Spalling brick

[kramm (Kenneth A. Ramm)]

@kshepard

I read that, but don’t understand it, Marcel. That article talks about "holes in the brick or mortar. In this damaged brick, there is a plane of weakness parallel to the face of the brick, located about 1/4 " in from the face, along which the face separates. But no holes.

How about this:

Brick is a reservoir cladding. It rains all morning, water soaks into the brick for a certain distance, maybe 1/4"-3/8" and then it stops raining. The brick starts to dry from the outside, but when the outside is dry, there’s still a thin layer of wet brick 1/4"-3/8" back from the brick face. The sun goes down, the temperature drops below freezing and the moisture along that thin layer turns to ice and expands, causing the brick facing to delaminate. The bricks are from different clay deposits or different firings, so there are small differences in performance characteristics that cause them to age at slightly different rates and some will delaminate sooner that others. Kind of like batch problems in asphalt shingles.

The problem with this theory is that there’s efflorescence visible on the remaining brick indicating that the moisture came from inside the chimney. So maybe unlined chimney is the problem combined with brick batch variations.

Efflorescence is salt, moisture is drawn to salt to balance or re-establish equilibrium to dilute the salt. As water evaporates it leaves more salt (efflorescence) on the outside, hence the cycle of hydrostatic pressure and sub-florescence which is deeper in the pores of the brick and mortar. So if the pressure is greater than the brick can stand and as in Marcels article and like I said look at your photos the mortar looks pretty good. One thing to note porous building materials in contact with the earth will pull the salt in and up and can travel a very long way, 6 miles by (hydrostatic pressure / osmosis). This will evaporate in the easiest place, if the chimney is encased in the home or painted then this would be the easiest place.

One thing to note as with Marcels post on mortar, this is why it is very important to use a strong lime based mortar when tuck pointing rock foundations or rock wall homes.