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 days, respectively. The ultimate strength of the cement is judged by the increase in strength between the two dates. A mini 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 the 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 Gin 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 11 on the gradu ated beam D, and the weights H on the counterpoise weight G.
The result will show the number of pounds required to break the specimen.
Cement is shipped in barrels or in cotton or paper bags.
The usual dimensions of a barrel are: length 2 ft. 4 in., middle diameter 1 ft. 4i in., end diameter 1 ft. 3} in.
The bags hold 50, 100, or 200 pounds.
A barrel weighs about as follows: Rosendale, N. Y 300 lb. net Rosendale, Western 265 " Portland 375 " A barrel of Rosendale cement contains about 3.40 cubic feet and will make from 3.70 to 3.75 cubic feet of stiff paste, or 79 to 83 pounds will make about one cubic foot of paste. A barrel of Rosendale cement (300 lb.) and two barrels of sand (71 cubic feet) mixed with about half a barrel of water will make about 8 cubic feet of mortar, sufficient for 92 square feet of mortar-joint 1 inch thick.
288 " " " 3S4 " " " 768 " " " A barrel of Portland cement contains ".bout 3.25 to 3.35 cubic feet-100 pounds will make about one cubic foot of stiff paste.
A barrel of cement measured loosely increases considerably in bulk. The following results were obtained by measuring in quan tities of two cubic feet: 1 bbl. Norton's Rosendale gave 4.37 cu. ft.
" Anchor Portland gave 3.65 " " Sphinx Portland gave 3.71 " " Buckeye Portland gave.. 4.25 " Preservation of Cements. Cements require to be stored in a dry place protected from the weather; the packages should not be placed directly on the ground, but on boards raised a few inches from it. If necessary to stack it out of doors a platform of planks should first be made and the pile covered with canvas. Port land cement exposed to the atmosphere will absorb moisture until it is practically ruined. The absorption of moisture by the natural cements will cause the development of carbonate of lime, which will interfere with the subsequent hydration.