The extent to which differences in moisture condition may effect the volume of the sand depends upon the position in which the sand is placed and the way it is handled in measuring. If dry sand in a bin, or a pile, be moistened with a small quantity of water, the sand will not appreciably swell in the pile, as the particles are held by the weight of the mass above—they are not free to move and the water fails to separate them. If the sand be loosened in moving to a new position, it will be found to have increased in volume and will not return to its former dimensions until it has become dry, or wet to saturation.
Proportioning by 1Peight.—In Germany it has been quite common to measure the material for mortar by weight. This has been applied in some instances in the United States, and reduces largely the vari ations in the proportions due to moisture. On important work it may frequently be possible to arrange for weight measurement without materially increasing the cost of handling the material.
The ratio of cement to sand is commonly arbitrarily fixed with reference to the particular use to which the mortar is to be put, without considering the character of the sand to be used. For ordinary masonry, or massive concrete, Portland cement is usually employed in 1 to 3 mixtures. When high strength is needed, as in reinforced concrete work, the mixture is 1 to 2. Under specially trying conditions, or sometimes when cement grout is being used, a 1 to 1 mixture may be employed. With natural cement, the mix tures are 1 to 2 for ordinary work and I to 1 where greater strength is needed. Natural cement is not used for reinforced concrete work. The choice of ratios has usually been well on the side of safety, and good results have been obtained in practice by this method, although equally good work at less cost might in many instances have been obtained by more careful study of the materials in proportioning the ingredients of the mortar.
In comparing the mortar-making qualities of various sands, it is found that the amount of cement necessary to make mortar of the same strength from different sands depends mainly upon the fineness and density of the sands. The office of the cement paste in mortar is to coat the grains of sand and fill the voids between them. In fine sand the surface to be coated with cement is greater than in coarse sand. Dense sand, with grains of varying sizes,
presents less voids to be filled than more uniform sand.
It is desirable that careful be given to the sands to be used in any important work before finally deciding upon the proportions of the materials, and that final judgment be based upon actual tests of the mortar itself.
Frequently a mixture of a fine with a coarse sand, or of crusher dust with sand may be so proportioned as to give economical results in the saving of cement, while at the same time improving the mortar.
33. Mixing Mortar.—In mixing mortar by hand a water-tight box or platform is used. The required quantity of sand is spread over the floor of the box and the cement distributed evenly over the sand. The cement and sand are then mixed together with a hoe or shovel until the cement is uniformly distributed through the sand, as shown by the even color of the mixture dry. It is important that the clay materials be very thoroughly mixed before water is added. A uniform mixture will not otherwise be obtained. When the mixing of dry materials is complete, water is acicled and the mass worked into a stiff paste. The quality of the mortar is materially affected by the vigor with which it is worked in bring ing it, to the proper consistency. After the water has been absorbed by the cement, vigorous working will make the mass more plastic, and working should continue until a permanent condition is reached.
Quantity of Water.—The quantity of water to be used in mixing mortar can be determined only by experiment in each instance— it depending upon the nature of the cement and sand, and the pro portion of cement to sand. The quantity of water used should be the least consistent with reducing the mortar to the required condi tion of plasticity by vigorous working. Additional water should not be used to save labor in working.
.Mixing should be quickly and energetically done, only such quantity being mixed at once as can be used before initial set takes place. A considerable quantity is sometimes mixed dry and left to stand until needed before adding water. If this is done with damp sand, the cement may be acted upon by the moisture in the sand to the injury of the mortar. Quick-setting cements are par ticularly liable to injury from this cause.