Spalling of Concrete

Spalling is the breaking, flaking or detachment of pieces of concrete from the surface, exposing the aggregate or the reinforcement beneath. The dominant cause in Indian conditions is reinforcement corrosion: chloride or carbonation reaches the steel, rust forms with 4-6× the volume of the parent metal, and the resulting bursting pressure cracks and pushes off the cover. Other causes are fire (entrapped moisture flashing to steam — explosive spalling), freeze-thaw, alkali-aggregate reaction, and mechanical/impact damage.

Spalling is both a durability and a structural concern — loss of cover accelerates further corrosion and reduces effective section. Assessment uses IS 13311 non-destructive methods (rebound hammer, ultrasonic pulse velocity) plus half-cell potential and cover-meter surveys. Repair follows the break-out / clean / passivate / re-profile sequence with polymer-modified or micro-concrete mortars. Prevention is adequate cover (IS 456 Table 16 by exposure), low w/c, good compaction + curing, and corrosion-resistant or coated bars in aggressive zones.