GYPSUM AND CEMENT BLOCK CONCRETE Gypsum Wall Blocks.—Blocks made by mixing gypsum plaster (see Section 37) with wood fiber or similar materials are used for partition walls in fireproof building construction. They are made 30 inches long, 12 inches high, and from 3 to S inches thick, with tapering openings through the block.
They are laid in the wall to break joints and cemented with mortar composed of gypsum cement plaster and sand, usually 1 to 3. They are not used for walls bearing loads, but form very light partitions, and have good soundproof and fireproof qualities.
The 3-inch blocks are used to a height of wall of about 12 feet, the 4-inch to 17 feet, and the 6-inch to 24 feet. The material may he cut with a saw, and plaster is applied directly to their surfaces.
The weights of walls of hollow gypsum blocks are approximately as follows: Thickness of block, inches 3 4 5 6 8 Weight of wall, lb per sq. ft 10 13 16 20 26 Three pounds per square foot is added for plaster upon each side of the wall.
69. Roofing and Floor Blocks.—Blocks of gypsum, similar in composition to the partition blocks, and reinforced with wire mesh, are made both in solid and hollow form for use in roof construction. They arc usually 3 or 4 feet in length and are used to span the open ings between purlins and form a solid deck upon which the roof covering may be placed. They are made with beveled edges, and are set with their lower edges in contact and the triangular openings between them filled with a grout of cement plaster. Blocks with heavier reinforcement for openings up to 10 feet in span are also now offered.
Floor blocks, to be used as fillers in reinforced-concrete floor con struction, are now available. These are designed to act as forms for the concrete, and require support at the ends of the blocks, which are 2 feet long. A spacer is placed between two adjoining blocks to hold the concrete for the web of the beam, forming a smooth surface on the under side upon which plaster may be placed. A section of floor constructed with these blocks is shown in Fig. 43.
70. Concrete Blocks.—Hollow building blocks of Portland cement concrete are frequently employed in building construction in the same manner as in solid concrete construction, given in Chapter V, and the concrete is proportioned and mixed in the same manner.
The blocks are usually made to set in the wall with the webs in a vertical position. Several patented forms are on the market which make blocks to bond in the wall in different ways and giving air spaces more or less effective as insulation against moisture and heat. Such blocks, when well made and properly set, make a sub stantial and durable building, and may be used in such manner as to give a pleasing appearance. The color of the blocks may be regulated by choice of the aggregate used upon their exposed faces. The use of coloring matter in the concrete has not usually been very successful, although there are mineral colors available which may be used without material injury to the concrete.
Metal molds are commonly employed, and concrete of rather dry consistency is compressed into them by tamping or by hydraulic pressure. This yields concrete of greatest strength and also makes a block which may be quickly removed from the mold. For orna mental work, sand molds are frequently employed, a wooden pattern being used in forming the mold, and the concrete poured in a wet The curing of the blocks is important in its effect upon the strength and durability of the concrete, which must not dry out during the period of hardening. After the blocks are removed from the molds, they are allowed to stand in the air until the cement has set, when they may be transferred to a steam chamber, where they are subjected to an atmosphere charged with steam at a temperature about 110° to 130° F. After two or three days in the steam, they may be removed to the open air, but should be sprinkled often enough to keep them continually damp for ten or twelve days. When a steam chamber is not employed, the blocks are cured in the open air, but should be kept wet for a longer period to give time for com plete hardening. The temperature to which they are subjected during hardening should never go lower than about 50° F.