Repair of Deteriorated Concrete

Reinforced concrete works thanks to a chemical balance before a mechanical one: the strongly alkaline environment of cement passivates the steel and protects it from corrosion. As long as that alkalinity holds, the reinforcement does not rust even in the presence of moisture. Decay begins when that protection is lost, and it happens by two routes. The first is carbonation: carbon dioxide from the air penetrates slowly from the surface and neutralises the alkalinity, advancing as a front. When the front reaches the reinforcement, passivation falls and corrosion starts. The second is chloride, typical of marine environments and of structures treated with de-icing salts: it penetrates and locally breaks the protective film even in still-alkaline concrete, producing deep pitting that reduces the bar's section far more than the surface suggests. The whole repair strategy follows from this distinction, because the two cases are addressed differently. Against carbonation it is enough to restore the cover and protect the surface; against chlorides, if the salt is already in, rebuilding the cover alone does not solve it, and deeper measures are needed. The mistake that makes the work useless is always the same: stopping at visibly damaged concrete. Demolition must extend until sound concrete is found and, above all, must reach behind the bar: if only the front is chipped away leaving the reinforcement resting on the old substrate, corrosion continues at the back where nobody has treated it. This is why so many repairs spall again within a few years, with the client convinced the product was poor.

It is the mechanism that explains all reinforced-concrete decay, and once clear it makes any deteriorated facade readable. Steel inside concrete is protected by the alkaline environment of the cement: as long as that alkalinity exists, a passivating film persists on the bar and prevents corrosion even in the presence of water. When the protection falls — through carbonation, that is progressive neutralisation by CO2 from the air, or through chlorides that locally break the film — the steel begins to oxidise. The decisive point is that the products of oxidation have a much greater volume than the metal they come from: the bar, swelling, exerts pressure from within that the concrete cannot take, because it is weak in tension. First cracks form parallel to the bar, then the cover spalls off in flakes, and at that point the reinforcement is directly exposed to air and water: decay accelerates instead of stabilising.

It is why work carried out to good practice still fails near the sea, and the reason is subtle. In carbonation decay advances as a uniform front from the surface: restoring the cover and protecting the surface turns the clock back, and the protection stops the front resuming. With chlorides it does not. The salt has already penetrated deeply and unevenly, and keeps acting even under a perfect repair: it breaks the passivating film at isolated points, producing localised corrosion. There is more, and it is the least intuitive trap: the freshly repaired zone, being alkaline and salt-free, becomes electrochemically different from the contaminated concrete around it, and that difference can accelerate corrosion precisely at the edges of the repair. The result is a repair that spalls in a ring around the patch. This is why, with chlorides, the right strategy is not a patch but an extended measure, decided at the diagnostic stage.