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Aircrete Blocks - Spares With Cracks


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#1 MikeGraham

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Posted 04 February 2016 - 02:42 PM

Hi,

I've got a pile of spare blocks left over from the extension I built last year, which I had planned to use on the garage this year, they are stacked on block paving, on their sides, piled about 15-20 high. Just brought the bin in, which lives next to the pile, and noticed that, not the bottom one, but the two directly above that, has about 5 hairline cracks in them (you can see them as parts are damper than other), vertical (in the sense that they are laid on their side). They have roofing felt over the top of them, but i wouldn't say they are bone dry all of the time.

Any ideas what might have caused this, and are they any good for use?

Hoping we haven't had a bad batch, some of the other blocks had vertical (when stood up properly) fissures (not really cracks) in them, we had one or two of them which got used in the build, not had any problems with them, and i don't think they are cracks as such, but any idea what could be going on? They were in from day 1, still a few with the same markings in on the pile.

Mike

#2 jsharris

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Posted 04 February 2016 - 04:39 PM

Could they have been frozen in the cold weather?

Water can fairly easily soak into these blocks and as they aren't that good in terms of tensile strength if it then freezes it could well expand and crack the blocks. It doesn't need to whole block to freeze, either, just a thin layer of ice on the outside will be enough to hold water in the block. Water has a maximum volume at around 4 deg C, so can expand inside as it cools and if the outer surface has frozen you could get an internal pressure build up that might be the cause.

#3 MikeGraham

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Posted 04 February 2016 - 04:49 PM

Its certainly a possibility, they aren't near a source of heat, and look to be wet most of the time, being bottom of the pile.

I spoke to the block manufacturer about this too, and they said something I wasn't expecting.

Apparently, the blocks are designed to be used and stacked in 1 way only, and if the blocks are laid on their side, they have only about 15-35% of their compressive strength as tensile strength. Now I thought tensile strength was more things being pulled apart, like tearing a piece of paper? When I first got these blocks they were stacked on their sides (which seemed the most sensible way of stacking them for stability) for around 23days in total.

How are the blocks on the outer leaf going to be faring? They are painted with breathable masonry paint, but other than that are open to the elements, and the extension is currently unheated bar the bit which escapes from the house. Hoping having them like they are isn't going to be causing them any damage? According to the blurb, these blocks are meant to be immune from the freeze/thaw cycle? And also closed cell.

#4 declan52

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Posted 04 February 2016 - 04:54 PM

If there are any tiny stones under the blocks they won't sit flat so you can get cracks as you stack more weight on top. Happens with all types blocks when you stack them up.

#5 MikeGraham

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Posted 04 February 2016 - 04:58 PM

I did sweep up before, but doubt i'll have done it perfect. Should I restack on the normal edge?? Obviously i'll bin any cracked ones, not worth it.

#6 jsharris

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Posted 04 February 2016 - 05:02 PM

View Postdeclan52, on 04 February 2016 - 04:54 PM, said:

If there are any tiny stones under the blocks they won't sit flat so you can get cracks as you stack more weight on top. Happens with all types blocks when you stack them up.

Makes a lot of sense. Pretty hard to keep things dead clean, and it's logical that the blocks aren't anywhere near as strong with a side load.

#7 MikeGraham

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Posted 04 February 2016 - 05:04 PM

OK, so worth restacking them to keep them in good shape? Mind they've been there for about 6months now

#8 MikeGraham

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Posted 04 February 2016 - 05:06 PM

One thing I don't get about the side loading is, surely the structure of the block is the same any way on? I'm sure I must be wrong about that, hoping I've not done anything really bad, more so to the ones which are already part of the extension. I've re-scoured the manuals, and no where does it warn about the way to stack them, just about keeping them dry, which I did.

#9 declan52

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Posted 04 February 2016 - 05:12 PM

Don't bin them they can be used for cuts. You can stack blocks on there edge or lying flat but just make sure they are clean so ly flat so all the weight transfers straight down to the ground. Just have a look to see if any are sitting up slightly on a few stones.
Another possible cause is the blocks where still green when they where delivered so hadn't fully hardened up.
I used them on my build and I thought they where much stronger than an ordinary block. Took a few more bangs with the hammer and bolster to break them.

#10 oz07

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Posted 04 February 2016 - 05:47 PM

They are harder to cut than normal blocks, also I thought according to manufacturers these were very resistant to thaw/freeze cycles?

#11 MikeGraham

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Posted 04 February 2016 - 06:06 PM

Dare I ask about hanging kitchen cabinets off of them when the plasterer has done dot and dab?

I'm guessing the weight in cabinet kept to a minimum, and also thought about injecting plasterboard adhesive in the gap between board and wall? Or maybe some decorative wooden posts under the cabinets on top of worktop to give extra support?

#12 declan52

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Posted 04 February 2016 - 06:09 PM

The fixing will have to be long enough to go into the blocks.

#13 MikeGraham

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Posted 04 February 2016 - 06:35 PM

Absolutely, they are going to be 100mm minimum to make a min 50mm penetration, want to want roughly in the centre of the block

#14 daiking

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Posted 04 February 2016 - 06:48 PM

I've been reading up on fixings for aircrete blocks. Nothing particularly weird but needs to be more than standard rawl plugs from what I've read.

Are you using anything specific?

(I'm sure it's covered in thread here as well)

Edited by daiking, 04 February 2016 - 06:51 PM.


#15 MikeGraham

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Posted 04 February 2016 - 08:24 PM

To be honest I think I'd just stick with manufacturers recommended ones, they all have tables showing the different fixings for various loads. I think the 'old fashioned' hammer through fixing tends to rate highly, Fischer. Spax bolts are OK for windows, but no good for kitchen cupboards.

I found in the past if you use the wrong fixing you can lose confidence, but use the right fixing and its as good as anything else

Edited by MikeGraham, 04 February 2016 - 08:25 PM.


#16 joe90

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Posted 05 February 2016 - 11:33 AM

Is it possible to get your plasterer to put a solid line of dobs at fixing height? I have had problems fixing cabinets where it's been done this way, if your fixing is in between dobs it tends to pull the plasterboard into the void.

#17 MikeGraham

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Posted 05 February 2016 - 03:16 PM

You could indeed yes, I'm going to inject some driwall adhesive into the plasterboard holes before drilling through to get around this.

#18 jsharris

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Posted 05 February 2016 - 04:30 PM

Better still, why not bin the dot'n'dab "plasterboard tent" and, after making the block wall airtight, fix the plasterboard with pink grip or similar. Quicker than dot'n'dab, avoids airflow behind the "plasterboard tent" and allows the plasterboard to sit tighter against the blockwork, reducing the problem with fixings potentially pulling the plasterboard into a void between the dabs.



#19 declan52

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Posted 05 February 2016 - 04:42 PM

Or even better mix some sand and cement and scratch coat the wall then skim finish it.

#20 jsharris

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Posted 05 February 2016 - 05:20 PM

View Postdeclan52, on 05 February 2016 - 04:42 PM, said:

Or even better mix some sand and cement and scratch coat the wall then skim finish it.

Without a doubt the best way. Dot'n'dab really should be outlawed, as anyone who's pointed a thermal imager at a dot'n'dab wall will probably confirm.