From a great number of tests of bars and unfinished cables, the committee considered that the average ultimate resistance and proof tests of chain cables made of the bars, whose diameters are given, should be such as are shown in the accompanying table. MASONRY MATERIALS OF CONSTRUCTION The materials employed in the construction of masonry are stone, brick, terra cotta, and the cementing materials used in the manufacture of mortars, namely, lime, cement, and sand. STONE Strength of Stone.-In ordinary buildings and engineering structures, stones are generally under compression. Occasionally, they are subjected to cross-stresses, as in lintels over wide openings. They are never subjected to direct tension. As a general rule, a stone should not be subjected to a greater compressive stress than one-tenth of the ultimate crushing strength, as found by experiment. The resistance to crushing varies within wide limits, owing to the great variety in the structure of the stones; the method of preparing and finishing the test pieces also affects the results; hence, the great variations found in the values given by different experiments. The accompanying table shows the average resistance of the principal building stones to crushing and to rupture when used as beams. Absorptive Power of Stone.-The absorptive power of a stone is a very important property, a low absorption generally indicating a good quality. The accompanying table gives the average percentage of water absorbed by stones. Durability of Stone.-The following rough estimate, based on observations made in the city of New York, indicates the number of years a sound stone may be expected to last without being discolored or disintegrated to such an extent as to require in. X2 in. The Size and Weight.-The dimensions of bricks vary considerably. The standard adopted by the National Brickmakers' Association is, for common clay brick, 8 in.X4 in. X2 in., and for face or pressed brick (clay) 8 in. X4 weight of a common clay brick is about 4 pressed-clay, enameled brick, about 7 lb. glazed bricks are made in two sizes: X4 in.; American size, 8 in. X2 dimensions for firebricks are 9 in. X4 sizes and forms are made to suit the required dimensions of the lime-sand bricks are 8 in. X4 The weight varies between 5 and 6 lb. lb.; that of a Enameled and English size, 9 in. X3 in. in. The usual in.; various in. X4 work. in. ×2 WEIGHT AND STRENGTH OF BRICK Weight Crushing Strength The in. Kind of Brick Pounds per Cubic Foot Best pressed-clay.. 150 Pounds per Square Inch 5,000 to 15,000 Common hard-clay. Soft-clay. 100 450 to 600 The accompanying table gives the approximate weight and resistance to crushing of brick. Requisites for Good Brick.-Bricks of good quality should be of regular shape, with parallel surfaces, plane faces, and sharp square edges. They should be of uniform texture; burnt hard; and thoroughly sound, free from cracks and flaws. They should emit a clear ringing sound when struck a sharp blow. A hard well-burned brick should not absorb more than one-tenth of its weight of water; it should have a specific gravity of 2 or more. The crushing strength of a brick laid flat should be at least 6,000 lb. per sq. in. The modulus of rupture should be at least 1,000 lb. per sq. in. CEMENTING MATERIALS Lime.-Common lime, commercially called quicklime, is manufactured by calcining, or burning, at a temperature of from 1,400° to 2,000° F., stones composed of pure or very nearly pure carbonate of lime. The product is practically pure oxide of calcium. It is prepared for use, converting it into calcium hydrate, by the addition of water. This process is called slaking. The quantity of water required in slaking lime is about one-third the volume of the lime. Lime weighs about 66 lb. per bu., or about 53 lb. per cu. ft. One barrel of lime, weighing 230 lb., will make about 24 bbl., or .3 cu. yd. of stiff paste. In 1-to-3 mortar, 1 bbl. of unslaked lime will make about 6 bbl. of mortar; or 1 bbl. of lime paste will make about 3 bbl. of mortar. For a 1-to-2 mortar, use is made of about 1 bbl. of quicklime to 5 or 5 bbl. of sand. Hydraulic Cements.-The hydraulic cements are divided into three main classes; namely, Portland cement, natural cement, and pozzuolana. These cements differ from the limes by not slaking after calcination. Portland cement is the product resulting from the process of grinding an intimate mixture of calcareous (containing lime) and argillaceous (containing clay) materials, calcining (heating) the mixture until it starts to fuse, or melt, and grinding the resulting clinker to a fine powder. Natural cement is made by calcining natural argillaceous or silicious limestones at a heat just below fusion and grinding the product to powder. Pozzuolana, or puzzolan, cement is a material resulting from grinding together, without subsequent calcination, an intimate mixture of slaked lime, and a puzzolanic substance, such as blast-furnace slag or volcanic scoria. The only variety of puzzolan cement employed extensively in American practice is slag cement. This cement is made by grinding together a mixture of blast-furnace slag and slaked lime. The slag used for this purpose is granulated, or quenched, in water as soon as it leaves the furnace, which operation drives off most of the dangerous sulphides and renders the slag puzzolanic. Portland cement may be distinguished by its dark color, heavy weight, slow rate of setting, and greater strength. Natural cement is characterized by lighter color, lighter weight, quicker set, and lower strength. Slag cement is somewhat similar to Portland, but may be distinguished from it by its lilac-bluish color, by its lighter weight, and by the greater fineness to which it is ground. Portland cement is adaptable to any class of mortar or concrete construction, and is unquestionably the best material for all such purposes. Natural and slag cements, however are cheaper, and under certain conditions, may be substituted for the more expensive Portland cement. All heavy construction, especially if exposed, all reinforced-concrete work, sidewalks, concrete blocks, foundations of buildings, piers, walls, abutments, etc., should be made with Portland cement. In second-class work, as in rubble masonry, brick |