Rebar corrodes inside concrete when the alkaline protection around it is lost: either the concrete has carbonated down to the steel, or chlorides (de-icing salt, seawater, process fluids) have reached it. Rust expands, cracks and then spalls the cover. A lasting repair starts with a diagnosis (carbonation depth, cover, chlorides, potential), removes all contaminated concrete around the steel, then protects the structure against the problem coming back.
A concrete spall that reveals rusted rebar is almost never an isolated defect: it is the visible sign of a mechanism progressing elsewhere in the structure. This guide is for maintenance managers of plants, wharves, industrial parking decks and process structures in Quebec and Ontario. It complements our article on concrete repair before coating, which focuses mainly on slabs.
Why does steel corrode inside concrete?
Steel does not rust in sound concrete because the highly alkaline pore solution forms a passive layer on its surface. Corrosion starts when that passivity breaks down, either through the loss of alkalinity caused by carbonation or through chlorides reaching the steel.
Once corrosion has started, it still needs water and oxygen to progress. That is why areas exposed to wet-dry cycles, splashing and leaks deteriorate first. The electrochemical mechanism is the same one described in our article on types of industrial corrosion: one area acts as an anode and corrodes, a neighbouring area acts as the cathode.
How do carbonation and chlorides work?
Carbonation advances slowly and evenly from the surface, while chlorides cause localized and often faster corrosion. Both mechanisms can occur on the same structure.
| Mechanism | Source | Form of corrosion | Typical structures |
|---|---|---|---|
| Carbonation | Carbon dioxide in the air reacting with the lime in concrete | General, over the whole carbonated zone | Indoor or sheltered structures, beams, columns, thin cover |
| External chlorides | De-icing salt, seawater, spray | Localized pitting, loss of section | Wharves, ramps, parking slabs, loading areas |
| Internal or process chlorides | Older admixtures, brines, chemicals | Localized, sometimes deep | Basins, chemical plants, brine rooms |
| Cracking | Shrinkage, loads, movement | Corrosion concentrated at the crack | Any cracked structure exposed to water |
Chlorides are particularly aggressive in Quebec, where de-icing salt enters buildings on vehicles and through loading docks. Our article on de-icing salt corrosion describes the same phenomenon on structural steel. In wastewater service the attack is different: see our article on concrete exposed to H2S.
Why does concrete spall?
Concrete spalls because rust occupies a larger volume than the steel it replaces. That expansion creates tensile stresses the cover concrete cannot carry: it cracks along the bars, delaminates, then falls off.
The progression is usually recognizable:
- Rust staining on the surface, sometimes aligned with the bars.
- Cracks parallel to the rebar, often fine at first.
- Delamination: the concrete sounds hollow under a hammer or chain drag, without having fallen yet.
- Spalling: the cover breaks off and the steel is exposed, with visible section loss.
Delamination is the most useful stage to detect, because it announces spalls before they become a falling-debris hazard for personnel.
How is rebar corrosion diagnosed?
Diagnosis combines a visual and sounding survey, measurement of carbonation depth and cover, chloride testing and, on larger structures, potential measurements. The goal is to map active areas and understand the cause, not just count spalls.
- Survey and sounding. Mapping of cracks, stains and spalls; hammer or chain-drag sounding to locate delaminated areas.
- Phenolphthalein. The indicator is sprayed on a fresh fracture or a core: concrete that is still alkaline turns pink, the carbonated zone stays colourless. The method is described in EN 14630, among others. Carbonation depth is then compared with cover.
- Cover. Bar position measured with an electromagnetic cover meter and checked by local opening. Thin cover often explains early corrosion.
- Chlorides. Testing of drilled powder at several depths, per ASTM C1152 or C1218, to see whether chlorides have reached the steel.
- Corrosion potential. Half-cell potential mapping per ASTM C876 to locate probably active areas; it is interpreted together with the other tests.
This diagnosis is part of our strategic advisory service; it then guides the choice between local repair, preventive protection and, if corrosion is very widespread, electrochemical solutions.
How is spalled concrete repaired?
The repair removes all delaminated or contaminated concrete around the steel, exposes the bars all the way around, cleans them to sound metal, then rebuilds the cover with a compatible repair mortar. A simple patch over rust fails quickly.
- Outline and saw-cut the repair perimeter to avoid feathered edges, following the principles of ICRI 310.1R.
- Remove concrete to beyond the steel, with hand tools or hydro blasting, which removes deteriorated concrete without micro-cracking sound concrete.
- Clean the rebar by abrasive blasting or power tools; assess section loss and add bars where needed, as advised by the structure's designer.
- Protect the steel with an anticorrosion primer if the repair system calls for one.
- Rebuild with a repair mortar or concrete, hand-applied, sprayed or formed, then cured per the data sheet.
Cracks that are not related to corrosion, or that must be sealed watertight, are treated by concrete crack injection. Active cracks caused by rust are not fixed by injection alone: the steel has to be treated.
Do corrosion inhibitors work?
Surface-applied migrating inhibitors can slow corrosion in lightly affected concrete, but they do not replace repairing spalled or delaminated areas. Their effectiveness depends on concrete permeability, cover and chloride level.
They are mainly used as a complement: around repairs, to limit new anodes forming at the edges (the incipient anode effect), or on areas that are still sound but exposed. When chlorides are high over large areas, cathodic protection or galvanic anodes embedded in the repairs are options to consider; our article on cathodic protection presents the principles.
Which coatings protect reinforced concrete?
Protective coatings slow the entry of water, chlorides and carbon dioxide, and so extend the life of a repaired structure. The system is chosen by exposure: water-repellent impregnation, anti-carbonation coating or waterproof membrane.
- Water-repellent impregnation (silanes, siloxanes). Penetrates the concrete and reduces water and chloride absorption without changing appearance; suited to vertical surfaces and splash zones.
- Anti-carbonation coating. An acrylic or elastomeric film that slows carbon dioxide diffusion while letting water vapour escape.
- Membrane or thick coating. Epoxy, polyurethane or polyurea on trafficked slabs and spill areas, with crack and joint treatment.
Our concrete protection page presents these solutions by type of structure, and our protective coatings page covers all our systems. A coating applied over concrete that already contains chlorides at the steel is not enough: repair comes first.
How do you plan the work and get a quote?
Plan by separating the urgent items (areas at risk of falling debris, significant section loss) from the repair and protection program. Send photos of the spalls and cracks, the age and use of the structure, its exposure (salt, water, process) and any inspection reports.
Our crews and qualified subcontractors work from LaSalle, including the Quebec region and the Côte-Nord, as well as Ontario. Pricing on consultation: use the request a quote form, reply within 24 business hours.
Frequently asked questions
Is phenolphthalein enough to diagnose corrosion?
No. It only shows carbonation depth. It has to be compared with cover and completed by chloride testing and observation of the rebar before drawing conclusions.
Can spalling concrete simply be repainted?
No. Paint hides the stains without stopping corrosion, which keeps spalling the concrete under the film. Delaminated areas must be repaired before any coating.
Why do new spalls appear around a repair?
This is often the incipient anode effect: the repair passivates the steel in the repaired area, and corrosion moves into the contaminated concrete next to it. Outlining wide enough, removing all contaminated concrete and adding anodes or an inhibitor limit that risk.
Does concrete always have to be removed behind the rebar?
In the corroded area, yes: the steel is exposed all the way around so it can be cleaned and fully encased by the mortar. Leaving contaminated concrete behind the bar restarts corrosion.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.