Composition.— Concrete here considered is composed of hydraulic cement (generally Port land cement) and a fine and coarse aggregate and water. Mortar is composed of cement, fine aggregate, and water. Subsequently the mixture sets by chemical action. Fine aggregate is usually sand but may be crushed stone or gravel screenings graded from fine to coarse up to Ys-inch size; the coarse aggregate is pebbles, broken stone or slag, up to a size de pending upon use. For buildings and bridges where steel reinforcing is used the maximum size should be not greater than Y2 inch. For concrete in large masses, such as piers, sizes up to 21/2 inches may be used. Under proper precautions large boulders may be embedded in the concrete. Occasionally banks of gravel are found of approximately the proper proportions of fine and coarse aggregate. For mass con crete cinders, composed of hard, clean, vitreous clinks free from sulphides, unburned coal or ashes may be used. The fine aggregate should be hard, free from organic matter; 5 per cent of clay, if fmely divided and nonadherent, may be allowed. Not more than 6 per cent of fine aggregate should pass a standard sieve having 100 meshes to the inch. Dirty sand when rubbed in the palm will discolor the hand. Avoid very fine sand. Sand or gravel is cleaned by washing.
Proportions.— For reinforced concrete con struction for structural concrete, engine foun dations, tanks, arches, a real nch proportion of 1 part cement to 6 parts of fine and coarse aggregates by volume; usually 1 cement, 2 sand, 4 broken stone or pebbles (1 :2:4). For reinforced columns up to 1 :1 :2. For ordinary foundations, walls of buildings, floors, side walls, use 1:2T/2:5. For heavy walls, piers and abutments, an ordinary mixture of 1:3:6. For unimportant work, in large mass, as ordinary foundations, backing, farm floors, a lean mix ture of 1:4:8 may be used if well handled. These proportions assume that the voids to be filled in the fine and coarse aggreg-ate approxi mate 50 per cent. In important work the most favorable proportions to yield the densest mix ture are determined by experiment. A grading of sizes of aggregate approximating an ellipse, vertex at zero size, is the ideal.
Mixing should continue for at least one minute in a batch concrete machine mixer. If mixed by hand ingredients should be turned at least six times on a watertight platform until homogeneous in color. The cement should be spread on the sand and the two well mixed by turning over three times. Then place this on the coarse aggregate, add the water and turn over. Generally the amount of water should be such as to yield a plastic concrete, whose surface will be horizontal in a container, but in which the coarse aggregate will not sink.
However, a dryer concrete may be tamped into place for foundations, walks, curbing, etc. Before mixing and placing concrete should not be allowed to set. Fresh concrete, as on a subsequent day, should not be placed on par tially set concrete until the old surface has been cleaned of laitance or dirt, roughened and wetted, and slushed with a mortar of 1 part cement to 2 parts fine aggregate.
Concrete should not be deposited in freezing weather unless special precautions are taken to heat the aggregate, the water and to protect the concrete. Salt is used to lower the freezing point of the water in amount of 1 per cent of the weight of the water for a temperature of 32° F. and an increase of one per cent for each degree of lower temperature to a maximum of 10 per cent. The strength of the concrete is not greatly affected. Salt should not be used in reinforced concrete. Wet concrete may be con tinuously placed under still water by use of a tremie or pipe with lower end buried in the flowing mass.
Usually forms of wood or steel are required to hold concrete to the desired shape while hardening. Accidents are likely when forms are removed too soon. Two weeks to one month in important structures are needed. A simple test is that a tenpenny nail will bend and not drive in concrete. Frozen concrete may be detected with a torch. Expert inspection is necessary where collapse is possible.
The following table gives the expected compressive strength of vanous con cretes at 28 days, in pounds per square inch: The expected strength of 1:2:4 concrete at 28 days is 2,000 pounds per square inch. The tousle strength is about one-tenth and the shearing strength about two-thirds the com pressive strength. True shearing failures are difficult to procure; usually tensile failures result.
The modulus of elasticity ranges from 2,500, 000 to 3,000,000 pounds per square inch. Usually in design it is assumed to be 2,000,000, or one-fifteenth that of steel. The bond or resistance to withdrawing steel or iron rods from 1:2:4 concrete at 28 days is: For plain and deformed rods at first slip, 260 pounds per square inch of contact surface; at ultimate strength, for plain rods, 400; for rods with deformed surface, 600 pounds per square inch. A plain rod should be embedded 40 diameters to develop its tensile strength. Concrete grows in strength with age. With compressive strength at 28 days is unity; 7 days, 0.60; 28 days, 1.0; 60 days, 120; 3 months, 125; 4 months, 1.30; 6 months, 1.35; 1 year, 1.45; 3 years, 1.50.