Why do bricks spall?

[noj]

It just seems to me that mortar hardness here is a red herring. Where it matters is in accomodating wall movement, which is a totally different matter.

Seems plausible that it's the hardness that is a key factor and possibly the breathability is less important - if there's flexibility in the mortar then everything can expand and contract a little in three dimensions before it spalls or cracks. Lock a brick or stone in a completely rigid straight jacket of cement and all expansion pressure has to be to the front which must contribute to pushing off the face.

Generally all these things are due to a combination of factors though... (hedging my bets)

 

[plasticpigeon]

That has been my hypothesis noj. Leading to the possibility that a weak cement mortar might be ok for old buildings.

 

[Penners]

Widely received wisdom strongly suggests that a lime mortar is best for old buildings. This policy seems to me to have been established by people who know a hell of a lot more than I do about such things! So it's one that I shall continue to follow.

 

[noj]

I'm firmly in the breathable, flexible camp myself these days. I can see the really nasty spalling on my chimney that's had a poured concrete liner and hard cement pointing, I've found the damp in the plinths and sole plate under the internal concrete and bitumen and gloss paint render.

It's a weekend of lime plaster finish coat for me (over nicely carbonating hemp lime) ready for limewash on the plinth and earthborn claypaint on the lath and plaster above.

20 odd years ago (in a different project) I was also in the received wisdom camp and so had an injected damp course installed with the obligatory waterproof render, I layered on lots of PVA and filled any gap with silicon sealants or cement and was endlessly on a quest for the latest miracle product that would last for a thousand years and never move or leak. Funny how things go full circle...

 

[Penners]

20 odd years ago (in a different project) I was also in the received wisdom camp and so had an injected damp course installed with the obligatory waterproof render, I layered on lots of PVA and filled any gap with silicon sealants or cement and was endlessly on a quest for the latest miracle product that would last for a thousand years and never move or leak. Funny how things go full circle...

The quality of received wisdom does, of course, depend on where the wisdom is received from!

 

[a twig]

I don't think anyone would question that all equally wet bricks and mortar will remain equally wet. No water moving.
But, if the Sun hits the bottom layers or top layers of brick whilst others are in the shade, they start to dry, moisture starts to move and the wall dries more quickly?

This. Spot on. No wall will dry evenly. Parts that are more exposed to wind/sun will dry quicker. Breathable mortar then allows the moisture movement, thus the overall wall dries quicker. With impervious mortar, the dry bits would dry and the wet bits stay wet.

The greater the difference in moisture levels of adjacent bricks, the greater the stresses caused by the relative expansion / contraction. With a hard/inflexible mortar it is then the brick that takes the punishment - hence spalling.


The relative importance of softness and breathability can thus be separated:

A mortar softer than the brick will reduce the physical damage caused by the relative stresses.

A breathable mortar helps to maintain a consistent moisture distribution in the wall, thus reducing the causes of the stress in the first place.


If anyone has the shed space and time, the central premise above can be tested.

1)Two single skin walls of soft reds, minimum 3 courses height. One impervious mortar, one breathable.
2) Hose them down until walls are saturated.
3) Take two heat lamps, one per wall, point them at the same ends of the respective walls.
4) Use moisture sensors to record levels at the "wet end" over time.
5) Using stress/strain gauges on a brick in the middle of each wall in the middle course, record strain/stress over time.

A years supply of jaffa cakes says that the breathable wall will win: i.e. moisture level in the wet end of the breathable wall will drop quickest and the stress/strain on the middle brick will be lowest.

 

[Penners]

I wonder what's on TV...

 

[Me!]

The bricks that are spalling the the most in my house are inside one side of an 'inglenook'-ish fireplace. The fireplace hasn't been used for decades, it is internal to the house (not on an end-wall etc.) and there are no obvious signs of damp. The bricks are soft red, laid with lime mortar and have probably been untouched for several hundred years. Despite my economical use of the central heating, there is no chance at all that they are ever subjected to any temperature even close to freezing point, and they are in a position that never gets direct sunlight.

 

[Roger440]

Dont underestimate the poor quality brick aspect.

20 years ago, i built a retaining wall for my garage from class B engineering brick. Just brick on edge on the top, no dpc, no plastic behind, no overhang at the top. Everything you shouldn't do. 20 years later, it is still perfect.

About 10 years ago, i built another to retain a raised are. Same position as the other (north facing face) Within 5 years, all the top bricks had spalled as had some on the face. Redid the top row, and within 2 years were starting to go again. Now much the same as before.

Same guys building it, same postion. Only difference was different brick supplier. And different absorbency rates. A wise man told to me took at the absorbency rates.

Not sure it helps much, (its hardly an old wall) but poor bricks are poor bricks. Im of the view on my wall that its just poor bricks.