To simplify the matter somewhat, the different classes of traffic to which roads are subjected may be divided into five groups, which may be called city, urban, suburban, highway, and country road traffic, respectively. City traffic is a traffic so great that no macadam road can withstand it, and is such as exists on the business streets of large cities. For such a traffic stone and wood blocks, asphalt, brick, or some such materials are necessary. Urban traffic is such as exists on city streets which are not subjected to continuous heavy teaming, but which have to withstand very heavy wear, and need the hardest and toughest macadam rock. Suburban traffic is such as is common in the suburbs of a city and the main streets of country towns. Highway traffic is a traffic equal to that of the main country roads. Country road traffic is a traffic equal to that of the less frequented country roads.
The city traffic will not be considered here. For an urban traffic, the hardest and toughest rock, or in other words, a rock of the highest wearing quality that can be found, is best. For a suburban traffic the best rock would be one of high toughness but of less hardness than one for urban traffic. For highway traffic a rock of medium hardness and toughness is best. For country road traffic it is best to use a comparatively soft rock of medium toughness. In all cases high cementing value should be sought, and especially if the locality is very wet or windy.
Rocks belonging to the same species and having the same name, such as traps, granites, quartzites, etc., vary almost as much in different localities in their physical road-building properties as they do from rocks of distinct species. This variation is also true of the mineral composition of rocks of the same species, as well as in the size and arrangement of their crystals. It is impossible, therefore, to classify rocks for road-building by simply giving their specific names. It can be said, however, that certain species of rock possess in common some road-building properties. For instance, the trap[8] rocks as a class are hard and tough and usually have binding power, and consequently stand heavy traffic well; and for this reason they are frequently spoken of as the best rocks for road-building. This, however, is not always true, for numerous examples can be shown where trap rock having the above properties in the highest degree has failed to give good results on light traffic roads. The reason trap rock has gained so much favor with road-builders is because a large majority of macadam roads in our country are built to stand an urban traffic, and the traps stand such a traffic better than any other single class of rocks. There are, however, other rocks that will stand an urban traffic perfectly well, and there are traps that are not sufficiently hard and tough for a suburban or highway traffic. The granites are generally brittle, and many of them do not bind well, but there are a great many which when used under proper conditions make excellent roads. The felsites are usually very hard and brittle, and many have excellent binding power, some varieties being suitable for the heaviest macadam traffic. Limestones generally bind well, are soft, and frequently hygroscopic. Quartzites are almost always very hard, brittle, and have very low binding power. The slates are usually soft, brittle, and lack binding power.
The above generalizations are of necessity vague, and for practical purposes are of little value, since rocks of the same variety occurring in different localities have very wide ranges of character. It consequently happens in many cases, particularly where there are a number of rocks to choose from, that the difficulty of making the best selection is great, and this difficulty is constantly increasing with the rapidly growing facilities of transportation and the increased range of choice which this permits. On account of their desirable road properties some rocks are now shipped several hundred miles for use.
There are but two ways in which the value of a rock as a road material can be accurately determined. One way, and beyond all doubt the surest, is to build sample roads of all the rocks available in a locality, to measure the traffic and wear to which they are subjected, and keep an accurate account of the cost both of construction and annual repairs for each. By this method actual results are obtained, but it has grave and obvious disadvantages. It is very costly (especially so when the results are negative), and it requires so great a lapse of time before results are obtained that it cannot be considered a practical method when macadam roads are first being built in a locality. Further than this, results thus obtained are not applicable to other roads and materials. Such a method, while excellent in its results, can only be adopted by communities which can afford the necessary time and money, and is entirely inadequate for general use.
The other method is to make laboratory tests of the physical properties of available rocks in a locality, study the conditions obtaining on the particular road that is to be built, and then select the material that best suits the conditions. This method has the advantages of giving speedy results and of being inexpensive, and as far as the results of laboratory tests have been compared with the results of actual practice they have been found to agree.
Laboratory tests on road materials were first adopted in France about thirty years ago, and their usefulness has been thoroughly established. The tests for rock there are to determine its degree of hardness, resistance to abrasion, and resistance to compression. In 1893 the Massachusetts Highway Commission established a laboratory at Harvard University for testing road materials. The French abrasion test was adopted, and tests for determining the cementing power and toughness of rock were added. Since then similar laboratories have been established at Johns Hopkins University, Columbia University, Wisconsin Geological Survey, Cornell University, and the University of California.
The Department of Agriculture has now established a road-material laboratory in the Division of Chemistry, where any person residing in the United States may have road materials tested free by applying for instructions to the Office of Public Road Inquiries. The laboratory is equipped with the apparatus necessary for carrying on such work, and the Department intends to carry on general investigations on roads. Part of the general plan will be to make tests on actual roads for the purpose of comparing the results with those obtained in the laboratory.
Besides testing road materials for the public, blank forms for recording traffic will be supplied by the department to any one intending to build a road. When these forms are filled and returned to the laboratory, together with the samples of materials available for building the road, the traffic of the road will be rated in its proper group, as described above; each property of the materials will be tested and similarly rated according to its degree, the climatic conditions will be considered, and expert advice given as to the proper choice to be made.