Why concrete alone is not enough
Concrete is made of cement, sand, small stones (aggregate) and water. It hardens into a stone-like block.
It is very good at compression. Compression means being squeezed or pressed. A concrete block can hold a very heavy weight on top.
It is poor at tension. Tension means being pulled or stretched. Concrete cracks easily when pulled. Its tension strength is only about one tenth of its squeeze strength.
A beam is a horizontal part. When a load sits on it, the beam bends. The top is squeezed and the bottom is stretched. So the bottom of a plain concrete beam cracks first.
Steel bars fix the weak side
Reinforced concrete (RC) means concrete with steel bars inside. The bars are called reinforcement or rebar.
Steel is strong when pulled. So we put the bars on the stretched side, near the bottom of a beam. The concrete takes the squeeze. The steel takes the stretch.
Steel and concrete work well together for three reasons:
- Concrete grips the rough steel bar (this is called bond).
- Both expand by almost the same amount when heated, so they do not tear apart.
- Concrete is alkaline. It covers the steel and protects it from rust and from fire.
Parts of an RC building
An RC building is built from a few simple parts. Each one carries load to the part below it.
- Slab: the flat floor or roof. It passes its load to the beams.
- Beam: a horizontal part. Main bars at the bottom fight stretching. Stirrups (loops) fight slanting shear cracks.
- Column: a vertical part. It is mostly squeezed. Vertical bars and ties (loops) hold the concrete.
- Foundation: a wide base that spreads the load on the ground.
A frame of beams and columns joined firmly together is called a rigid frame (rahmen). Walls are often only filling.
Good points and weak points
Good points: it can be moulded into any shape, it is strong and lasts long, it does not burn, it is cheap because the materials are easy to find, and it resists wind and earthquakes when well designed.
Weak points: it is heavy, it takes days to harden (curing), poor quality work can leave hidden gaps, and cracks let water in and rust the steel. Water rusting steel makes it swell and break the concrete cover.
That is why the cover (the concrete thickness over the bars) must be enough, and why we cure the concrete with water.
Try it
In the 3D: go to the last step. Slide the load to 70% with no steel and watch the beam crack and break. Now tick "Add steel bars" and slide again.
At home: take a slab of dry clay or a thick chalk stick. Hold both ends and bend it gently. It snaps at the bottom edge first. Now press chalk end to end. It does not break. This is concrete: strong in squeeze, weak in stretch.
Key formulas and definitions
- Compression = squeezing; Tension = stretching; Shear = sliding/cutting
- Concrete tension strength is only about 1/10 of its compression strength
- RC = concrete (takes compression) + steel bars (take tension)
- Beam: main bars at the bottom, stirrups around them
- Cover = concrete thickness over bars (protects from rust and fire)
Worked examples
1. A simply supported beam carries a load in the middle. Which side stretches and where should the main bars go?
A load in the middle bends the beam downward. The top is squeezed, the bottom is stretched. Main bars go near the bottom.
2. A concrete block holds 30 units of squeeze but only 3 units of stretch. Compare the two.
30 ÷ 3 = 10. It is 10 times stronger in squeeze than in stretch. So steel is added where stretch happens.
3. A balcony slab is fixed at the wall and hangs out in the air with a load on the free end. Where should the main bars go: top or bottom?
A hanging (cantilever) slab bends downward at the free end, so the TOP stretches. Main bars go near the top, not the bottom. The bar goes where the stretch is.
Common mistakes
- Putting steel at the bottom of every part. Steel goes where the part stretches, and that is the top for a cantilever.
- Thinking steel is added to make concrete squeeze better. It is added for stretching (and for ties and shear).
- Saying steel and concrete "stick" only by glue. The ribs on the bar give the grip (bond).
- Ignoring cover. If cover is too thin, rust reaches the bars and the concrete breaks.