Spalling, Paint & Repointing
Spalling is a shallow delamination, bulging, and dislodging of thin slices of a material. In the case of a masonry chimney spalling is the delamination and dislodging of thin slices of the clay brick material which often extends into the mortar joints. See Figure SPR-1.
Spalling is the result of excessive moisture freezing and thawing just below the surface of the brick or mortar. Excessive moisture in the masonry construction freezes and expands, cracking the brick or mortar just below the surface, pushing a thin layer of the material off the face. The various stages of spalling can be seen in Figure S-2.
The excessive moisture is the result of surface runoff infiltrating the brick or mortar or leakage of the surface runoff into already existing cracking, especially at the chimney cap.
Since paint is a sealant it seems that painting the brick is a way of keeping moisture out of the masonry. However, paint is more likely to keep moisture from escaping the masonry, and thus, increasing the spalling. There are better sealant for masonry, ones that allow moisture to evaporate fort the brick and mortar.
Spalling commonly doesn’t occur or appear suddenly, but is a progressive deterioration of the masonry construction. The progressive nature is commonly evidenced by the worn and discolored edges at the cracking, the paint coverage at the cracking, and the paint coverage on the exposed surfaces of the mortar and the brick (Figure S-4).
To reduce the risk of spalling the masonry must be regularly cleaned of all loose material and the joints re-pointed. Re-pointing makes the masonry more resistant to water infiltration, and thus, reduces the risk of freeze-thaw damage. This should be performed before the masonry appears to be losing its water-resistance. Although the service life of masonry depends largely on the quality of the materials and the workmanship, cleaning and repointing are usually required approximately every 20 years.