159. Whether the tensile test should be applied to neat cement briquets or to those prepared from sand mortars has been a disputed point, but there are now but few authorities who recommend the use of the neat test exclusively. When tests for soundness are not carefully made, the behavior of the cement in neat briquets gives, perhaps, a better idea as to the reliability of the cement than do sand tests, but otherwise the sand test is a better index of the value of the cement. The principal objection to the sand test is that the use of sand introduces another cause of variation in the results obtained by different experimenters. This objection has considerable weight, because it is impracticable to find sand in widely separated localities which is absolutely the same in composition and physical properties; but two cements which appear to be of equal value when tested neat may exhibit quite different characteristics when used with sand, and it is believed that this fact far outweighs the objection noted. As soon as regularity in sieves is established, the size of the sand grains may be regulated. The chemical and physical properties of the sand and the shape of the grains is a more difficult matter. The crushed quartz that is used in the manufacture of sandpaper was recommended by the Committee of the American Society of Civil Engineers of 1885, and if some care is taken to select that which is clean and made from pure quartz, there is little difficulty in obtaining a uniform product of this kind.
The German Normal Sand is obtained by washing and drying a natural quartz sand. In various parts of Germany sand answering the purpose may be found. Some tests made in this country to compare the "normal" German sand with American crushed quartz have shown the sand to give a somewhat higher strength, while other tests have shown an opposite result.1 A few of these tests are given in Table 27.
Strength of Mortar, 1 Cement, 3 Sand, Obtained at Laboratory Number
7 7 28 28
218 253 317 334
173 219 341 300
201 211 281 283
Per Cent, of water Used.
. . .
Mr. Max Gary has stated that "the Russian standard sand gives markedly lower, and the Swiss sand considerably higher, strength than the German".
It is not to be concluded from what has preceded that one must make mortar tests with a "standard" sand only. On the contrary, one may obtain valuable results by using in tests the sand which it is proposed to use on the work. The only point to be insisted upon is that a cement shall not be rejected on account of the poor quality of the sand used in testing. It is thus very desirable that a certain proportion of the tests be made with a pure quartz sand, and by making parallel tests with the natural sands, the coefficient of the latter may be obtained. In any case it is necessary, in order to obtain comparable results, to sift the sand used for tests.
1 Article by Clifford Richardson, Engineering Record, Aug. 4, 1894.
The American practice in using crushed quartz is to reject the coarser particles by a sieve having 20 meshes per linear inch (holes about .03 inch square) and to reject the finer particles by a sieve of 30 meshes per linear inch (holes about .02 inch square). The size of grain of German normal sand is practically the same. In using a natural sand it is not necessary to use this size of grain, but it is better to do so, or at least to use some definite size or definite combination of sizes; as, for instance, one-half of 20 to 30 (passing holes .03 inch square and not passing holes .02 inch square) and one-half 30 to 50 (passing holes .02 inch square and failing to pass holes .012 inch square). Such a method will permit of duplicating a given size of grain at any time, while if the sand is used as it occurs in nature, considerable variations will be found. The effect of the quality of sand on the strength obtained is discussed in Chapter XI.