Spalling is a result of water with mineral salts entering brick, concrete or natural stone and forcing the surface to peel, pop out or flake off. Spalling concrete is due to a natural deterioration process called carbonation. Carbon dioxide in the air diffuses into the concrete and reacts with the alkalis in it. The concrete becomes carbonated allowing embedded steel to corrode and expand to exert a force on the surrounding concrete causing the concrete to bulge and crack. Spalling should be repaired as soon as possible before embedded steel corrodes further and damages larger areas where it becomes known as "concrete cancer"  Eventually, spalling can cause crumbling and destruction of a structure.

WIKIPEDIA ENTRY FOR SPALLING