ответить на вопросы concrete is a most necessary material in modern architecture. this very old material was used by the ancient romans. they knew the art of building huge concrete roofs supported only by walls but this art was last in the middle ages. and now concrete plays an important part in the history of architecture. concrete is a mixture of a power called cement, made from clay and chalk, water, sand and broken brick or gravel when mixed and left for a time, this mass becomes as hard as stone a concrete beam, like stone, can crack if heavily weighted, but we know a way - unknown to the ancient romans - of strengthening concrete. before pouring the mixture, one can place long steel rods or a steel mesh into the trough. we call this ferro-concrete, and we can mould it into any shape. concrete is as plastic as clay and it is much easier to get unusual shapes from concrete than from stone or brick. one can use concrete to model a building into different forms as readily as a child uses plasticine to do the same thing. ответьте на вопросы к тексту : 1) what is concrete? 2) what role does concrete play in construction building? 3) what is concrete made of? 4) the use of what makes concrete stronger? 5) what are comparative characteristics of concrete?