In practically all forms of masonry construction, cement is called upon to resist compression. In consequence of this fact, the opinion is somewhat general that the greatest amount of information would be obtained by compressive tests. But the compressive strength of cement is so much greater than its tensile strength, that when failures occur, they are likely to be due to other forms of stress. In short, the ratio of the compressive strength to the crushing force it is likely to be called upon to resist, is usually much greater than the corresponding ratio in tensile strength.
142. There is no doubt that compressive tests are of much interest and value, especially so since the use of concrete and steel in combination has become general, but as yet the facilities for making, the test are not available without considerable expense. This is on account of the larger force required (the compressive strength being six to ten times the tensile) and because the uniform distribution of the stress over the surface of the specimen, and the accurate recording of the force exerted, are even more difficult than the corresponding operations in tensile tests. Prof. Sondericker,1 in a paper read before the Boston Society of Civil Engineers, describes an apparatus in which he seems to have overcome a part of these difficulties.
A convenient specimen for compressive tests is a cube measuring two inches on a side. The specimens are prepared and treated in the same way as briquets for tensile tests. Before testing, two opposite faces of the cubes are usually ground so as to be true planes, parallel to each other, or the opposite sides may be faced with plaster of Paris, though this is not recommended. Grinding two surfaces to true planes increases very much the work involved in testing, so that several tensile tests may be made in the time required to make one compressive test.
Although tests of compressive strength are of interest from a scientific point of view, it is not considered that they would give much greater information concerning the relative qualities of cements than is given by tensile tests, and therefore they need not be included in an ordinary series of acceptance tests.
Although cement is frequently called upon to withstand a shearing stress, tests of this kind are very seldom made. Some of the difficulties encountered in compressive tests are also present in tests of shearing. Prof. Cecil B. Smith made quite an extended series of shearing tests by cementing together three bricks, the middle one projecting above the other two, and the pressure being so applied as to avoid any transverse stress. It is evident that by this method the adhesive strength is also brought into play. Shearing tests need not be included in normal tests of quality.