Nowadays a building’s framework is made of reinforced concrete and of structural steel. Concrete is made by mixing together small stones, sand, cement and water. The coarse stones used in the mix give the concrete its strength. The sand is needed to fill the gaps between the stones. The cement, mixed with just enough water to make it into a paste, covers the surface of all solids, and binds the entire mixture into single mass. It is used less water to make mixing concrete denser and stronger. It is a difficulty here. Dry mixing concrete is not so easy to stir as one that is fairly wet and sloppy. When it is really strong concrete, it is mixed with the necessary minimum of water and placed in the forms. After this it is vibrated with electrically vibrated bars. The mixture is tipped or piped into forms (wooden mold) of the shape required.
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The sand is needed …
to stir as one that is fairly wet and sloppy
to make mixing concrete denser and stronger
to give the concrete its strength
to fill the gaps between the stones
