Let’s examine the different forces a concrete beam or column endures:
- Compression, from weight applied directly to the structure.
- Bending, from tensile loads.
Concrete can handle compression very well, because it’s basically made of sand and gravel. It can withstand compressive loads of 3000 - 6000 psi.
However, concrete doesn’t handle tensile loads as well. Concrete’s malleable strength is a mere tenth of its compressive strength.
When you examine the bend, the inner part has visible compressive strain, but the outer area has tensile strains, which means that a concrete beam is pretty ineffective when it comes to bending.
However, steel has much more tensile strength, and embedding a steel support into a concrete beam distributes the tensile load and increases the flexibility of the structure as a whole.