Spalling is the result of reinforcing steel either being misplaced too close to the edge of a concrete slab or from the concrete being left uncoated and receiving excessive weathering.
New concrete is naturally very alkaline. This high alkalinity actually acts to protect reinforcing steel from corrosion. The effect of weathering of bare concrete is that concrete slowly has its alkaline materials washed out of it. In areas of acid rain in highly industrialised countries the process will be accelerated. As this happens the steel will begin to corrode. When steel rusts it takes up to 3 times the volume the unrusted steel did. This is difficult when the steel is embedded in concrete but as the steel rusts it eventually forces the concrete to crack, exposing new reinforcing steel and fresh concrete to the weather. The process repeats itself with the situation continually worsening, hence the nickname concrete cancer.
Slight rust stains coming out of an otherwise innocent looking concrete surface are often the first symptom of spalling. Such areas must be repaired immediately.
A heavy weathertight system of Resene X-200 on new concrete will help prevent this type of problem.
Spalling is repaired by digging out the cracked concrete, cutting off rusted steel, priming the remaining exposed area and then concreting over the hole. If damage is structural an engineer should be consulted.
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