mortars are made with one part typical Portland cement in proportions of 1:3 and 1: 4, and in addition a 1:3 mortar is made from sand screened to one size between Nos. 30 and 40.sieves and from stone screenings sifted to one size between Nos. 10 and 20 sieves. These mortars are molded into tensile briquets, 2-inch cubes, and transverse test pieces of 1-inch cross section 13 inches long.

STRENGTH TESTS.

Apparatus. An improved Fairbanks shot machine is used for the tension tests, and a 40,000-pound capacity oil-pressure hand-operated machine, shown in Pl. III, A (p. 22), is used for the compression tests. The transverse tests are made either on the 10,000-pound wire tester, which is fitted with transverse tools, or on a long-lever 2,000pound Tinius Olsen & Co. machine with a special bearing made by the same company, as shown in Pl. III, C. When transverse specimens are being tested, the heavy counterpoise is replaced by a light wooden one loaded with shot. This requires a greater movement of the poise to balance the same load, and thus permits the small loads sustained by the beams to be more accurately measured than with the original counterpoise.

Methods. The methods recommended by the special committee on uniform methods of the American Society of Civil Engineers are used in the mixing and molding. The test pieces are placed in the testing machine upon their sides (with reference to the position in which they are molded).

Computations. The only computations necessary are those for obtaining the unit strengths from the gross loads read at the machine and the averages of the three tests in each case.

DENSITY TESTS.

Apparatus.--The implements used in the density tests are shown in Pl. VIII, A. Two sizes of molds are used in order to determine which size gives the more uniform results. After this has been decided, it is proposed to abandon the one giving the less consistent results. The molds are sections of wrought-iron pipe capped at one end. One is 3 inches in diameter and 4 inches deep, while the other is 4 inches in diameter and 5 inches deep, inside dimensions. The diameter of the tamper head is one-half the diameter of the cylinder in each case, so that when the tamper is moved around the inner circumference of the cylinder and kept in contact with the mold the entire surface of the mortar in the cylinder will be tamped. The table top is of slate.

Method. The required proportions of the dry material are carefully weighed out, and water is added to form a normal consistency. The

mixing is continued for two minutes and the mortar placed in the molds in layers about 1 inch thick and thoroughly and uniformly tamped. The top surface is troweled off even with the top of the mold and allowed to stand one-half hour. The weight of the full mold is then taken. The amount of shrinkage of the mortar from the top of the mold is measured by a steel rule.

Computations. The weight of each ingredient in the mixture multiplied by the ratio of the weight of the mortar in the mold to the total weight of the mixture gives the weight of the cement and sand entering the cylinder. The absolute volume of the sand and cement in the molds is computed by dividing their weights in grams by the respective specific gravities. The sum of these absolute volumes is then divided by the volume of mortar in the cylinder, and thus the density obtained.

The values are recorded on Form C.

Form C. {

Station.

Sand reg. No.

UNITED STATES GEOLOGICAL SURVEY.
STRUCTURAL-MATERIALS TESTING LABORATORIES.

No.

Approved:

MORTAR
REPORT.

Cement reg. For parallel cement tests, see Cement report on.. sand passed through No. sieve and retained on No. Proportions of mortar by weight, ......; by volume, Water used

Temperature at Tension-water...... F.; air.. ° F. Compression-water..
Flexure-water..... ° F.; air..... ° F. Shear-water.....

mixing.

sieve.

-per cent.

° F.; air.... ° F. ° F.; air..... ° F.

The large material, crusher-run stone, pit-run gravel, etc., is made into concrete with typical Portland cement, using different proportions, and the concrete investigated as to its compressive strength, its modulus of elasticity, and its density.

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 1-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 31⁄2 by 14 inches at the ends and weighing 12 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