TESTS OF STEEL.

Method. As may be seen in the tables (pp. 38, 40-47), every rod used for reinforcement is tested. The tests include the determination of the yielding point as seen from the drop of the beam, the elastic limit obtained by the divider method, and the ultimate strength, elongation, reduction of area, and breaking strength. The elastic limit is determined on a gage length of 8 inches, one point of the dividers resting in a punch mark and the other marking on chalk rubbed on the surface of the test piece. The elongation is measured on the gage length of 8 inches. In addition to the above, the modulus of elasticity is determined on every tenth bar, using the Henning extensometer with electric contact. The percentage of carbon, phosphorus, and sulphur in every bar is determined in the chemical laboratory.

A view of the steel ready for testing is shown in Pl. XVIII, A.

Computations.—A record of steel tests is kept on a log sheet similar to Form E (p. 34). After the computations have been made the results are entered in Form O. The unit stresses at different loads are calculated by dividing the gross load by the area of the test piece. The percentage elongation in 8 inches and the reduction of area at fracture are also calculated.

The modulus of elasticity is determined by dividing any unit stress below the elastic limit by the unit elongation at that stress. The elongations as obtained by the two micrometers on either side of the test piece are averaged and divided by the gage length to obtain the unit elongation.

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 -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