Prestressed concrete - tension in steel cables

[fog37]

Hello Everyone,
I was learning about prestressed concrete and how it works. Concrete is very strong under compression but weak under tension. In the case of prestressed concrete, the steel bars are pulled and become elongated.

• How much do the steel bars elongate when they are pulled?

• The concrete then poured around pulled steel bars under tension. The tension on the steel bars is later released once the concrete dries. That puts the concrete under compression since it gets squeezed by the steel bars trying to return to their original length. The final concrete slab ends up bulging upward (convex shape) on one surface (that surface is under tension) and bulging inward at the other surface (which is in a state of compression). Wouldn't that tension at one of the surface make the concrete weak? Why would it make it stronger? I see how, once a load is applied to the concrete slab, the convex surface under tension becomes flat and so does the concave surface under compression.

Thanks!

 

[Baluncore]

How much do the steel bars elongate when they are pulled?

There is no advantage in stretching the bars beyond the elastic limit.

When the external tension is removed, the concrete is compressed by the steel. To optimise the result you must get the ratio of steel to concrete right. Where you place the steel in the slab is dependent on the forces the slab must handle. A cantilever will have more steel at the top, a beam between two supports will have more steel at the bottom.

Concrete may shrink slightly during the curing process which makes tensioning more difficult. There are a few types of concrete that expand and so tension the steel.