AXIAL COMPRESSION For concentric compression on a plain concrete pier, the length of which does not exceed 4 diameters, or on a column reinforced with longitudinal bars only, the length of which does not exceed 12 diameters, 22.5 per cent of the compressive strength may be allowed.
Allowable stresses for concrete in reinforced work are given in Chapter VI.
Cement.—The strength of concrete varies nearly in proportion to the cement contained by it, and the ratio of cement to aggregate should be selected to give the strength needed in the particular work in hand.
The Committee suggests: For reinforced concrete construction, one part cement to a total of six parts of fine and coarse aggregates measured separately should generally he used. For columns, richer mixtures are preferable. In massive masonry or rubble concrete a mixture of 1 : 9 or even 1: 12 may be used.
These proportions should be determined by the strength or other qualities required in the construction at the critical period of use. Experience and judg ment based on observation and tests of similar conditions in similar localities are excellent guides as to the proper proportions for any particular case.
Sire of Aggregate.—The values given above are suggested for concrete as used in reinforced work and correspond to materials broken to rather small maximum sizes. Stones of larger maximum dimensions ordinarily show somewhat higher strengths. A stone broken to 2 2-inch maximum size will sometimes show strength 20 to 35 per cent higher than the same stone broken to 1-inch maximum size, if both be properly graded.
Consistency.—The mushy consistency recommended in the report of the Committee is most convenient for ordinary use in concrete work, particularly in reinforced work. In massive work, a stiffer plastic consistency gives a slightly higher strength if the concrete is properly compacted in placing. The strength rapidly decreases as the quantity of water is made greater, so that the materials begin to separate and considerable laitance forms.
Strength is gained more slowly by concrete mixed wet than by that mixed with less water. A stiff plastic concrete may have con
siderable more strength at seven days than a wetter mushy concrete though the difference will have largely disappeared in twenty-eight clays.
Growth in Strength.—It is customary to use tee strength at twenty eight days in fixing the stresses to he allowed on concrete in struc tures. This strength would usually be attained before maximum loads could be applied. The strength of concrete under normal conditions continues to increase through a considerable period. Tests have shown that average concrete may be expected to reach about twice the twenty-eight-day strength in two or three years if kept from becoming too dry. Specimens kept dry show a con siderably smaller increase, and may ultimately gain but little more than the twenty-eight-day strength.
Grading of llggregates.—W ith the same ratio of cement to total aggregates, the strength of concrete is greater when the aggregates are graded to give more dense mixtures. The amount of cement required to give a definite strength is less for well-graded aggregates than for those giving more porous concrete. In some instances it is possible, by sifting the aggregates into several sizes and recombin ing in proper proportions, to reduce considerably the quantity of cement to reach the required density and strength, with a material saving in cost. In one case, with a poorly graded stone, it was found that if the stone be sifted into sizes and recombined, 1 : 2 : 6 concrete of the graded stone gave as much strength as 1 : 2 :4 concrete of the ungraded material. The saving in cost of cement amounted to SO cents per cubic yard, with an estimated additional cost of mixing of about 20 cents. Such results are obtained only for material which is badly graded originally, and probably for average materials the saving in cost of cement would not he enough to pay for regrading. On any important work, however, it is worth while to examine carefully the materials with reference to their granulometric composition.