Various tests have shown ultimate bond strengths for ordinary concrete front about 200 to 700 pounds per square inch of surface area of bar. In general, for concrete as commonly employed in structural work the unit bond resistance for plain bars may be from 200 to 300 pounds per square inch.
Twisted and deformed bars are made in a number of forms for the purpose of increasing the bond strength, and are extensively used in reinforced concrete work; their raised projections or uneven surfaces give a mechanical bond and carry considerable more load before finally yielding than plain bars, although initial slip may occur under about the same stresses.
102. Reinforcing Steel.—The Joint Committee on Concrete makes the following recommendations' concerning steel for reinforce ment bars: The Committee recommends as a suitable material for reinforcement, steel of structural grade filling the requirements of the Specifications for Billet-Steel Concrete Reinforcement Bars of the American Society for Testing Materials.
For reinforcing slabs, small beams or minor details, or for reinforcing for shrinkage and temperature stresses, steel wire, expanded metal, or other reticu lated steel may be used, with the unit stresses hereinafter recommended.
The reinforcement should he free from flaking rust, scale or coatings of any character which would tend to reduce or destroy the bond.
The Specifications of the American Society for Testing Materials are given in their Book of Standards or may be obtained in reprints from the Secretary of the Society.
On important work, it is common to purchase steel subject to these specifications, and to submit steel to careful inspection at the mills.
Engineers differ as to the advisability of using " hard-grade " steel for reinforcement. As a concrete beam usually gives way when the yield point of the steel is reached, through the cracking and crushing of the concrete, the yield point may he considered as the ultimate strength for concrete work, and some engineers prefer to use hard-grade steel on account of its high yield point. Medium steel is, however, usually preferred as less expensive and less likely to he brittle. When hard-grade steel is used, either high carbon or cold deformed material, it should be carefully tested, as it is more variable in quality than medium steel, but when meeting the specifications is a superior material.
For ordinary reinforced concrete work, mild steel as commonly found upon the market is usually employed. It is desirable to subject this to the cold bending test, which is the most important test for reinforcing stool, and upon failure the material should always be rejected.
103. Ratio of Moduli of Elasticity.—The modulus of elasticity of a material is the ratio of unit stress to the corresponding unit deformation, within the elastic limit of the material.
When two materials with different moduli of elasticity, like steel and concrete, are combined in a structural member so that they must act together, as in a column, they will each be extended or compressed to the same amount, and the unit stress carried by each material will be proportional to the modulus of elasticity of the material.
When a beam is loaded so as to cause it to bend, it is lengthened on the convex and shortened on the concave side. Tests of reinforced concrete beams show that ally plane section of the beam before bending remains approximately plane when bent, and that the amount of extension or shortening is proportional to the distance from its neutral surface. In such beams the stresses upon steel at the same distance from the neutral surface are pro portional to the moduli of elasticity of the materials.
In the discussion of stresses in any structural member of steel and concrete subject to deformation, it is therefore necessary to know the ratio of the moduli of elasticity of the two materials in order to determine the amount of stress carried by each.
The modulus of elasticity of steel is practically the same for the different grades and is independent of the ultimate strength or yield point. An average value is about 30,000,000 lb./in., and this value is usually employed in reinforced concrete computations.
The modulus of elasticity of concrete is not a constant, but varies with the stress, becoming less as the stress becomes greater. For small stresses, within the limits of allowable working stress, however, the variation is very small, and the modulus of elasticity may be taken as constant without appreciable error. The formulas in common use are based upon the assumption of a constant modulus of elasticity, and variation of stress in beam design proportional to distance from the neutral axis.