Does Rebar Rust in Concrete? Corrosion Explained

Rebar does not normally rust inside good concrete. Fresh concrete is strongly alkaline, and that chemistry forms a thin protective (passive) film on the steel that stops corrosion. Rebar starts to rust when something breaks that protection: chlorides from road salt or seawater, carbonation of the concrete, or cracks and thin cover that let water and oxygen reach the bar. Once it starts, the rust expands, cracks the concrete and eventually pops off the cover.

That is why the questions "does rebar rust in concrete?" and "is rusty rebar OK to pour?" have different answers. The first depends on the concrete around the bar for decades. The second is mostly about whether the bar still meets its standard on the day of the pour.

Why rebar is protected inside concrete

Concrete is made with cement that releases alkaline compounds as it hydrates. Steel sitting in that highly alkaline environment develops a microscopically thin oxide film on its surface, and that film is stable. As long as it stays intact, the steel corrodes so slowly that it does not matter.

Two things keep the film intact:

• The concrete cover. The layer of concrete between the bar and the surface is a physical barrier. It slows down how fast water, oxygen, chlorides and carbon dioxide can reach the steel.
• The concrete quality. Dense, well-cured concrete with a low water-to-cementitious materials ratio has fewer connected pores, so contaminants travel through it more slowly.

So "rebar does not rust in concrete" really means "rebar does not rust in concrete that is thick enough, dense enough and uncracked enough for its exposure".

What makes rebar start to rust

Chlorides

Chlorides are the main cause of rebar corrosion in Ontario. They come from de-icing salt tracked into garages and splashed onto balconies, ramps, bridges and sidewalks, and from seawater in marine work. Chlorides work their way through the cover over years. When enough reach the bar, they break down the passive film locally, even though the concrete is still alkaline, and corrosion starts. It often appears as pitting, which can eat into a bar's cross-section faster than general rust.

Carbonation

Carbon dioxide from the air slowly reacts with the concrete from the surface inward and lowers its alkalinity. When that carbonated zone reaches the bar, the passive film is no longer stable. Carbonation is slow in good concrete, and it is slowest when the concrete stays continuously wet. CSA A23.1 points to soffits of suspended slabs and balconies, and exposed vertical surfaces that see little rain, as the conditions where carbonation risk is highest, and it limits the water-to-cementitious materials ratio for some concrete with thin cover to address it.

Cracks, honeycombing and thin cover

Anything that shortens the path to the bar speeds things up. Wide cracks, honeycombed concrete, bars that sagged or moved during the pour, and chairs that let a bar sit too close to the surface all put the steel within easy reach of water and salt. Cover that is right on the drawings but wrong in the slab is one of the most common causes of early corrosion. Our guide to rebar placement tolerances covers how close is close enough.

What happens when rebar rusts in concrete

Corrosion needs moisture and oxygen as well as a broken passive film, so it is worst in concrete that gets wet and dries out repeatedly. Once it is under way:

1. Rust expands. Rust takes up several times the volume of the steel it forms from. Inside concrete, there is nowhere for it to go.
2. The concrete cracks. The expanding rust pushes on the cover until it cracks, usually along the line of the bar.
3. Staining appears. Brown rust stains bleed out of the cracks.
4. The cover spalls. Pieces of concrete break off, exposing the bar. The cracks now let in even more water and salt, so corrosion accelerates.
5. The bar loses section. Over time the steel area drops, and with it the member's capacity. This is when corrosion becomes a structural problem rather than a cosmetic one.

Spalling balconies, parking garage soffits with exposed bar, and cracked, rust-stained concrete steps are all the same process. If you see these on a structure, have it assessed by an engineer before you decide on a repair.

Is rusty rebar OK to use?

Yes, within limits. A brown coating of surface rust on new bar is normal and is not a reason to reject it.

The Reinforcing Steel Institute of Canada (RSIC) Manual of Standard Practice, referencing CSA A23.1 (Clause 6.1.6 in the 2019 edition), says reinforcing steel with rust is acceptable provided the minimum dimensions, including deformation height and bar diameter, still meet the CSA standard. Its guidance on receiving bar is that bars with rust, mill scale or both are acceptable without cleaning or brushing, as long as the dimensions, deformation height and mass of a cleaned sample are not less than CSA G30.18 requires. The RSIC manual also notes research indicating that rust improves bond strength.

What is not fine:

• Loose, flaking rust or scale that comes off in your hand. Knock it off before the pour.
• Heavy pitting where the bar has visibly lost section. If in doubt, a wire-brushed sample can be checked against the standard.
• Mud, oil, form release and other contaminants, which reduce bond. These should be removed before concrete placement.

Why rebar is always rusty

Rebar is plain carbon steel, stored in yards and on sites in the open. It picks up a light rust within days of rolling. Coated bar (epoxy or galvanized) and GFRP do not, which is part of how you can tell them apart on a site.

How long does rebar last in concrete?

There is no honest single number. In dry interior concrete (what CSA A23.1 calls Class N), with the right cover, steel bar can last for the life of the building. In a parking deck that sees salt every winter, with thin cover and cracks, corrosion damage can show up far sooner.

The factors that decide it are the ones the designer controls:

• Exposure class, which reflects chlorides, freeze-thaw and moisture
• Cover, set by CSA A23.1 Table 17. For example, 20 mm for slabs and walls in dry interior Class N concrete, but 60 mm for chloride exposure classes such as C-1 and 75 mm for concrete cast against earth
• Concrete quality, through mix design limits tied to the exposure class
• Crack control, through bar size, spacing and joints
• The bar itself, plain or corrosion resistant

Our post on concrete cover for rebar under CSA A23.1 explains the exposure classes and the full cover table.

Options when corrosion is a real risk

Where chlorides will reach the concrete, the designer may combine better concrete and more cover with membranes or sealers, or specify a more corrosion-resistant bar:

Each of these changes the design, not just the bar. Coated and GFRP bar affect lap lengths, bending and handling, so a switch needs the engineer's sign-off. For most buildings above ground and out of the salt, plain black bar with correct cover and good concrete is the right answer.

Getting durable reinforcement on your job

Corrosion problems are cheap to prevent and expensive to fix. The prevention starts at the detailing stage: bars detailed with the right cover for the exposure class, chairs at the right heights, and bent bars that fit the forms so they do not end up against the surface.

We detail, cut and bend steel rebar and supply GFRP bar for corrosive exposures. We don't supply epoxy-coated, galvanized or stainless bar; those come from specialist suppliers. Send your drawings, or see our rebar products.