Brick /lime /water/ ice. A novice’s question?

[michael]

I’m still a bit of a novice on the lime front and I need to get my head around something.

As I understand it Lime mortar is able to absorb more water than cement but unlike cement it dose not trap the water and allows it to evaporate away safely i.e. Breathability.

Again as I understand it, the damaging factor with cement mortar is that the water it traps within the brickwork is susceptible to freezing and it is this expansion/contraction of ice particles within the structure that damages brickwork.

My question is, how quickly can a large quantity of water (say after a rain storm) evaporate away from a brick /lime mortar wall?

If after a few days of heavy rain you’re suddenly met with a spell of cold freezing nights (before the water has had a chance to evaporate away from the brick/lime mortar wall) wouldn’t the increased volume of water absorbed by the lime (compared with that of cement) and therefore the increased volume of ice particles within the wall, be equally if not more damaging to the structure?

 

[Lime]

Well the proof of the pudding is in the eating.
My house is over 100 years old and the lime mortar shows signs of weathering but no part is particularly bad.
The worst areas are facing south-west so presumable that is rain and sun rather than frost.
The bricks are medium-soft red in the front and hard yellow (gildenburg) bricks at the back.
None show any sign of weathering.

As a matter of interest, I owned a very large property years ago next to a church yard.
The yew trees had 1000s of sparrows living in them and I used to see them cleaning their beaks on several quite hard bricks on the house.
In just a 2 year period they scapped away at least an inch into the surfaces.
This gives the appearance of erosion by weather but it is nothing of the sort.
Again, my nextdoor neighbour rebuilt his front garden wall 10 years ago and some of those bricks now have crescent like shaped beak scrapping marks on them.
(the birds stand in one place and move their heads from side to side making a smooth furrow.)
If you did not know what caused it you could easily think is was the brick failing.

 

[Andrew Bodenham]

A very interesting question, Michael - I wish I could give you a definitive answer backed up by research, but unfortunately I can only give you a guess and hunch.

Firstly I feel that a long hard freeze following rapidly on from heavy driving rain is the sort of weather that any building materials will find uncomfortable. I wonder how cold (and for how long) it needs to be to freeze water beyond the surface layer? I would have thought that a few mm into the wall the rate of change of temperature would be much slower than on the surface. However, that's another matter.

I think that lime mortar is preferable simply because of its relative softness compared to the brick. If there is ice expansion then the softest material in the wall is more likey to suffer, and this should be the mortar. This is not always the case when hard cement mortars are used for pointing (particularly) old bricks. The mortar is essentially a sacrificial element in the structure, which is why periodic repointing is (or should be) the norm.

From my observation cement pointed (or bedded) brickwork is often riddled with capillary cracks anyway, which must allow fairly deep penetration of water into the brickwork. It's my understanding that softer lime mortars are more accomodating of movement and also have some self-healing capability by re-crystalising cracks. The overall effect being a more uniform absorbency across the wall.

Fortunately I live in a hard limestone house.

Now we hope for somebody with some real expertise to shed a bit more light on the topic...

 

[Andrew Bodenham]

Those sparrows sound a bit scary. And why do they need such sharp beaks? I'd always regarded sparrows as rather benign little things - I shall treat them with more respect from now on.

 

[JoceAndChris]

It is an intriguing question. My 250 yr old limestone house (isolated on a hillside) has been pulverised by the weather. So in the 60s someone clapped cement render all over it, to avoid making difficult and costly repair. I guess these old buildings just need constant love to stop nature destroying them.

 

[Jon Maine]

Fortunately, it is relatively less frequent for a hard frost to fall hot on the heels of a heavy rain storm ( although it has happened here at least twice this year so far ).

The cement pointed wall is as pointed out likely to be riddled with hairline cracks. This lets a lot of moisture in but not out.

We all know anything wet transmits heat more quickly than something drier. a warmer wall will be more resistant to frost damage.

Also hard cement mortar is often harder than the stone and therefore will fail after the stone does.

 

[middi]

The properties of a soft red brick must not be confused with the fact that a modern hard brick abosorb less water therefore the soft red should, in theory, suffer more damage when frozen.
As we all know frozen water will expand ,and depending on the extent of freezing, can exert temendous hydraulic pressures that will distort the hardest of steel.

It is all a question of air/water absorbing ratio. A wall can be constructed with the hadest of bricks and cement mortar to make it " wateproof" and this works extemely well as long as EVEYTHING is wateproof, if a small quantity of water enters any damaged area where freezing occurs there will be an immediate hydraulic effect on the immediate surronding area that can result in damage.

The opposite is true with a very porous soft brick and lime mortar scenario. This brick can absorb enough water to DOUBLE it's weight and the average lime mortar can actually TREBLE it's weight, realistically, unless one immerses these materials in a bucket of water for about half an hour there will still be some microporous air gaps in the materials. If the balance of this kind of brick and mortar is correct it is highly unlikely that even in our rain ridden Country these building materials will ever become totally saturated to the extent that all air molecules are extruded. It is these air molecules which act as a buffer to counteract the hyraulic process that causes the damage. Any respite between the persistant rains the drying process immediately follows.

This is the main reason why an incorrect mix of materials will result in extensive damage on our traditional buildings.

 

[DaveBrigg]

Now that the more severe frosts have passed, it's time to inspect the damage that the cold weather has caused to our 'new' render. We followed the CO's advice, with two coats of hydraulic lime mortar professionally applied. The work was finished in August, and most of the damage occured during the couple of nights in January when temperatures dropped to -8. The picture shows the render where the front wall joins the corner of the house; no prizes for guessing which side is which. Fortunately the flaking seems to be mainly in the top coat.