Aerated Blocks and bond strength

[Will A]

Hi,

 

I've read a number of times that vertical cracking in aerated blocks is often caused by the mortar being stronger than the blocks, this causing the blocks to crack if they shrink, however why is the compressive strength of the mortar a factor in this?

surely its the shear bond strength which would need to be greater in the mortar to prevent the block from cracking when it shrinks.

[Mike]

Aerated blocks just tend to crack, even when laid in accordance with the manufacturer's instructions, including the correct mortar mix. Better to avoid them if you can.

[Russell griffiths]

I would not use aircrete blocks if you gave them away free. 

[Chanmenie]

I laid all my internal walls with durox aircrete blocks and a 6:1 mortar mix, didn’t have a single crack 

Edited November 25 by Chanmenie

[saveasteading]

3 hours ago, Russell griffiths said:

would not use aircrete blocks 

I do. Because I am both feeble and a poor bricklayer, i have infilled a redundant door with them. 3 years on , no cracks.

 

And the most recent use was to jack up the site caravan, because a dozen go on a trolley and into the car.

 

They are not strong.

More importantly they have little interlock. If concrete blocks crack , they have a very jagged interface on the crack, and resist displacement.  Aerated blocks dont have stones so this doesn't happen, and cracks will grow.

[Will A]

My neighbor has the same blocks and none are cracked, both houses built just a few months apart in 2017, my blocks started to crack this year, probably 80% of the blocks to the gables have cracked.

 

However I suspect that the cause of the cracking is due to water ingestion, I can see a darkening around the edges and moving to the center of the blocks which have cracked, most blocks are jumbo length 162mm and contain between 2 and 4 cracks.

 

So my blocks likely shrunk, what I don't understand is why they cracked if the shear bond strength of the mortar and the blocks are both 0.15N/mm, the compressive strength shouldn't be a factor since its the shear strength which holds the block in place and causes it to crack when it shrinks.

 

Even if these blocks shrink but didn't crack wouldn't this then leave gaps between the block and mortar joints?

[saveasteading]

 

 

 

1 hour ago, Will A said:

shear bond strength of the mortar and the blocks are both 0.15N/mm, the compressive strength shouldn't be a factor 

If the blocks and mortar shrink, then something has to give.  

Concrete shrinks after manufacture, bricks expand.

They can be delivered still sopping wet or still hot from the oven.

Leaving them to cure before use will overcome this.

A month sitting on site is a nuisance and a cash flow burden but it works. Alternatively ensure that they are coming from stock, not straight from the factory.

 

I have boldly/ riskilly deleted movement joints on the basis of the materials being matured on site. Proved right or got away with it?

[Chanmenie]

That’s handy to know, my blocks had been sitting on site for several months before I laid them.

I’d heard the main reason for cracking is too strong a mortar mix, so on building sites the labourers won’t do two separate mixes so the bricks and blocks get laid with the same same 4:1 or 5:1 mix which is too strong for aircrete blocks 

[saveasteading]

8 minutes ago, Chanmenie said:

the labourers won’t do two separate mixes

And they don't understand the subject anyway (why would they?) and they fill the mixer by bucket or shovel, not scientifically.

 

An expert can tell me, but isn't a high cement mix much smoother and sticker and easier to use?

 

The story used to be of a bricky having  a pocket full of washing powder, allowing a surreptitious handful in the mix. Perhaps that was preferable to high cement?

[Mike]

1 hour ago, Chanmenie said:

That’s handy to know, my blocks had been sitting on site for several months before I laid them.

Theoretically that helps, but in practice they may still crack.

[Gus Potter]

On 25/11/2024 at 17:57, Will A said:

I've read a number of times that vertical cracking in aerated blocks is often caused by the mortar being stronger than the blocks

Agree.

 

On 25/11/2024 at 19:45, Mike said:

Aerated blocks just tend to crack, even when laid in accordance with the manufacturer's instructions, including the correct mortar mix. Better to avoid them if you can.

Disagree as there is a lack of understanding.. still in the building trade and by designers who have practical site experience.

 

On 25/11/2024 at 19:53, Russell griffiths said:

I would not use aircrete blocks if you gave them away free.

Its your money.. spend as you wish.

 

On 25/11/2024 at 21:27, Chanmenie said:

I laid all my internal walls with durox aircrete blocks and a 6:1 mortar mix, didn’t have a single crack

Can be achieved.. so there!

 

Ok to sum up.

 

Look after your Thermalite et all blocks when they get delivered to site. Use the right strength of mortar with plasticiser. Use a brickie that is properly trained.

 

 

 

21 hours ago, saveasteading said:

More importantly they have little interlock. If concrete blocks crack , they have a very jagged interface on the crack, and resist displacement.

Now we are getting technical. We call this aggregate interlock.. same thing happens in concrete floor slabs and how you can design joints in slabs taking advantage of the aggregate interlock.

 

14 hours ago, Will A said:

most blocks are jumbo length 162mm and contain between 2 and 4 cracks.

This is a clue.. but there are loads of other factors.

 

3 hours ago, Chanmenie said:

I’d heard the main reason for cracking is too strong a mortar mix, so on building sites the labourers won’t do two separate mixes so the bricks and blocks get laid with the same same 4:1 or 5:1 mix which is too strong for aircrete blocks 

Here lies a big clue!

 

3 hours ago, saveasteading said:

An expert can tell me, but isn't a high cement mix much smoother and sticker and easier to use?

 

The story used to be of a bricky having  a pocket full of washing powder, allowing a surreptitious handful in the mix.

Depends on what kind of cement you buy. Most brickies unless properly trained have no clue, I caught one the other day with a bottle of Fairy liquid lying beside the mixer.. used to entrain air.. this is a no no!

 

 

 

 

 

[SteamyTea]

I am surprised that the 'lime brigade' have not made a suggestion, they claim so much of it.

 

10 hours ago, Gus Potter said:

We call this aggregate interlock

That is interesting.

I firmly believe that it is not the cement, or lime, itself that is the problem, but more a case of what it is mixed with and in what ratios.

[Brickie]

I’d suggest a 1:1:6 mix is about right for aerated. In summer months,I ideally like to wet the top of the course below before bedding out,to kill the initial suction grab much the same way as old school plasterers would. 

[Will A]

 

These are the gable walls, the black lines are where the crack run

[Mr Punter]

Get on to the manufacturer and see what they have to say.

[Will A]

59 minutes ago, Mr Punter said:

Get on to the manufacturer and see what they have to say.

They said that the blocks shouldn't shrink 5 years after the drying out process would have completed and advised that I have a structural inspection performed, my house is under NHBC warranty so they agreed to have one done, not that I hold much hope of that report being unbiased, once I have that I'll have my own performed