Walk along any exposed coastal stretch and you can date the buildings by their concrete. Rust staining bleeding down a soffit. Spalled corners with reinforcement bars visible and flaking. Patch repairs that have themselves begun to fail at the edges.
Almost none of that is a structural design failure. The beams were sized correctly and the reinforcement was calculated properly. What failed was durability — specifically, the concrete's ability to keep chloride ions away from the steel for the intended life of the building.
This is worth understanding precisely, because the fix is cheap at construction stage and extremely expensive afterwards.
The mechanism
Steel inside concrete is normally protected. Fresh concrete is strongly alkaline, and in that environment a thin passive oxide film forms on the reinforcement and stops it corroding. As long as that alkaline environment holds, the steel is stable indefinitely.
Two things destroy it.
Chloride attack. Chloride ions — from sea spray, salt-laden air, or contaminated aggregate or mixing water — diffuse through the pore structure of the concrete. When the chloride concentration at the steel surface passes a threshold, the passive film breaks down locally and corrosion begins. This is the dominant mechanism in coastal Sri Lanka.
Carbonation. Atmospheric carbon dioxide reacts with the concrete over time, progressively reducing its alkalinity from the surface inward. When the carbonated zone reaches the steel, the passive film is no longer sustained. This is slower and generally secondary to chloride attack near the coast, but it matters inland.
Once corrosion starts, the process is self-destructive. Rust occupies substantially more volume than the steel it came from. That expansion cracks the concrete from the inside, the cracks let in more chloride and oxygen, and the rate accelerates. Spalling follows, then loss of section, then loss of the bond between steel and concrete.
Corrosion damage is not linear. There is a long initiation period during which nothing is visible, followed by a short propagation period during which the damage becomes obvious and severe. By the time you can see it, the initiation phase has already been over for some time.
Cover is the primary defence
Cover is the thickness of concrete between the outer face and the nearest reinforcement. Chloride has to travel through it to reach the steel, and diffusion time increases sharply with distance. Doubling the cover more than doubles the time to initiation.
Design codes set nominal cover by exposure class. The relevant point for work here is that a coastal or marine-influenced environment is a severe exposure class and demands substantially more cover than a sheltered internal one. A member that would take 25 millimetres of cover in a dry internal environment may require 50 millimetres or more in a coastal exterior condition.
The exposure class must be assessed element by element, not building by building. A single structure can contain internal elements in mild exposure, external elements in severe exposure, and elements in a splash or tidal zone requiring more still.
Cover only exists if the spacers hold it
This is where the theory meets the pour, and where most of the real-world failures originate.
Cover is delivered by spacers — small blocks or plastic chairs holding the reinforcement cage away from the formwork. Specified cover of 50 millimetres and actual cover of 15 millimetres, because the cage sagged or was walked on, gives you the durability of a 15 millimetre cover. The drawing is irrelevant at that point.
What actually protects cover on site:
- Spacers at adequate spacing, so the cage cannot sag between them. Too few spacers is the most common defect.
- Spacers of the right material. Concrete spacers should be of at least the same quality and density as the surrounding concrete — a porous spacer is a direct path for chloride to the bar. Plastic spacers must be the type designed not to create a continuous void.
- A pre-pour inspection with a cover check as a hold point. Someone measures, someone signs, and the pour does not start otherwise.
- Protecting the cage from traffic. Reinforcement that has been used as a walkway is reinforcement that has been displaced.
- A cover meter survey after striking, on critical elements, to confirm what was actually achieved.
The mix matters as much as the cover
Cover buys distance. The concrete's permeability determines how fast chlorides travel across that distance. A dense, low-permeability concrete can be many times more resistant than a porous one at the same cover.
The levers:
Water-cement ratio. The single most important variable. Lower water content gives a denser, less permeable pore structure. Water added on site to improve workability is directly buying a shorter service life, and it is common. Specify the maximum water-cement ratio, and achieve workability with a plasticiser rather than a bucket.
Supplementary cementitious materials. Ground granulated blast-furnace slag, fly ash and silica fume all substantially reduce chloride diffusion and improve chloride binding. Where available, a well-designed blended mix is one of the most cost-effective durability measures available.
Cement content and type. Adequate cement content for the exposure class, and a cement type appropriate to any aggressive ground chemistry identified in the site investigation.
Aggregate and water cleanliness. Chloride can arrive in the mix itself. Aggregate washed in or dredged from seawater, or mixing water drawn from a brackish source, contaminates the concrete from day one and no amount of cover will help. This needs to be checked, not assumed.
Curing is not a formality
Concrete gains its final pore structure during hydration, and hydration needs water. Concrete that dries out early — which in this climate happens quickly on a hot, breezy day — never develops the dense surface layer it was designed to have. And the surface layer is precisely the part doing the durability work.
Seven days of continuous, effective curing is a reasonable minimum for durability-critical elements, longer for mixes containing significant supplementary cementitious material, which hydrate more slowly. Effective means the surface stays wet: ponding, continuous wet hessian, or a curing membrane applied properly. Hosing the slab twice a day is not curing.
Curing is free apart from labour and attention. It is also the item most quietly dropped when the programme tightens.
A short specification checklist
For a coastal or marine-influenced project:
- Exposure class assessed and stated per element.
- Nominal cover specified accordingly, with fixing tolerance.
- Maximum water-cement ratio specified and enforced; no site water addition.
- Supplementary cementitious materials considered in the mix design.
- Aggregate and mixing water tested for chloride content.
- Pre-pour cover inspection as a formal hold point.
- Curing regime specified with duration and method, and supervised.
- Cover meter survey on critical elements after striking.
Every item on that list is inexpensive during construction. Every one of them is the reason a building is sound at fifty years instead of being repaired at fifteen.
