Rebar does not prevent concrete from cracking. Concrete shrinks as it dries and cracks when it is pulled or bent, and steel inside it can't stop that. What rebar does is take over once the concrete cracks: it carries the tension, holds cracks tight and keeps the pieces working together. That's why rebar is essential in structural members and optional in many slabs, where joints, the base and the mix matter more.
Below is what rebar actually does, why concrete cracks anyway, and when you need it in Ontario.
Why concrete cracks, rebar or not
Concrete is very strong in compression and weak in tension. CSA A23.3 (the Canadian concrete design standard; 2004 edition cited here) uses a modulus of rupture of 0.6 times the square root of the compressive strength for design. For ordinary 25 MPa concrete, that works out to about 3 MPa, roughly one eighth of its compressive strength. Anything that pulls or bends concrete past that point cracks it.
Plenty of things do. The Ontario concrete industry's reference guide (OCCDC) lists the main causes of shrinkage and cracking as:
- Settlement of the subgrade under a slab
- Chemical shrinkage of the concrete as it hydrates
- Temperature and moisture changes in the concrete
- Loads applied to the surface
- Restraint, where the concrete can't move freely as it expands or contracts
Restraint is the big one for slabs. A slab on the ground wants to shrink, the base and anything it's cast against hold it back, and the tension that builds up cracks it. Steel in the slab is part of the restraint too. Reinforcement doesn't remove the shrinkage, so the cracks still come; they just come as more, finer cracks instead of a few wide ones.
What rebar actually does in concrete
Rebar has three different jobs, and it helps to keep them separate:
- Carries tension in structural members. In a beam, the bottom of the span is pulled apart. The concrete there cracks at low load and the bars carry the tension, while the concrete at the top carries compression. Without the steel, the beam breaks at the first crack. This is the main reason rebar exists; our post on what rebar is explains how the two materials work together.
- Controls crack width. Bars crossing a crack hold the two sides together, so the crack stays narrow. Narrow cracks let in less water and salt and keep slab panels level. CSA A23.3 sets a minimum amount of slab reinforcement in each direction (Clause 7.8.1 in the 2004 edition) and calls for more where crack control is essential (Clause 7.8.2).
- Ties pieces together. If a slab cracks right through or the base settles a little, rebar keeps the pieces from separating or stepping out of level.
None of these jobs is "stop the crack from happening". In fact, a properly reinforced beam is expected to have fine cracks on its tension face under load. That is how it works.
Position matters for the second and third jobs. Shrinkage cracks start at the top of a slab, so slab reinforcement is useful only if it's held up in the slab on chairs, not lying on the base. See where rebar should sit in a concrete slab.
When is rebar needed in concrete?
In Ontario, the answer comes from the 2024 Ontario Building Code (O. Reg. 163/24) and, for anything outside Part 9, from the engineer's design. Sentence 9.3.1.1.(3) says reinforced concrete must be designed to Part 4, apart from the specific Part 9 foundation and ICF rules. Here is how that plays out on a typical house or small building:
| Element | Rebar required? | Where the rule comes from |
|---|---|---|
| Strip footings under Part 9 | Not by default | Subsection 9.15.3 has no rebar requirement for plain footings |
| Floor slab on ground (basement, garage) | Not by default | Section 9.16 has no reinforcement requirement; structural slabs go to Part 4 |
| Poured foundation wall within table limits | Not by default | Table 9.15.4.2.-A |
| Flat ICF foundation and above-grade walls | Yes | Article 9.15.4.5 and Subsection 9.20.17 |
| Reinforced block foundation walls | Yes | Table 9.15.4.2.-B |
| Beams, suspended slabs, columns, stairs | Yes, designed | Part 4 and CSA A23.3 |
| Retaining walls, piers, slabs that carry walls or columns | Yes, designed | Part 4 |
| Driveways, patios, walkways | Not required | Contractor's or designer's choice |
Our posts on footing rebar and slab-on-grade rebar go through those rules in detail.
The pattern is simple. If the concrete spans, cantilevers, retains soil or carries concentrated loads, it needs rebar and a design. If it sits fully supported on good ground and only carries light, spread-out loads, plain concrete with proper joints is often acceptable, and rebar becomes a choice to control cracking.
Whatever the code minimum, the permit drawings govern. If the drawings show bars in a footing or slab, those bars are a permit condition.
What happens if you don't put rebar in concrete?
It depends which of the two groups the concrete is in.
Where rebar was required, leaving it out is a structural defect. A beam, lintel, suspended slab or retaining wall without its steel can crack through and fail, sometimes suddenly. It will also fail inspection, and fixing it usually means removing the concrete.
Where rebar was optional, plain concrete still does its job, but you give up crack control:
- Cracks that form between joints can open wider over time, especially through freeze-thaw cycles.
- Panels on each side of a crack can shift or settle independently, leaving a trip edge on a walkway or driveway.
- Wider cracks let water and de-icing salt into the slab and the base.
That trade-off is why many builders reinforce slabs that the code doesn't require them to, particularly garage floors, slabs over backfill and driveways. Fibres are another option for some of these slabs; our post on whether fibre concrete replaces rebar covers where fibres can and can't stand in for bars.
What actually controls cracking
If your real goal is fewer and less visible cracks, these matter more than steel:
- Joints. The OCCDC guide's rules for slabs on grade: joint spacing of no more than 24 to 36 times the slab thickness and no more than 4.5 m, panels as square as possible (length no more than 1.5 times the width), joints at least one quarter of the slab depth, and joints at re-entrant corners. Joints tell the slab where to crack.
- A uniform, compacted base. Settlement under a slab cracks it whether it's reinforced or not.
- The right concrete. Less water in the mix means less shrinkage. For exterior flatwork and garage floors, Article 9.3.1.6 of the 2024 OBC sets a minimum strength of 32 MPa, and Article 9.3.1.7 caps the water to cementing materials ratio at 0.45.
- Curing. Concrete that dries too fast at the surface cracks early. Keep it moist and protected for the curing period the specification calls for.
- Isolation joints where a slab meets walls, columns and existing concrete, so it isn't locked in place.
Rebar then does its part on top of that: holding the cracks that still form tight and keeping the slab in one piece.
Getting the right rebar for your job
If your drawings call for reinforcement in footings, walls, slabs or structural members, send them to us for a take-off and quote. We detail, cut and bend 10M to 55M bar, tag it to your drawings and deliver across the GTA and Ontario. See our rebar products for straight bar, fabricated shapes and GFRP.
Frequently asked questions
Does rebar prevent concrete from cracking?
No. Concrete cracks as it shrinks and as it is loaded, with or without steel. Rebar controls the cracks once they form: it holds them tight, keeps the pieces together and carries the tension the concrete can't. Joints, a good base, the right mix and curing decide how much cracking you get.
Does rebar make concrete stronger?
Not the concrete itself. Rebar makes a concrete member able to resist bending and tension, which plain concrete does poorly. A reinforced beam, slab or wall can carry loads that would break the same piece of plain concrete.
Is rebar necessary in concrete?
Only in some places. Structural members such as beams, suspended slabs, columns, retaining walls and ICF walls need it. Under Part 9 of the Ontario Building Code, ordinary strip footings, floor slabs on ground and many foundation walls can be plain concrete.
What happens if you don't put rebar in concrete?
Where rebar was required, the member can fail under load and won't pass inspection. Where it was optional, such as a patio or a basement floor, the concrete still works, but cracks are free to open wider and slabs on each side can shift or settle unevenly.
