What causes concrete to spall?

The most common cause of concrete spalling is reinforcement corrosion. As steel rebar rusts, it expands up to seven times its original volume, generating enormous internal pressure that cracks and blows off the concrete cover. This corrosion is triggered by carbonation (CO2 slowly neutralising the concrete's alkalinity) or chloride ingress (from road salts, de-icing chemicals, or marine environments). Other causes include freeze-thaw cycling, alkali-silica reaction (ASR), fire damage, and poor-quality original construction (insufficient cover, high water-cement ratio). Early signs include hairline cracks, rust staining, hollow-sounding areas, and flaking surfaces.