225. One of the problems presented in the inspection of cement is to foretell the ultimate relative strengths of two samples from the results of short time tests. Formulas have been presented purporting to solve this problem, such formulas being based on the assumption that the strength gained at the end of months or years is a function of that developed in a few days. In fact, the raison d'etre of tensile or other short-time strength tests for the acceptance of cement, rests, in a sense, upon this same assumption.

The value of strength tests as one of the guides in determining in a short time the probable quality of a cement is unquestioned. One is apt, however, to seek too close an agreement between the results of such tests and the actual quality of the cement. It would be easy to select examples illustrating the harmony between short and long time tests; but it will be of greater value to show, rather, some of the many exceptions to such a rule, and thereby emphasize the fact that it is only by a close analysis of all of the information obtainable concerning a sample, and a general knowledge of the behavior of the different grades of cement, that one may hope to arrive at a tolerably accurate opinion.