Oompressive Stbfnoth of Stone Concrete

OOMPRESSIVE STBFNOTH OF STONE CONCRETE. The results of experiments to determine the crushing strength vary materially with the form of the test specimen and also with the condition of the pressed surface. Usually the test specimen is a cube, but occasion ally prisms much taller than broad have been used. The latter give much smaller results than the former. The smoothness of the pressed surface materially affects the results for compressive strength, and unfortunately there is no agreement as to the method of preparing the surface. Probably the best method is to give the surface a coat of neat cement paste or of plaster of paris, and turn it upside down upon a sheet of plate glass. If there is plenty of time for the cement to set, it is more scientific to use the cement; but the plaster of paris is more commonly used, because of its more rapid set.

Dow's Experiments.

Table 29 shows the results of a series of experiments made by A. W. Dow, Inspector of Asphalt and Cement, Washington, D. C.* Each result is the mean of two 1-foot cubes, except those for one year; which are the mean of five. Owing to the friction of the hydrostatic press with which the tests were made, the results are 3 to 8 per cent too high. With the natural cement the water used was 0.317 cu. ft. (20 lb.) per cu. ft. of rammed concrete, and with portland cement 0.24 cu. ft. (12 lb.)—in both cases including the moisture in the sand. The sand contained 4.4 per cent of water, which increased the volume of the sand and made the mortar slightly richer than as stated.

Rafter's Experiments. Mr.

George W. Rafter, under the direction of the State Engineer of New York, tested 542 1-foot cubes of concrete in which five brands of portland cement and one of natural were used.t The sand was pure, clean, sharp silica, con taining 32 per cent of voids. The aggregate was sandstone broken to pass a 2-inch ring, and had 37 per cent voids when tamped. In half the blocks the mortar was a little more than enough to fill the voids; and in the other half the mortar was equal to about 80 per cent of the voids.

In making the cubes whose strength is summarized in Table 30, three brands of portland cement were used which meet the specifi cations of Table 13, page 81. The mortar for these cubes was mixed

as "dry as damp earth," and the test specimens were stored under water for four months and then buried in sand. The age when tested ranged from 550 to 650 days, the average being about 600.

The cubes were crushed on the U. S. Watertown Arsenal testing machine. The individual results agreed well among themselves.

The cubes summarized in Table 30 were stored under water. An equal number of companion blocks stored in a cool cellar gave 82 per cent as much strength; those fully exposed to the weather, 81 per cent; and those covered with burlap and wetted several times a day for about three months and afterwards exposed to the weather, 80 per cent.

The cubes of Table 30 were mixed as "dry as damp earth." Companion blocks, of which the mortar was mixed to the "ordinary consistency used by the average mason," gave 90 per cent as much strength; and those mixed to "quake like liver under moderate ramming," 88 per cent.

The cubes containing mortar practically equal to the voids in the broken stone were 3 per cent stronger than those containing mortar equal to about 80 per cent of the voids.

Kimball's Experiments.

Mr. George A. Kimball, Chief Engineer of the Boston Elevated Railway Co., made 372 1-foot concrete cubes which were tested at the U. S. Arsenal at Watertown, Mass.* The results are summarized in Table 31. The mixing of the concrete for the test specimen was done as nearly as possible like that of the concrete which went into actual construction. The consistency was such that with the richer mixtures water barely flushed to the surface under severe ramming, while with the leaner mixtures the surface merely appeared moist. The cubes were stored in a room the temperature of which was usually about 40° F., although it ordinarily varied from 28° to 50° and twice fell to 20°. When the temperature of the room was below 32° the cubes were covered with burlaps. Five different brands of portland cement were used. In the first line of the table, each result is the mean of fourteen tests, and in the remainder of the table each result is the mean of twenty-six.