10. Physical Tests. A test made by crushing a block of stone in a testing machine is apparently a very simple and conclusive test, but in reality the results are apt to be inconclusive and even decep tive. This is due to the following reasons, among others: (a) The crushing strength of a cube per square inch is far less than that of a slab having considerably greater length and width than height.
(1)) The result of a test depends very largely on the preparation of the specimen. If sawed, the strength will be greater than if cut by chipping. If the upper and lower faces are not truly parallel, so that there is a concen tration of pressure on one corner, the apparent result will he less.
(c) The result depends on the imbedment. Specimens which are rubbed and ground with machines that will insure truly parallel and plane surfaces, will give higher results than when wood, lead, leather, or plaster of-paris cushions are employed.
(d) The strength of masonry depends largely on the crushing strength of the mortar used and the thickness of the joints. Other things being equal, an increase in the crushing strength of the stone (or brick) which is used does not add proportionately to the strength of the masonry as a whole; and if the mortar joints are very thick, it adds little or nothing. Since the strength of the masonry is the only real criterion, the strength of a cube of the stone is of comparatively little importance.
In short, tests of two-inch cubes (the size usually employed) are valuable chiefly in comparing the strength of two or more different kinds of stones, all of which are tested under precisely similar con ditions. A comparison of such figures with the figures obtained by others will have but little value unless the precise conditions of the other tests are accurately known. Under any conditions, the results of the tests will bear but little relation to the actual strength of the masonry to be built.
11. Quarry Examinations. These are generally the surest tests, and should never be neglected if the choice of stone is a matter of great importance. Field stone and outcropping rock which have withstood the weather for indefinite periods of years, can usually be relied on as being durable against all deterioration except that due to acids in the atmosphere, to which they probably have not been subjected in the country as they might be in a city. On the other hand, however, large blocks of stone can seldom be obtained from field stones. If a quarry has been opened for several years, a. com parison of the other surfaces with those just exposed may indicate the possible disintegrating or discoloring effects of the atmosphere. A stone which is dense and of uniform structure, and which will not disintegrate, may be relied on to withstand any physical stress to which masonry should be subjected.