Reinforced concrete is a partnership: concrete carries compression and shields the steel, steel carries tension. The shield works because the same alkaline chemistry that makes the paste cementitious also keeps the embedded steel coated in a stable passive film. Chloride ions are the most common way that film gets compromised, and the story of how they arrive is the story of most serious infrastructure deterioration.
How chlorides travel
Chlorides arrive dissolved in water, whether from deicing salt dissolved in meltwater, marine spray, or salt fog. Their transport into concrete follows the same paths as the water itself:
- Diffusion through saturated paste, driven by the concentration difference between outside and inside.
- Capillary suction carrying salt solution inward during wetting.
- Permeation under hydraulic pressure where water is pushed through.
- Rapid entry through cracks and joints, where transport rates can be orders of magnitude higher than sound paste.
Over time, a chloride profile develops: high concentration near the surface, decreasing with depth. The shape of that profile depends on the exposure pattern (continuous wet, wet-dry cycles) and the concrete's transport properties.
The threshold concept
Steel in alkaline concrete is passive, meaning it corrodes at negligible rates. Chlorides at the steel surface destabilize that passive film once their concentration crosses what the literature calls the chloride threshold. Three things are worth knowing about the threshold:
- It is conventionally expressed as a ratio of chloride to cementitious material content (or to hydroxyl concentration), and reported values span a range rather than a single number.
- It depends on context: pH, moisture availability, and temperature all influence when corrosion initiates.
- Crossing the threshold starts corrosion; it does not instantly destroy the member. Propagation takes time, and the visible symptoms (rust staining, cracking along bars, spalling) lag the chemistry by years.
Cover depth is the first line of defence
The distance between surface and steel is the chloride's travel distance. More cover means longer time to accumulation, all else equal. This is why cover depth is a durability parameter, not just a constructability detail, and why corrosion damage clusters where cover is thin (corners, tie locations) or compromised (cracks above bars).
Testing is how you see the invisible
Because surface appearance cannot reveal chloride content at depth, assessment relies on sampling:
- Chloride profile testing measures concentration at several depths through the cover, on powder or cores extracted from the structure.
- The profile shape allows estimation of the surface chloride load and the rate of inward movement.
- Combined with cover depth measurements, a profile indicates whether the steel is currently at risk and how the situation is trending.
FHWA's bridge corrosion research program has long used chloride measurement as a core diagnostic for exactly this reason: the decision to protect, repair, or monitor is a function of the chloride distribution, not the visual condition alone.
Where ASTM C1202 fits (and does not)
The Rapid Chloride Permeability Test (ASTM C1202) is commonly used to characterize concrete's resistance to chloride penetration. It measures electrical charge passed through a saturated specimen, which correlates with transport resistance but is not a direct measurement of chloride diffusion. That nuance matters when comparing materials or treatments, and our dedicated article explains what the test does and does not establish.
What this means in Atlantic Canada
The region's bridges, ports, parking structures, and coastal buildings face combined salt sources: road deicing, marine environment, or both. For asset owners the practical sequence is consistent:
- Map where salt and moisture concentrate on the structure.
- Where the structure is older or shows early symptoms, get chloride profiles before choosing a strategy.
- Treat the outcome as data: corrosion management decisions made on measured chloride distribution age far better than decisions made from surface appearance.
Reading a chloride profile, in plain language
A chloride profile report shows concentration (usually as percent by mass of cementitious material, or kg/m3) at several depths from the surface. Three features tell the story:
- Surface concentration reflects the exposure: salted and marine structures carry high surface values.
- The slope. A steep profile means chloride is arriving faster than it penetrates; a flat slope means ions have been moving inward for a long time. Comparing profiles from different years gives the rate directly.
- The value at the steel depth. This is the number the assessment turns on: compare it against the applicable threshold context for the structure's materials and exposure.
Two practical notes. Sample the hot spots and the quiet zones both: the difference between them is exposure information. And expect variability between cores; single-core conclusions are fragile, which is why profiles come in sets.
How the profile changes the decision
The same garage deck can produce very different conclusions depending on the profile:
- High chlorides at the surface, low at the steel: the ion supply is arriving; protection of sound concrete is the conversation (the preserve category).
- Chlorides at the steel above threshold context in localized areas: corrosion is likely initiating; potential mapping and a repair strategy follow.
- Chlorides uniformly above threshold at the steel across a wide area: the structure is in corrosion management, and the strategy is repair-and-manage, not prevention.
Each profile shape implies a different budget conversation. That is the whole value of the testing: it sorts structures that look similar into genuinely different situations.
What this article does not tell you
It does not provide threshold numbers for design (those come from the applicable codes and specifications), nor does it evaluate any specific protective product or claim. It also cannot tell you what any particular structure's chloride profile looks like; only sampling can.
Where to go next
Why Reinforcement Corrodes Inside Concrete continues from this article into the corrosion process itself, and Why Parking Garages Deteriorate and Concrete in Ports and Marine Environments show the exposure in context. What ASTM C1202 Actually Measures covers the lab side.