MIXING THE CONCRETE. The value of the concrete de pends greatly upon the thoroughness of the mixing. Every grain of sand and every fragment of aggregate should have cement adher ing to every point of its surface. Thorough mixing should cause the cement not only to adhere to all the surfaces, but should force it into intimate contact at every point. It is possible to increase the strength of really good concrete 100 per cent by prolonged trituration and rubbing together of its constituents. The longer and the more thorough the mixing the better, provided the time of mixing does not trench upon the time of set, or the working does not break and pulverize the angles of the stone. Further, uniformity of mixture is as important as intimacy of contact be tween the ingredients. Of course thoroughness of mixing adds to the cost, and it may be wiser to use more cement or more con crete, and less labor.
Concrete may be mixed by hand or by machinery The latter is usually the better method; since the work is more thoroughly and more quickly done, and since ordinarily the ingredients are brought into more intimate contact But the concrete employed for pavement foundations is usually mixed by hand, apparently because either of the expense of transporting the concrete con siderable distances, or of the difficulty of continually moving the mixing machine. However, several machines have been intro duced very recently which give promise of success in mixing con erete for pavement foundations.
The concrete is usually mixed on a board platform 8 or 10 feet square, although a metal plate, to inch thick, possesses some decided advantages. It lasts longer, is easier to shovel from and to wheel upon. The ingredients of the concrete are stored on the parking or in the space to be paved. If the materials are hauled upon the subgrade after it is finished, it will usually be necessary to cover the trackway with plank to prevent cutting up the sub grade. The materials of the concrete are wheeled upon the mixing board, mixed, and cast with shovels directly into place.
The mixing is usually done about as follows: The sand to be mixed in a batch of mortar is spread evenly over the middle of the mixing board, and the dry cement is spread evenly over the sand, then the two are thoroughly mixed with hoes or shovels. As the
mixing proceeds the necessary water is added, preferably with a spray to secure greater uniformity and to prevent the washing away of the cement. The mass should be worked until it is of a uniform consistency. The broken stone, having previously been sprinkled but having no free water in the heap, is next added. The whole is then turned until every fragment is covered with cement. Some contractors use shovels instead of hoes, claiming that the former are more economical. The effectiveness of the shovel varies greatly with the manner of using it. It is not sufficient simply to turn the mass; but the sand and cement should be allowed to run off from the blade in such a manner as thoroughly to mix them. It is difficult to get laborers to do this with the shovel, which is one reason why many contractors prefer hoes. When it is supposed the mixing will be done with shovels, the ber of times the material shall be turned is specified; but the fication is not definite, since so much depends upon the manner of doing the work. Often it is specified that the sand and cement shall be turned twice, and that the mortar and stone shall be turned four times exclusive of casting into place. The concrete appears wetter each time it is turned, and should appear too dry until the very last. LAYING THE CONCRETE. After being mixed the concrete is shoveled from, the boards into place. The proper thickness of the layer is indicated either (1) by the tops of small stakes driven at intervals of 4 or 5 feet each way, whose position is determined by measuring down from a string stretched from curb to curb (see Fig. 108, page 347), or (2) by a curved template having one end upon the curb and the other end upon the opposite curb or upon a scantling near the middle of the street. The concrete is brought to the proper elevation with a shovel or a rake, and then tamped.