MIXING CONCRETE. The value of the concrete depends greatly upon the thoroughness of the mixing. Every grain of sand and every fragment of aggregate should have cement adhering to every point of its surface. Thorough mixing not only causes the cement to adhere to all the surfaces, but forces it into intimate contact' with the other ingredients at every point. The longer and more thorough the mixing the better, provided the time does not trench upon the time of set or the working does not break and pulverize the angles of the stone. Uniformity of the mixture is as important as intimacy of contact between the ingredients. The mixing has been thorough if the mass has a uniform color, if no uncoated particles of sand or stone are visible, if the mortar is distributed uniformly throughout the mass, or if the consistency of the concrete is uniform throughout. Lean mixtures require more mixing than rich ones, and dry mixtures require more than wet ones.
Concrete may be mixed by hand or by machine. The latter is the better; since the work is more quickly and more thoroughly done, and since ordinarily the ingredients are brought into more intimate contact. If any considerable quantity is required, machine mixing is the cheaper; but on small jobs hand mixing is the cheaper. More careful inspection is required to secure thorough mixing by hand than by machinery; and for this reason machine mixing is preferred for thin walls and for reinforced work, since in these cases a single batch of poorly mixed concrete may materially affect the strength or water-tightness of the structure.
However, in discussing the relative merits of hand-mixed and machine-mixed concrete it is important to state the kind of machine with which the comparison is made, since there is as much difference between the products of different machines as between good and poor hand mixing.
it is more accurate to measure the quantities by shoveling them into bottomless boxes set upon the mixing board. These boxes may be 6 or 8 inches deep and have other dimensions to give the required proportions of concrete.
For the best results the sand should be evenly spread upon the mixing board, and the requisite amount of cement should be formly spread over the sand. These should he turned with the shovel until thoroughly mixed. It is not sufficient to simply turn the mass with the shovel, but the sand and cement should be allowed to run off from the shovel in such a way as to thoroughly mix them. If skilfully done, two turnings will usually give a uniform mixture. Some engineers add water to the cement and sand, thus forming a mortar before adding the stone; while others mix the dry cement and the sand with the stone before adding the water. It is claimed that the latter requires a little less labor and gives equally good concrete. Some engineers add the water with a spray to secure greater uniformity and to prevent the washing away of the cement, and depend upon the appearance of the concrete to secure the right consistency; while others measure the water in buckets and pour nearly all of it on the dry materials before beginning to turn the wet mixture, reserving a little water to wet the dry spots as the turning proceeds. In either case the stone should be dampened before being mixed.
The mixed cement and sand, either before or after wetting, may be shoveled upon the broken stone previously spread evenly on the mixing board; or the cement and sand, either before or after wetting, may be spread evenly on the board and the broken stone dumped on it. In any case the mass should be turned and re-turned until every fragments is covered with mortar.