Concrete of three degrees of consistency is in somewhat common use: dry, quaking, and wet concrete (see § 334-37). The first is the most common, and the last the least. Dry concrete is most advantageous to the menu facturer, since the block can be removed from the form as soon as it is moulded, and hence fewer forms are required.
No statement is required here concerning the proportions and the method employed in mixing the materials, as these subjects have already been considered in preceding portions of this volume (see 4 3:38-41).
Ordinary concrete blocks are usually moulded in metal forms, but ornamental blocks, as capitals, balus trades, cornices, etc., are successfully cast by pouring liquid concrete into sand moulds, although the process is expensive on account of the labor of making a new mould for each piece. In making the ordinary plain building block, the dry mixture is always tamped, usually by hand and occasionally with a pneumatic tamper; plastic concrete is usually tamped, and occasionally pressed; and the wet mixture is poured. Tamping is the most common, and pressing the least. It is impossible to make a dense block by direct com pression unless the pressure is applied to the face of a comparatively thin layer, which makes the method impracticable, except for a two piece block (I 514) in which the pressure is applied to pieces of no great thickness.
Moulding Machines. There are a great number of machines on the market for facilitating the moulding of concrete blocks, which differ according to the plasticity of the concrete used, the method of consolidating the block, and the conveniences for removing the cores and handling the block. For advertisements of such ma chines, consult the advertising pages of engineering journals, pro ceedings of engineering societies, etc. Blocks have been made successfully by tamping by hand in wooden moulds.
In the early history of the concrete building block industry, it was customary to give the block a rich mortar face, partly to secure a more dense and more impervious surface, and partly to aid in forming the imitation rock-face then much in vogue; but now the facing mortar is frequently omitted, a satisfactory surface being obtained by using a wet mixture and "spading" the face (§ 353). The so-called rock-face was very unsatisfactory, being at best only a dull and monotonous imitation of pitch-faced natural stone. It is now conceded by architects and the better manufac
turers that a plain face is the most satisfactory for concrete blocks. Some really handsome structures have been built wholly or in part of concrete blocks having plain faces.
If desired, the face of the block can be dressed with a stone cutter's tool; but concrete is considerably more difficult to work than equally hard natural stone, probably because the fragments of the aggregate are not held as firmly as the grains of the natural stone, and hence there is a slight movement of the aggregate under the chisel which materially decreases the effectiveness of the blow. The surface of the block may be treated by either of the processes described in § 359 and 360.
The color of the face may be varied by selecting different-colored aggregates or by using artificial coloring matter (see § 362).
Since the chief use of concrete blocks is for buildings, it is important that they should be waterproof. Blocks may he made impermeable by any of the methods described in § 366 81. The penetration of water from the outside to the inside is reduced by placing an air space opposite each web member between the back and the face of the block (§ 514), and is prevented by mak ing a continuous horizontal air space (Fig. 36).
It is important that the blocks should be properly cared for while the cement is hardening. They should not be allowed to dry too rapidly; and, particularly, the sun should not be allowed to shine upon the unseasoned block, as it not only will dry the block out unduly but will also make it spotted. The blocks should be kept in a humid atmosphere or should be sprinkled three or four times a day. When a dry concrete is used, the block should be sprinkled more frequently and more copiously and for a longer time than when a wet concrete is used. It is advantageous to cover the blocks with straw, excelsior, or burlaps, to retain the moisture and help secure uniformity of color and prevent hair cracks. While hardening, the blocks should not be in contact with one another.
Ordinarily, blocks should not be placed in the wall until they have seasoned for three weeks, and preferably four.