It is a serious problem caused by rusted stains reinforcing from within the steel. As steel rusts, it can expand up to 7 times its original size causing the surrounding concrete to be displaced. As the steel pushes the concrete away, more water gets to the steel expediting the process. The process is generally due to:
- Poorly treated reinforcing steel being used in the original pour of the slab
- The ends of reinforcing being too close to the surface allowing water to seep through concrete and react with the steel
- Incompatible metals being used in close proximity to each other, thus causing a reaction
- Fractures in the concrete allowing water to penetrate the concrete and react with the steel
Spalled concrete can cause a safety hazard. Concrete cancer and delaminated concrete should be treated immediately as deferring the treatment will inevitably lead to increased problems into the future. Similarly, treating the visual aspects such as rendering over the steel are short term solutions as the rusting process will continue below the surface causing the steel to again displace the concrete and in some cases, rust so badly the steel eventually needs replacement.