A six-storey reinforced-concrete car park, built in 1974, was showing the classic signs of its age: spalling soffits, rust staining bleeding through the surface, and delamination over the busiest bays. The owner wanted it kept in service, not demolished. The route from those symptoms to a working retrofit scheme is a good illustration of how assessment and intervention fit together.
Reading the symptoms
The visible distress told a consistent story. Spalling - where the concrete cover breaks away - happens when reinforcement corrodes: the rust occupies more volume than the steel it came from, and that expansion bursts the cover off. Rust staining is the same process announcing itself before the cover lets go. Delamination over the most-used bays pointed to where the de-icing salts carried in on vehicles had done the most damage.
Car parks are a textbook durability problem. They are exposed, they collect chloride-laden water from vehicles, and the decks are thin with the cover always under pressure - exactly the conditions for chloride-induced corrosion.
Assessment
Symptoms suggest; testing confirms. The investigation used a combination of techniques to turn appearance into data:
- Half-cell potential mapping to locate where the reinforcement was actively corroding, not just where the surface looked bad.
- Chloride profiling - sampling at depths to see how far the chloride front had penetrated. It confirmed corrosion had progressed past the first layer of reinforcement.
- Cover meter surveys to measure the actual cover, which explained the whole pattern: cover was routinely around 15 mm against a specified 40 mm.
That cover finding is the crux. With a quarter of the intended cover, the chlorides reached the steel far sooner than the design assumed, and the corrosion the survey found was the inevitable result.
The scheme
Rather than demolish a structurally repairable building, the intervention targeted the actual mechanism:
- Localised breakout and patch repair back to sound concrete, removing the contaminated and delaminated material and reinstating the section.
- CFRP plates bonded to restore flexural capacity in the worst-affected spans, where corrosion had reduced the effective reinforcement - a strengthening method that adds little weight and depth.
- A migrating corrosion inhibitor plus an anti-carbonation (and chloride-resisting) coating to slow the ongoing deterioration and protect the repaired structure going forward.
The combination treats both the damage already done (patch and CFRP) and the cause that would otherwise continue (inhibitor and coating).
Keeping it in service
The constraint that shaped the delivery as much as the engineering was that the car park had to stay open. The whole job was staged so that no more than one level closed at a time, sequencing the breakout, repair and coating floor by floor around live use.
The lessons that travel
- Diagnose the mechanism before specifying the cure. The cover survey, not the spalling, explained the problem - and a repair that ignored the chloride cause would simply fail again.
- Repair and protect together. Patching without addressing the corrosion driver buys only time; the inhibitor and coating are what make the repair durable.
- Assessment of an existing structure is its own discipline - half-cell, chloride profiling and cover survey turn a worrying appearance into a defensible scheme. The related precast-vs-in-situ framework and crack-control article cover the durability thinking that, designed in from the start, prevents exactly this kind of premature deterioration.