SHEAR AND TENSION SECTION.

OUTLINE OF INVESTIGATIONS.

Shear. A schedule of the tests on the strength of concrete in shear is given in the subjoined table (p. 67). The test pieces are cylinders 6 inches in diameter and 18 inches long. The molds are similar to those for making the regular compression cylinders, differing only in size. The testing apparatus is operated between the heads of the Universal testing machine, and the three cutting tools are arranged to shear out a section ranging from 12 to 2 inches in thickness. Only the top half of the 2-inch piece is used. The 4- and 6-inch pieces can be used with the bottom piece so as to incase the whole section. The object of these bottom pieces is to prevent any bending action in the test piece for the larger sizes. The results thus far obtained seem to indicate that this precaution is unnecessary. Pl. XIX, A, shows a

test piece in the machine ready for testing. The diameter of the bearing surfaces is 6 inches and the 1-inch space between the test piece and the castings is filled with plaster of Paris. A spherical bearing block is placed on the top of the cutting tool. The bed plate is slotted so that the end support of the cylindrical test piece may be adjusted for any width of cutting tool.

Tension. A schedule for determining the modulus of elasticity and strength of concrete in tension is shown in the table (p. 67). The test pieces will be of the flat dumb-bell shape, about 8 feet long over all, having a stem 8 by 8 inches in cross section. One of these test pieces is shown in fig. 5, and the grip or apparatus for testing it is shown in fig. 6. The grip is constructed by bending an 8-inch channel so as to inclose the head of the test piece and to bear against the inner surfaces of the head. The two sides of the channel are kept from spreading apart by plates extending from side to side. One of the plates is riveted to the flanges of the channel; the other is held

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by bolts so that it can be taken off for the purpose of placing and removing the test piece.

FIG. 6.-Apparatus for concrete tension tests.
Shear and tension tests proposed for 1907.

Concrete (proportions by volume).

(days).

Number of pieces.

MIXING, MOLDING, AND STORAGE.

The specimens for the shear and tension tests are mixed, molded, and stored under the same conditions as in the case of beams (pp. 48–53).

COMPUTATIONS.

The unit shearing stress is found by dividing one-half the shearing load by the area of a vertical section of the test piece. For the tension tests the dimensions of the section and the distance of the break from the center will be given, besides a complete record of the usual information regarding the making of the test pieces. The unit strength and the initial modulus of elasticity will be calculated from the results.

A series of parallel tests will be made, including a cylinder for compression tests, so that values for a particular lot of concrete will be obtained, giving the strength in shear, in tension, and in compression, and also the modulus of elasticity for tension and for compression. In the report a complete record will be given of all the materials used in making the test pieces, together with a record of the conditions governing the making, storing, and testing.

BUILDING-BLOCK SECTION.

OUTLINE OF INVESTIGATIONS.

In the building-block section an extensive series of investigations on the properties of building blocks is under way. Five different types of building blocks are used. They are made up of different proportions and different aggregates and are tested at different ages. With each series of blocks a set of cylinders 8 inches in diameter and 16 inches in length are made, and these are stored under the same conditions as the blocks. The blocks are subjected to strength tests in cross bending and compression, and to fire tests to determine what aggregates and proportions offer the greatest resistance to fire.

A large number of mortar blocks have been made and many of these have been tested. The schedule of tests now under way comprises the manufacture on five different types of block, mixed in proportion of 1 part typical Portland cement to 2, 4, and 8 parts Meramec River sand, and of damp, medium, and wet consistencies. The strength tests cover periods of 28, 90, 180, and 360 days. This series necessitates the making and testing of 720 blocks and 360 cylinders.

In the case of blocks made of concrete, which include aggregates of limestone, gravel, granite, and cinders, proportions of 1:2:4, 1:2:5, and 1:3:6 are used. For convenience in handling, all volumes are reduced to weight and all materials are charged into the mixer by weight.