The reinforcing steel usually consists of small bars of such shape and size that they may easily be bent and placed in the concrete so as to form a monolithic structure. To distribute the stress in the concrete, and secure the necessary bond between the steel and concrete, the steel required must be supplied in comparatively small sections. All types of the regularly rolled small bars of square, round, and rectangular section, as well as some of the smaller sections of structural steel, such as angles, T-bars, and channels, and also many special rolled bars, have been used for reinforcing concrete. These bars vary in size from 1/4 inch for light construction, up to 1 1/2 inches for heavy beams, and up to 2 inches for large columns. In Europe, plain round bars have been extensively used for many years; and in the United States also, they have been extensively used, but not to the same extent as in Europe; that is, in America a very much larger percentage of work has been done with deformed bars.

140. Plain Bars

With plain bars, the transmission of stresses is dependent upon the adhesion between the concrete and the steel.

Square and round bars show about the same adhesive strength, but the adhesive strength of flat bars is far below that of the round and square bars. The round bars are more convenient to handle and easier obtained, and have, therefore, generally been used when plain bars were desirable.

141. Structural Steel

Small angles, T-bars, and channels have been used to a greater extent in Europe than in this country. They are principally used where riveted skeleton work is prepared for the steel reinforcement; and in this case, usually, it is desirable to have the steel work self-supporting.