311. Concrete is simply a class of masonry in which the stones are small and of irregular shape. The strength of the concrete largely depends upon the strength of the mortar; in fact, this dependence will be much closer than in the case of other classes of masonry, since it may be stated as a general rule, that the larger and more perfectly cut are the stone, the less will the strength of the masonry depend upon the strength of the mortar.
In deciding, then, upon the proportions of ingredients to use in a given case, the quality of the mortar should first be considered. If the concrete is to be subject to but a moderate compressive stress, the mortar may be comparatively poor in cement; but if great strength is required, the mortar must be of sufficient richness; while if imperviousness is desired, the mortar must also possess this quality and be sufficient to thoroughly fill the voids in the stone.
The usual method of stating proportions in concrete is to give the number of parts of sand and aggregate to one of cement. These parts usually refer to volumes of sand and stone, measured loose, to one volume of packed cement. However, there is no established practice in regard to this and a "1-2-5 concrete" may mean five volumes of loose stone to two volumes loose sand to one volume loose cement, or any one of several combinations.
This method of stating proportions leads to confusion unless one is careful to explain what is meant by such an expression as " 1-3-6 concrete." The evils of similar methods of stating proportions in mortars, and the desirability of fixing upon some standard system of weight or volume, have already been pointed out. The only circumstances under which such expressions as the above may be used with propriety are when one wishes to give only an approximate idea of the character of concrete used.
1 Portions of this article were contributed to Municipal Engineering by the author, and appeared in that magazine, May, 1899.
From tests of strength it is known that to obtain the strongest concrete with a given quality of mortar the quantity of the latter should be just sufficient to fill the voids in the aggregate. The strength is notably diminished if the mortar is deficient, and is also impaired by a large excess of mortar. This last statement is subject to one exception: if the mortar is stronger than the stone, then an excess of mortar does not weaken the concrete. This case, however, should never be allowed to occur, since it is evident that the strength of the stone should be at least equal to the required strength of the concrete. Further, the ordinary uses of concrete are generally best served by a compact mixture containing as few voids as possible.
For these reasons, then, one should consider concrete not as a mixture of cement, sand and stone, but rather as a volume of aggregate bound together by a mortar of the proper strength. The volume of voids in the aggregate, the per cent, of this volume filled with mortar, and the strength of this mortar become then the important considerations in proportioning concrete. When thus considered, it is an easy matter to determine the required volume of mortar for a given volume of stone, and the amount of cement and sand required for a given volume of mortar has already been considered.
The bulk of a given quantity of broken stone is not so variable as the volume of sand. The volume of the stone, and consequently the voids, will vary with the degree of packing, but the packing is not influenced appreciably by the amount of moisture present.
The proportion of voids in the broken stone may be obtained as follows: Find the weight per cubic foot of the broken stone in the condition in which the volume of voids is sought, being careful to use a measure holding not less than two or three cubic feet. Also obtain the specific gravity, and hence the weight per cubic foot of the solid stone. Then one, less the quotient obtained by dividing the weight per cubic foot of the broken stone by the weight per cubic foot of the solid stone, will be the proportion of voids in the aggregate.
For example, suppose the weight per cubic foot of the broken stone is 102 pounds. The specific gravity of the solid stone determined in the ordinary manner is found to be 2.724. Then weight per cubic foot of solid stone is 62.4 X 2.724 = 170 pounds and 1 — 102/170 = .40, voids in stone.
Another method is to fill a vessel of known capacity with the stone to be used, and to pour in a measured quantity of water until the vessel is entirely filled. The volume of water required indicates the necessary amount of mortar to use. The stone should be moistened before placing in the vessel, to approximate more nearly its condition when used for concrete, and to avoid an error from absorption of the water used to measure voids.
314. As to the degree of jarring or packing to which the stone should be subjected in filling the measure, if the stone is filled in loose, and it is proposed to ram the concrete in place, the amount of mortar indicated will be a little more than the required quantity. If the concrete is to be placed without ramming (as in submarine construction), the amount of mortar indicated will not be too great. On the other hand, if the stone is shaken down in the vessel to refusal, the voids obtained will be less than the amount of mortar which should be used, because it is not possible to obtain a perfect distribution of mortar in a mass of concrete, and because the concrete will usually occupy a greater space than did the stone when shaken down. And again, for perfect concrete, pieces of stone should be separated one from another by a thin film of mortar, and hence the volume of the concrete will be greater than the volume of the stone measured in a compact condition without mortar. A deficiency of mortar is usually more detrimental than an excess. It is safer, therefore, to measure the voids in the stone loose, or when but slightly packed, and make the amount of mortar equal to, or a trifle in excess of, the voids so obtained.
If in the case of broken stone all of the fine particles are used, or if gravel which contains a considerable amount of sand is employed, then this fine material or sand must be considered as forming a part of the mortar. This will not change the method of obtaining the amount of mortar required for such broken stone or gravel, but it will change the composition of the mortar used. Thus, suppose we have a gravel ten per cent, of which is sand (grains smaller than one-tenth inch in diameter) and we find the voids to be thirty-three and one-third per cent. To three cubic yards of this gravel we will add one cubic yard of a one-to-three mortar. The voids will be filled, but instead of having three cubic yards of stone imbedded in one cubic yard of a one-to-three mortar, we will in reality have a little less than that amount of stone imbedded in a mortar composed of one part of cement to about three and three-tenths parts sand.
In the paragraphs just preceding, an attempt has been made to indicate the general principles to be applied in proportioning the materials in concrete. To decide on the actual proportions of the ingredients to use for a given purpose, one must have clearly in mind the strength that will be demanded and any special condition to which the concrete is to be subjected. A reference to Art. 57 concerning the strength of concrete, will be of service in deciding on the proper proportions to use in a given case.