FORM OF BLOCK. Concrete blocks may be classified as solid and hollow. The first con crete blocks were solid, but that form is not now much used, since the hollow blocks are cheaper and give better insulation against moisture and heat or cold. The hollow space usually runs vertically from the top to the bottom of the block, but in a few cases horizontally from end to end. Fig. 34 shows the top view of three common arrangements of vertical air spaces, and Fig. 35 is an end view of a wall showing the most common form of horizontal air spaces. - The blocks are usually made 12 inches high although they are some times 8 inches; and the other dimensions are about as shown in Fig. 34 and 35. The special advantage claimed for the Miracle block, Fig. 34, and also for the block shown in Fig. 35 is that each web is backed by an air space, and hence no portion of the solid concrete extends from the front to the back of the wall, the object sought being to prevent capillary attraction from drawing water to the interior of the building.
Concrete blocks may also be classified as one-piece, two-piece, or three-piece blocks. Fig. 34 and 35 are examples of one-piece blocks. Fig. 36 shows the top view of a wall built of two-piece blocks. The block shown in Fig. 36 may be laid either with or without the continuous horizontal air space as shown. The con tinuous horizontal air space is to prevent the passage of water from the outside to the inside of the wall. The objects sought in the two-piece blocks are to overcome some of the difficulties encountered in the manufacture of one-piece blocks and also to afford better insulation against moisture and heat or cold. The two-piece and the three-piece blocks • can be moulded by pressure much more satisfactorily than the one-piece block. Fig. 37 shows a method of arranging the two-piece blocks to make a three-piece wall. The three-piece wall may be laid with continuous horizontal air spaces similar to those shown in Fig. 36.
There are numerous other forms of blocks, but the above are representative.
Several manufacturers make a bead on one side or on one end or on both sides and ends of a block, and a groove on the opposite side to give additional resistance to prevent one block's sliding horizontally on another. The beads and grooves are not shown in
Fig. 34 to 37.
In blocks for buildings the object of the air space is to cheapen the product, and also to insulate the wall against the passage through it of water or heat; but in structures requiring great strength, the chief object of the air spaces is to peimit a thorough bonding of the work by the filling of the hollow spaces with concrete after the blocks are set in position. For an example of the use of the latter principle in the piers, parapet walls, railings, etc., of a concrete arch, see Transactions of American Society of Civil Engineers, Vol. lix, pages 193-207, particularly 199-200; or Engineering News, Vol. lvii, pages 497-503.
In the ordinary commercial building blocks the hollow spaces are formed by removable metal cores (see § 521) ; but in the large blocks used in more massive engineering construction, the air spaces, have been formed by inserting paraffinated paper bags filled with sand.
No statement is required here concerning the materials other than to say that almost universally portland cement is used in concrete blocks. Either gravel or broken stone may be employed. The size of the largest pieces is usually not more than inch in diameter for the smaller hollow blocks, and 1 inch for the larger hollow blocks; while of course larger pieces may be used for solid blocks.