Stone.

I've got a Victorian semi, and the brick walls go above the roof. On top of those walls is a coping of pre-cast concrete lengths - each one a couple of feet or so long - replaced along with the roof some years ago.
At the end of this wall (front of the house) is a typical Victorian cast stone. Which included a short length of this coping stone - but all in one, so would tend to retain the others. This stone is three brick courses high (at the front) and extends about 2.5 ft back into the wall.
And is spalling badly. One worse than the other - and of course it's the one between the two houses, on the party wall.
When the roof was replaced about 30 years ago, the roofers rendered this stone - and that has all fallen off, bit by bit. Luckily no-one was injured by the falling chunks.
The house is scaffolded for painting, so I have reasonable access. But only on my side of this central stone.
In an ideal world, I'd remove it. Make up some shuttering and cast up a new one. But I don't fancy doing any damage to next door's roof - and I can't get access to that side from the scaffolding anyway.
I've got a load of lime mortar made specifically for fixing this sort of stone. It would be possible to fix shuttering to the existing stone to remake the coping part. But just how well would the lime mortar hold?
I'll take some pics and put a link to them if it would help.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On 7/29/2017 2:31 PM, Dave Plowman (News) wrote:

Sounds like a plausible idea. The other thing you might think about is ways to improve the keying. For example you could drill some holes in the face to be repaired, insert plastic wallplugs, and then put in some screws protruding a certain amount from the surface. I think I would use stainless steel screws.
Are you talking about pure lime mortar, or one containing some cement as well? My suspicion is that while simple lime/sand is fine for repointing and filling cracks, you might want something a little bit stronger here.
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That was in my mind too. One thought was to cut the coping part off with an angle grinder and cut down one of those readily available cast ones to the correct length. And fit it to the original stone in some way as well as with the mortar, like screws.
Sadly, there's nothing in the instructions about what metal is best with lime. SS would be easy if it is good.

It's the stuff Conserv supply ready mixed to repair this sort of stone - by their website. In what little I've read they caution about adding cement - ever.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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More than a small proportion (exact figure unknown to me) of cement turns it into relatively-weak well plasticised cement mortar. Especially it gives it the undesirable property of being impervious. But it is somewhat mechanically stronger.
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Roger Hayter

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On Saturday, 29 July 2017 19:00:07 UTC+1, Roger Hayter wrote:
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that's true when the cement strength equals/exceeds lime strength. At 5% ce ment it's not, the cement just enables a quick (weak) set. The reason most cement/lime mixes are deprecated is that ratios other than 1:1 have been fo und to be at risk of premature failure in practice. The other reason is tha t as you say non-low cement content changes the properties of the mortar fo r the worse.
If adding 5% makes the difference between getting the job done or not I'd d o it, and accept the degree of risk. If you can get it done a better way, g reat. Adding 5% is an often accepted tradeoff.
NT
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On Saturday, 29 July 2017 19:00:07 UTC+1, Roger Hayter wrote:
s

me

use

h
to

l
h
as

ing

re.

-
that's true when the cement strength equals/exceeds lime strength. At 5% ce ment it's not, the cement just enables a quick (weak) set. The reason most cement/lime mixes are deprecated is that ratios other than 1:1 have been fo und to be at risk of premature failure in practice. The other reason is tha t as you say non-low cement content changes the properties of the mortar fo r the worse.
If adding 5% makes the difference between getting the job done or not I'd d o it, and accept the degree of risk. If you can get it done a better way, g reat. Adding 5% is an often accepted tradeoff.
NT
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snipped-for-privacy@gmail.com wrote:

Worth adding some fibres to the lime mortar?
<http://cornishlime.co.uk/products/associated-building-products/hair-fibres-mesh
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Yes - was thinking about that. When the rain stopped I had another look. What I think I'd like to do is re-make the coping stone top to this one. It would be easy to fit some shuttering and cast on top of the remains. After removing all the loose bits, the rest appears sound.
The traditional stuff was horse hair. I wondered about using glass fibre - since that is easily obtianed. And is used in those lightweight concrete tiles you get for say a roof terrace.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Looks like a fibreglass mesh might be the easiest way?
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*He who dies with the most toys is, nonetheless, dead.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On 30/07/2017 11:22, Dave Plowman (News) wrote:

http://www.wickes.co.uk/Sika-No-Crack-Concrete-Admixture/p/154064

https://www.youtube.com/watch?v=H81sjhIC76o

--
mailto: news admac {dot] myzen co uk

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I now know why a present day builder avoids lime mortar like the plague.
Takes ages to set. Fine in the days of a horse and cart, I suppose.
--
*War does not determine who is right - only who is left.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Wed, 09 Aug 2017 13:57:46 +0100

I'm fairly sure most builders who /do/ avoid it like the plague simply don't understand it and don't want to learn.
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On 31/07/2018 21:18, Rob Morley wrote:

We've got lime mortar & plaster in parts of our house. It's better against the timber frame, and it fits the age of the building.
But however slow it doesn't take a year (less 6 days) to set!
Andy
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On 29-Jul-17 2:41 PM, newshound wrote: ...

I suspect that stainless steel would not have enough oxygen reaching it to keep the protective surface coating intact. That would mean it will rust just as quickly as mild steel. I would use a non-ferrous metal bonded to the existing stonework with epoxy resin.
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Colin Bignell
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On 7/29/2017 7:57 PM, Nightjar wrote:

I don't think I agree. In an alkaline environment there is nothing present that is going to degrade the protective oxide, so why do you need any additional oxygen. In any case, in a "breathable" mortar there's always going to be some water (and hence oxygen) present by diffusion from the outside.
Excuse me if I digress, but I did have one interesting instance in the past where austenitic stainless steel was corroding at an alarming rate in an alkaline environment. My current house has thick rubble-filled random limestone walls built straight on the underlying limestone. And I have a 100 metre hill rising within a kilometer behind the house. The previous owner, towards the end of his life, dealt with the chronic rising damp problem by putting (untreated) studding on the end wall, stapling thick polyethylene sheet over this, then plasterboarding and plastering. (The exterior surface was cement rendered with the traditional bell-cast above a few inches of exposed stonework).
As a result, when we moved in the dry rot had reached many of the first floor joists, as well as taking out the staircase in one corner and the window frame in the other.
Being in those days totally dependent on a commercial mortgage, I had no choice but to follow the advice of an approved "damp specialist". They ripped out the studding and plasterboard, replaced the staircase, window frames and doors, put in an electro osmotic damp proof course, and re-rendered inside with waterproof sand/cement rendering, topped with gypsum plaster. (They did use Sirapite on another wall).
This of course sealed in all the rising damp that was forced up by the fact that the normal water table was at least half way up the wall. In due course, the gypsum plaster blew out. Also, the new radiator fell off the wall when the screws rusted through.
At this point I thought I would be clever. I got four lengths of 8 mm austenitic (18-8) stainless steel studding about 15 inches long, set these into the wall, and hung the radiator brackets on them. (At this time I had small children, no money, and was just living with the rising damp).
After a while I decided I needed to sort the damp properly, so I moved the radiator to a better location, and stripped off all the interior rendering to allow the wall to dry out properly. (The exterior rendering was on the site boundary and faced a garage forecourt, I didn't really want to tackle that wall). I was interested to find very substantial corrosion on the "stainless" studding that had been holding up the radiators. But by that time, I had also discovered that 90% of the metal in standard galvanised electrical back boxes had also vanished on another really wet wall. On checking the polarity of my "electro osmotic" damp proof course (which was made from titanium wire) I decided that the iron loss in both the mild and the stainless steel had actually been caused by the electrochemistry because the back boxes, and also the radiator and hence the brackets and supporting studs were well earthed. Reverse electroplating, if you like. Provided the wall was wet (and therefore conductive) enough, current from the protection system was just converting any earthed metal into ions.
With all the rendering stripped, I found after a few months that I got a little bit of salt formation on the bottom couple of feet, but the rest of the wall remained dry. So I made the top of the wall a "feature", had it properly pointed by an expert, and fitted well ventilated wainscotting over the bottom three feet or so. And that has completely cured my rising damp.
I have a few photos documenting this work. I do wonder, from time to time, whether I ought to submit something to the DIY Wiki.
Going back to Colin's original suggestion of non-ferrous metal for the OP's problem, I will confess to wondering whether to suggest brass screws rather than stainless. The trouble is, readily available brass screws will just be simple copper/zinc. Perhaps you need the 1% of tin that is in "Admiralty brass" to improve the corrosion resistance. Or perhaps you need something moving further into the gun-metals and bronzes: not necessarily so easy to find, short of going to chandlers with many pound notes.
I still think that the readily available A2 stainless should work. Disclaimer: I'm not a proper chemist and happy to be corrected.
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On 29/07/2017 21:05, newshound wrote:

Sounds like it would make a nice article - also we have not got much along those lines in there at the moment, so it would be handy...
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John.
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On 7/30/2017 1:03 PM, John Rumm wrote:

Thanks. What do I need to do? Get access privileges from someone? I've never updated a Wiki but I (sort of) know my way around markup languages.
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On 30/07/2017 22:37, newshound wrote:

Drop me an email with your preferred account name, and I will create an account for you.
Wiki markup is pretty straight forward for the basic things. But even if you just slap in text and upload the photos, there will be someone along who can help you tart it up.
Getting started instructions here:
http://wiki.diyfaq.org.uk/index.php/Creating_and_Editing_Articles
(we use the same software as wikipedia; "Mediawiki" - so there are tons of help pages out there)
--
Cheers,

John.
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On 29-Jul-17 9:05 PM, newshound wrote: ...

Which is more likely to cause problems in the long term; assuming it could rust and being wrong or assuming it won't and being wrong?
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On 7/31/2017 10:06 AM, Nightjar wrote:

My eminent old colleague Jack Harris advised on repairs to St Paul's, although I can't immediately locate details of the replacement metal parts.
https://www.theguardian.com/world/2009/mar/05/obituary-jack-harris-scientist
However, the very long studs which now hold together the corners of the tower of York Minster are certainly made from austenitic stainless steel.
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