It is seemed firm, homogeneous and it will not change, when see the concrete that became hard.
However, this concrete is only the mixture of the water, air bubble, sand with which sizes different and cement when they are mixed.
Although each substance constitutes a temporary form according to each friction power, they are resisting being mixed with other substances in fact. Then, if a moderate vibration is given to the fresh concrete kneaded and mixed by vibrator, liquefaction will raise concrete density, unnecessary mixing air will be removed, and the beautiful concrete structure (product) with high and intensity with which aggregate was distributed equally will be done.
Also, it can be said what combined the aggregate that does not consider a chemical change as concrete with the cement and water that carries out a chemical change and hardens. Only the alkalinity and the close nature which does not let pass the water even the atmosphere of concrete of the portion which covers the outside of a steel rod are protecting the steel rod which carries out a chemical change.