic in units is desirable, as it permits the traffic of a road to be expressed in one number.
Before this French method can be applied to the traffic of our country it will be necessary to modify considerably the mode of rating. This, however, is a matter which can be studied and properly adjusted by the Office of Public Road Inquiries. It is most important to obtain a record of the average number of horses and vehicles and kind of vehicles that pass over an earth road in a day before the macadam road is built. The small cost of such a record is trifling when compared with the cost of a macadam road (from $4,000 to $10,000 per mile for a fifteen-foot road), in view of the fact that an error in the selection of material may cost a much larger sum of money. After a record of the traffic is obtained, if the road is to be built of crushed rock for the first time, an allowance for an immediate increase in traffic amounting at least to ten or fifteen per cent had best be made, for the improved road generally brings traffic from adjoining roads.