NATURAL STONE AND ITS CHARACTERISTICS 1. From the constructor's standpoint, any stone is good which will fulfil certain desired characteristics. These various charac teristics are not found combined in the highest degree in any one kind of stone. It is essential to learn to what extent these various desirable characteristics are combined in the various types of stone which are quarried. At the same time, it should not be forgotten that stones of the same nominal 'classification vary greatly in the extent of -their desirability. The chief characteristics to be con sidered by the constructor are Cost, Strength, and Appear ance. Although in some cases this represents the order in which these qualifications are desired, in other cases the order is indefinitely varied. For example, in a high-grade public building or monument, a good appearance is considered essential, regardless of cost. In a subsurface foundation, appearance is of absolutely no importance.
2. Cost. The cost of ally stone depends on its intrinsic valua tion in the quarry, the cost of quarrying and dressing, and the cost of transportation from the quarry to the site of the structure. The cost of transportation is often the most important, and this consideration frequently decides not only the choice of stone but even the type of construction—whether stone masonry or concrete. To give a rough idea of the cost of stone quarrying, a few values arc quoted from Gillette's "Handbook of Cost Data." In one instance the cost of quarrying granite, exclusive of rental of plant, rental of quarry, and cost of stripping off the upper soil, averaged about $4.50 per cubic yard. In another instance the cost of quarrying rubble amounted to $1.S0 per cubic yard. The cost of explosives was not included in this estimate, but it should not have increased the cost to over $2.00 per cubic yard. Tit another instance the cost of quarrying gneiss amounted to $3.55, not including explosives and teaming. Even these items should not have made the total cost more than $4.25 per cubic yard.
3. Durability. Under many conditions the most important qualification is durability. The lack of it is also the most seriously disappointing quality. Rocks which have remained hard and tough for unnumbered ages while covered by earth from air and frost, will disintegrate after a comparatively few years' exposure.
Atmospheric Influences. A very porous stone will absorb water, which may freeze and cause crystals near the surface to flake off.
Even though such action during a single winter may be hardly per ceptible, the continued exposure of fresh surfaces to such action may sooner or later cause a serious loss and disintegration. Even rain water which has absorbed carbonic acid from the atmosphere will soak into the stone, and the acid will have a greater or less effect on nearly all stones. Quartz is the only constituent which is absolutely unaffected by acid. The sulphuric acid gas given off by coal will also affect building stone very seriously.
Fire. Natural stone is far less able to withstand a conflagration than the artificial compositions such as brick, concrete, and terra cotta. Granite, so popularly considered the type of durability, is especially affected. Limestone and marble will be utterly spoiled, at least in appearance if not structurally, by a hot fire. Sandstone is the least affected of the natural stones.
Hardness. The durability of a stone is tested by its resistance to abrasive action in pavements, door-sills, and similar cases. The value of trap rock for macadam and block pavements is chiefly due to this quality.
4. Strength. In some structural work (as, for example, an arch) the crushing strength of the stone is the primary consideration. The average crushing strength of various kinds of stone will be quoted later. The tensile strength should never be depended on, except to a very limited extent as a function of the transverse strength. Even this is only applicable to such cases as the lintels over doors and windows, the footing stones for foundations, and the cover stones for box culverts. It is usually true that a stone which is free from cracks and which has a high crushing strength also, has as much transverse strength as should be required of any stone.
5. Appearance. It is seldom that an engineer need concern himself with the appearance of a stone, provided it is satisfactory in the respects previously mentioned. The presence of iron oxide in a stone will sometimes cause a deterioration in appearance by the formation of a reddish stain on the outer surface. It usually happens, however, that a stone whose strength and durability are satisfactory will have a sufficiently good appearance, unless in high-grade archi tectural work, where it is considered essential that a certain color or appearance shall be obtained.