Rebar carries the tension that concrete cannot. Concrete is very strong when it is squeezed but weak when it is pulled or bent, so steel bars are cast into the parts of a slab, beam, column or footing that will be pulled. The concrete takes the compression, the steel takes the tension, and the bond between them lets the two act as one material: reinforced concrete.
That is the short answer. The rest of this post explains why concrete needs the help, how the steel and concrete work together, the specific jobs rebar does in different parts of a building, and what it does not do.
Concrete's weak spot: tension
Concrete is specified by its compressive strength: 25 MPa, 32 MPa and so on. Its tensile strength is a small fraction of that. CSA A23.3 (the 2004 edition we checked) takes the modulus of rupture, the stress at which plain concrete cracks in bending, as 0.6 times the square root of the compressive strength for normal-density concrete. For 25 MPa concrete that is about 3 MPa, roughly an eighth of its compressive strength.
Worse, that small tensile strength disappears once the concrete cracks, and concrete cracks easily: as it dries and shrinks, as temperatures swing, and as loads bend it. A plain concrete beam fails at its first serious crack, suddenly and without warning.
Steel is the opposite. Common Canadian rebar (CSA G30.18 Grade 400W) has a specified yield strength of 400 MPa in tension, more than a hundred times the cracking stress of that 25 MPa concrete. Put the two together and each covers the other's weakness.
How rebar and concrete work together
Reinforced concrete only works because a few properties line up:
- Bond. Canadian rebar is deformed, with ribs rolled onto the surface. The concrete locks around those ribs, so when the concrete tries to stretch, the bar is pulled along with it and takes the load. That is why bars need enough length past the point where they are needed, called the development length, and why they are lapped rather than butted end to end.
- Matching movement. Steel and concrete expand and contract with temperature at nearly the same rate, so seasonal temperature swings do not tear the bond apart.
- Protection. Fresh concrete is highly alkaline, and that alkalinity forms a protective film on the steel that stops rust. The concrete cover over the bars keeps that protection in place. When chlorides from road salt or carbonation over many years break it down, the bars can corrode. Our post on whether rebar rusts in concrete covers how that happens.
Rebar only helps where it sits. A bar in the middle of a slab does little for bending, and a bar lying on the ground under a slab does almost nothing. That is why cover, chair height and bar position appear on every structural drawing.
The jobs rebar does in different elements
Tension is the headline, but rebar does several jobs depending on where it is.
| Element | What the rebar does |
|---|---|
| Suspended slab | Carries bending tension at the bottom between supports and at the top over supports |
| Beam | Bottom bars carry bending tension; top bars work over supports; stirrups resist shear |
| Column | Vertical bars share the compression load and resist bending; ties hold the bars and confine the concrete |
| Footing | Spreads column or wall loads, resists bending as the footing is pushed up by the soil, ties corners together |
| Foundation and retaining wall | Resists soil pressure pushing the wall sideways |
| Slab on grade | Mostly crack control; holds shrinkage and temperature cracks tight |
Bending
When a slab or beam spans between supports and carries load, it sags. The bottom stretches and the top squeezes. The bottom bars hold the stretching face together. Where a slab or beam is continuous over a support, the pattern flips and the top bars do the work.
Shear
Near supports, beams develop diagonal tension that can split them at an angle. Stirrups wrapped around the main bars cross those potential cracks. See our post on stirrups and ties for how they differ.
Compression and confinement
Columns are mostly in compression, which concrete handles well, but bars still add capacity, resist the bending that every real column sees, and give a ductile failure instead of a sudden crush. Lateral ties stop the vertical bars buckling outward and hold the concrete core together.
Shrinkage and temperature
Concrete shrinks as it dries and moves with temperature. Lightly placed bars, called shrinkage and temperature steel, spread that movement over many fine cracks instead of a few wide ones. CSA A23.3 (2004) requires structural slabs to carry at least 0.002 times the gross concrete area in each direction for this reason.
Rebar controls cracks; it does not prevent them
This is the most common misunderstanding about rebar. Reinforced concrete still cracks. In fact, the steel cannot start working in tension until the concrete around it has cracked. What rebar does is keep those cracks narrow and stop them from opening, separating or letting one side drop relative to the other.
Two things decide how well it does that:
- Enough steel. Too little and the bars yield at the first crack, which then opens wide.
- The right depth. Bars need to be near the face that cracks, with the specified cover. For slabs, our post on where rebar should sit in a slab explains why crack-control steel in a slab on grade belongs in the upper part of the slab, not on the bottom.
Joints, curing and a good base matter just as much. Rebar cannot fix a slab poured on poorly compacted fill.
Why steel yields first: ductility
Engineers deliberately design reinforced concrete so that, if it is ever overloaded, the steel yields before the concrete crushes. Yielding steel stretches a long way before it breaks, so an overloaded beam sags, cracks visibly and gives warning. A beam with too much steel would fail by the concrete crushing, suddenly.
CSA A23.3 builds that in from both ends. It sets minimum steel so a member does not fail the moment it cracks, and limits how much tension steel a beam can carry so the steel still yields first. Columns have their own limits, with A23.3 (2004) requiring vertical steel of at least 1% and not more than 8% of the gross area. Our post on choosing rebar sizes for beams and columns goes into those limits.
What rebar does not do
- It does not make weak concrete strong. The mix, placing and curing still decide compressive strength and durability.
- It does not help if it is in the wrong place. Bars on the ground, bars with no cover, and bars missing at laps all lose most of their value.
- It is not always required. Under Part 9 of the Ontario Building Code, many slabs on grade and plain strip footings in houses have no rebar requirement, though designers and builders often add some. See do footings need rebar.
- It is not structural steel. Rebar works inside concrete. Beams, columns and posts made of steel shapes carry load on their own. Our post on structural steel vs rebar explains the difference.
Getting the right rebar for your concrete
The amount, size and position of rebar in each element comes from the structural design, and the bar list turns that design into steel on a truck. Send your drawings through our quote request form and we will prepare a free take-off and quote, then detail, cut and bend the bar from 10M to 55M, tag it by bar mark and deliver it across the GTA and Ontario. We also supply GFRP bar where corrosion or magnetism rules steel out. See our rebar products for what we supply.
Frequently asked questions
What is the purpose of rebar in concrete?
Rebar carries the tension that concrete cannot. Concrete is strong when squeezed but weak when pulled or bent, so steel bars are placed where the concrete will be pulled. Rebar also resists shear, helps columns carry load, holds cracks tight and makes failures gradual instead of sudden.
Does rebar stop concrete from cracking?
No. Concrete still cracks from shrinkage, temperature changes and load. Rebar keeps those cracks narrow and stops them from opening into wide, separating cracks, provided it is the right amount and sits at the right depth.
Why do they use steel for rebar?
Steel is strong in tension, bonds well to concrete through the ribs on the bar, expands and contracts with temperature at nearly the same rate as concrete, and is protected from rust by concrete's natural alkalinity. Where corrosion or magnetism is a problem, fiberglass (GFRP) bar can be used instead.
Does all concrete need rebar?
No. Some elements, like many slabs on grade and plain strip footings in houses under Part 9 of the Ontario Building Code, have no rebar requirement. Structural slabs, beams, columns, walls and many footings do need it, as set by the design.
