What is concrete spalling and why does it happen?

Concrete spalling, often referred to as 'blown' concrete, occurs when the surface of the concrete cracks, breaks, or flakes off, exposing the aggregate or reinforcement steel underneath. The primary cause is the corrosion of the embedded steel rebar. As steel rusts, it expands up to seven times its original volume, creating immense internal pressure that forces the surrounding concrete to break away. Other causes include freeze-thaw cycles, where water enters capillary pores, freezes, and expands, as well as 'concrete cancer' (Alkali-Silica Reaction). Ignoring spalling can lead to severe structural weakness and falling debris hazards.