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Green Building Bible, Fourth Edition
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  1.  
    We've had a very strange failure on our render. Nobody seems to know what it might be. So I thought I would draw on the knowledge available here.

    One one wall, mainly in one area, in the topcoat, there are a series of very round render failures, where the render has come away from the wall. It sounds hollow when you tap it and the round 'disk' comes away from the wall very easily. See photos.

    The bottom coat was a sprayed, meshed, NHL2 with limestone aggregate and silica aggregate (the latter to help the spray pump). The top coat is a coloured, proprietary, lime render, supplied in bags.

    Does anybody have any ideas? Any leads would be very gratefully received.
      IMG_20170919_150806862.jpg
  2.  
    This is the most serious area:
      IMG_20170919_151032775.jpg
  3.  
    Here's a close up of a blown one that has not been knocked out.
      IMG_20170919_151246836.jpg
  4.  
    No idea at all
    Just a thought - some sort of contamination in the top coat preventing adhesion. Thought based on colour difference.
    Is the area affected approx. the area of one bags worth?
    • CommentAuthortony
    • CommentTimeSep 20th 2017
     
    I would say either over working of the top coat or drying out too fast
  5.  
    @Peter, yes, we were thinking of some kind of contamination. The spots are perfectly round as if some kind of impurity has been carried through by capillary action.
  6.  
    Posted By: tonyI would say either over working of the top coat or drying out too fast

    I'm not sure about that. It's on a wall that gets morning sun, and was done on Friday. It wasn't particularly hot or windy since then, and the labourer said she noticed it while dampening the wall on Sunday.
    • CommentAuthortony
    • CommentTimeSep 20th 2017
     
    Delamination caused by overworking with wooden float then,

    It looks to me like the basecoat was not scratched to provide a key for the top coat.
  7.  
    Posted By: tonyDelamination caused by overworking with wooden float then,

    It looks to me like the basecoat was not scratched to provide a key for the top coat.

    If so, why is it only in this one area of the wall? The basecoat was well scratched all over, I saw it.
  8.  
    I've found this article from the Rose of Jericho which says: 'Sulphate Attack
    Hydraulic lime renders are vulnerable to the soluble sulphates of various bases. Sodium
    sulphate for example present in groundwater and some unwashed aggregates will react
    with any free lime to produce calcium sulphate. This more than doubles the solid volume
    and expansion and cracking occurs.
    Calcium sulphate will react with hydrated calcium aluminates, particularly tri-calcium
    aluminate, present in many hydraulic limes, to form calcium alumino-sulphate. This
    again more than doubles the solid volume and expansionary damage results. A hydraulic
    lime render failure involving massive expansion in the south of England has been
    attributed to sulphate attack.
    This largely explains why only washed sands and additives with a low sulphate content
    should be used with hydraulic limes (and cements).'

    However, if it is impurities, these are likely to have been in the basecoat. Yet surely they should have manifested themselves there, rather than in the top coat?
    • CommentAuthortony
    • CommentTimeSep 20th 2017
     
    Over working then
  9.  
    There was also a hail storm the following day, so that is a possibility, though I'm not sure the pattern matches the exposure to the weather.
    • CommentAuthorMike1
    • CommentTimeSep 20th 2017
     
    I'm not a plastering expert, but due to the circular pattern I wonder if it may be either over-trowelling, or trowelling before the coat had adequately bonded.
    •  
      CommentAuthordjh
    • CommentTimeSep 20th 2017
     
    Posted By: ComeOnPilgrimThe basecoat was well scratched all over, I saw it.

    I don't see any sign of it in that picture where the topcoat has come off?

    Have you asked the lime supplier?

    How was the topcoat compressed onto the basecoat?

    Find a local expert and get them to take a look.
  10.  
    Update: it looks like the issue was with the basecoat as it matches up with the scaffolding lift that was in place at the time.
  11.  
    I've followed up a lot of leads. So far, nothing definite. One suggestion is unslaked lime in the basecoat.
  12.  
    Looks like over sponging of the top coat. ie sponge float to wet when put on top coat and has drawn lime to the surface making mix weaker.
    •  
      CommentAuthordjh
    • CommentTimeSep 21st 2017
     
    Posted By: ComeOnPilgrimOne suggestion is unslaked lime in the basecoat.

    I think unslaked lime (i.e. quicklime) is unlikely in a packaged product but it's quite possible for powdered lime in bags to start carbonating and so become less useful. Whetehr that's a possible cause here, I've no idea.
  13.  
    Posted By: djh
    Posted By: ComeOnPilgrimOne suggestion is unslaked lime in the basecoat.

    I think unslaked lime (i.e. quicklime) is unlikely in a packaged product but it's quite possible for powdered lime in bags to start carbonating and so become less useful. Whetehr that's a possible cause here, I've no idea.

    The strange pattern of perfect circles seems to be a clue, ie, expansion below the surface. The Scottish Lime Centre Trust say they have experienced unslaked lime in the particular bagged product that we used for the basecoat.
  14.  
    Posted By: djh
    Posted By: ComeOnPilgrimThe basecoat was well scratched all over, I saw it.

    I don't see any sign of it in that picture where the topcoat has come off?

    Have you asked the lime supplier?

    How was the topcoat compressed onto the basecoat?

    Find a local expert and get them to take a look.

    I've asked the lime supplier and provided them with details.

    The render was sprayed on.

    Other than the possibility of unslaked lime, the experts over spoken to aren't really sure.
  15.  
    Are you saying the topcoat was sprayed on but not leveled with a float. It seems very smooth for a spray coat. If it is just a spray coat then its a very poor mix.
    •  
      CommentAuthordjh
    • CommentTimeSep 22nd 2017
     
    Posted By: ComeOnPilgrimThe render was sprayed on.

    Even if it is sprayed on, it still needs compressing afterwards, and probably more than once as it firms up over a few days.
  16.  
    Posted By: renewablejohnAre you saying the topcoat was sprayed on but not leveled with a float. It seems very smooth for a spray coat. If it is just a spray coat then its a very poor mix.

    It was sprayed on and then levelled with a float as it firmed up.
  17.  
    I previously thought that the explanation could lie in the aggregate because we took new deliveries for the final wall. However, the wall on which the trouble started was not the last to be rendered. This means that it had the same render as most of the rest of the house, yet there were no issues elsewhere.

    One cause might be the heat from the sun, but the marks do not line up with where one would expect the heat to arise. Areas that are shaded by the scaffolding have issues, but areas that were not shaded do not seem to.
  18.  
    Also updating this thread. I'm now sure the cause was unburnt lime in the undercoat supplied by a certain Portuguese lime supplier. From time to time, I see 'bubbles' appearing in the wet lime. You can work them out, but I suspect this wasn't done with our top coat render.

    Whisper me if you'd like to know more details.
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