Plain, unreinforced concrete has a strikingly lopsided set of structural strengths: it resists compression, being squeezed or pushed together, quite well, but it resists tension, being pulled or stretched apart, only very poorly, tending to crack under tensile stress at a small fraction of the load it can handle in compression. Reinforced concrete solves this by embedding steel reinforcing bars, commonly called rebar, directly within the concrete before it cures, since steel happens to have the opposite strength profile: it resists tension excellently, letting the two materials cover for each other's opposite structural weakness within a single composite element.
A beam under load is simultaneously compressed and stretched
When a horizontal structural beam bears a load, it doesn't experience a single uniform kind of stress throughout its cross-section — the beam's upper region, curving slightly downward under the load, is compressed, while its lower region, stretching to accommodate that same curve, is placed under tension. A beam made purely of plain concrete would perform reasonably well handling the compression on its upper side but would be vulnerable to cracking and failure on its lower, tension side, well before the material's compressive strength was ever actually challenged.
Placing steel where the tension is lets each material do only what it's good at
Reinforced concrete addresses this directly by positioning steel reinforcing bars specifically within the regions of a structural element that will experience tension under load, most often nearer a beam's lower edge, letting the embedded steel carry the tensile stress that plain concrete alone couldn't reliably handle, while the surrounding concrete continues carrying the compressive load it's already naturally well suited to. The result is a composite structural material considerably stronger and more versatile than either concrete or steel would be on its own for this kind of application, which is exactly why reinforced concrete became, and remains, one of the dominant structural materials in modern civil engineering.
What we're still unsure about
The basic compression-versus-tension logic behind reinforced concrete, and its dominant role in modern structural engineering, are well established, foundational civil-engineering knowledge. What's more genuinely an active area of ongoing engineering research is how to extend reinforced concrete's working lifespan under real-world conditions, since steel rebar can corrode over time when moisture and chlorides reach it through cracks or porous concrete, and once that corrosion begins it can progressively weaken the very reinforcement the structure depends on — researchers continue actively developing corrosion-resistant reinforcement materials and protective techniques, without a single universally adopted solution that fully eliminates this long-term durability concern.
This sits inside Concrete & Steel Design, one of seven topics in Civil Engineering, one of four domains in Engineering, one of seventeen subjects the app can quiz you on.