aircrete block fixings

[Tul]

Aircrete blocks!! I am building a new build and finding it a bugger to make decent fixings, so far I have found 3" wood screws are my best bet screwing into a small pilot hole. I am thinking of using injected resin for stronger points or using a wood plate with several screws and then fixing to that. What do you builders do now you have to use these things to meet SAP? Also expansion cracking under windows etc should I ignore or repair?

Thanks in advance

 

[simonjay]

Aircrete blocks!! I am building a new build and finding it a b*****r to make decent fixings, so far I have found 3" wood screws are my best bet screwing into a small pilot hole. I am thinking of using injected resin for stronger points or using a wood plate with several screws and then fixing to that. What do you builders do now you have to use these things to meet SAP? Also expansion cracking under windows etc should I ignore or repair?

Thanks in advance

What are you trying to fix ?

Several fixings spread over an area can help distribute the load.

A screw into the block (no pilot hole required) is OK for light loads, and loads at right angles to the screw where there is no pullout force. I fixed the wall plate restraint straps this way. When you do this, keep pressure on the screw so it does not chew up the thread it has just cut.

There are special hammer-in fixings for aircrete but I didn't have much success with my aircrete blocks that was the slightly denser version.
I have found good old frame fixings are actually fine. You could also get long rawlplugs or cut the ends of frame fixings if needed. Make sure you get ones with quite a knobbly profile and drill the hole a half mm smaller for a tight fitting.

Or resin fixings for more strength (make sure all the dust is removed). I've also heard of folks who squirt in a bit of no-more-nails before banging in a plug or frame fixing just to be sure.

You should run expanded metal reinforcement in the mortar every few rows and below / above openings so expansion cracks do not run too far.
Having said that, I had a corner that cracked nearly all the way up. Folks say don't get them wet to avoid this, but have you ever seen a building site that does not get rained on ?
But as long as you are building a cavity wall, the wall ties keep it all together I guess.
And, expansion cracks are generally not straight across a block but at funny angles that restrict movement too much.
I also replaced the mortar in a few places where shrinkage had pull the block away.

Also, you don't HAVE to use them, as long as the overall wall meets the U values - the walls might be quite thick though. And bear in mind there is a maximum cavity size for normall wall ties.

Simon.

 

[Tul]

Thanks for that, I used woodscrews myself for the wall plate ties. I haven't tried standard frame fixings so I'll give that a go. Too late for the expamet I'm afraid for the cracking, but wondering if I should run repairs like stitching in cracked brick or might I just create another crack elsewhere? I was thinking of using some wall ties horizontally across the cracks but it's all well tied to a dense block outer so I might just leave as is.

 

[Emeraude119]

Tul - The fixing technique for aircrete blocks is different to that required for aggregate blocks. nevertheless a sound fixing can be obtained using ordinary
plas plug type fixings.
Use a wood bit in the drill for the pilot hole, not a masonry bit. They are just not needed for aircrete.
Any fixing must penetrate the block by at least 50mm (more if possible) so take account of the thickness of the finish when selecting a plug length.
Do not use a hammer action drill, Again just not needed for aircrete.
And now the important bit, use a pilot hole drill bit which is at least 1 or 2 mm less in diameter than that of the plug.
You can tell a fixing is going to work, if it can then be pushed using thumb pressure by about a third to a half its length into the hole but which then needs to be tapped flush with the wall surface with a hammer.
There are also dedicated aircrete fixing types e.g. Fischers GB twist lok anchors, which are specialized finned plas plug type fixings.
Resin anchors don't seem to work so well in aircrete blocks, especially if the hole has not been properly blown out to clear the swarf, and slight recession of the resin occurs on drying which means the fixing may not necessarily pull out, but may also never be tight.
The strongest fixing in an aircrete block would be something like Fischers
FIM cement grout anchors, as if these pull out, they usually take most of the surrounding block out with them.

 

[Tul]

Thanks for that. It's a while since my post and since then the house is complete. I ended up using insulated plasterboard dot and dabbed to the aircretes. I tend to use plasterboard fixings for most things. The method of an undersized drill hole and standard rawlplug works as well as anything, the only other advice I'd pass on is Nip up - don't overtighten. My curtain poles are still OK after 2 years.