Rebar is steel put into concrete to do the job concrete cannot
Short for reinforcing bar. Steel rod, usually ribbed, tied into a cage or a mat and cast into concrete so the two set as one thing. Reinforced concrete is not concrete with steel added for extra strength. It is a composite in which each material does the half of the work the other cannot.
Concrete is brilliant in compression and poor in tension
Squash concrete and it will take an enormous amount before it fails. Pull it apart and it gives up early, roughly a tenth as strong, and it fails suddenly rather than gracefully. That single asymmetry is the entire reason reinforcement exists.
Now bend something. Anything supported at its ends and loaded in the middle goes into compression along the top and tension along the bottom. The top is fine, because compression is what concrete does. The bottom is where an unreinforced beam cracks and drops, and it is where the steel goes.
Which is why the height of the bar matters more than the amount
If steel only earns its place where the concrete is being pulled, then a bar in the wrong part of the section is close to useless. Bars lying on the ground before a pour, meaning to be lifted into place and then not lifted, are the classic version, and the slab ends up with its reinforcement in the compression zone doing almost nothing.
That is what chairs and spacers are for. They look like site clutter and they are the thing holding the design together, which is also why walking a mat of steel flat during a pour is a real problem rather than a tidiness complaint.
Cover is a clock
The concrete over the steel is doing two jobs at once: keeping air and water off it, and keeping heat off it in a fire. Both are about distance, which is why cover is specified rather than left to judgement.
Steel in good concrete does not corrode, because the concrete is alkaline enough to hold a protective film on it. What eats that away is carbonation from the air, and chlorides from salt, both of which travel inwards from the surface over years. So the cover is not really a dimension, it is how long the structure has before the front arrives at the steel. When it does, the bar rusts, rust occupies more space than steel, and it pushes the face off. Spalling concrete on an old car park soffit is that, mid-process.
Ordering it
Rebar is bought by weight and cut to a schedule, and the rebar weight and bar schedule tool turns a list of bars into something a supplier can actually price and cut from, which is a different document from a quantity. If you are pricing the concrete around it, the concrete calculator does the volume, and the mix reference covers what the grades mean.
What none of them will do is tell you what reinforcement your slab needs. That is a design output rather than a lookup, and it belongs to whoever is signing the drawing.
Common questions
Why can concrete not just be made stronger instead?
Because the weakness is not a quality problem, it is what the material is. Concrete is roughly ten times stronger in compression than in tension, and making it stronger in compression barely moves the tension figure. A stronger mix in an unreinforced beam still cracks in the same place under the same bending, just at a slightly higher load. Steel is the fix because steel is excellent in tension, which is precisely what concrete is not.
Why is rebar ribbed rather than smooth?
So it cannot slide. The whole arrangement depends on the steel and the concrete moving together, and a smooth bar under load can pull through the concrete around it while the concrete stays put, at which point the reinforcement is doing nothing. The ribs, properly called deformations, mechanically key the bar into the concrete so a force in one is a force in the other. It is also why a bar caked in loose rust or mud is a genuine problem rather than a cosmetic one.
What is cover, and why does everybody go on about it?
The distance from the face of the concrete to the outside of the steel. It matters for two separate reasons: the concrete protects the steel chemically from corroding, and it protects it from heat in a fire. Too little cover and the bar rusts, which is worse than it sounds because rusting steel expands and pushes the concrete off, exposing more steel. That is what spalling on a car park soffit is: not the concrete failing, the steel inside swelling.
Can I just put the bars anywhere in the slab?
No, and getting the height wrong is the commonest way a slab ends up weaker than an unreinforced one would have been. Steel only works where the tension is, so a bar sitting on the ground at the bottom of a slab that spans between supports is in roughly the right place, and the same bar in the top of that slab is doing very little. Over a support the tension flips to the top and so does the steel. That is what chairs and spacers are for, and why they are not optional site clutter.
Is mesh the same thing as rebar?
Same idea, different product. Fabric reinforcement, the welded sheets, is a grid of smaller bars supplied in sheets for slabs where the reinforcement is light and spread out. Loose bar is used where the steel needs to be placed deliberately: in beams, in columns, around openings, and wherever the quantity is set by a calculation rather than by a nominal spec. Plenty of jobs use both.
Does rebar rust inside the concrete?
Not while the concrete is doing its job. Fresh concrete is strongly alkaline, and that alkalinity keeps a passive film on the steel that stops it corroding. What breaks it is carbonation, where the surface gradually reacts with air and loses alkalinity, or chlorides getting in from de-icing salt or seawater. Both take years, and both travel inwards from the face, which is why the depth of cover is effectively a clock.