Two separate zones, the tension zone and the compression zone, form in a beam that is bent.

Tension Zone: When a beam bends under load, it experiences tensile (or stretching) forces in this area.

It usually happens where the beam bends downward at the bottom of the simply supported beam.

Since concrete is weak in tension, steel reinforcement, or rebar, is inserted in this zone of reinforced concrete beams to withstand tensile stresses.

Compression Zone: As the beam bends, compressive (squeezing) forces are applied in this zone.

Usually, it happens in the upper part of the beam that is simply supported.

Together, the two zones balance the forces applied to the beam.