It's tempting to think of "strength" as a simple variable and that various materials all fall somewhere on a spectrum of strong to weak. The reality, of course, is much more complex. Different materials are strong in specific situations, but weak in others. For a really robust material, you have to start mixing and matching. Reinforced concrete is a perfect example.
Concrete is great at putting up with stress in the form of compression. It can take a lot of squeezing abuse before it gives up. It's terrible at handling tensile stress though, and is not suited to fight against forces that would threaten to pull it apart.
This is a particular problem in construction, as Practical Engineering explains, because concrete in a building almost always experiences both kinds of stress. If you put weight on a concrete beam from above, its top will hold up heroically against the compression. Its underside, however, will experience severe tensile stress as the beam deforms. This creates a situation that engineers often refer to by the technical term "bad."
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Rebar helps to alleviate this issue because, unlike concrete, rebar is pretty great taking tensile stress. As a result, rebar inside of concrete both strengthens the resulting mixture by making its strength more all-around, and also decreases the speed at which failure occurs, giving engineers crucial time to spot a disaster before it happens.
Watch Practical Engineering explain with some handy visual examples, take a moment to appreciate what's underneath you the next time you stand on a bridge:
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