Aerated block - cracking?

[Moonshine]

I am looking to use aerated blocks as the internal leaf of a fully filled external cavity masonry wall.

However I have heard that they can crack easily and also you have to dot and dab onto them.

How secure are screw fixings into them?

Has any one got any practical experience of using them in this way and issues they had?

Edited September 30, 2020 by Moonshine

[Oz07]

I've got good method of fixings. They are bad for cracking. 

[joe90]

I hate them and used ordinary blocks fir my inner skin (but went for 200mm cavity). I too have come across many cracked aerated blocks but perhaps they were laid badly. I have had problems fixing heavy items to them.

[Carrerahill]

1 hour ago, Moonshine said:

I am looking to use aerated blocks as the internal leaf of a fully filled external cavity masonry wall.

However I have heard that they can crack easily and also you have to dot and dab onto them.

How secure are screw fixings into them?

Has any one got any practical experience of using them in this way and issues they had?

I don't like them, they are basically made from a fines only "concrete" mousse, i.e. the slurry has heaps of air entrained into them. They are usually more expensive than ordinary blocks so unless weight was an issue I'd always just use block, they are not as strong, they are actually worse for the environment than ordinary blocks due to the high energy costs for preparing the materials for use in the "concrete". You also can't get a reliable structural fixing into them either as under high loads the bearing area tends to crush opening up the hole over time, perhaps something can be done with epoxy however the block is so wimpy you'd never trust anything of any importance on it.

 

 

[Moonshine]

48 minutes ago, Carrerahill said:

They are usually more expensive than ordinary blocks so unless weight was an issue I'd always just use block

 

you get a better U-value performance from them, which drew me to wanting to use them, though i may ditch them for a standard lightweight block for a 0.01 increase in U-value

[Mr Punter]

I prefer medium density aggregate blocks.  They are inexpensive, easy to fix to, easy to render / plaster.  Have you worked out the difference in heating costs with the aero ones?  I suspect insignificant.

[Moonshine]

37 minutes ago, Mr Punter said:

I prefer medium density aggregate blocks.  They are inexpensive, easy to fix to, easy to render / plaster.  Have you worked out the difference in heating costs with the aero ones?  I suspect insignificant.

 

I haven't but the wall u value goes from 0.17 to 0.18 and I am more concerned with the overall SAP score

[Mr Punter]

If you have a decent SAP person you will pass.  There is no test at the end other than airtightness.  Sometimes using different lintels can help the SAP rating for thermal bridging.

[MAB]

12 hours ago, Moonshine said:

I am looking to use aerated blocks as the internal leaf of a fully filled external cavity masonry wall.

However I have heard that they can crack easily and also you have to dot and dab onto them.

How secure are screw fixings into them?

Has any one got any practical experience of using them in this way and issues they had?

 

It is important that the mortar mix is correct and not too strong or it can cause cracks.

There is a golden rule in the building industry that states ‘mortar should never be stronger than the material it is joining’.

Some good advice here:-

https://skill-builder.uk/aircrete-blocks

 

These Fischer Duopower wall plugs work well:-

https://www.screwfix.com/p/fischer-duopower-wall-plugs-6mm-100-pack/1030P?tc=KT1&ds_kid=92700048793290424&ds_rl=1249413&gclid=EAIaIQobChMI3oXXyb6R7AIV0-ntCh10cQq9EAQYASABEgIxmPD_BwE&gclsrc=aw.ds

 

 

[MJNewton]

4 hours ago, MAB said:

These Fischer Duopower wall plugs work well:-

https://www.screwfix.com/p/fischer-duopower-wall-plugs-6mm-100-pack/1030P?tc=KT1&ds_kid=92700048793290424&ds_rl=1249413&gclid=EAIaIQobChMI3oXXyb6R7AIV0-ntCh10cQq9EAQYASABEgIxmPD_BwE&gclsrc=aw.ds

 

As do Fischer's UX range that expand into whatever shape is required to suit 'problem' materials. I struggled with the cheapest of cheap generic wall plugs for years until discovering these...

Edited September 30, 2020 by MJNewton

[farm boy]

Thought I would add to this thread.

I ve used 3.6kn aerated blocks on my internal leaf of my cavity walls and I have fine vertical cracks in lots of places, they are worse on the sw wall. Nothing showing on the external brickwork. Not impressed with them, but would not of achieved an acceptable U value on walls without them.

 

House is built on concrete raft so not due to settlement but thermal expansion or moisture content change. Planning to wet plaster, may put expanded metal mesh over the cracks first and dot and dab the sw wall.

Cannot believe my house has cracks before I have had chance to finish the dam thing!

 

[Wagas]

Are Fibolites aerated? I used them on a previous build and had one or two cracks but nothing major 

[Moonshine]

8 minutes ago, Wagas said:

Are Fibolites aerated? I used them on a previous build and had one or two cracks but nothing major 

 

No they are ultra lightweight aggregate blocks.

Good thermal values for them, may look to use them rather than besblock ultralites

[tonyshouse]

Aerated blocks crack and can shift around , they always crack, and sadly let air leaks in. Avoid using them in your own house. 

[Digmixfill]

I've got a few cracks in the celcons here. The walls that where put up when the blocks were wet have cracks, the walls built with blocks that were fully dry are OK.

It appears as though they shrink as they dry out.

[Brickie]

On 30/09/2020 at 19:29, MAB said:

 

It is important that the mortar mix is correct and not too strong or it can cause cracks.

There is a golden rule in the building industry that states ‘mortar should never be stronger than the material it is joining’.

Some good advice here:-

https://skill-builder.uk/aircrete-blocks

 

These Fischer Duopower wall plugs work well:-

https://www.screwfix.com/p/fischer-duopower-wall-plugs-6mm-100-pack/1030P?tc=KT1&ds_kid=92700048793290424&ds_rl=1249413&gclid=EAIaIQobChMI3oXXyb6R7AIV0-ntCh10cQq9EAQYASABEgIxmPD_BwE&gclsrc=aw.ds

 

 

+1

[joe90]

I did not use airated blocks as I have seen so many crack on various jobs, Mortor may account for some but also fixing to them is not rubbish. 

[Digmixfill]

Mortar was the same 5:1 for all walls. Only the wet blocks have cracked so far. I'm going to keep an eye on the rest to see if that stays the case.

I'm trying to move blocks indoors in rotating stacks to dry them before using them. It's only a matter of time before I forget to replenish a stack and have to use moist blocks.

[Conor]

This explains why our builder stored all the blocks under weighted DPM...

[tonyshouse]

I never used or will use aerated blocks, too many cracks, even slamming a door will cause them to crack, thermal movements, waekness points, under windows, above doors, beside lintels all common for cracking

 

in contrast, medium density blocks virtually no cracks. - ten years on too 

[Digmixfill]

I've already bought all the blocks I think I'll need - celcon standards. Committed.

I'm hoping to have wet plaster when it's all done, if not I'll parge. Hopefully the current cracks will not be an issue for me.

[Brickie]

11 hours ago, Digmixfill said:

Mortar was the same 5:1 for all walls. Only the wet blocks have cracked so far. I'm going to keep an eye on the rest to see if that stays the case.

I'm trying to move blocks indoors in rotating stacks to dry them before using them. It's only a matter of time before I forget to replenish a stack and have to use moist blocks.

Personally I’d use 1:1:6 (cement:hydrated lime:sand). 
The lime should make the mortar a little more forgiving with those blocks. 
Might be worth talking to your architect/engineer/bricklayer about introducing some movement joints too. 

[Digmixfill]

The longest wall is 4.4m. I wouldn't have expected to need a movement joint in that. I thought they were only required at >= 6m ?

I am the bricklayer There are some pictures of my finest amateur work in the thread here

 

You can see in the pictures there that some of the blocks are very wet, much darker than the dry blocks.

 

If it is the blocks shrinking as they dry would the lime cause the cracking to be in the perps and not the blocks?

 

[MarkyP]

I used lightweight aggregate blocks, avoid aerated. Aerated is one reason why the volume builders dot and dab -  to hide the cracks. Can you not widen the cavity to improve the uvalue if needed and add more insulation? Wet plaster best for air tightness and a nice solid finish, good to fix to. My brickie said he really doesn't like aerated, very high suction and mortar can dry before it's cured in summer. Cracking seems to be brittle nature of the blocks, they have standard compressive strength 3.6n 7n, etc,  but weak flexural strength.

[Digmixfill]

10 hours ago, MarkyP said:

Can you not widen the cavity to improve the uvalue if needed and add more insulation?

I'm already above DPM all around and lower slab is in. Have to make the best of what I have.

 

10 hours ago, MarkyP said:

My brickie said he really doesn't like aerated, very high suction and mortar can dry before it's cured in summer.

With my very limited experience of them I concur. During the warmer months I find that I have to dampen the block surface I'm laying on before mortar gets near them.