The quantity of concrete is less than the sum of the fine and coarse aggregate, since the fine aggregate fills the voids in the coarse aggregate. Fullers' rule is: The number of barrels of Portland cement required for a cubic yard of concrete is the sum of the parts of the ingredients divided into 11. Let c equal num ber of parts of cement; s equal number of parts of sand; g equal number of parts of broken stone or gravel; P equal number of barrels of Portland cement to cubic yard of concrete.
Then 11= 11 Then P X g X W= num ber of cubic yards of sand required for one 3.8 cubic yard of concrete; of cubic yards of stone or gravel required for one cubic yard of concrete. A barrel of Port land cement weighs 376 pounds. Here 3.8 is the number of cubic feet of loose sand per barrel of cement. For a proportion of 1:2:4 a cubic yard of concrete requires 1.57 barrels of cement; 0.44 cubic yards of sand; 0.88 cubic yards of stone.
Safe working stresses are (in per cent of compressive strength) : For on con crete, 33 per cent; for axial compression, piers and plain columns, 22.5 per cent; for com pression in extreme fibre of• beams, 32.5 per cent; for bond, plain bars, 4 per cent; deformed bars, 5 per cent. A tensile stress of 40 to 50 pounds per square inch, when shrinkage cracks are not likely, and the conditions are not critical, may be allowed. Coefficient of expan sion and contraction of concrete with tem perature for one degree Fahrenheit is 0.0000055 to 0.0000060. Expansion joints must be pro vided about every 30 feet in plain concrete. By use of steel expansion is reduced, and distrib uted with a number of fine cracks that do not seem to affect the structure. Concrete shrinks when hardened in air and swells when hardened in water. Sudden change in dimen sion of sections should be avoided, or fillets should be used.
Destructive all cases concrete should be impervious to water. Steel embedded in good watertight concrete is preserved from corrosion above or below water level. Stray electric currents may cause corrosion of steel hut the concrete is practically immune. Sea water, entering porous concrete, is likely to cause disintegration. Frost acting on water in porous concrete will cause spalling. Strong acids and organic acids and rapid crystalliza tion of alkali salts under wet and dry condi tions attack concrete. Mineral oils have no
action.
Concrete has low heat conductivity, and is a good fireproofing material. Dehydration begins at 500° F. and is completed at 900° F; but is confined to outer surfaces of two to three inches, in conflagrations. The most severe conflagration of the Edison factories at West Orange, N. J., 9 Dec. 1914, in which temperatures of from 1000-2500° F. were de veloped, showed reinforced concrete buildings (in this case of defective design) to withstand such extreme fires satisfactorily; columns and beams suffered spalling; some failed; the cost of restoration was from 10 to 15 per cent.
Imperviousness of concrete to water is best ensured by a dense concrete mixed wet. Dry or porous concretes are rendered less pervious by various integral compounds or surface coat ings; many of these are of temporary effect. Hydrated lime, in amount of 10 per cent of the cement, has been recommended.
Structures, such as retaining walls, bridge floors, subways, basements, are waterproofed by a membrane made up of one or more layers of fabric impregnated with asphalt, or coal tar pitch, cemented together with asphalt or coal tar pitch preparation. The fabric may be a wool felt, an asphalt felt or burlap. From two to six layers are used often. The membrane is protected from abrasion or other injuries by a covering of asphalt mastic or paving brick.
For damp-proofing, coal tar pitch, either dis solved in benzene or applied hot, is painted on like a paint. Surface of concrete must be dry. A coal tar paint when made up of one part kerosene, four parts Portland cement, 16 parts refined coal tar, is very effective and may be applied cold.
Cost of Cost of coal tar when applied in three coats is cent for material and /2 cent for labor per square foot. Cost of membranes is from 20 to 45 cents per square foot, including protection. For surface work a mixture of concentrated lye and alum in proportions one pound lye to five pounds alum is used successfully by the United States army engineers. Paraffine ap plied hot is sometimes used. A wall may be impermeable, but not damp-proof, for the water may enter by capillary action into minute openings and eventually may render damp the air of a room where there is little circulation of air. The secret of success is dense concrete laid continuously.