What is the 'halo effect' in concrete repair?

The halo effect (incipient anode formation) happens when a patch repair is done *without* treating the surrounding steel. The new patch is highly alkaline, while the old surrounding concrete is carbonated. This electrochemical difference creates a battery effect, accelerating corrosion in the steel *just outside* the patch. The result is a new ring of spalling around the repair. We prevent this using sacrificial anodes or managing the interface carefully.