Chloride Attack

Chloride ions (from marine environments, de-icing salts, or contaminated aggregates in older concrete) migrate through the concrete cover until they reach the reinforcement. Above a critical concentration, they destroy the natural alkaline passive film that protects the steel, and corrosion accelerates rapidly.

• 01Marine and coastal exposure (splash and spray zones)
• 02De-icing salts on car park decks and bridge structures
• 03Chloride-contaminated aggregates in older (pre-1977) concrete
• 04Industrial chloride process contamination

• Localised rust staining and cracking along the line of reinforcement
• Spalling of concrete cover with exposed corroded bars
• Delamination detected by hammer-tap survey
• Pitting corrosion of the reinforcement itself

• T · 01Cover surveys with a cover meter to establish depth to steel
• T · 02Half-cell potential mapping to locate active corrosion cells
• T · 03Core sampling with chloride ion titration to a profile
• T · 04Resistivity testing to gauge corrosion risk

Repair is a combination of removing the source and treating the mechanism. Chloride-contaminated concrete around the affected bar is broken out to a minimum of 25 mm behind the steel, the reinforcement is cleaned to bare metal (SA 2½) and primed with a zinc-rich passivating coating. The area is reinstated in an EN 1504-3 R4 repair mortar. For widespread contamination, galvanic or impressed-current cathodic protection is advised.