Concrete's strength where steel cannot work
Concrete is excellent at resisting crushing forces, which allows it to form thick compression-bearing walls and columns efficiently. Its high strength-to-weight ratio in compression makes it ideal for bearing walls that must carry many stories. Unreinforced concrete does fail under tension, where flexibility or stretching is required. Adding steel bars exploits the material pairing: concrete protects the steel from rust and fire, while steel prevents cracks from spreading.
Steel placement and reinforcement strategy
Rebar is positioned where tension develops: in the bottom of beams where floors sag, in the outer face of walls where bending occurs, and wrapped around columns to resist lateral sway. The amount and spacing of steel bars is engineered so that concrete stress and steel stress both stay within safe limits. Proper cover depth (concrete encasing the rebar) prevents water and air from reaching the steel, which would cause corrosion and loss of bond. Reinforced concrete combines two weak materials into a composite strong in both directions.