Many great dams are made of concrete. Thousands of miles of roads and sidewalks are made of concrete. There are concrete floors and foundations in many buildings. Concrete is one of our most important building materials.
   Cement, water, sand, and crushed rock or gravel are mixed together to make ordinary concrete. Usually big machines do the mixing. The rock "batter" is then poured into wood or steel molds which give the concrete the shape wanted. When the concrete hardens, the mold can be taken away. A concrete wall or walk or dam can be made in much less time than it would take to build it of blocks of stone.
   Concrete can also be made into blocks and then built into buildings. Many small houses are made of concrete blocks.
   There are special kinds of concrete for special uses. Sometimes steel rods are run through it. Then it is called reinforced concrete. Concrete floors may have asphalt added so that they will be more comfortable to stand on. For lightweight concrete, rock materials lighter than sand and gravel are used. Pumice, a very light rock formed from lava, is one. Vermiculite is another. Vermiculite concrete may weigh only one-eighth as much as concrete mad,e with sand and gravel.
   Concrete is strong, and it lasts well. It will not burn. Termites and borers cannot eat it. And concrete is cheap, compared to many other materials. No wonder it is popular.