Lime mortar for underpinning

[88v8]

Well, not quite underpinning...

An elm wall plate about 12' long and 6"h x 4"w, is damp/soft because the wall below it has no means of breathing. I plan to rectify this by taking out the top course of flat stone below and inserting weathered coping to provide runoff, and at the same time slip in a couple of layers of slate to isolate the beam from the wall beneath. I am conscious that this is akin to underpinning and has to be done sectionwise, probably in 18" runs.

So, question: how long should each 18" run be left to cure before it will bear load and permit removal of the adjacent section.
I am employing a good specialist stonemason, who will be using NHL3.5 and sharp sand.
I just want to check that he doesn't get too carried away in terms of speed.

Ivor

 

[robgil]

When I underpinned an internal wall I made sure nothing relied on the mortar alone to hold it up, slate and old quarry tiles were pretty good to wedge in between the bricks.

 

[biffvernon]

I think that's right. The load is taken by the stone, not the mortar. The mortar's function is just to stop things wobbling about - you should be able to do it with mud if done well, or nothing if done very well, so the lime mortar is just belt and braces on trousers that are already elasticated.
/coat

 

[MdB]

I think you are underestimating the strength of lime...

NHL 3.5 has a strength of 3.5E6 Pascals after 3 months. If I have done my sums correctly, this is equivalent to the mortar holding two standard bricks apart supporting 9 tonnes. 9 tonnes per brick will support a wall 250m high (at point of failure). With the standard safety factor of 10, this is still a wall 25m high (not including other loads such as floors and roofs).

Having said all of that, it probably isn't a bad idea to put in some packers if possible. The important thing though is to ensure that there isn't any air pockets or voids in the mixture as this will obviously weaken it.

As for how long to wait, this suggests that over half the final strength is gained after the first week. Personally I wouldn't want to do the next section in less than this time.

 

[biffvernon]

Yes, and that's all well and good in civil engineering, but for domestic architecture you don't need high compressive strength in the mortar. You don't even need the bricks to be held apart - they just have to kept from wobbling. In a mortar with a graded aggregate fraction, the big bits take the load and the little bits hold the big bits in place and the lime could as often as not be replaced with mud. Lime's good because it does not wash away in the rain.

 

[robgil]

Any pics? I did a few bricks at a time allowing 24 hours inbetween, probably around 4 or so.
If I remember I will load up some photos after tea.

 

[MdB]

Yes, and that's all well and good in civil engineering, but for domestic architecture you don't need high compressive strength in the mortar. You don't even need the bricks to be held apart - they just have to kept from wobbling. In a mortar with a graded aggregate fraction, the big bits take the load and the little bits hold the big bits in place and the lime could as often as not be replaced with mud. Lime's good because it does not wash away in the rain.

That's the point I was trying to make (badly..).

Lime mortar is strong enough by itself to hold the wall up so you don't need to bridge the gap with slate or stone, you can just use mortar (or mud for that matter). Large stone to large stone touching isn't needed as you get small stone to small stone touching within the mortar aggregate. (Gap size is of course important to this statement with the larger gaps needing additional stone work to help with slumping and the like.) There isn't anything wrong with large stone to large stone contact (as far as I can see) apart from the difficulty sometimes in getting rid of voids.

I've taken enough of my wall to bits to see quite well how it was constructed. There is very little stone to stone contact with there being a good layer of mud between each. With a strong stone such as granite, I can see stone to stone contact as not being a problem but with our horrible weak sandstone point contact is likely to cause failures (of which there are many anyway). With a layer of mud in between, rather than point to point, the forces on the stone are more distributed which helps reduce the point loading.

 

[robgil]

I dont think strength was what the OP is trying to research , more how long it will take to become strong enough to hold the wall up as it is repaired in sections?

This is how i did mine.

 

[biffvernon]

I dont think strength was what the OP is trying to research , more how long it will take to become strong enough to hold the wall up as it is repaired in sections?

And my point was that it will be strong enough by tea time.

(Note, I take no responsibility if your house falls over.)

 

[88v8]

Thankyou all.

Caramba! robgil, you have some major stuff going on there :shock:

Had a longer conversation with the mason yesterday. Not being online in our cottage, I hadn't seen your replies, but he intends to use slate wedges and pack the joint tight, with only a thin bed of mortar. He is actually going to use NHL5 beneath the beam to get a higher strength, He also thought that one day should suffice between sections.

This brings me to a supplementary question: the slate bed, I envisaged as a dpc beneath the elm beam, should the slate be below the drip course or above it?

I had thought below the beam, as it's the beam I'm trying to isolate rather than the drip course, and also because the beam is treen and of irregular profile, and the pieces of slate will more easily conform to the irregularity.

Ivor