CONCRETE BUILDING BLOCKS 29. The growth of the concrete block industry has been rapid. The blocks are taking the place of wood, brick, and stone for ordi nary wall construction. They are strong, durable, and cheap. The blocks are made at a factory or on the site of the work where they are to be used, and arc placed in the wall in the same manlier as brick or stone. There are two general types of blocks made—the one-piece block, and the two-piece block. The one-piece type consists of a single block, with hollow cores, making the whole thickness of the wall. In the two-piece type, the front and back of the blocks are made in two separate pieces, and bonded when laid up in the wall. The one-piece blocks are more generally used than the two-piece blocks.
30. Size of Blocks. Various shapes and sizes of blocks are made. Builders of some of the standard machines have adopted a standard length of 32 inches and a height of 9 inches for the full sized blocks, with width of 8, 10, and 12 inches. Lengths of 8, 12, 16, 20, and 24 inches are made from the same machine, by the use of parting plates and suitably divided face-plates. Most machines are constructed so that any length between 4 and 32 inches, and any desired height, can be obtained.
The size of the openings (the cores) varies from one-third to one-half of the surface of the top or bottom of the block. The building laws of many cities state that the openings shall amount to only one-third of the surface. For any ordinary purpose, blocks with 50 per cent open space are stronger than necessary.
31. Material. The material for making concrete blocks con sists of Portland cement, sand, and crushed stone or gravel. Owing to the narrow space to be filled with concrete, the stone and gravel are limited to one-half or three-quarters of an inch in size. At least one-third of the material, by weight, should be coarser than inch. A block made with gravel or screenings (sand to i-inch stone), with proportions of 1 part Portland cement to 5 parts screenings, will be as good as a block with 1 part Portland cement and 3 parts sand. These materials will be further treated under the headings of "Port land Cement," "Sand," and "Stone." 32. Proportions. The proportions generally used in the making of concrete blocks, vary from a mixture of 1 part cement, 2 parts sand, and 4 parts stone, to a mixture of 1 part cement, 3 parts sand, and 6 parts stone. A very common mixture consists of 1 part cement, 23; parts sand, and 5 parts stone. A denser mixture may be secured by varying these proportions somewhat; that is, the maker may find that he secures a more compact block by using 2i parts sand and 4i parts stone; but a leaner mixture than 1 : 2 : 5 is not to be recommended. In strength this mixture will have a crushing resistance far beyond any load that it will ever have to support. Even a mixture of 1: 3: 6 or 1: 31: 7 will be stronger than necessary to sustain any ordinary load. Such a mixture, however, would be
porous and unsatisfactory in the wall of a building. Blocks, in being handled at the factory, carted to the building site, and in being placed in the wall, will necessarily receive more or less rough handling; and safety in this respect calls for a stronger block than is needed to bear the weight of a wall of a building. For a high-grade water-tight block, a 1 : 2 : 4 or a 1 : 2 : 4 mixture is generally used.
33. Proportion of Water. Blocks made with dry concrete will be soft and weak, even if they are well sprinkled after being taken out of the forms. Blocks that are to be removed from the machine as soon as they are made will stick to the plates and sag out of shape, if the concrete is mixed too wet. Therefore there should be as much water as possible used, without causing the block to stick or sag out of shape when being removed from the moulds. This amount of water is generally S to 9 per cent of the weight of the dry mixture. To secure uniform blocks in strength and color, the same amount of water should be used for each batch.
34. Mixing and Tamping. The concrete should be mixed in a batch mixer, although good results are obtained in hand-mixed con crete. The tamping is generally done with hand-rammers. Pneu matic tampers, operated by an air-compressor, arc used successfully. Moulding concrete by pressure is not successful unless the concrete is laid in comparatively thin layers.
35. Curing of Blocks. The blocks are removed from the machine on a steel plate, on which they should remain for 24 hours. The blocks should be protected from the sun and dry winds for at least a week, and thoroughly sprinkled frequently. They should be at least four weeks old before they are placed in a wall. If they arc built up in a wall while green, skrinkage cracks will be apt to occur in the joints.
36. Mixture for Facing. For appearance, a facing of a richer mixture is often used, generally consisting of 1 part cement to 2 parts sand. The penetration of water may be effectively prevented by this rich coat. Care must be taken to avoid a seam between the two mixtures.
Blocks are made with either a plane face, or of various orna mental patterns, as tool-faced, paneled, rock-faced, etc. Coloring of the face is often desired. Mineral coloring, rather than chemical, should be used, as the chemical color may injure the concrete or fade.
37. Cost of Making. The following is quoted from a paper by N. F. Palmer, C. E.: Blocks 8 by 9 by 32 inches; gang consisted of five workmen, and foreman; record for one hour, 30 blocks; general average for 10 hours, 200 blocks. The itemized cost was as follows: 1 foreman @ $2.50 $ 2.50 5 helpers @ 2.00 10.00 13 bbls. cement ® 2.00 26.00 10 cu. yds. sand and gravel ® 1.00 10.00 Interest and depreciation on machine 2.00