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What purpose do iron rods in concrete serve? Do these iron rods impart any strength to the concrete apart from defining the framework for the concrete to solidify upon initially?

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up vote 8 down vote accepted

To expand a bit on Ignacio's answer: he's refering to pre-stressed concrete. The idea is to put the metal rods (rebars) under tension then allow the concrete to set around them. Once the concrete has set you release the tension on the rebars and the rebars then exert a compression force on the concrete. This allows the concrete to deal better with stretching forces. As Ignacio says, un-prestressed concrete is weak under tension.

You also see unstressed metal grids being used when concrete is being poured. I'm not sure what benefit you get from these except that they stop cracks in the concrete from compromising the whole structure.

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Hm. I've never seen stressed metal grids... I think. What we see hereabouts are just a grid with a smaller rod at right-angles every X cm to keep the rods from splaying out. – Everyone Jul 5 '12 at 17:04

Concrete is strong against compression but weak against tension. Rebar compensates for this.

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