EngiMetric Arena / Diagnose the Failure
Diagnose the corrosion staining on a marine facade
Competency Fault diagnosis primary Evidence interpretation supporting Constraints and trade-offs supporting
What you should take away: Use a chloride-ingress model to distinguish chloride-induced corrosion from carbonation, sulphate attack and structural cracking.
The situation
What is happening
A reinforced-concrete facade 200 m from the sea shows rust staining and hairline cracking over the reinforcement after about 20 years. Cover is 40 mm, the surface chloride concentration is 0.8 % by mass of cement, the diffusion coefficient is 8 mm2/year and the corrosion threshold is 0.05 %. The design life is 50 years. Choose the diagnosis the durability model supports.
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Concrete cover | 40 | mm | To the reinforcement. |
| Surface chloride | 0.8 | % by mass of cement | Marine exposure. |
| Diffusion coefficient | 8 | mm2/year | Chloride transport through concrete. |
| Corrosion threshold | 0.05 | % by mass of cement | Threshold at the steel. |
| Design life | 50 | years | Required service life. |
Assumptions this challenge makes
- Chloride transport is represented by a single Fickian diffusion coefficient.
- The surface chloride concentration is constant over the exposure period.
- The threshold is a single value at the steel; real thresholds vary with cement type and exposure.
Your call
Your answer
Hints
- Start with the transport mechanism. What is moving through the cover, and what drives it in a marine environment?
- Compare the time for chloride to reach the threshold at the steel with the 50-year design life. Also compare the cover with the cover the model says is required.
- Match the visual symptom: rust staining over reinforcement is steel corrosion. The question is what depassivated the steel.
Your answer Engine result
Where that comes from
Each limit, checked
Why
- The chloride-ingress model gives a chloride concentration at the steel of about 0.126 % at 50 years, well above the 0.05 % threshold.
- The threshold is reached at about 29 years, inside the 50-year design life. The cover required to survive the design life is about 53 mm.
- At 40 mm, the cover is about 13 mm short. That is a durability deficit, not a construction tolerance issue.
- Chloride-induced corrosion produces rust staining and cracking along the reinforcement, which matches the reported symptom. Carbonation, sulphate attack and overload cracking have different signatures and are not the governing mechanism on this evidence.
- The practical conclusion is that the cover is insufficient for the exposure. Repair and protection options must address both the existing chloride profile and the remaining design life.
What this challenge does not establish
- The model uses a single diffusion coefficient and a constant surface chloride; real exposure varies with wind, wetting and drying.
- The threshold is a single value; real chloride thresholds depend on cement type, pH and steel potential.
- Carbonation, sulphate attack and structural cracking are not modelled and must be assessed separately.
- The diagnosis is not a condition survey. Cover depth, chloride profiling and half-cell potential testing are required before repair design.