Why do bricks spall?

[LadyArowana]

On a more serious note it has made me wonder whether it is only the cycle of frost shattering that causes brick to fail like this. Next time I am out and about on my hot little island I shall try and find something that is old and made of brick and see how it's doing.

 

[DJH]

Yes, quite right about the protection afforded by overhangs and projections which explains why spalling rarely happens in these protected locations.

I reckon it's difficult without chemical analysis to determine the presence of a small amount of cement in lime mortar and I know, from experience, that even the most reputable conservation firms are susceptible to the mason' labourer throwing in the odd shovel of cement into the mix... :shock:

I had a conversation only last week with a time served plasterer (an excellent tradesman by the way) who told me that when he started his apprenticeship he used to knock up the lime mortar in the clamp, leave it for a few months and was then told to throw in a shovel of cement to 'harden it up' before using it! It's always gone on I'm afraid and it always will unfortunately...

Doug

Good afternoon Lady A...

 

[DJH]

Hot little island!! The mind boggles... :shock: :shock: :lol:

Doug

 

[plasticpigeon]

Like Middi has mentioned earlier, he has used mortar that is a cement hydrated lime mix as it is cheaper than hydraulic lime but similar in other properties. I was suggesting that perhaps it is relative hardness of the pointing mortar to the masonry that determines whether it is the mortar or masonry that fails, and not whether it has cement in it! That would make sense to me anyway.

 

[DJH]

I'm not sure that it is the consistency of mortar that causes bricks to spall. If they are soft or under fired beneath a hard face and continually subjected to cycles of saturation and frost then they will spall eventually. I suppose, as FF says, that a soft mortar will hold moisture to the detriment of the brick when it freezes imposing the same force on the face of the brick as frozen water does when trapped between hard mortar and brick.

It's a just going to happen.

I would never introduce cement into a lime mortar mix. Does anyone remember the so called soft masonry cement called Walcrete that was used once. I think it's no longer available but was thought to be an alternative to hard setting cement setting a lot softer for masonry.

I've just Googled this...perhaps Lady A might pop round and have a look...

http://www.lafarge.com.my/wps/portal/my/2_3_B_1-Detail?WCM_GLOBAL_CONTEXT=/wps/wcm/connectlib_my/Site_my/AllProductDataSheet/ProductDatasheet+Exemple_1270807362578/ProductDatasheet+EN

Doug

 

[LadyArowana]

Does anyone remember the so called soft masonry cement called Walcrete that was used once. I think it's no longer available but was thought to be an alternative to hard setting cement setting a lot softer for masonry.

I've just Googled this...perhaps Lady A might pop round and have a look...

http://www.lafarge.com.my/wps/portal/my/2_3_B_1-Detail?WCM_GLOBAL_CONTEXT=/wps/wcm/connectlib_my/Site_my/AllProductDataSheet/ProductDatasheet+Exemple_1270807362578/ProductDatasheet+EN

Doug

It doesn't look like they sell it in Singapore. Not surprised really, they love steel glass and poured concrete, anything in bags and designed for use with bricks or blocks probably not going to be much of a big seller. The current crop of residential projects are all looking for marketing gimmicks, so we have some where you can take your car up in the elevator and another one with cantilevered swimming pools for some of the units :roll: A long way from Tipperary

 

[middi]

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?

The thing if FF as we know water gets absorbed from the whole face of the brick under raining condition and it will not travel much further or past any hard mortar which beds the brick in situ. The brick will remain wet a lot longer and will slowly re-suck water from within the " hole" which is in until it is dry. Whilst the edges of the brick will be sitting in a near impermeable barrier, this water will be re-sucked back towards the centre of the brick (ie - it will not evaporate from the top or bottom edges only). If the brick exposure is unfavourable and frost sets before this water can be evaporated then the obvious will happen. It should be noticed that there are countless walled gardens build in soft reds and soft lime based mortar still in good working order ( and lots of Victorian house still in good order) If these soft brick built houses or walls were constructed using impermeable mortar with those bricks there would be a very different story
to tell. Also it has to be remembered that not all " soft" bricks or stones behave in the same manner, their performance is relative to orientation to the elements and exposure or firing ( if any) of the bricks.
Again, due to the above, softer , permeable mortar is a lot more favourable than harder mortars.
In the end is all a matter of balance and judicious use of building materials which should complement each other's function and use.

Middi

 

[Flyfisher]

I agree that a brick laid with cement mortar will not be able to dry out by water migrating through the mortar, but a brick does not exist within a wall on its own and is surrounded by other bricks and those bricks will also become equally wet during rainfall. Water will migrate from wet areas to dry areas but would water from one wet brick migrate to another equally wet brick? So, even if the mortar was totally permeable to water, I'm struggling to understand how water would migrate from a wet brick, through wet mortar (which is equally exposed to rainfall) and into an adjacent wet brick.That's the thing I'm struggling with - not the fact that lime mortar is permeable and cement mortar is not.

As for using soft mortar with soft bricks, that seems eminently sensible to me, but more on a physical basis rather than concerns over water permeability. I imagine that soft brick is more subject to slight movement than hard brick and so a matching mortar that can allow a certain amount of mortar would seem sensible. I understand this is why soft lime renders are so appropriate for timber-frame buildings, which are also subject to seasonal movement. Wrap them in a hard cement render and the render will inevitably crack as the frame moves throughout the seasons, whereas a soft lime render will move with the frame.

I wonder if there has been any formal research into these issues?

 

[malcolm]

Some modern bricks here from a wall built around 1960s. I didn't hammer at the render - it fell off by itself. I guess they had figured out how to make bricks by then so they shouldn't have been the problem. I think the trouble was an impermeable concrete render which cracked at some point letting water in. The bricks couldn't dry out from any face.

Bricks prevented from drying out from just a few faces (rather than all faces with the failed concrete render) would presumably have fared better. Taking that further would suggest allowing the bricks to dry from as many faces as possible might be even better.

There might be modern 'conservation' rules that don't let you prevent water getting on the wall in the first place, in that case let it breath as much as possible.

I knocked this wall down. A sub-50 year life is sad but that's the modern world for you.

 

[88v8]

Roof tiles spall, except that there we call it delaminating.
Neither cement nor lime involved. Just over or underfired.

Ivor

 

[DJH]

Exactly..

Doug

 

[middi]

Look here chaps, nothing lasts for ever !!!.
Some building materials are more suitable than others in certain prevailing conditions;
Also, some time tested methods of putting period buildings together have taught us a thing or two.
The scientific approach as to what is the correct way to build a wall or a roof has little meaning to the brick layer or roofer, he/she would ( like myself !!!!) rely entirely on observation and some intuition gathered from experience as to what has appeared to work in the past or what does not.
( Unless some of you, like FF, have some degree or other in some computer studies or other disciplines and start asking some "intelligent" questions as to why certain building materials behave in a certain way. No brick laying graduates on this site I am afraid :wink: )
On the other hand, I have deducted ( by my modest education and observation) that ( soft) roof tiles appear to "spall" a lot more on roof elevation that are covered by moss etc. and on the north or east elevations which leads me to think that when wet , and subsequently subjected to frosts, a certain amount of water is retained within the tiles which leads to frost damage. It has to be observed that glazed or hard cement tiles do not spall easily ( or at all) as they do not retain water . The above should convince some sceptics why it is important to use mortar which is ( softer ?) or similar density of the bricks or stones which is used upon.
My " non scientific" explanation concluded, you are now allowed to use the scientific approach to take the P**S ( :wink: )

Middi

 

[skier-hughes]

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?

 

[CliffordPope]

We seem to be talking about water migration within a wall, from brick to brick. I don't see how the hardness of the mortar can have anything to do with that, unless hardness is coincident with non-absorbancy?

If water saturates one brick, the water will start to run down onto the next. If there is a (hard?) impervious ledge, such as cement, won't that direct the water outwards? Wouldn't a soft permeable mortar absorb the water and ensure that the next brick got saturated too?
If the whole wall is saturated, why would the mortar properties affect the rate of evaporation?

By the converse argument, if the mortar were softer than the bricks, supposing softer = more absorbant, then the water would be drawn into the wall, rather than evaporating on the surface. So the whole hard/soft argument seems to me only to work if the mortar is at exactly the same hardness and absorbancy as the bricks, as if the wall were built of one continuous brick.

I think the poster who said that some bricks simply weren't very good has got it right. Some bricks and some stone have a limited life.

 

[Penners]

I still say that Middi has this one nailed.

A damp brick surrounded by cement mortar only has one available face from which to give off its moisture, whereas enrobed in lime mortar it has the opportunity for its moisture to migrate into that mortar and be given off from there. So the cement-clad brick stays damper, and thus spalls more easily in the frost.

That may be something of an over-simplification, but it works for me!

 

[CliffordPope]

That's only looking at one brick in isolation. In normal situations all the bricks get wet, so there is no point or tendency for water to migrate to a drier spot. If the water can't evaporate from the brick face it's got to run off or soak inwards. I don't see what difference the "hardness" of the mortar makes.
(Except of course that hard impervious mortar inside the wall would stop the water penetrating any further than one brick-depth !)

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.

 

[middi]

For the sceptical posters regarding hard ( or cement ) impermeable mortar answer this:-
Let us assume, for example, that the only " mortar" substitute choice you have in laying your soft brick wall is between impermeable cling film or tissue paper, bearing in mind susceptibility to frost damage if water does not evaporate uniformly from the wall face and at a reasonable fast rate ?.
Which of the two suggested mortars would be your choice and why.

Middi

(from my observations it does not take a gipsy to forecast that the cling film application as mortar will hold water much longer than the tissue paper)

 

[plasticpigeon]

My front garden wall is made of soft sandstone. Before I knew any better I got some local builders (apparently stone masons) to rebuild the wall. They were very bad masons, but at least they built the wall in lime. One stone they dropped and cracked in two, so they stuck it together with resin (anchorfix). It is interesting to note that around the resin the stone is degrading far faster than on the rest of the stone and far faster than next to the mortar joint, even though I think the NHL 3.5 is still a bit hard for this application. I believe this is because some water can get between the stone and resin and when it freezes and expands it breaks the stone rather than the resin. I shall take a picture and see what people think.

 

[Flyfisher]

Interesting discussion!

I'm not trying to refute observations (I know from grim experience that Middi's example of north facing roof tiles is very true, having scrapped many, many more spalled pantiles from our north facing roof than the south facing side) but I have this annoying habit of always wondering why. As a pragmatist, I'll use lime mortar with soft bricks, but I confess to feeling ill at ease with the 'black magic' aspect of doing so.

I can't seem to avoid thinking that all bricks in a brick wall are (broadly) equally affected by the weather. Thus, when one gets wet, they all get equally wet and when they dry out they all dry out at the same rate, and I have difficulty imagining how this wetting and drying can be substantially affected by the type of mortar used. I accept that Impermeable mortar will limit the drying areas of any brick to the exposed face (because water cannot migrate through the mortar), but I'm thinking that permeable mortar will itself become wet, due to the same rain that made the bricks wet, and therefore will not draw any water away from the bricks because there will be no 'dampness gradient' along which the water will travel.

I suppose a double-skinned wall with permeable mortar might help dry bricks by wicking water into the inner layers of mortar, but that would require the bricks to be totally saturated, which I suspect is a rare event in normal circumstances (eg unless a broken gutter or something similar is drenching the wall).

I'm in no way trying to build a case for using hard cement mortars in soft brick, or other, period applications, just trying to really understand why it is such a good thing to use soft lime mortar - other than observation and experience (which should never be ignored!). So yes, I would use soft lime mortar to build a soft brick wall, but I'd be doing it blind (which few would dispute if they saw my bricklaying! :lol: )

 

[CliffordPope]

Cling film, obviously.
It will have no tendency to absorb water, so will not distract the wet bricks from trying to dry out by the water either draining out or by evaporation.
Tissue paper will absorb water - why on earth would you want to add to the wetness problem by introducing something absorbant?

Which would dry out faster - a carpet spread on polythene sheet, or one on soggy underfelt?