Brick face spalling from sealant 1

[ArneG]

I have a building with clay bricks from the 1940’s. The brick has exhibited full faceshell spalling (entire face approximately 1/16” to 3/16” thick peeling off). This spalling randomly affects 3 or 4 areas of approximately 50 square feet. This spalling is probably due to the brick being sealed with a non penetrating sealer in the past. The wall parapet flashing has been replaced to reduce moisture behind the brick, and wicks will be installed in this phase of construction. There is limited area of efflorescence, no efflorescence is visible at the spalled bricks. We are in a freezing climate.

This leads to my conclusion that moisture is being trapped behind the sealant, freezing and popping the faceshells off the bricks. If the wall is tuckpointed it will allow the wall to “breathe more” reducing the moisture behind the sealant and reducing the chance that additional faceshell spalling will occur.

My questions are:

1) Is there another viable option other than stripping the sealant that will reduce the chance of faceshell spalling?

2) Is there a standard way to seal the new bricks to match the existing without sealing the mortar joints and locking in the moisture?

Thanks for your help,

Arne

 

[Ron]

What you are describing is a fairly common deterioration pattern in older brick. It usually has nothing to do with whether they were sealed or not, and in most cases, sealers only last a couple of years anyway.

The usual cause of this problem is stratification fatigue caused by wetting/drying cycles. The mechanism for this is that moisture absorbs into the brick during a "damp" cycle, and the brick expands due to the moisture to a finite depth. The brick with no moisture or little moisture at greater depth, expands less. Then the brick dries out (dry cycle)and shrinks to some depth, while the "steady-state" (inner)portion of the brick stays the same. At this interface of wet-dry cycling, the brick fatigues and eventually spalls.

To mitigate this, I would use a penetrating methacrylate sealer.

 

[JAE]

Ron,

Being a building built in the 40's, would the mortar tend to be softer (i.e. higher lime) and thus would this effect you describe still be as prevalent as with a brick wall with more modern, harder mortar?

 

[Ron]

JAE...yes. I recently evaluated one for a church built in early 1900's. They used a lime putty mortar and had similar issues. I did a petrographic examination on the mortar and the brick. I found the mortar to be more consistent than I had expected, but still somewhat friable. The brick had a "crust" on the outside from the firing but was relatively soft on the inside. That was common with the methods of firing brick in earlier production.

I was able to polish the mortar, telling me that the compressive strength was at least in the 300 to 500 psi range, based on testing I've done in the past.

 

[dik]

Ron, It is a fairly common problem with improper sealers and masonry renovation. Sealers can trap moisture within the brick and freezing can cause exfoliation of the outer brick skin. This and sandblasting the brick surface for cleaning are two of the more common faults of renovation. Another big problem with the use of sealers is the discolouration that can occur.

JAE, the lime in early mortars as Ron notes relied on impurities in the lime itself to provide hydraulic/pozzolithic properties. By the 40's processed lime was 'pure' enough that it relied on the masonry cement or portland cement additives to provide pozzolans. Lime mortar for early brickwork was soft and porous to permit 'wicking' of moisture through the mortar joints (not the brick); many early buildings did not have vapour barriers. Softer mortar promoted failure to occur through the mortar joint and not the brick; it was just a matter of re-pointing and not replacing brick units. Another common repair error with early brickwork is using 'hard' Portland cement mortars . They lack the ductility and the ability to wick moisture.

Dik

 

[ArneG]

I contacted the BIA and their opinion was that it probably was a sealant related issue. This is more common in Minnesota than it would be in non - freezing states because a very small amount of moisture can "pop" the surface sealant off.

I did not state that the brick behind the peeling layer does not sluff off significant dust when brushed, which may indicate that it is not experiencing fatigue to to cyclical moisture conditions.

Tentatively we are planning on tuckpointing the entire building with a higher lime content "softer" tuckpointing mortar to allow the bricks to breathe a bit better. We will also add additional weeps to the wall. The wall had a new parapet/roof system installed a couple years ago that should help reduce moisture behind the wall surface.

The remaining issue is how to get the new bricks to match the existing bricks without sealing them. I've got a meeting with the restoration contractor later this week to work that out.

I've also got the owner trying to figure out what product was used to seal the building, and how long ago it was sealed.

Tricky stuff.

Thanks for your replies, they are all VERY helpful.