Spalling is the breakdown of masonry surface layers in response to high temperature or internal mechanical pressure sometimes caused by corroding reinforcing steel. Spalling can often be a warning sign of more extensive structural damage extending into the building.
In concrete, the physical appearance of spalling may include striated lines, discoloration of the masonry, pitting, or rough texture to the concrete that remains attached to the slab.
Small spalls, or pop outs, can occur at edges where bar ends terminate near the concrete surface or at other localized areas of shallow concrete cover. Larger spalls arise near embedments and where long sections of reinforcing steel, with insufficient protection, run parallel to the concrete surface.
In brick, spalling maybe be caused by a mortar of excessive hardness. Any movement in the wall, from a settlement, thermal expansion, or vibration, is deflected by the mortar, instead of being absorbed by it. This pressure then transmits back to the brick, ultimately leading to spalling.
Spalling may also result from deterioration of the masonry structure’s interior anchoring system. Water penetration can lead to metal reinforcements and anchoring to erode, causing undue internal pressures.
The only practical way to accurately repair spalling masonry is to replace it. Replacement may require removing individual bricks or even an entire window sill.