The briquettes are then im mersed in water, where they should remain constantly covered until tested. If they are exposed to the air, the water may be carried away by evaporation and leave the mortar a pulverant mass. Also, since the mortar does not ordinarily set as rapidly under water as in the air (owing to the difference in temperature), it is necessary for accurate work to note the time of immersion, and also to break the briquette as soon as it is taken from the water. Cement ordinarily attains a greater strength when allowed to set under water, but attains it more slowly.
Age of Briquette for Testing. It is customary to break part of the briquettes at the end of seven days, and the remainder at the end of twenty-eight days.. As it is sometimes impracticable to wait twenty-eight days, tests are often made at the end of one and seven clays, respectively. The ultimate strength of the cement is judged by the increase in strength between the two dates. A mum strength for the two dates is usually specified.
Testing the Briquettes. When taken out of the water the briquettes are subjected to a tensile strain until rupture takes place in a suitably devised machine. There are several machines on the
ket for this purpose. Fig. 4 represents one which is extensively used. To make a test, hang the cup F on the end of the beam D, as shown in the illustration. See that the poise R is at the zero mark, and balance the beam by turning the ball L. Fill hopper B with fine shot, place the specimen in the clamps N N, and adjust the hand wheel P so that the graduated beam D will rise nearly to the stop K.
Open the automatic valve J so as to allow the shot to run slowly into the cup F. Stand back and leave the machine to make the test.
When the specimen breaks, the graduated beam D drops and closes the valve J, remove the cup with the shot in it, and hang the counterpoise weight G in its place.
Hang the cup F on the hook under the large ball E, and proceed to weigh the shot in the regular way, using the poise It on the gradu ated beam D, and the weights II on the counterpoise weight G. The result will show the number of pounds required to break the specimen.