Occasionally the weight of a unit of volume of the sand and the cement is determined, and the proportions of the mortar are nominally adjusted according to weight, although the actual proportioning is done by volumes. This is practically no better than one of the two following methods.
2. By Volumes of Packed Cement and Loose Sand. This method consists in mixing one barrel of packed cement as received from the manufacturer with one or more barrels of loose sand. The measuring is done by emptying a barrel of cement upon the mortar board and then filling the barrel full of sand one or more times. This method is inaccurate, since the volume of a barrel of cement is not the same as packed by different makers. Further, the presence of moisture affects the volume of sand considerably more than its weight (see 196), and the volume of the sand varies with the method of handling it in the measuring; and hence, for these reasons also, this method is not very accurate.
Not infrequently in the use of this method both heads of the barrel were knocked out, the barrel was set upon the mortar board and filled with sand, and then the barrel was lifted up and the sand spilled out. Notice that this procedure uses more sand than that described above.
The method of proportioning by volumes of packed cement and loose sand was more convenient formerly when cement was usually shipped in barrels than now when cement is generally shipped in bags, for then the barrel was always at hand for use in measuring the sand. When the cement is shipped in bags, about the only prac ticable way of applying this method is to assume the weight of a cubic foot of packed cement, or what is the same thing, assume the number of cubic feet in a barrel, and then measure the sand in cubic feet. A cubic foot of portland cement as packed in a barrel usually weighs a little over 100 pounds, but recently it has become common to assume for convenience that a cubic foot of packed portland cement weighs 100 pounds, and a cubic foot of natural cement 75. The proportioning is then done by mixing a bag or barrel of cement with a certain number of cubic feet of sand, which is virtually weighing the cement and measuring the sand by volumes.
American portland cement packed in the barrel weighs about 100 pounds per cubic foot, and dry loose sand about 90 pounds, and therefore equal parts of cement and sand by weight, say 100 pounds of cement to 100 pounds of dry sand, would be equivalent to 1 cubic foot of packed cement to 1.11 cubic foot of loose sand, or 1
volume of packed cement to 1.11 volumes of loose sand. Natural cement weighs about 75 pounds per cubic foot as packed in a barrel, and hence a 1 : 1 natural cement mortar by weight is equivalent to 1:0.8 by volumes of packed cement and loose sand.
3. By Volumes of Loose Cement and Loose Sand. A volume of loose cement is mixed with one or more volumes of loose sand. This method was much more common formerly when cement was usually shipped in barrels than now when it is nearly always shipped in bags. Then it was common to fill a wheelbarrow with loose cement and fill one or more wheelbarrows equally full of sand. As far as the sand is concerned, this method is as inaccurate as the second; and in addition, it is subject to great variations owing to differences in the fineness and compactness of the cement. This is the most inaccurate of the three methods.
At present a modification of this method is in common use. A trial bag of cement is emptied into a wheelbarrow to show how full the wheelbarrows should be filled with sand, the actual measuring being done by emptying a bag of cement on the mortar board and adding one or more equal volumes of sand. By this method there is no uncertainty in measuring the cement; and there is no serious objection to this procedure provided it was distinctly understood that the cement was to be measured loose and not compacted.
When cement was shipped in barrels the proportioning was occasionally done by throwing into the mortar box one shovelful of cement to one or more shovelfuls of sand. This is very crude, and should never be permitted.
Since American portland cement weighs about 90 pounds per cubic foot when shoveled loosely into a box, and since sand weighs the same, a 1:1 portland cement mortar by weight is equivalent to a 1:1 mortar by volumes of loose cement and loose sand. Since natural cement weighs about 56 pounds per cubic foot when shoveled into a box, a 1:1 natural cement mortar by weight is equivalent to a 1:0.6 by volumes of loose cement and loose sand.