STEEL FOR REINFORCING CONCRETE 137. Steel for reinforcing concrete is not usually subjected to so severe treatment as ordinary structural steel, as the impact effect is likely to be less; but the quality of the steel should be carefully specified. To reduce the cost of reinforce 1-concrete structures, there has been a great tendency to use cheap steel. It has been generally recognized in the design of reinforced concrete, that the yield point or elastic limit of the steel shall be considered as the failing point. It has been shown by beam tests, that when the yield point of the steel the beam sags because of the stretching or slipping of the steel, and the top of the beam is likely to crush.
135. Quality of Reinforcing Steel. The grades of steel used in reinforced concrete range from soft to fairly hard. These grades of steel may be classified under three heads: soft, medium, and hard.
Soft steel should have an ultimate strength of 50,000 to 60,000 pounds per square inch, and an elastic limit of 2S,000 to 35,000 pounds per square inch. The elongation should be 25 per cent in S inches; and the specimen should bend cold 180 degrees flat on itself, without fracture on the outside.
Medium steel, ordinary market steel, has an ultimate strength of 60,000 to 70,000 pounds per square inch; and the elastic limit ranges from 3.5,000 to 40,000 pounds per square inch. The elonga tion should be 22 per cent in S inches, and the specimen 5110111(1 bend cold around a diameter equal to the thickness of the piece tested. This steel is manufactured and sold under standard conditions, and usually it can safely be used without being tested.
Hard steel, better known as high-carbon steel, should have an ultimate strength of 85,000 to 105,000 pounds per square inch; and the elastic limit should be from 50,000 to 65,000 pounds per square inch. The elongation should not be less than 10 per cent in S inches for a test piece to inch in diameter. A test piece A- inch in thick ness should bend 100 degrees without fracture, around a diameter equal to its own. The high steel has a larger percentage of carbon than the medium steel, and therefore the yield point is higher. This steel is to be preferred for reinforced-concrete work; but it should be thoroughly tested, as many engineers object to it on account of its brittleness and the poor quality of the material from which it is sometimes rolled. On account of its higher elastic limit, a smaller
percentage of steel is required; and when rolled under proper specifi cations and inspection, high steel is more economical for use than low-carbon steel.
In high-carbon steel, the chemical properties should conform to the following limits: Phosphorus not to exceed 0.06 per cent.
Sulphur not to exceed 0.06 per cent.
Manganese not to exceed 0.80 nor less than 0.40 per cent.
Carbon not to exceed 0.65 nor less than 0.45 per cent.
In comparing the two processes of making steel, the products of Bessemer steel found in the general market are apt to be extremely irregular in their composition, although they may be rolled into like forms and sold for the same purpose. Open-hearth products pur chased in the open market and designed to serve the same purpose, are more uniform in quality. Test specimens cut from different parts of the same Bessemer steel plate, often show a wide difference in their mechanical properties. In the open-hearth steel, this wide difference is not found, this grade of steel being more homogeneous than the Bessemer plates.
139. Types of Reinforcing Steel. 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 sup plied 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 4 inch for light construction, up to 11 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.