The method recommended by M. H. LeChatelier for testing soundness requires the use of a cylindrical mold, about 1 1/4 inches in diameter and of about the same height, which is made of thin metal and slit along a generatrix. The mortar is to be placed in the mold as soon as made, and immersed at once in cold water; the mold is held firmly by a clamp, and a flat plate at either end of the mold retains the mortar in shape until set. When setting has taken place, the mold is unclamped and the widening of the slit indicates the expansion of the mortar. If desired, the swelling may be increased and hastened by transferring the mold and its contents to hot water as soon as the cement is set. The same writer has suggested a modification of the hot test by placing briquets in cold water and gradually heating to near the boiling point, this temperature being maintained for six hours.
1 "Notes on the Testing and Use of Hydraulic Cement," by Fred. P. Spalding.
Various other tests have been suggested, such as the effect of regaging; withstanding immersion as soon as gaged; allowing large thin cakes to harden in air and striking them to obtain a musical sound. Most of these tests, however, are worthy of passing notice only.
There are but few experiments to show that a cement which will actually fail and disintegrate when properly used, may still pass the cold water neat pat test; yet there is no doubt that inferior cements may pass this test perfectly, "inferior cements" being those which will not give the best results in practice, though they do not disintegrate.
Cement is at present used in a very crude way, and it is only in exceptional cases that a poor quality of material may be detected in the completed structure. This is sufficient reason why so few failures can be found in cement work which may be attributed to a poor quality of cement. But in the more economical manner in which this material is, even now, beginning to be used, it is absolutely essential to know what its future behavior will be. That the cement will never be exposed to hot water in actual use, is a weak argument against hot water tests. It must be remembered that the chief object of testing cement is to arrange the various products in their true order of merit, and any system which will effect this result is perfectly legitimate. On the other hand, it is due to the manufacturers that a test which will occasionally reject perfect cements should not be adopted when it is possible in any other way to detect poor products with certainty.
132. It is possible that the temperature used and recommended by Mr. Faija is sufficiently high to detect unsoundness or a tendency to "blow." It has never been clearly proved that it is not, but the higher temperature of 70° to 100° C. has appeared to meet with greater favor. The writer made a few experiments to compare results obtained with mixtures of Portland cement and lime when using the temperature of 110° Fahr. (43° C.) with those obtained in water at 190° Fahr. (88° C), and in water at the ordinary temperature of 60° to 65° Fahr. (16° to 18° C). Quicklime, in proportions varying from one to ten per cent., was added to the cement, and seven pats were made from each mixture of cement and lime.
These pats were subjected to the following treatment: —
Pat No. 1, placed in vapor of water at 110° F. when made.
" 2, " " " 110° F. when set.
" 3, " " " 110° F. after 24 hours.
" 4, " " " 190° F. when made.
" 5, " " " 190° F. when set.
" 6, " " " 190° F. after 24 hours.
Above six pats immersed in the hot water after three hours in vapor.
Pat No. 7, placed in cool water when set.
When no lime was added, pats 1, 2 and 3 revealed no defects; pats 4 and 5 showed small cracks in two days, but pat No. 6 still adhered to the glass after eight days. Pat No. 7 was perfect after two months. With 2 per cent, lime added to the cement, pat No. 1 was slightly warped and cracked, and Nos. 2 and 3 were off glass; Nos. 4 and 5 were cracked and warped; No. 6 was off glass, and No. 7 became detached from glass after two months, but was otherwise perfect. With 4 per cent, lime, all the pats failed, the one in cool water being off glass, cracked and warped after one day.
It must be remembered that the free lime occurring in cement is of a different character from the quicklime added in these tests, because the former contains impurities and has been calcined at a very high temperature, and would therefore slake more slowly. It has been said that as small an amount as 1 per cent of free lime in cement is dangerous. If this is true, and it probably is, the temperature of 110° Fahr. would seem to be inadequate to quickly indicate a tendency to "blow".
133. Some of the results obtained by M. Deval have already been given (§ 127). Mr. Maclay made similar tests on several samples of Portland cement, using a temperature of 200° Fahr., but these tests only permit of comparing the strength acquired in cold water in seven and twenty-eight days with the strength in hot water at ages of from two to seven days. Long time tests, showing that the cements which give low results in hot water and normal results in cold water on short time tests, give in reality a low strength at the end of six months or more, have been almost entirely lacking until very recently.
Table 40, § 226, gives some of the results obtained by the author in hot tests and long time cold tests on Portland cement. It is seen that the hot test at 80° C. indicated, in seven days, the inferior quality of sample W, although it gave normal results in cold water up to twenty-eight days; the two year tests with mortars containing two parts or more sand, show it to be inferior. If we attempt to carry the analogy too far, however, we fall into the error which placed the hot test in disrepute for several years, that is, we must not expect that the strength in cold water after a long time will be exactly proportional to the strength developed in hot water in a few days.