Chloride-induced corrosion is the dominant durability problem in reinforced concrete exposed to de-icing salts or seawater, and it is unusual among deterioration mechanisms in being genuinely quantifiable. Chlorides move through the concrete cover by diffusion, they accumulate at the reinforcement over time, and corrosion begins when the concentration at the steel crosses a threshold. Each of those quantities can be measured on cores taken from the structure, which means the question of when corrosion began is answerable with far more rigour than in most deterioration matters.
Passivity and what breaks it
Reinforcement in sound concrete is protected by a passive oxide film maintained by the high alkalinity of the surrounding paste. That film is stable indefinitely as long as the environment stays alkaline and chloride-free. Chloride ions reaching the steel in sufficient concentration break the film down locally, and corrosion begins in pits rather than uniformly — which is why chloride attack can cause substantial section loss at a location while the surrounding steel looks sound.
What a chloride profile is
Powder samples are taken at successive depths from the exposed surface, and chloride content is measured at each depth — acid-soluble measurement under ASTM C1152 for total chloride, water-soluble under C1218 where the free fraction is of interest. Plotting concentration against depth produces a curve that is high at the surface and falls with depth, and the shape of that curve carries the diffusion history of the structure.
Fitting the profile gives a diffusion coefficient
The profile can be fitted to a diffusion model, yielding an apparent diffusion coefficient and a surface concentration for this concrete in this exposure. Those two parameters, together with the age of the structure, describe how chloride has been moving. They also permit projection: the concentration at any depth at any past or future date follows from the same fit, which is what converts a set of measurements into a timeline.
Cover depth is the other governing variable
Because concentration falls steeply with depth, small differences in cover produce large differences in time to corrosion. Cover is also among the most variable things on a construction site, and measuring it — with a covermeter across the structure and directly at cores — usually reveals a distribution rather than a value. Areas of low cover corrode first, which is why deterioration so often maps onto construction variability rather than onto exposure.
The threshold is a range, not a constant
The chloride concentration at which corrosion initiates depends on the concrete's alkalinity, its moisture state, the cement type, and whether the reinforcement is carbon steel or a corrosion-resistant alloy. Commonly cited threshold values are useful starting points and should not be treated as precise. Sensitivity analysis across a plausible threshold range, reported as a range of initiation dates, is more defensible than a single date derived from a single assumed value.
Initiation and propagation are separate phases
Corrosion beginning is not the same as damage appearing. After initiation, corrosion products accumulate and expand, generating tensile stress in the cover until it cracks and delaminates. That propagation phase can run for years, and its duration depends on corrosion rate, cover and concrete quality. Distinguishing the two phases matters, because the date corrosion started and the date damage became visible can be far apart, and different obligations attach to each.
Measuring what is happening now
Half-cell potential mapping under ASTM C876 indicates where corrosion is probable across a structure, and corrosion-rate measurement estimates how fast it is proceeding. These are present-condition measurements rather than history, and they complement the chloride profile rather than substituting for it: the profile establishes when, the potential map establishes where, and the rate measurement establishes how quickly the remaining service life is being consumed.
Ruling in or out the other mechanisms
Chloride is not the only route to reinforcement corrosion, and not the only cause of cracking and spalling. Carbonation lowers alkalinity from the surface inward and is checked with an indicator on freshly fractured concrete. Alkali-silica reaction, sulfate attack and freeze-thaw damage all produce distress that can resemble corrosion damage superficially. Petrographic examination of cores under ASTM C856 is what distinguishes them, and it is worth doing even where chloride is the leading candidate.
What to preserve and record
Cores with their orientation and exposed face identified, and their locations mapped against the structure. Cover measurements across a representative area rather than at the damage alone. The exposure history — de-icing practice, marine exposure, drainage and leakage — and the original mix design, specified cover and construction records. Repairs also matter: a patch changes the local chloride and moisture regime and can accelerate corrosion in the concrete adjacent to it.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.