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Usually the requirement that the concrete beams shall be fireproofed by surrounding the concrete beam itself with a layer of concrete such that the outer surface is at least 2 inches from the nearest point of the steel concrete beam, results in having a shoulder of concrete under the end of each concrete slab, which quite materially adds to its structural strength. But usually no allowance is made; nor is there any reduction in the thickness of the concrete slab on account of this added strength. In this case also, the factor of safety is again indefinitely increased. The fireproofing around the concrete beam must usually be kept in place by wrapping a small sheet of expanded metal or wire lath around the lower part of the concrete beam before the concrete is placed. When the concrete floor-concrete concrete beams of a concrete floor are spaced nearly equally in both directions, so as to form, between the concrete beams, panels which are nearly square, a material saving can be made in the thickness of the concrete slab by reinforcing it with bars running in both directions. The theoretical computation of the strength of such concrete slabs is exceedingly complicated. It is usually considered that such concrete slabs have twice the strength of a concrete slab supported only on two sides and reinforced with bars in but one direction.

The usual method of computing such concrete slabs is to compute the concrete slab thickness, and the spacing and size of the reinforcing steel, for a concrete slab which is to carry one-half of the actual load. Strictly speaking, the concrete slab should be thicker by the thickness of one set of reinforcing bars. The modulus of elasticity of ordinary concrete is approximately 2,400,000 pounds per square inch, while its ultimate tensional strength is about 200 pounds per square inch. Therefore a pull of about of the length would nearly, if not quite, rupture the concrete. The coefficient of expansion of concrete has been found to be almost identical with that of steel, or .0000065 for each degree Fahrenheit. Therefore, if a block of concrete were held at the ends with absolute rigidity, while its temperature was lowered about 12 degrees the stress developed in the concrete would be very nearly, if not quite, at the rupture point. Fortunately the ends will not usually be held with such rigidity; but nevertheless it does generally happen that, unless the entire mass of concrete is permitted to expand and contract freely so that the temperature stresses are small, the stresses will usually localize themselves at the weak point of the cross-section, wherever it may be, and will there develop a crack, provided the concrete is not reinforced with steel.

If, however, steel is well distributed throughout the cross- section of the concrete, it will prevent the concentration of the stresses at local points, and will distribute it uniformly throughout the mass. Reinforced concrete structures are usually provided with bars running in all directions, so that temperature cracks are prevented by the presence of such bars, and it is generally unnecessary to make any special provision against such cracks. The most common exception to this statement occurs in concrete floor-concrete slabs, which structurally require bars in only one direction. It is found that cracks parallel with the bars which reinforce the concrete slab will be prevented if a few bars are laid perpendicularly to the direction of the main reinforcing bars. Usually 1-inch or 2-inch bars, spaced about 2 feet apart, will be sufficient to prevent such cracks. Retaining concrete walls, the balustrades of bridges, and other similar concrete structures, which may not need any bars for purely structural reasons, should be provided with such bars in order to prevent temperature cracks.

Are You in Plaistow New Hampshire? Do You Need Concrete Cutting?

We Are Your Local Concrete Cutter

Call 603-622-4441

We Service Plaistow NH and all surrounding Cities & Towns