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The large material, first, has all the 4-inch material screened out and is then made into the following concretes:

(1) Using Meramec River sand in proportions of 1:3:6, 1:2:4, in such amounts that the cement is 10 per cent in excess of the amount required to fill the voids in the sand and the mortar 10 per cent in excess of the amount required to fill the voids in the stone, and in proportions which will produce the greatest density as determined by the yield test, when the cement is first one-ninth and second onesixth of the total aggregate. Meramec sand is a bar sand of excellent and uniform quality, donated by a company operating on the Meramec River near St. Louis, Mo.

(2) Using the 4-inch screenings in place of the sand in proportions of 1:3:6, 1:2:4, and in the proportion producing the maximum densities, as with the Meramec sand. The concrete is mixed in a one-half cubic yard Chicago cube mixer. This mixer is equipped with a charging hopper and a direct-connected motor. Water is supplied from a barrel which rests on a platform scale, so that the amount of water used may be weighed. The barrel is fed from a faucet and discharges through a quick-closing faucet into a large funnel. The water passes from the funnel to the mixer through a 2-inch hose. The concrete is molded into cylinders 8 inches in diameter and 16 inches long and into 6-inch cubes. Both cylinders and cubes are tested for compressive strength, and on the cylinders the modulus of elasticity is also determined. The cubes and cylinders are tested at 28, 90, 180, and 360 days, three similar pieces being tested at each age.

STRENGTH TESTS.

Apparatus.-The cube and cylinder molds are shown in Pl. VIII, B. They are of cast iron, with the inner surfaces machined. The clamp screws are of brass. In testing the cylinders and cubes, a 12-inch spherical bearing block (shown in Pl. VIII, C) is used to give a uniform distribution of the load.

Method. (a) Molding: The concrete is made of medium consistency (see description of consistencies under "Beam section," pp. 49-50), and the tamping is done by hand in 3-inch layers, using tampers 34 by 11 inches at the ends and weighing 124 pounds each. The greatest care is exercised to insure the uniform tamping of all test pieces. The cylinders and cubes are permitted to remain in the molds for twenty-four hours; then they are placed in a moist room. This room is lined with waterproof paper, and either steam or water may be sprayed into the air from a number of spraying nozzles at the ceiling. The specimens are sprinkled with water at regular eight-hour intervals. (b) Testing: The cubes are centered in the testing machine on a spherical bearing block and bedded top and bottom with asbestos

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D. TYPICAL CONES FORMED BY RUPTURE OF CONCRETE CYLINDERS.

board one-eighth inch thick (Pl. X, A, p. 48). The testing of the cylinders is described under the next heading.

MODULUS OF ELASTICITY IN COMPRESSION.

Apparatus.-The compressometer used in measuring the vertical deformations of the cylinders during testing is shown in Pl. IX, C. The gage length is exactly 12 inches. The micrometers measure to one ten-thousandth inch, and the contact is read by listening for the making and breaking of the electric circuit in the usual telephone operator's receivers (shown at the left in Pl. IX, C), which are held to the ears of the operator by clamps.

Method. The ends of the cylinders are smoothed off with plaster of Paris at right angles to the axes a short time before testing. This insures an even distribution of the breaking load over the ends. For this purpose a rigid table is used, the top of which is horizontal and consists of a heavy piece of plate glass about 1 foot square. After the glass has been oiled, plaster of Paris is spread on it about onefourth inch thick, into which, before it sets, the cylinder is forced, so that all but a layer about one-sixteenth to one-eighth inch thick is squeezed out. The sides of the cylinder are made vertical by means of a spirit level. After the plaster at one end has set, the other end is treated in the same way. The cylinders thus prepared are placed in the testing machine without further bedding.

The load is applied continuously at a speed of about one-fortieth inch per minute, and readings of the gross deformations are taken at 5,000-pound intervals or at about 100 pounds per square inch. The load at first crack, the ultimate load, and the appearance of the ruptured cylinder are recorded, the latter in the form of a sketch. In Pl. IX, A and B, are shown typical forms of failure. In the example shown in B the rupture has occurred by shearing through the aggregate. Pl. IX, D, shows three typical cones from broken cylinders.

DENSITY TESTS.

Apparatus. The tests for density of concrete are made in cylinder molds of wrought-iron pipe similar to those used for the determination of the density of cement mortar, except that they are 8 inches in diameter and 9 inches long, or 10 inches in diameter and 12 inches long.

Method. The test is made in the same manner as that for density of mortar. In the series of tests now under way the proportions of cement and large and small aggregates are varied in order to obtain the mixture that will produce concrete with the greatest density. The ratio between the cement and the total aggregate is kept constant (1:9), while the ratio between the small and large aggregate is varied.

In another series about to be started this ratio of cement to total aggregate will be 1:6. Four or five trials are usually sufficient to determine the proportions giving a concrete of the greatest density. The results of these tests are recorded on Form D.

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Form E is the log sheet upon which the micrometer readings in tests of modulus of elasticity are recorded, and Form F is used at the time of molding the test pieces. A record is made in the batch report of the basis upon which the proportioning was done, whether by volume, maximum density, or percentage of voids.

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Computations. The proportions by weight entered on the batch report are determined from the weight per cubic foot of the material and the weight of the moisture contained in it and correspond to the proportions by volume.

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