STRENGTH OF PLAIN CONCRETE Compressive Strength.—There are several variables which affect the strength of concrete. The quality and quantity of cement, the kind of aggregates and their grading, the proportions of fine to coarse aggregate, the thoroughness of mixing, the consistency, the degree of compacting, and the conditions under which hardening takes place, all have an influence upon the strength of the resulting Thorough mixing and careful placing and compacting of the concrete should be obtained in all work. Carelessness always reduces its strength, and is wasteful and unnecessary.
The Joint Committee on Concrete recommends that the materials be proportioned to secure as nearly as possible a maximum density. " The fine and coarse aggregates should be used in such proport ions as will secure density. These proportions should be care fully determined by density- experiments and the grading of the fine and coarse aggregates should be uniformly maintained, or the por portions changed to meet the varying sizes." They also recommend
that " the materials be mixed wet enough to produce a concrete of such consistency as will flow sluggishly into the forms, and about the metal reinforcement when used, and which, a-t the same time, can be conveyed from the mixer to the forms without separation of the coarse aggregate from the mortar." For such concrete, the Committee suggests the following values of ultimate strength in compression as those which should be obtained for the materials and proportions listed. the ratios given being those of cement to the volumes of fine and coarse aggregates measured separately. This ultimate strength is that developed at an age of twenty-eight days, in cylinders S inches in diameter and 16 inches long of the consistency described above, made and stored under laboratory conditions.