Healthy concrete protects the steel inside it. Most of the spalling we repair starts when that protection is lost.
The passive layer
Fresh concrete is highly alkaline, which forms a thin protective film on the reinforcement. While that film survives, the steel barely corrodes, even in a harsh climate.
Two ways the protection is lost
Carbonation: carbon dioxide from the air slowly lowers the alkalinity of the concrete, working inward from the surface.
Chlorides: salts from sea air, groundwater or old contaminated materials reach the steel and break down the film locally, even in alkaline concrete.
Once steel starts to rust it expands, and the concrete in front of it has nowhere to go but off.
What that means for repairs
A good repair deals with the cause, not just the broken concrete:
- Remove concrete behind the bar, not just in front of it
- Clean and treat the steel, and replace it where section loss is significant
- Use compatible repair mortars, applied and cured properly
- Protect the surrounding area with inhibitors or coatings so corrosion does not just move next door
KEY TAKEAWAYS
- Spalling is usually a symptom of corroding reinforcement.
- Testing for carbonation depth and chloride levels guides the repair design.
- Patch repairs without protection often fail around their edges.
