Chloride-ion contamination is the buildup of chloride ions inside concrete, mostly from deicing salts tracked in on vehicles or from marine air, that eventually corrodes the reinforcing steel.
Concrete normally protects steel with a highly alkaline environment that keeps a thin passive film on the bars. When enough chloride reaches the steel, that film breaks down and the steel rusts. Rust takes up more space than steel, so it cracks the concrete from inside, leading to delamination and spalling. It is the leading cause of deterioration in parking structures in cold and coastal climates.
How it is found
- Visual surveys for rust staining, cracking and spalls.
- Sounding with a hammer or chain drag to find hollow, delaminated areas.
- Concrete samples tested for chloride content at the depth of the steel.
- Half-cell potential surveys to map where corrosion is active.
How it is prevented and treated
- Dense, low-permeability concrete with good cover. See concrete.
- Penetrating sealers and traffic-bearing membranes, maintained on schedule.
- Good drainage, so salty water does not sit on the deck.
- Coated reinforcing, corrosion inhibitors or cathodic protection in severe exposures.
- Routine washdowns, especially after winter.
The Parkonomics view
Salt is the slow enemy of every garage in snow country. A sealer reapplied on time costs a fraction of the concrete repair it prevents.
Related terms: Spalled, Concrete, Expansion Joint, Conventionally Reinforced Concrete, Post-Tensioned Concrete, Life-Cycle Cost