Why use reinforcement in Concrete

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 The mix of concrete and steel in reinforced concrete is a common and highly efficient building material used in the building industry on a daily basis, but how does it work.

Concrete is strong in Compression

and weak in Tension

Steel is strong in Tension

and weak in Compression.

The combination of the components obviously complement each other but how does a concrete slab become affected by compression and tension forces.

Let say that a concrete slab is used as the floor slab in a multi-storey building. It will be supported at both end by the walls of the building.

The self weight of the slab as well as the dead and live loads that are placed on the floor will try to make the slab bow in the middle.

When this occurs the top of the slab is placed into compression and the bottom of the slab in tension

So it stand to reason that the reinforcement is placed in the bottom section of the slab in this case.

What would happen if the support was moved in from the edge of the slab similar to what would happen in the case of a cantilevered balcony.

Now the weight of the overhang  will cause the top of the slab to be in tension over the support while the bottom of the slab is still in tension between the supports.

While it is good to have a basic understanding of the relationship between concrete and steel it is the job of the structural engineer to work out the size of the concrete slab or beam and placement and size of the reinforcement.