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Reinforced concrete

Reinforced concrete is a combination of two of the strong-' est structural materials, concrete and steel, either of which is used separately. Consequently, the resulting material has the advantages of both.

Concrete has poor elastic and tensional properties, but it is rigid, strong in compression, extremely durable under and above ground, and in the presence or absence of air and water, it increases in strength with age, it is fire resistant, and suitable for any architectural or engineering forms.

Steel has great tensional, compressive and elastic prop­erties but is not durable below ground or if exposed to moisture, it loses its strength with age or exposure to high temperature. Steel is also fragile in appearance, giving an impression of weakness rather than strength. When strength is the most important consideration great care must be taken to produce a concrete that will be homogeneous and as dense as possible. Homogeneity is not possible without thorough mixing. To obtain strength and density, the aggregate must be sand and gravel, or crushed rock clean and free from or­ganic matter, and carefully graded from the smallest to the largest particles.

For general work the most suitable proportions of cement and aggregate are: 1 part cement, 2 parts sand, and 4 parts gravel or crushed rock.

Let us consider what is the effect of the addition of steel reinforcement to a concrete.

Steel is a material which does not undergo shrinkage on drying, and therefore the steel acts as a restraining medium in a reinforced concrete member; shrinkage causes tensile stresses in the concrete which are balanced by compressive stresses in the steel. Now the steel itself deforms under the induced stresses so that there must be a resultant shrinkage moment in a reinforced concrete member.

An economic quantity of steel, however, can be so disposed as to distribute the cracking under shrinkage stresses and thus render it innocuous. This is the problem for the designer of a reinforced concrete building, and it demands careful consideration on a proper design basis, both of the quantity and of the distribution of the reinforce­ment.

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