145. It is probable that the earliest rupture tests of cement were made by submitting rectangular prisms to a bending stress; but such tests have long held a place subordinate to trials of tensile strength. A mass of masonry, taken as a whole, is very apt to be subjected to a bending stress, but it is a question whether a transverse test on a small specimen gives any-better idea of the ability of a large beam to carry its load, than do simple tensile and compressive tests.
1 Jour. Assoc. Engr. Soc, Vol. vii, p. 212.
In Engineering News of December 14, 1893, appeared an article giving the comparative results obtained in tensile and transverse tests. The tensile specimens had an area of one square inch at the smallest place, and the transverse specimens also had an area of cross-section of one square inch. It was found that the modulus of rupture computed by the common formula f = 3Wl/2bd2 from 1.1 to 3.8 times the tensile strength developed by the briquets. Some comparative tests made at St. Mary's Falls Canal are discussed in Art. 56.
146. The objections to transverse tests are: 1st, if the specimens are made but one inch in cross-section, it is difficult to handle them without injuring them, and if the section is made much larger than one inch square, a much greater amount of cement is required to make the specimens and more room required to store them; 2d, it would seem that the results obtained might be less trustworthy than those in tensile tests because of the greater influence of the outside layers, which are subjected to the greatest accidental variations, on the apparent strength of the specimen. On the other hand, it may be said that, when no testing machine is at hand, the apparatus requisite to make a crude test may easily be improvised. All that is required is a rectangular wooden mold, three knife edges, and a pail with a quantity of sand or water.
147. When it is a question of making tests of transverse strength accurately and rapidly, the apparatus required is no more simple than the apparatus for tensile tests. In the construction of metal molds in large quantities it makes little difference whether the form requires curved or straight lines. As far as breaking is concerned, there is a certain force to be applied, and a machine that will answer for one test may also be used for the other. In the matter of clips, there may be a slight advantage as to simplicity in a clip designed for transverse breaking.
In making transverse tests the author has used a form two inches square and eight inches long. By placing the end supports five and one-third inches apart, the modulus of rupture by the formula f = 3Wl/2bd2 becomes equal to W, the center load applied.
148. Finally, it may be said that there is little objection to substituting transverse tests for tensile tests, although no evident advantage would be gained. It would also seem that there is no object in making tests for quality by both transverse and tensile tests, though from a scientific standpoint comparative tests of transverse and tensile strength are of great interest.