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Reinforced Concrete

steel, bond, strength, bars, stresses, materials and construction

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REINFORCED CONCRETE Object of Reinforcement.—Conerete and steel are frequently combined in structural work in two distinct types of construction: 1. Structural Jlenth'rs of Steel Encased in this type of construction, the steel member is designed to carry the loads, and the concrete is used for protection of the steel against the weather or fire, or sometimes to give lateral stiffness to the member.

2. Reinforced in which the load-carrying member is made of concrete, the steel being used to strengthen the concrete by taking stresses that the concrete is unfitted to resist.

Structures of the first type, in which the concrete is used to give stiffness to the structure, are often classed as reinforced concrete, although reliance is placed upon the steel alone for carrying the loads. These are not, however, designed in accordance with the theories of reinforced concrete.

The advantages to be gained by combining steel and concrete are due to the fact that concrete is extremely weak and uneconomical when subjected to tension, but has much greater strength and is a convenient and economical material for resistance to compressions, while steel must be made of special forms satisfactorily to carry compression, but may be used for resisting tension in the form of ordinary bars.

In structural forms, such as beams, in which both tensile and compressive stresses are developed, the combination of the two materials offers an economical means of construction when the con ditions are favorable, and the use of this type of construction bas been rapidly extending during the past few years.

In the use of concrete and steel in combination, the following properties of the materials are important: 1. When st-eel bars are imbedded in concrete, the concrete adheres to the steel and develops a considerable bond strength, which may be relied upon to make the two materials act together.

2. Concrete acts as a protection to the steel against rust. In a number of instances in removing concrete structures, it has been found that the steel, after being embedded for several years, was in good condition and free from rust. To form an efficient protection

the concrete must be mixed rather wet, so that the steel is completely covered with it coating of mortar.

3. The coefficients of expansion for the two materials are so nearly the same that no stresses need be considered as resulting from differences of expansions or contractions clue to changes in temperature.

4. Changes in dimension occur in unreinforcecl concrete during hardening (see Section 85) and with variations in moisture condi tions. When these changes are restrained by reinforcement, the concrete seems to adjust itself to the situation, adopting permanently the form in which it is held, without being placed under appreciable stress.

101. Bond Strength.—The stresses carried by the steel in a reinforced concrete structural member must usually be trans mitted to the steel through the bond between the steel and concrete. Tests and experience show that plain steel bars imbedded in concrete develop considerable bond strength which may be relied upon to hold the bars permanently in place in resisting stresses which tend to separate them from the concrete. Experiments upon the adhesion of plain bars to concrete show that the bond strength is approximately proportional to the area of surface contact, and varies with the quality of the concrete, being nearly proportional to the strength in compression.

When tests are made by pulling a bar of steel out of a block of concrete in which its end has been embedded, the compression of the concrete may influence the results, and the bond strength shown he greater than would be developed in a bean where both steel and concrete are under tension. When the length of the bar embedded in the concrete is considerable, the bar may begin to slip at the sur face of the block before the resistance of the more deeply embedded part is fully brought into play, and the bond resistance per unit of surface area be less than for shorter lengths. In tests at the University of Wisconsin no differences in unit bond strengths was found between 6-inch and 12-inch embedments.

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