OFFICIAL REPORT ON REINFORCED CONCRETE As an official pronouncement representing a summary of approved practice in reinforced con crete work, the following extracts from the re port of the Committee on Reinforced Concrete of the National Association of Cement Users will be found of great interest and practical value: 2. Uses. Concrete, plain and reinforced, is recognized as an established material for construction. Because of its fire-resisting qualities, strength in compression, and relatively low cost, concrete is adapted not only to struc tures where stresses are principally compressive, such as foundations, dams, retaining walls and other walls, piers, abutments, short columns and arches ; but with the use of metal reinforcement properly placed, it is also available for a variety of structures and structural forms where both compression and tension exist, as in beams and slabs.
3. Resistance to External Resistance. Concrete is incombustible and resists the action of ordinary fires, so that it may be safely used for fireproof ing purposes. The thickness of the protective coating required depends upon the probable duration of a fire which is likely to occur in a structure. For ordinary con ditions it is recommended that the metal in girders and columns be protected by a minimum of 2 inches of con crete, that the metal in beams be protected by a minimum of inches of concrete, and that the metal in floor slabs be protected by a minimum of 1 inch of concrete. In monolithic concrete columns the concrete to a depth of inches may be considered as a protective covering and not included in the effective section when computing the strength.
4. When proportioned to attain greatest density, not leaner than one part cement to two parts fine aggregate, to four parts coarse aggregate, and mixed to a rather wet consistency, concrete is impervious under ordinary conditions. Long walls to resist water pressure must be effectively reinforced to prevent cracks due to temperature contraction or shrinkage, or must be waterproofed by asphaltic or similar preparation, or the length may be broken by vertical construction joints sealed by metal flashing.
5. Corrosion of Metal Reinforcement. Steel embedded in good, dense concrete will not corrode under ordinary conditions, whether located above or below fresh or sea water level. On the other hand, if concrete is porous, it may permit moisture to penetrate and corrode the metal.
6. Sea Water. To best resist sea water, concrete must be proportioned to secure maximum density and mixed thoroughly to obtain impervious concrete.
7. Acids. Concrete of first-class quality thoroughly hardened is affected appreciably only by strong acids which seriously injure other materials. A substance like manure, because of the acid in its composition is injurious to green concrete, but after the concrete has thoroughly hardened it satisfactorily resists such action.
8. Oils. When concrete is properly made and the sur face carefully finished and hardened, it resists the action of such oils as petroleum and ordinary engine oils. Cer tain oils which contain fatty acids appear to produce injurious effects.
9. Alkalis. In the arid regions, alkaline ground water in some cases has been known to injure concrete and most other structural materials. For use under such conditions, concrete should be especially rich, dense, and impervious.
10. Reoponsibility and Supervision. The users of cement will recognize the great difference, in both de sign and construction, between plain concrete work such as sidewalks, roads, foundations, heavy mass work, etc., and reinforced concrete structures. Plain concrete serves the requirements admirably for work where tensile stresses do not occur to any extent. In reinforced con crete structures, steel must be added to the concrete to make a safe structure ; and the proper sizes, dimensions, and arrangement of concrete and steel to obtain safety, strength, and durability, require the knowledge and expe rience of the engineer. Many of our failures in reinforced concrete construction are the results of improper or inadequate design, and many others are due to incom petent supervision.