Not infrequently the proportions are stated as 1 part of cement to a certain number of parts of sand, without stating whether the parts are to be determined by weight or by volume, or whether the cement is to be measured packed or loose. The examples in the above discussion show the differences possible when the method of proportioning is not stated; and Table 22, page 120, shows inciden tally the relative amounts of cement required by the three methods of proportioning. Indefiniteness in stating the proportions is likely to cause misunderstandings between the engineer and the con tractor.
The only definite and accurate method of proportion is by weight, for the amount of cement in a given volume of either packed or loose cement is liable to considerable variation, and the volume of sand is materially affected by the presence of even a small amount of mois ture—particularly if the sand is fine. Weighing each unit of sand adds some complications and expense; but it secures definiteness of results and in a work of any magnitude may save enough cement to pay for the extra trouble. If the sand for each batch is not weighed, the weighing of an occasional batch will serve as a valuable check upon the method of proportioning actually employed.
The proportions com monly employed in practice are: for portland cement 1 : 2 or 1 : 3; and for natural cement 1 : 1 or 1 : 2. The specifications do not usu ally define which method is to be employed in the proportioning; and hence the same mortar may be designated by very different pro portions by different persons. Further, the proportions are usually fixed without regard to the quality of the sand to be used, although the difference between two sands may make more than a unit differ ence in the proportions. For example, a 1 : 3 mortar with one sand may be better than a 1 : 2 mortar with another sand (see Table 18, page 89).
is sometimes prescribed. If skilfully done, twice turning with a shovel will thoroughly mix the dry ingredients, although four turnings are sometimes specified, and occasionally as high as six. It is very important that the sand and cement be thoroughly mixed. When thoroughly mixed the mass will have a uniform color.
The dampness of the sand is a matter of some importance. If the sand is very damp when it is mixed with the cement, sufficient moisture may he given off to cause the cement to set partially, which may materially decrease its strength. This is particularly noticeable with quick-setting cements.
The dry mixture is next shoveled to one end of the box, and water is poured into the other. The sand and cement are then drawn down with a hoe, small quantities at a time, and mixed with water until enough has been added to make a stiff paste. The mortar should be vigorously worked to insure a uniform product. When the mortar is of the proper plasticity the hoe should be clean when drawn out of it, or at most but very little mortar should stick to it.
Cements vary greatly in their capacity for water (see § 161), the naturals requiring more than the portlands and the fresh-ground more than the stale. An excess of water is better than a deficiency, particularly with a quick-setting cement, as .its capacity for com bining with water is very great; and further an excess is better than a deficiency, owing to the possibility of the water evaporating before it has combined with the cement. On the other hand, an excess of water decreases the strength of the mortar. If the mortar is stiff, the brick or stone should be dampened before laying; else the brick will absorb the water from the mortar before it can set, and thus destroy the adherence of the mortar.
It is customary to mix mortar for use upon the work considerably wetter than for experiments in the laboratory. A wet mortar is more easily mixed, is less likely to deteriorate from the loss of water by vaporization, and is less likely to be damaged by the absorption of water by the stone. Of course, a great excess of water makes the mortar weak and porous, and difficult to keep in the joints. In hot dry weather, the mortar in the box and also in the wall should be shielded from the direct rays of the sun.