Choice of Concrete Mixtures

CHOICE OF CONCRETE MIXTURES Having learned how to select good materials and how to proportion them so as to obtain con crete of any desired strength up to maximum, the next important requirement for the concrete worker is that he shall know how to decide upon the proper mixture to use for any particular kind of work.

Mixtures of greatly different strength and greatly different cost, as already explained, can be made from the same materials, by simply combining them in different proportions. The particular mixture to use will depend princi pally upon the kind of service that will be required of the concrete—upon whether it will have to bear heavy or light loads and withstand great or small stresses; whether or not it will be subjected to hard and rough wear; whether it is to be covered or left exposed, above ground or underground; whether it will have to with stand water pressure, etc. If a man, for ex ample, builds a concrete sidewalk from the kitchen steps to the back gate, he will naturally require a different mixture from that he would use in making a water-tank or cistern; and for the face of the sidewalk he will hardly use the same coarse and cheap mixture that will serve very well for the sub-base. It is important, therefore, to know how to serve the purposes of economy as well as structural stability, in deter mining what mixtures shall be used in the differ ent kinds of construction and even in different parts of the same work.

Kinds of Mixtures Concrete mixtures are classified in two differ ent ways—(1) as to consistency, or relative wet ness; and (2) as to richness, or relative quantity of cement.

Wetness of Mixture.

Classified according to relative wetness, there are three kinds of mix ture commonly used in concrete work—namely, Very Wet, Medium, and Dry. Their distin guishing features are as follows: Very Wet Mixture. Concrete wet enough to be mushy and to run off a shovel in handling. Used for reinforced work, such as thin walls, floors, or other thin sections, reinforced columns, tanks, conduits, etc. Practically no ramming is necessary, but spading should be done next to the forms, to insure burial of the coarser aggre gate and the formation of an unbroken face by the cement grout.

Medium Mixture.

Concrete just wet enough to make it jelly-like, so that it will quake on tamping. Used for some reinforced work, but chiefly for foundations, floors, and massive con structions, such as retaining walls, piers, abut ments, arches, etc. Ramming with tamper or treading with feet necessary to remove air bubbles and fill voids. A man stepping on a pile of this concrete would sink ankle-deep. Spading also necessary for same reason as in the very wet mixture.

Dry Mixture.

Concrete like damp earth. Used for foundations, etc., where it is important that the concrete shall set as quickly as possible. Should be placed in layers 4 to 6 inches deep at a time, and thoroughly tamped until the water comes to the surface. In dry concrete, it is only by thorough tamping that the danger of the formation of pockets which would weaken the concrete can be avoided. Careful spading should also be done next the forms.

The drier the mixture, the sooner the con crete will set. Any quantity of water that falls below the minimum necessary for thorough hydration of the cement, is, of course, not to be allowed in any case, for then the concrete would be weak and crumbly; but beyond this point the addition of more or less water does not seem to have any very important bearing upon the ulti mate strength of the concrete, provided the mix ing is thoroughly done and the concrete properly deposited. A liberal use of water, however, besides increasing plasticity and ease of work ing, renders easier the insuring of a thorough mixture and the securing of great density. In reinforced concrete work, a dry mixture is not at all satisfactory, since it is more difficult, and hence more costly, to handle than a wet mixture, will not penetrate and fill all spaces between the reinforcing steel members so as to insure the thorough bond that is required, must be pro tected with greater care from the sun or from drying too quickly, and, unless great skill is used in spading it, will show voids or pockets in the face of the work when the forms are removed. The prevailing tendency in present-day practice, therefore, is toward the medium or very wet mixtures.