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Cement Mortar 32

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CEMENT MORTAR 32. Proportioning Mortar.—In specifying the proportions of ingredients for cement mortar to be used in construction, it is usual to give the ratio of parts of cement to those of sand by volume The relative proportions of sand and cement to be used in any instance depend upon the nature of the work and the necessity for developing strength or water-tightness in the mortt r. The pro portions commonly used in ordinary work are: for natural cement, one part cement to one part or two parts sand; for Portland cement, one part cement to two parts or three parts of sand. In common practice these ratios are chosen without reference to the particular materials used and the resulting mortars vary widely in character.

Good sand in a to 3 mortar frequently shows greater strength than a poorer one mixed 1 to 2, and gives equally good results in use.

The methods of measuring materials also vary, and the relative quantities of cement and sand in the mortar differ correspondingly.

Measuring Cenwnt.—Cement should always be measured by weight, on account of the variation in volume of the same quantity of cement with different degrees of compactness. In specifying proportions by volume, therefore, it is always desirable to state the weight of cement to be taken as unit volume.

Portland cement is usually packed in wooden barrels or in canvas bags. A barrel of cement contains 376 pounds of cement, while a bag contains 94 pounds, or one-quarter barrel. Natural cement is ordinarily packed in barrels of 282 pounds, or bags of 94 pounds (one-third barrel) each.

Portland cement as packed in barrels weighs a little more than 100 pounds per cubic foot. A cubic foot of cement paste requires from 95 to 110 pounds of cement. It is common to consider a cubic foot of Portland cement to weigh 94 pounds in porportioning mortar. A bag of cement is then mixed with 2 cubic feet of sand to form 1 to 2 mortar, or with 3 cubic feet of sand to form 1 to 3 mortar. This assumes the volume of a barrel of cement to be 4 cubic feet. This is the recommendation of the Joint Committee of the Engineer ing Societies. Some engineers use 3.8 cubic feet as the volume of a barrel, or 100 pounds as the weight of a cubic foot.

In the same way, 70 pounds is frequently used as the weight of a cubic foot of natural cement. This makes the volume of a sack of natural cement 1 cubic feet. A barrel of natural cement would then have the same nominal as a barrel of Portland, 4 cubic feet. The actual volume-weight of natural cement varies con siderably for different brands.

Measuring Sand. It is usual to measure sand by volume. The method of measuring to be used in any particular instance depends upon the method of mixing and handling the mortar. Very com monly the measuring is done in the barrow or bucket in which the sand is carried to the mixer or platform. Measuring boxes with out bottoms are often employed to set on the mixing platform, and after filling are removed, leaving the measure of sand. Whatever method of handling the sand is employed, it is important that care ful attention be given to securing the correct proportion of sand for the mortar.

Effect of Jloistarc.---In proportioning mortar by volume. the moisture content of the sand may be a matter of importance. Damp sand weighs less per unit. volume than dry sand. When sand is moistened with a small quantity of water, the grains of sand are coated with a thin film of water, which separates the grains, causing the sand to occupy more space than when dry. When the amount of water becomes sufficient to coat all the grains of sand (about 4 to 7 per cent with ordinary sands), a maximum effect is reached, and an increase in amount of water beyond that point causes a reduc tion of volume. At. saturation (10 to 20 per cent of water), it becomes slightly less in volume than when dry.

The solid content in a given volume of moist sand is less than that of the same volume of dry sand, and a mortar mixed with the moist sand will be richer in cement than that mixed with the same sand when dry. This effect is greater with fine then with coarse sand. A given volume of sand measured dry may contain 10 per cent to 13 per cent more solid material than the same volume of the same sand measured in a moist condition.

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