Reinforced concrete combines two materials that solve each other’s weaknesses: concrete is strong in compression, while steel rebar is strong in tension. This matters because beams, slabs, columns, and foundations in buildings and bridges must safely carry loads without cracking or collapsing. A plain concrete beam can crack quickly when it bends because the stretched side cannot resist much tension.
Adding steel bars in the right locations lets the member carry much larger bending forces.
Key Facts
- Concrete is strong in compression but weak in tension, so tensile stress is usually carried by steel rebar.
- For a simply supported beam with a downward load, the top region is in compression and the bottom region is in tension.
- Bending stress increases with distance from the neutral axis: sigma = My/I.
- The neutral axis is the line inside the beam where bending stress changes sign and is approximately zero.
- Steel and concrete work together because they bond well and have similar thermal expansion rates.
- Rebar is usually placed near the tension face, with concrete cover to protect it from fire, corrosion, and bond failure.
Vocabulary
- Reinforced concrete
- A composite construction material made of concrete strengthened with embedded steel reinforcement.
- Rebar
- Steel reinforcing bar placed inside concrete to carry tensile forces and improve ductility.
- Neutral axis
- The line or surface in a bent beam where longitudinal bending stress is zero.
- Compression zone
- The region of a bending member where the material is being squeezed by compressive stress.
- Concrete cover
- The thickness of concrete between the outside surface and the nearest reinforcing steel.
Common Mistakes to Avoid
- Putting the main rebar in the middle of a beam, which is wrong because bending tension is greatest near the outer tension face, not at the neutral axis.
- Assuming concrete carries tension as well as compression, which is wrong because concrete cracks at relatively low tensile stress.
- Ignoring the neutral axis, which is wrong because it explains why one side of a beam compresses while the opposite side stretches.
- Forgetting concrete cover, which is wrong because exposed or shallow rebar can corrode, lose bond strength, and fail fire protection requirements.
Practice Questions
- 1 A simply supported reinforced concrete beam has a downward load at midspan. Identify which side of the beam is in compression and which side is in tension, and state where the main rebar should be placed.
- 2 A rectangular beam has bending moment M = 18,000 N m, second moment of area I = 8.0 x 10^-5 m^4, and a point 0.15 m from the neutral axis. Calculate the bending stress using sigma = My/I.
- 3 Two beams have the same dimensions and concrete strength. Beam A has steel rebar near the bottom face, while Beam B has the same steel near the neutral axis. Explain which beam resists positive bending better and why.