Page:Popular Science Monthly Volume 37.djvu/268

256 the same way have shown that these were unreliable as final tests of strength, because the briquettes had not hardened sufficiently, and the table would place inferior cements above those of much greater strength because the inferior develops its ultimate strength much sooner. But a comparative test of the same cements when mixed with sand in equal parts was also made, and is of very great value and probably perfectly reliable, for the tests were then made at the end of ninety days, so giving the slow-setting cements time to develop their strength.

Thus the one which in the test applied to clear cement broke at 38 pounds now sustained 152 pounds, an increase of four hundred per cent; while the one that was the strongest at the end of seven days now broke at 204 pounds, an increase of only fifty-four per cent. The one that showed the greatest tensile strength of all, at the end of ninety days, the Milwaukee cement, 290 pounds, broke at only 96 at the end of seven days. An experiment made with a briquette taken at random, that had been made six months and exposed to the air at least half that time, strikingly showed the same fact, for it broke only under a strain of 636 pounds. This test was made simply to show the writer the method of using the testing machine.

The United States Government had a series of tests made a few years ago, using the cements commonly sold in the West, and giving in each case the mean result of seventy-five tests. The table is so interesting that we give it entire.

Cement is far more often called upon to resist a crushing than a tensile strain. A large number of tests has been made to determine the weight required to crush a cube one inch in each dimension. When mixed with sand in equal proportions, the best cements will sustain a crushing weight of upward of a ton, the specimen having been allowed to harden for ninety days, while the poorest do not sustain quite half a ton, and even when mixed with three parts sand to one of the cement, the Milwaukee, which tests have shown the best, sustains over eleven hundred pounds. These tests show conclusively that structures well built of mixed cement