Cracks in aerated blocks - celcon and thermolite

Discussion in 'Building' started by Rob broom, 14 Apr 2021.

  1. Rob broom

    Rob broom

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    Hi!

    Would anyone be kind enough to offer some advice?

    i am currently working on a 1 & 1/2 story extension to my house (single story with a room in the roof).

    I’ve followed the instructions from the architect, structural engineer and BCO throughout and havnt cut corners!

    ground underneath was solid, with no clay.

    concrete in footings is 650mm wide and 400mm deep and and around a metre underground.

    All foundations were specified by a SE and My BCO came and inspected the footings prior to concrete fill and he also inspected when I was at DPC, and was happy with everything.

    I have finished all of the blockwork and I am now working on the roof.

    I have recently noticed a few cracked aerated blocks, mainly centrally under the windows.


    The cracks are hairline, consistent in width and are less than 1mm.

    I have been building the walls since September last year, on and off, with them being exposed to the elements all that time.

    I am hoping that the cracking is just caused by the usual settlement and thermal reasons.

    I can still see two courses of blocks below DPC all around the extension and there are no problems in the foundations.

    I just wanted to know if anyone else has seen this before, particularly under windows and can either put my mind at risk or suggest a fix? Should I chop the cracked blocks out and replace them? Or just dot and dab over them and forget them?

    Many thanks

     
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  3. stuart45

    stuart45

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    Shrinkage cracks are common with AAC blocks.
     
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  4. Rob broom

    Rob broom

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    Thanks.

    yeah, I’ve read that a million times but don’t ever see anyone mentioning the issue happening under windows.

    one crack is not near a window, it is in a larger area of wall at ceiling height and passes through through 5 courses of blocks in total. Going through three blocks and 2 mortar joins. No corresponding crack in the outer leaf I might add.

    I have got it under 2 windows. I’m hoping that that is just due to the settlement of the loading at either side of the window reveal?m that has caused this block to let go?

    As I am no where near fitting windows yet, I can do any rectification work required, so if people think I should swap out the broken blocks for new ones, I will. and maybe chase in some mesh into the mortar bed either side of the new block and re point perhaps?

    Thanks again!
     
  5. ^woody^

    ^woody^

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    Everyone who has laid these and has a wall of these blocks has seen these cracks.

    I've got a wall here, its five blocks long I stored the blocks correctly, mixed the mix correctly, laid the blocks correctly, protected it after building correctly and still the damn thing cracked.

    I've built other walls and included ex-met bed reinforcement and still the wall cracked. Winter, summer, dry, wet, fresh open, old stock, banged about, kid gloves you name it, there is no solution the blocks are complete shiite.

    You can't win with these things, just embrace it. Its a once only shrinkage crack, no need to replace blocks.
     
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  6. Rob broom

    Rob broom

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    Ok cool!

    Thanks for that woody!

    reassuring that it is common!

    Hopefully that will let me sleep a bit tonight!

    Where the cracks run through multiple courses, going through the vertical mortar joints (above and below the cracks in the blocks), is it worth spending half an hour taking the mortar out and repointing?

    cheers
     
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  8. stuart45

    stuart45

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    I've found that using a 1/2/9 OPC/lime/sand seems to help keep the cracks in the joints more than going through the blockwork.
     
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  9. Rob broom

    Rob broom

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    Ok cool!

    Will bear that in mind for future mixes!

    cheers
     
  10. Rob broom

    Rob broom

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    Thinking a bit more about it, I’m hoping that the cracks are under the windows, because the walls are weaker there.

    Being that that the crack has an easy place to start in a window because the top block doesn’t have one on top of it to strengthen the join.

    The cracks under my window reveals all start in the mortar join of the upper most course under the window. It then passes directly through the next course below, splitting that block (thin/hairline), then a little way down the next mortar join.

    the weather was rather miserable when I was building those section, although I never actually worked in the rain!

    My “rookie” confession here is that the blocks were not kept under a sheet and dry in storage and were open to the elements! Maybe that has lead to them being rather wet when I was laying them and lead to shrinkage?

    fingers crossed that’s all it is!
     
  11. Notch7

    Notch7

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    You wouldn't want blocks too dry as they would suck too much on the mortar.

    Water in the blocks certainly is a key factor in subsequent shrinkage, but it's the weakness of the blocks rather than anything you've done wrong.

    I've seen loads of building works and pretty much all lightweight blockwork gets cracks in it - sometimes floor to ceiling.
     
  12. Rob broom

    Rob broom

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    Cool!
    Happier now!!

    just spoke to my BCO and he said that I shouldn’t worry too much!

    he’s coming out to site tomorrow morning to see my roof etc.. so he will see it with his own eyes then!


    I’m now going around the whole build with a fine tooth comb, and I’ve found a hairline stair step ladder crack one place in the top 4 courses of of the exterior wall.
    It’s in the mortar, and those blocks were only laid within the last to three weeks.
    They are south east facing, so I’m hoping that that is just a little bit of shrinkage again?

    or ground settlement maybe?
     
  13. DIYnot Local

    DIYnot Local

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