Why do bricks spall?

[plasticpigeon]

I am totally convinced that cement pointing ruins old brickwork as I have observed so much damaged and spalled old brickwork where it has been pointed in cement. The reason normally cited for this is that lime pointing allows moisture evaporation out of the bricks. I've always thought this sounds a bit unlikely. I'm sure there are times when there is a freeze after a heavy rainfall without enough time for evaporation in between to make a difference. It seems more likely to me that water can get between the brick and the mortar, and when it freezes and expands, it damages the brick if the mortar is harder than the brick. What do others think.

 

[JoceAndChris]

Well, it's interesting to look at our brick haha which has no mortar at all and gets buffeted by the winds and weather all the time. Many of the bricks have simply crumbled away and the most weather receiving part of the haha has lost around 3 courses. All the exposed bricks are cracked with fine lines pitted and extremely damaged. This leads me to conclude that brick is an inferior building material really, we have dry stone walls here and they're fine.

Then in our kitchen we have exposed brick with cement pointing and they really have spalled. It's partially sunk so any moisture really has nowhere to go except push the face of the brick off. I spoke to Phil about getting Skillingtons to remove all the cement pointing, he said that as the bricks are in such a fragile state it would do more harm than good. They plan to mortar in new ones where bricks have fallen away and lime plaster the lot - bad idea to have exposed brick underground.

 

[88v8]

'Yes, it's the received wisdom that hard pointing leads to spalling of the base material, and I dare say it can contribute.
However, our cement pointed since 1972 back wall has not spalled at all.

I'm inclined to think that some stone and some bricks just are not very good - too soft, too porous, the water gets in and frost blows the face off, and can continue so doing until there is nothing much left.
The cottage up the road - cement pointed long ago - had some very eroded/spalled stonework - was it the pointing or the stone? Only some stones, others adjacent not all.

An experiment with lime in part of a wall and cement in part would be instructive. Perhaps one of our younger members would like to undertake and report back every few years.

Ivor

 

[Flyfisher]

I must confess that I've never fully understood the 'sacrificial pointing' theory.

Yes, hard, impermeable, cement-based pointing would not absorb much water and therefore be fairly resilient to freeze-thaw damage, and soft, absorbent, lime-based pointing would absorb water and therefore be susceptible to freeze-thaw damage. That much I can understand, but why would any freeze-thaw effect act preferentially on only lime-based pointing?

Can a relatively small amount of lime-based pointing really draw away all the moisture from a relatively large brick and thus protect it from freeze-thaw damage?

Wouldn't both the brick and the lime pointing become equally wet and therefore equally at risk of freeze-thaw damage?

Is it possible that cement-based pointing is more resilient to damage and therefore becomes noticeably 'proud' of spalled brickwork, whereas lime-based pointing erodes at a similar rate to the bricks and so the overall effect is less noticeable?

 

[middi]

It is quite easy really to understand why cement mortar should never be used with soft brick or stone wall construction.
It is also quite easy to understand why ( even as a rule of thumb) it is always advised that the construction mortar should never be harder that the brick or stone that it used for constructing these.

In the first scenario it must be accepted that hard cement mortar is, more or less and depending on mix ratio, not very breathable and therefore water penetration and ( eventual) evaporation is very slow. When this mortar is used with soft bricks or type of stone, the water that penetrates their exposed faces will accumulate above and below the mortar bed that is in contact with the brick or stone creating a water film which is very slow to evaporate. If this water should freeze an expansion will occur and the first thing to give way is the softer material, ie- the ( softer) bricks or stones.
If this construction mortar is of the breathable type ( lime ), this type of mortar will aid the " suction" process from the brick faces and quickly evaporate it to the elements with hardly any expansion damage to the construction materials.
With the passing of time pointing of the mortar maybe required due to the slow deterioration of the pointing " faces" but no damage to the softer bricks ( or other ) will occur.

AS to the second scenario, there is no point at all for the building mortar to be harder than the construction materials for reasons explained above.

Middi

 

[philpjuk100]

I walked down a street in newark during the bad winter (2011?) which has nice Victorian semis down one side,the front garden walls seem to be built from hard smooth bricks and a soft lime mortar.The pavement was covered with the front of spalled bricks on looking closer the walls that had spalled badly seem to have had the soil level raised on the house side of wall so probably causing the bricks to be damp.I passed down there again this morning and the damage is quite bad,some bricks missing about 3/4" (did not have a camera with me so no pics).

 

[Penners]

Middi's explanation is definitive (as always).

 

[middi]

Middi's explanation is definitive (as always).

Thank you Penners. Your thoughts are of great comfort, it proves to me that my brain is still " ticking" in the right direction following my mishap. :wink:

Middi

 

[DJH]

I think it matters not a jot what sort of pointing is used on either stone or brick walls. If either are soft then they will spall regardless of the hardness of the mortar joints and pointing.

Soft sandstone will soften and spall over time when exposed to the elements and especially if it is not laid in the correct bed joint orientation. Soft or under fired brick, (a common problem) will also spall in the elements and those bricks with a hard face hiding the soft under fired inner brick will also spall in frosty weather. The different expansion properties of the hard face against the soft inner cause them to part company revealing the soft inner to further erosion. The common solution to this problem on more humble dwellings, along with the need to hide cheap and non-matching bricks, was to render with a coat of cement based render. On houses with pretensions a coat of stucco with ashlar joints was used to cover sub-standard work liable to disintegrate and give the appearance of quality ashlar.

Non coursed random and rubble masonry was invariably coated with lime wash without the need for a render, as the mortar was finished flush with the surface of the stone and could take the wash easily. Where a render was used it was still whitewashed anyway. Building mortar was not intended to be exposed to the vagaries of the weather as it would wash away without a protective coat of whitewash and, if used, a coat of render that was also whitewashed.

Hard ashlar was laid with joints rarely exceeding a few millimetres of putty lime and the stones were dressed and laid correctly.
We often see pictures of spalled brickwork with residual cement pointing still in place and the spalling is blamed on this residual pointing whereas it’s blatantly obvious that the bricks are soft and are spalling on their own.

If an exposed wall of soft stone or soft brick becomes saturated, doesn’t dry out for whatever reason and is then subjected to frosty conditions then spalling will more than likely happen.

That’s what I reckon…cement is often the fall guy but in some cases if the underlying mortar is dry during or after extended sunshine and dry weather it can in fsct prevent ingress of water to the absorbent mortar behind keeping it dry. Soft lime mortar can absorb and retain moisture for a long time even when the sun is shining and evaporation is taking place. Retaining moisture it can freeze quicker and exacerbate spalling by expanding and jacking any friable soft surface from brick or stone.

Just my thoughts on the subject…if the masonry and bricks are soft then they will spall at some time.

Doug

 

[middi]

We take it then, Doug, that you would have no objection in constructing a wall of soft red bricks ( or porous stone ) with cement based mortar.

Middi

 

[DJH]

No, I'm not saying that. I just think that it's not correct that cement based mortar pointing is the only reason that bricks and masonry spall. If they are soft and porous then they will spall whatever the mortar used.

Doug

 

[Flyfisher]

That was my thinking as well. Unless a lime mortar can actively reduce water retention in soft bricks, how can it protect the bricks from frost damage?

 

[middi]

The reality is ( as I mentioned earlier) that the mortar must not be harder or denser than the brickwork.
When one considers that the array of bricks, stones or blocks are extremely varied in density the appropriate mortar should be used to lay these in order to avoid excess water being trapped longer than necessary within the masonary, there is a wide differential of evaporation between bedding mortar and brickwork and harder bedding mortar will trap water for much longer in the top and bottom faces of the " brick". We are talking about balance here.
I have often used 8:2:1 ( 8 of sand, 2 hyd.lime, 1 ( white) cement ) to lay some soft red bricks with no ill effect 25 years later. ( only because it is cheaper than other " pure" lime materials mortar but the porousity is extremely good and does exactly the same job.)
On the other hand, using a solid cement mortar for engineering bricks ( including waterproof) is the order of the day.
Without a doubt, getting the balance incorrect will lead to problems on exterior masonary , especially where frost are concerned.

Middi

 

[DJH]

Waterlogged stone and brick will spall when trapped moisture within freezes irrespective of the mortar used. The process can be exacerbated by the mortar used be it lime mortar, cement or a mixture of both, unfortunately.

Doug

 

[Flyfisher]

I think I'm having trouble with the idea of the mortar 'trapping' moisture in the bricks.

I have no doubts about cement mortar/render being able to trap water (I've seen enough of it in real life - unfortunately!) but when we're considering the weathering of an unrendered brick wall, I don't really understand how the mortar can trap water when everything is exposed to the air and when the surface area of the exposed brick is far greater than the surface area of the exposed mortar.

Assuming there is no internal source of water, the only way water can get into the brick is from rainfall and soft brick will, of course, absorb a certain amount of water when it rains. When the rain stops falling the absorbed water will evaporate from the surface of the brick. If it freezes before complete evaporation then the brick may be damaged. Can lime mortar really speed up the drying out of a wet brick when it will be just as wet from the same rainfall?

It would be interesting to build three brick pillars as an experiment. One with cement mortar, one with lime mortar and one with no mortar. Assuming the one with no mortar does not get blown over, I wonder which pillar would suffer the worst effects of weathering?

 

[plasticpigeon]

But if the water gets trapped between brick and mortar and freezes, it makes sense that the mortar should be softer than the brick.

 

[Flyfisher]

Yes, that makes some sense, but do bricks only spall at their edges where they meet the mortar? Doesn't the entire face of the brick spall - as we might expect when a wet soft brick is exposed to freezing conditions?

 

[DJH]

Yes, fair enough PP but to the detriment of the mortar? It can't, in the long run, be good to have mortar exposed to cycles of soaking and freezing without an adverse effect on it after all, the face of mortar has nearly always been struck to help prevent ingress of water and to direct it away from the joint. In an ideal world neither the brick or the mortar should be capable of absorbing water. The mortar should maybe have an amount of white cement in it if it is to be as successful as the wall middi built...

I have often used 8:2:1 ( 8 of sand, 2 hyd.lime, 1 ( white) cement ) to lay some soft red bricks with no ill effect 25 years later.

Is that success because of the cement in the mortar I wonder?

Doug

 

[malcolm]

I have a perfectly good wall that hasn't been pointed in well over 100 years and doesn't need re-pointing. There isn't any cement in the mortar at all. :roll:

It does make sense to me that breathable mortar will help the bricks dry out quickly and reduce spalling. Though unprotected soft reds like in my garden wall will probably spall in the end whatever you do with them. Cement repairs to that wall are most annoying to me because the bricks won't be reusable when I rebuild the wall.

Bricks spall when they get wet and freeze. Nobody has mentioned reducing the amount of water getting onto the bricks in the first place. Houses do this with roof overhangs, gutters, drip things in the window sills etc. Even garden walls often have a bit of corbelling or tile sticking out near the top to divert drips.

 

[LadyArowana]

Nobody has mentioned reducing the amount of water getting onto the bricks in the first place.

It was at this point someone thought of waterproof paints wasn't it :shock: