- Issue Time
Steel + Concrete = Safe
Steel only = Expensive
Concrete only = Are you mad?
Everyone knows that concrete is damn good in compression while steel is great in tension. Steel is also good in compression but because it has very high strength, the cross section generally becomes small and the smaller the cross section the more are the chances of buckling, while concrete on the other hand has low strength so we require larger cross sections and in doing so we overcome the buckling effects.
Now, the important question is why only steel? Why not aluminium?
The reason is, when both the material are heated they possess a similar coefficient of thermal expansion, concrete has of the order of 14 x 10^-6 while steel has 12.6 x 10^-6. So when it gets hot, both the material experience almost similar strains and there is no internal stress formation. Now one more question comes to mind is, what would happen even if internal stresses are formed? Well, those small strains can create tensile stresses in concrete, once these stresses are induced then concrete cracks and once the concrete cracks the bond strength between steel and concrete is reduced and once it is lost then the material is as good as an independent one. So this gives a great advantage of using steel rather than anything else in concrete.
Second reason is the availability of steel and the way we can recycle it. We have huge furnaces which can process and recycle iron and it is available in quite an abundance.
Even if we had used aluminum would it have been better? No, because aluminium is more flexible than steel but it does not show any yield point.
In fact it shows a very non linear behavior and there is no such effect as strain hardening. Also aluminium loses strength quite rapidly just after 80 degrees of heating. And in case of earthquake we rely on strain hardening of steel which helps us predicting the behavior.
So in all, all the points are in favor of steel so why not use it rather than anything else.