Why does rebar or other wire/steel strengthen concrete?
In: Engineering
Concrete has good compressive strength, but very little tensile strength and somewhat limited shear strength (ability to take forces sideways through the member). This is important as when you have a beam or slab it’s trying to bend, one side (top/bottom) will be in compression and the other will be in tension.
Further, the tensile strength it does have has a brittle failure mode. There are no warning signs until it fails, which is not great.
So we add steel reinforcement. The steel bars are great in tension. They can withstand enormous forces for their size, and when they do fail they (normally) do so in a ductile manner – the beams/slabs will deflect like crazy so everyone can GTFO before a building falls on them.
This allows us to build slabs and beams, rather than having to have everything be arch or dome systems which take up a LOT of space and get very heavy so aren’t practical for buildings with multiple stories unless you want a really really expensive structure.
Concrete is very, very difficult to compress. It can be broken, if the concrete is put in tension. When you have a beam that’s carrying a load, the top side is being compressed but the bottom is in tension.
Steel is very strong in tension, but it can be bent. Steel in concrete is the best of both worlds. The steel takes the tension load, so the concrete doesn’t crack, and the concrete prevents the steel from bending because to bend the steel you’d have to compress the concrete.
Concrete is really good at deal with compressive forces, you can put a massive amount of weight pushing against concrete or cement and it will be fine
But what if you were to make a cement house? Well it can take the weight of the walls pushing downward, but the walls may also want to lean out a little bit meaning that the corner experiences a pulling force rather than a squishing force that its good at and you get cracks
This is where steel comes in. Steel has really good tensile strength meaning its great at resisting those pulling forces without breaking. The combination of steel and concrete gives you something with good compressive strength (concrete) and good tensile strength (rebar/steel)
So the fundamental weakness of concrete is when it cracks. Under all other circumstances concrete is better then steel for most stuff, but when it cracks, it can flake and separate and become weak. Steel rebar and wire are more resistant to separation, slowing the formation of the cracks and keeping the concrete together if it does crack. Basically concretes one weakness is steels strength
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