66. Block Construction.—In the construction of walls of ordi nary hollow rectangular blocks, the blocks are usually laid so as to break joints and extend through the walls. They should be so placed that the vertical webs in each course are directly above those in the course below. Such construction is shown in Fig. 38, bi using tile with horizontal cells, jamb blocks and corner blocks are made with the cells vertical. When very light walls are used, longitudinal reinforcement, consisting of thin band iron or of special forms of wire mesh, is placed in the joints. This is necessary for 2-inch partitions or for 3-inch partitions more than 10 feet high.
Tiles with vertical cell openings are made by some makers. Fig. 40 shows construction with standard tiling of this type.
Portland cement mortar, or mortar of lime and cement, is used in laying hollow blocks. In walls which are to carry considerable loads, Portland cement with 10 to 15 per cent of hydrated lime by volume (4 to 6 per cent by weight) should be used in 1 to 3 mortar with well-graded sand. For walls which are not to carry loads, a larger amount (equal volumes) of lime may be used. The surfaces of tiles are often grooved to aid the adhesion of the mortar in the joints. When the finish of the wall is to he plaster or stucco, the surface of the tile is grooved to hold the plaster. If brick veneer is to he applied, or if the surface of the tile is to he used for exterior finish, a smooth finish may be desirable.
Floor Construction.—The method of using hollow blocks in flat arch floor construction is shown in Fig. 41. These arches vary from about 3 to 6 feet in span and from 6 to 12 inches in depth. The blocks required consist of the skewback, the fillers and the key-block.
The skewbacks are usually made of such form as to enclose the bottom of the I-beam for fire protection.
Such arches are now commonly made by the end-construction method in which the cell openings run lengthwise of the arch. The blocks do not break joints, but form a series of independent arches side by side. A number of different shapes are offered for these arches by different makers, lighter weight being obtained than with side-construction arches for the same strength.
Hollow blocks are frequently used as fillers in reinforced con crete floors, the blocks filling spaces between the webs of the T-beams of concrete, as shown in Fig. 42. Blocks 12 inches wide are usually employed for this purpose, the depth depending upon the span and loading of the floor.
Strength of Block Masonry.—Comparatively few data are avail able upon the strength of constructions of terra-cotta blocks. A very carefully constructed wall of natco tile (see Fig. 40) was tested by R. W. Hunt & Company. The wall was 36a inches long, S inches
thick, and 12 feet V inches high, and was twenty-eight clays old when tested. It failed under a load of 436,000 pounds, giving a compression of 3110lb. 'in.'' on the net section of the web, or about 1500 of gross area. Tests of a wall of Denison tile (see Fig. 39) faced with brick, forty-two clays old, was made at the labo ratory of the Bureau of Standards. This wall was 5 feet, 1 inch in length, 12z inches thick, and 31 feet high. It carried a load of 6S6,000 pounds, or about 900 of gross area.
Good dense tiling should have a crushing strength of 3000 to 6000 of net section. When hid in masonry the allowable load is usually not more than one-fifteenth of the ultimate strength of the block. Carefully laid masonry of good quality hollow blocks may be allowed to carry a load of 200 of net section of block, or in general about 5 tons per square foot of gross area.
67. Architectural Terra-cotta.—Terra-cotta for ext erior finish or ornamental work is usually made from a mixture of clays, carefully selected to secure the desired qualities. The clay is ground, mixed, tempered, and worked to a proper condition of plasticity. It is then formed into the desired shapes in plaster molds or by hand, modeled as may be necessary, and dried. After drying, it is given a surface treatment, by spraying with a liquid upon the surface, which determines the kind of finish to be given in burning and its color.
The blocks of terra-cotta may have a length up to 30 inches, and depth of G to 10 inches, with height according to the requirements of the work. They are constructed as hollow shells with webs about 11 inches thick, and cells 6 inches or less in width. These blocks are built into the body of the wall by bonding the masonry into and filling the cells.
Several kinds of surface finish are used for terra-cotta. Standard terra-cotta is that in which no special finish is applied, leaving the block somewhat porous. Vitreous terra-cotta has a spray applied to the surface which causes the surface material to vitrify during burn ing, making the material non-absorbent. Glazed terra-cotta has an impervious coating of glaze upon the surface. When the glaze is deadened, it is called mat-glazed. A variety of colors are available for use with this material, and make its use possible in a wide range of artistic designs.
Terra-cotta of good quality is one of the most durable materials for use in the trimming and ornamentation of masonry structures. Being practically non-absorbent, it is not affected by frost, or by the gases in the atmosphere. The facility with which it may he worked into desired forms makes it a desirable material for artistic design.