When stone is to be used to span openings and carry transverse loads, its strength is important and care should be taken in its selec tion. 'I'1ie ability of stone to resist cross-bending stresses is mainly dependent upon its tensile strength. Tests of transverse strength may serve to detect brittleness and lack of toughness or uniformity in the texture of the stone.
Tests for Compressive Strength.—The compressive strength of stone is determined by measuring the loads necessary to crush small blocks cut from the stone. The results of such tests vary with the sizes and shapes of the blocks tested and the methods of placing them in the testing machine. It is necessary in comparing the strengths of different stones to use a standard form and size of speci men and standard method of test Mg. It is usual to use small cubes, 2 inches on the edge. The size does not seem to very greatly affect the resistance per unit area, but it is desirable to use blocks of the same size in snaking comparative tuts.
The shape of the block is Highly important in its effect upon the results of such tests. When subjected to compression, materials of this kind break by shearing on planes making angles of about 30° with the direction of the compressing force. The ratio of the height of the specimen to its lateral dimensions is therefore important. The strength of the flat slab is much greater than that of a cube, while a prism whose height is greater than its breadth will show less strength on the test.
The test of small specimens gives no indication of the actual strength of the stone in large masses, and tests of this kind can be of value only as indicating the quality of the material, through comparison with the results of similar tests applied to other stones.
The method of preparing the specimen may be quite important in the results of a test. When the dressing is clone with hand tools, the shocks frequently have the effect of weakening the internal structure of the stone. This effect with small specimens may amount to a decrease of 30 or 40 per cent as compared with the strength of sawed blocks. The use of sawed test pieces is desirable in such work.
The manner of placing the specimen in the testing machine is also important. It is essential that the test piece be accurately centered in the machine, and that it be evenly in contact with the pressing surfaces, in order to distribute uniformly the compressing force over the area of the block. If the surfaces of the test piece
he carefully ground to parallel planes, and the piece carefully centered in the machine in exact contact with the metal surfaces, the best results will he obtained. This method, however, involves con siderable labor in preparation of the specimen, and is expensive. The more common method is to set the specimen in a thin bedding of plaster of paris between the plates of the machine and leave it under light pressure for a few minutes, to allow the plaster of paris to set, before applying the load. This method, if carefully handled, gives uniform results, although the strength shown is somewhat less than that obtained by using ground surfaces.
A block of stone may have much less strength in one direction than in another. Most rocks have planes of cleavage in one direc tion in which they split more easily than in other directions. These planes are usually parallel to the natural bed of the rock and are known as the rift of the rock. Care should be taken to place the test specimen on its natural bed, or in such position that the com pression is applied in a direction normal to the rift.
Compressive results of tests upon building stones show a wide variation in compressive strengths of different samples of the same classification, as well as between different classes of stone.
Hard limestones usually show crushing strengths of 8000 to 12,000 although softer stones of good quality may run from 3000 to 6000 Sandstones used in building vary in compressive strength from about 4000 to 15,000 The better grades of stone usually reach 9000 to 12,000 Granites of good quality should show a crushing strength of 10,000 to 20,000 Transverse of transverse strength are usually made on a small bar of stone, 1 inch square in section, and the method of preparing the specimen is important in its effect upon the results of the test. Comparatively little data exist concerning the strengths of stone under transverse loadings. The following table gives approximate values which have been obtained for ordinary stone used in building.