ROCKS FOR ROAD BUILDING.
Properties Required. The surface material for broken stone roads must bind together into a solid surface capable of bearing the loads which come upon it and of resisting the wear of the traffic.
A stone to be durable in the surface of a road should be as hard and tough as possible. The qualities of toughness and resistance to abrasion are of more importance than hardness and resistance to crushing. A stone may be hard and brittle and quickly pound to pieces in a road surface, or it may have a high crushing strength and grind away quickly under abrasion, as is the case with some varieties of sand stone. If, however, it be too soft, it may crush under the loads coming upon it, and thus lack in durability.
A stone for a road-surface must also resist well the disintegrating influences of the atmosphere. It should be as little absorptive of moisture as possible in order that it may not be liable to injury from the action of frost. Many limestones are objectionable on this account.
The material of a road-surface should also be uni form in quality; otherwise the wear of the surface will not be even, and depressions will appear where the softer material has been placed.
As the under parts of the road are not subject to the wear of the traffic, and have only the weight of the loads to sustain, it is evidently not important that the foundation or lower layers be of so hard or tough a material as the surface; and hence it is frequently pos sible, by using an inferior stone for that portion of the work, to greatly reduce the cost of construction.
The binding of the road-surface into a compact mass capable of resisting the wear of traffic depends largely upon the cementing properties of the material. By the cementing power of the stone is meant that property which enables the fine dust to act, when wet, as a cement and bind the fragments of rock composing the road-surface firmly together. This is perhaps the most important quality of the material, and a high cementing value is always desirable. The tenacity with which the fragments of rock are held together is perhaps more important in the wear of the road than the resistance to wear of the fragments themselves. The powdering of the cementing material in dry weather sometimes causes the loosening of the stones and the raveling of the broken-stone surface. This is more apt to occur where the road metal is hard and resistant to wear than where it grinds up more rapidly.
The character of the material which will give the best in a road-surface depends upon the local conditions under which the road is to be built and the traffic to which it is to be subjected. Under heavy
traffic, hard and tough road metal is necessary to good results. Under lighter traffic, a softer rock may sometimes be better if it is coupled with good binding properties.
* " 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 parkway, 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, the cheapest results would be obtained. Such a rock would so effectively resist the wear of a light traffic that the amount of fme dust worn off would be carried away by wind and rain faster than it would be supplied by wear. Conse quently the binder supplied by wear would be insuffi cient, 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." The selection of material for road metal is commonly determined rather by the cheapness and convenience of location than by its desirability for the purpose. In most instances this is of necessity the case, and the availability of material in vicinity of the work makes possible the construction of the road. It is, however, frequently possible, by judicious selection of materials, to greatly improve the results obtained in such work, and while the selection of a stone for road construction will of course always depend largely upon what is to be obtained in the locality of the work, the importance of a thoroughly good material in the road surface is so great in its effect upon the durability and cost of repairs of the road that it may frequently be found economical, on roads subjected to a considerable traffic, to bring a good material a considerable distance rather than to use an inferior one from the immediate vicinity. It may also be suggested in this connection that in many instances railway transportation over a consid erable distance may be small compared with wagon transportation over a short distance, and the impor tation of good material may add but slightly to the aggregate cost of the work.