In selecting stone for use as lintels, or where it is to carry trans verse loads, it is desirable that the stone be tested in blocks of size comparable to those in which it is to be used. The results of tests upon small specimens is not of much value for this purpose.
The table on p. 59, by Herbert F. Moore, is taken from Merri man's " American Civil Engineer's Pocket Book." 42. Durability of Building Stone.—That stone should be durable under the conditions of use is evidently one of the most important points to be considered in the selection of material for use in con struction. The situation in which the stone is to be placed and the climatic or other conditions which may affect the durability should therefore be carefully considered. Local conditions have frequently been overlooked in selecting stone, with disastrous results. The White House at Washington is of sandstone which requires frequent painting. The obelisk, in perfect condition after long exposure in Egypt, began to disintegrate almost immediately when set up in New York City. The Parliament House, built of stone selected with the greatest care, is not able to resist the disintegrating in fluences of the London atmosphere.
The range of changes in temperature, presence of moisture and gases in the atmosphere, and the action of winds and dust are the principal causes of deterioration in stones used in structures.
Expansion and contraction clue to changes in temperature create an almost continual tendency to motion among the particles of the stone, an effect which is felt mainly at the exposed surfaces where expansions are very unequal, and may cause the scaling of the surface layers. Surfaces exposed to the direct rays of the sun are most affected from this cause. In a number of instances, scal ing of the surfaces on the south side of buildings has been observed, when the less exposed sides were free from it.
Frost Action.—When stone saturated with water is frozen, the expansion of the liquid in freezing causes a heavy internal pressure, which may be greater than the tenacity of the stone. In the climate of the Northern United States this is commonly one of the most active causes of disintegration of building stones, and the ability to resist frost action is of chief importance. The results of the action
of frost on a stone depend upon the porosity of the stone and upon the texture and toughness of the material.
Granite usually absorbs not more than 1 per cent of water, and is not often appreciably affected by frost. Sandstones and lime stones may absorb from about 2 to 12 or even 15 per cent. Ordi narily, a good stone that does not absorb more than 4 or 5 per cent of water may be expected to stand frost well. Some more porous stones have also shown well in use. A porous stone of coarse texture is more apt to resist• frost action than one of fine texture. Moisture escapes more readily and the stone is less likely to be saturated when frozen.
Fire Resistance.—Any building stone may be injured if subjected to high heat as in the case of serious fires. This injury is intensified by contact of water when so heated. Unequal expansions and sud den surface contractions are likely to cause internal stresses beyond the strength of the stone.
Granites are apt to split and spall badly on the surface and usually show poor fire-resisting qualities. Limestones usually resist fire better than granite until the heat becomes sufficient to drive off the carbonic acid. At high heats they are destroyed. When suddenly cooled by water, limestone is likely to spall badly. Sand stones usually withstand fires better than other building stones, sometimes coming through severe fires without serious injury. They are, however, likely to spall and crack under the combined action of a hot fire and water.
Chemical Agencies.—Hock to be durable in use as building stone must be capable of resisting changes due to the presence of water and gases in the atmosphere.
Certain ingredients in the rock may be soluble in water carrying acids in solution; limestones commonly weather in this way, the carbonate of lime being somewhat soluble in water containing car bonic or sulphurous acid, hence these stones are usually liable to surface deterioration in cities. The extent of such deterioration is greater for the more absorbent stones.