Reinforced concrete is a composite material in which concrete’s relatively low tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength or ductility.
source/image(PrtSc): Practical Engineering
More destructive testing to answer your questions about concrete.Concrete’s greatest weakness is its tensile strength, which can be less than 10% of its compressive strength. Watch the video from Practical Engineering for more info:
So, we often reinforce it to create a composite material strong against all types of stress. This video briefly touches on conventional rebar and prestressed/post-tensioned reinforcement.
The reinforcement is usually, though not necessarily, steel reinforcing bars (rebar) and is usually embedded passively in the concrete before the concrete sets.