What Is Rebar and Why Is It Used in Concrete?

Rebar (short for reinforcing bar) is ribbed steel bar cast into concrete to give it the tensile strength it lacks. Concrete is excellent at resisting crushing but cracks when it is pulled or bent. Steel bars placed where those tension forces occur carry them instead, so a reinforced beam, slab or wall can span, cantilever and carry loads that would break plain concrete. That combination, reinforced concrete, is the basis of almost every foundation, parking structure and concrete building in Canada.

Rebar meaning and definition

In Canada, rebar means deformed carbon steel bar made to CSA G30.18. "Deformed" refers to the ribs rolled onto the surface. Bars are sized in metric designations from 10M to 55M, where the number is roughly the bar diameter in millimetres:

Nominal values per CSA G30.18. Most building rebar is Grade 400W, meaning a 400 MPa minimum yield strength with weldable chemistry; 500W is used where a design calls for higher strength. Our posts on Canadian rebar sizes and rebar grades cover the details, including how to read the markings rolled onto each bar.

How rebar works in concrete

Concrete is strong one way and weak the other

Concrete's tensile strength is a small fraction of its compressive strength. CSA A23.3 (2004 edition cited here) takes the modulus of rupture of concrete, its strength when bent, as 0.6 times the square root of the compressive strength. For ordinary 25 MPa concrete, that is about 3 MPa, roughly one eighth of what it can take in compression.

Think of a concrete beam spanning between two walls. Under load it sags: the top is squeezed and the bottom is stretched. The top is fine. The bottom cracks almost immediately. Put steel bars near the bottom and, once the concrete there cracks, the bars carry the tension while the concrete above carries the compression. Each material does the job it's good at.

The ribs make them work together

For that to happen, load has to pass between the steel and the concrete. Smooth bar would slip. The ribs (deformations) on rebar lock into the hardened concrete so the two move together. The RSIC Manual of Standard Practice requires all reinforcing steel, including column spirals, to be deformed. It's also why bars need enough length beyond the point where they're needed, called development length, and why bars are overlapped at splices. The required lengths come from CSA A23.3 and the drawings.

The concrete protects the steel

Steel rusts in air, but fresh concrete is highly alkaline, which protects embedded steel from corrosion. That protection only works if there's enough good concrete around the bar, which is why drawings specify concrete cover, the distance from the bar to the surface. Our post on concrete cover for rebar lists the CSA A23.1 values.

Steel and concrete also expand and contract by similar amounts with temperature, so they don't tear apart as the seasons change. That compatibility is a big part of why reinforced concrete has worked so well for so long.

Why rebar is used: its main jobs

Rebar does more than "make concrete stronger". In a reinforced concrete structure, it has several distinct jobs:

• Carrying tension from bending, in beams, slabs, footings and walls.
• Resisting shear, through stirrups that wrap the main bars in beams.
• Confining columns, where ties or spirals wrap the vertical bars and hold the concrete core together under heavy load.
• Sharing compression in columns, along with the concrete.
• Controlling cracks from shrinkage and temperature, by holding them tight. Rebar doesn't stop concrete cracking; our post on whether rebar prevents cracking explains the difference.
• Tying the structure together, so pours placed at different times, and elements such as walls and footings, act as one.

Common rebar uses

Not every piece of concrete needs rebar. Under Part 9 of the 2024 Ontario Building Code, ordinary strip footings, floor slabs on ground and many poured foundation walls can be plain concrete, while anything that spans, retains soil or carries concentrated loads is designed with reinforcement. Our post on whether rebar prevents cracking includes a table of where Part 9 does and doesn't require it.

Types of rebar

Most rebar is plain "black" carbon steel bar. Where corrosion from de-icing salt, sea water or chemicals is a concern, designers may specify epoxy-coated, galvanized or stainless steel bar, or glass fibre reinforced polymer (GFRP) bar, which doesn't rust at all. Each has its own standard and handling rules; our guide to types of rebar in Canada compares them.

Rebar vs mesh, fibres and structural steel

Three other products get mixed up with rebar. Welded wire mesh is a grid of smaller wires, used mostly for light slabs. Fibres (synthetic or steel) are mixed into the concrete itself and mainly help control cracking; only some types carry meaningful load after the concrete cracks. Structural steel (beams, columns, HSS) carries loads on its own rather than inside concrete. Rebar is the one designed to be embedded in concrete and work with it as a single material, which is why structural concrete drawings call for bars, not mesh or fibres, wherever the concrete has to carry tension.

From drawings to bars on site

Rebar on a real project rarely shows up as plain stock lengths. The usual path is:

1. Structural drawings show bar sizes, spacing and details.
2. Detailing. A rebar detailer turns those into placing drawings and a bar list with every bar's size, length, shape and mark. See what rebar detailing is.
3. Fabrication. Bars are cut to length and bent to shape in a shop, then tagged and bundled by location.
4. Delivery and placing. Bars arrive on site sorted for the crew, are set on chairs or blocks at the right cover, then tied and inspected before the pour.

Getting rebar for your project

We detail, cut, bend, tag and deliver rebar from 10M to 55M, including dowels and stirrups, and supply GFRP bar for corrosion-prone work. Send us your drawings for a free take-off and quote, or see our rebar products.