BULGARIA. A land of contrasts is Bulgaria. Sofia, the capital, is situated in about the same lati tude as Chicago, and like that city it has a cold winter. But in summer Sofia is the center of a smiling land of sunshine and flowers. Crocuses, are provided by putting in double columns and double beams separating the structure into units. Because of its cheapness, its adaptability, and the ease with which it can be worked, reinforced concrete is one of the most popular of modern build ing materials.
Concrete is made by mixing a cement, usually portland cement, with water, sand, and broken stone, pebbles, or gravel. From the mixing machine this is poured into forms of wood or steel, where it is allowed to harden. If no great side stress is to be resisted, no steel reinforcement is necessary, and the concrete is called massive concrete. In important struc tures. the supporting frame is left on for two to four weeks, to give the con crete time to harden sufficiently. It is considered hard enough when a tenpenny nail will bend when one attempts to drive it into the concrete.
This material is also cast into hollow blocks, which are used like blocks of stone for building houses and other small structures. Concrete is also widely used in stucco, a mixture of concrete, gypsum, and sand. This is thrown up on a surface of lathing and makes a durable and cheap sur facing material. (See also articles on Brick and Tile; Cement; Concrete; Iron and Steel.)