It has been thought that briquets, made to test cement that is to be used in air, should be hardened in the same medium in order that the tests should more nearly approach the conditions of use. Several points, however, should be borne in mind in interpreting the results obtained with air-hardened specimens. In actual work the mortar is usually in a large mass, or is protected from the influence of a warm, dry atmosphere, so that it remains moist for a long time, whereas a briquet placed in the open air is much more affected by changes in atmospheric conditions. If the briquets are allowed to harden in a room, such a small quantity of mortar may become quite dry in a few days, and, unless the amount of moisture in the air is regulated, another source of variation is introduced in the tests.
It has been found impossible to obtain uniform results from briquets made as nearly alike as possible and stored side by side in the air of the laboratory. The regular acceptance tests should, therefore, it is thought, be made in the ordinary manner, but if cement is to be used in locations where it is likely to become very dry, a few special tests should be made to assure one that the brand of cement in question is one that will yield good results in such exposure. It may be found that certain kinds or brands should be entirely avoided for use in such locations. A few tests of this character are given in Tables 72 and 73, §§ 359, 360. The results in any given line of the table are from briquets made the same way but treated differently in the method of storing. It is seen that these brands harden well in dry air. The effect of the amount of water used in gaging appears to follow somewhat the same law, whether the briquets are stored in air or water.
A method more nearly approaching conditions that frequently prevail in practice is to bury the briquets in damp sand. Table 120, §409, gives the results obtained with a large number of briquets stored in this way. While the results are somewhat more irregular than those for water-hardened specimens, since the conditions cannot be made so nearly uniform, yet this method gives better results than dry air storage.