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 reinforced-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 jailing point. It has been shown by beam tests, that when the yield point of the steel is reached, the beam sags because of the stretching or slipping of the steel, and the top of the beam is likely to crush.

138. 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 28,000 to 35,000 pounds per square inch. The elongation should be 25 per cent in 8 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 35,000 to 40,000 pounds per square inch. The elongation should be 22 per cent in 8 inches, and the specimen should 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 8 inches for a test piece 3/8 to 3/4 inch in diameter. A test piece 1/2 inch in thickness 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 specifications 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:

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 purchased 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.