Some experiments made to compare the strength of briquets which were alike in all other respects, but were immersed in different tanks in which the water had not been frequently renewed, showed very clearly the possible variations from this source. Natural cement briquets, neat, and with one and two parts sand, gave, when immersed in one of the tanks, only from 40 to 60 per cent, of the strength attained in another tank by briquets entirely similar.
To store briquets in running water is going to the other extreme; this appears to be the best method, at least for short-time acceptance tests, provided the temperature can be regulated. However, in some cases where this has been adopted, the strength of the briquets is said to have fallen off very much after four or five years. Whether this is due to the action of running water is a very interesting point, and a valuable one from the practical standpoint of the use of cement, but it has not yet been thoroughly investigated.
195. It appears from the foregoing that variations in the temperature and freshness of the water in which the briquets are immersed is an uncertain contingent, and therefore that all such variations should be carefully avoided. As a matter of convenience, the tanks may well be maintained at 60° to 70° Fahr., but if one does not care for a comparison of his results with those obtained in other laboratories, then any other constant temperature between 40° and 75° may be adopted. The water in the tanks should be renewed at least once a month, and preferably once a week.
When the cement under test is to be used for constructions in the sea, some of the briquets should be stored in sea water to indicate the behavior in this medium. Many tests have been made in this way by several experimenters, but the varied results obtained only indicate the different effects of such treatment on different samples of cement. One of the effects of storing in sea water has been touched upon under the head of consistency of mortar, where it is shown that porous briquets may disintegrate in this medium. A small specimen like a briquet will of course be more quickly affected than a large mass of concrete, but on the other hand, the concrete in work is likely to be more porous than the briquet. The effect of sea water upon cement will be taken up in another place.