WHY DO WE PROVIDE STEEL IN CONSTRUCTION :
RCC (Reinforced Cement Concrete) is like a marriage. Both concrete and steel (i.e. rebar) need each others during structural elements are designed to withstand the load.
As we knows, concrete is formed by the mixture of Portland cement, aggregates(fine and course) & water. And this results in stone like artificial material which posses a great strength to resist the compression load .
Whereas the Stones have been already used by the Greek as well as other civilizations to resist/beam the loads. However its use was limited mainly to the columns (Compression Member).
Imagine if we have a concrete beam with moment which is long enough to be bent. Now, if we apply force strong enough, then it will eventually break.
Now whenever the concrete is tensioned, its molecular configuration doesn’t allow it to resist the force and so it breaks.(Why concrete is weak in tension). This happens to all of the stone like materials as they are brittle.
In 1853, a French industrialist known as Coignet realized that the introduction of steel in the form of rods or grids will confer concrete the missing property it lacked to become a material that was able to resist the compression and tension at the same time.
Fig. : A modern reinforced cement concrete beam.
Steel provides a ductile quality to this partnership that will resists all the tensional stress in the structural elements that concrete by itself can’t.
Hence, In this way a new material can be obtained and new possibilities to design the concrete structures.
WHY STEEL ONLY ?
Now, one might ask why all of this is necessary if there are already steel structural elements available which can be easily fabricated.
Only Steel = Expensive
Only Concrete = Are you mad ?
Steel + Concrete = Safe
Everyone knows that the concrete is quit good in compression whereas steel is great in tension. Also Steel is good in compression but as it has a very high strength, the cross section will become smaller. And we all knows the smaller the cross section the more are the chances of buckling. Whereas concrete on the other hand have low strength so the c/s will be quite larger. Thus to strike the middle path we uae combination of both steel and concrete.
Now,another question is
why only steel? Why not aluminium or other material ?
The reason behind this is, when both(aluminium and steel) the material are heated they possess a similar coefficient of the thermal expansion, concrete has of the order of 14 x 10^-6 whereas steel has 12.6 x 10^-6. Therefore when it gets hot, both the materials experience almost same strains resulting in no internal stress formation.
Now once again a question arises, what would happen even if the internal stresses are formed ? Well, those small strains can create the tensile stresses in concrete, once these stresses are induced then concrete cracks and once the concrete is cracked the bond strength between the steel and concrete is reduced and once it is lost then the material is as good as an independent one. Hence this gives a great advantage of using the steel rather than anything else in the concrete.
Second reason is availability of steel and the way we can easily recycle it. We have huge furnaces which are able to process and recycle the iron and it is available in quite an abundance.
Even if we had used aluminum would it have been better ? No, because aluminium is quit more flexible than the steel but it doesn’t show any yield point.
In fact it(aluminium) shows a very non-linear behavior and there is no such effect as strain hardening exist in case of aluminium. Also aluminium loses its strength quite rapidly just after 80 degrees of heating. And in case of the earthquake we rely mainly on strain hardening of the steel which helps us predicting the behavior.
So in all, all the points are in favor of steel so we use it rather than anything else.