Spalling is a form of masonry deterioration in which the outer section of a stone, brick or mortar breaks away in a localized area. The term spalling sometimes can be a general one, refering to any type of coming away of the surface, such as exfoliation or delamination.
The cause can be either freezing and thawing cycles of moisture trapped within the masonry or salt crystal pressures. Another cause is mortar of excessive hardness. Any movement in the wall, from settlement, thermal expansion, or vibration, is deflected by the mortar, instead of being absorbed by it. This pressure then transmits back to the masonry, ultimately leading to spalling.
Spalling can also result from the masonry structure’s interior anchoring system deteriorating. Water penetration can lead to metal reinforcements and anchorings to corrode, causing undue internal pressures.
Chipping is the separation of large or small masonry fragments from masonry. It typically occurs at mortar joint corners. The cause of chipping can be improper renovation and repairs (using too hard a mortar), or accidental mechanical impact (falling trees or vandalism).
Before attempting a repair to the spalling, the root cause should be determined and, as far as possible, remedied. If the spalling is on stonework and damage is not severe and deep, it is best to leave it as is. Spalled bricks should be replaced.
Repairing Stone Spalling
Significantly damaged stone may be repairable. There a couple of methods to choose from- Dutchman repair and plastic composite repair.
A Dutchman repair is piecing in a small patch or chip of either natural stone or imitation stone of precast concrete into the damaged area of stone.
The repair piece is bonded to the underlying masonry with an epoxy adhesive or specialized masonry repair adhesive.
The technique is similar to glueing back together a broken dish or vase. Both surfaces should be clean and free of dust, dirt and grime. Keep the join between the old and new surfaces as narrow as feasible so as to maintain the appearance of one continuous surface. Also take care in matching color and texture.
Plastic Composite Repair
In this spalling repair technique, a cementitious mortar-like mix is applied in layers to the deteriorated area in order to reconstruct the missing surface.
Care and skill is required to successfully repair using this method; done improperly the patching can shrink, crack and separate, leaving the stone even more damaged, or it may simply look shoddy, making the damage more visible. For this reason, the work should only be undertaken by an experienced craftsman.
See Also: Stacked Stone Veneer