Lakhasly

Since all parts of a structure are either in tension or in compression, or both, the materials used to build structures must, first of all, be strong in tension or in compression, or in both. In nature there are many kinds of materials strong in compression: stones of all kinds, for example. Among stones, marble is one of the strongest and that is why many columns and walls were built of marble in ancient times. The Romans, who were great road and bridge builders, built their arch bridges of stone, and the Egyptians built their pyramids out of stone. There are many man-made materials that behave like stone. Bricks, made out of burned clay, are so strong in compression that we can use them to build columns or walls 25 to 30 stories high. Concrete is a man-made material, a mixture of water, sand, small stones, and a gray powder called cement, that hardens to full strength in four weeks. It is very strong in compression because it is very compact: the grains of sand fill the spaces between the stones and the paste of sand. However, like bricks and stone, concrete is weak in tension. cement and water fills the space between the grains Stone, brick and concrete are good materials to build columns and arches with, but they are not well suited for beams as some part of a beam is always in tension. Heavily loaded beams of concrete show vertical cracks near the bottom because the tension there pulls the concrete apart.
A common natural material strong both in compression and in tension is wood. Even stronger than wood is the man-made material, steel. It is equally strong in tension and compression. Aluminum is three times lighter than steel and as strong in tension and compression, but it is much more expensive than steel. Concrete and steel can be combined to increase the strength of a concrete beam. Steel bars are added to the concrete and they prevent cracks in the concrete beam from are happening or opening wider if they do occur. Such beams are said to be made of reinforced concrete and are now used in all concrete buildings. Reinforced concrete is a very good and inexpensive material because it uses a large amount of cheap concrete and a small amount of expensive steel. It is also a fire-resistant material. Steel is very strong at normal temperatures, but if a fire develops in a steel- framed building, the columns and beams, even if covered with fire-insulating materials, may get hot, and if their temperature rises above 700° Fahrenheit, they lose their strength and melt. Concrete is a good insulating material. It protects the steel from melting for a long time and so prevents it from failing longer.