Concrete spalling is often termed concrete cancer. Its main symptom is concrete flaking from the surface of the affected area accompanied by nearby staining.
Concrete spalling is caused when steel reinforcement within a concrete slab begins rusting. When steel rusts, it expands approximately 5 times its original size causing concrete displacement and flaking. On becoming damaged, more water can enter the affected area, speeding up the process.
The exact cause of concrete cancer often depends on the unique circumstances of each case, however, several causative factors are often at play. These include the steel being too close to the surface allowing water to react with it, poor quality steel, incompatible metals used close to each other causing a reaction, and damage to concrete allowing water to enter the slab and come into contact with the steel.
Spalling concrete puts owners of property at risk of negligence law suits. Damage can occur through concrete pieces falling and injuring people or property. Further, concrete spalling that is not attended to in a timely manner will only lead to more repair costs as the problem spreads.
Concrete spalling cannot be repaired simply by rendering over the affected area. It is best to consult with a qualified engineer to work out the best course of action.