Can I mix standard and aircrete blocks?

[aa44]

I need to build an internal wall which will have a 45 degree corner in it. I want to use standard concrete blocks for most of it because I have read all those nasty stories about plaster cracking on aircrete walls. It seems that it would be a lot easier to do the 45 degree corner with something like aircrete where I can saw the blocks to the exact shape required. Similarly, where the wall will join the existing external wall, I will need to cut blocks on alternate courses and it would be easier to use the aircrete blocks for the cuts.

Is there any reason why I shouldn't mix the two types of block like this?

 

[noseall]

There is no problem with mixing the blocks, just ensure all are kept dry.

 

[aa44]

Thanks for that.

Why do you say keep them dry? I thought that you were supposed to wet them before use to stop them sucking all the water out of the mortar?

 

[stuart45]

The wetter they are the more they shrink.

 

[noseall]

It is true to say a dampened block will be easier to lay than an excessively dry fella.

However blocks that are saturated will be subject to minor shrinkage when they inevitably dry out.

It is not such a problem with small areas of masonry but can be problematic with a large slab of a wall.

Just make sure the muck is workable, i.e. quite wet, but obviously not running off the spot!

 

[Emeraude119]

Hi aa44 - The conductivity value of an aircrete block will be much lower than the conductivity value of an aggregate bock. Therefore at the interface between the two material types, there will be a significantly different response to internal temperature changes.
Not so much of a problem if you intend to use a dry lined finish over the interface of the two materials, but if you use a wet plaster finish, it is likely that a crack will occur at that interface. (and may re occur every time the internal temperature of the building changes, e.g. when the heating system is turned on / off each year)
It might be worth considering using a dry lined finish, but if you must use a wet plaster finish, the use of a proprietary render mesh, over the interface of the two block types, might help to avoid cracking at the the junction in question.
An alternative `might' be to form a movement joint at the junction, but this must necessarily come through whatever finish is used, where a movement joint / finishes stop bead must typically be used. so probably not the best solution from a visual point of view.

 

[Emeraude119]

Thanks for that.

Why do you say keep them dry? I thought that you were supposed to wet them before use to stop them sucking all the water out of the mortar?

aa44 Masonry product of all types have different suction rates, from engineering bricks which are technically waterproof. to aggregate blocks, which are deemed moderate suction products, to aircrete blocks which are deemed moderate to high suction backgrounds.
Simple tests can be carried out on the day to test the background suction of a wall . and appropriate steps taken to control / modify the suction level,
There is a big difference however, between `docking' blocks (Soaking them before laying) and damping down a built wall prior to applying a wet finish.
Docking often leads to significant cracking ocurring in the walls of newly constructed buildings, which is why it is not recommended.
slight dampening of blocks in very hot conditions might be viable but not more than this (however the Code of Practice for masonry states that blocks of any type that are hot (or wet) should not be used.
To control background suction on a built wall, a `light' spray of water can be used (but only in the area that is about to be plastered / rendered, as no one yet has found a way to plaster a wall in seconds, so if the whole wall is sprayed, parts of it are likely to be drying out again by the time a the plasterer finally gets to it)
Alternatively an internal bonding agent applied in accordance with the manufacturers instructions could be used. Generally PVA types are OK for internal work, but for external work, or rendering / plastering, in wet areas, e.g. changing / shower rooms, an SBR bonding agent should be used

 

[trowlerman]

dry line the wall is the way to go