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

[Gus Potter]

On 29/11/2024 at 15:17, Will A said:

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

Help ma boab ( Scottish expression for surprise / aghast)!

 

This needs investigating properly.

On 29/11/2024 at 17:31, Mr Punter said:

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

Agree. Send them your marked up photo as I think this will grab their attention. Also send a photo unmarked.

 

On 29/11/2024 at 18:41, Will A said:

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

See what the NHBC and the manufacturer say first.. let them do some leg work before you get an independant SE involved.. which may not be required... but you may want a second opinion anyway.. will be cheeper for you if the donkey work is done by those who build it and underwrote it.

 

Question is.. that's the bit you can see.. what about the rest that you can't see! Sorry to say it but..

 

 

[Brickie]

Is there a steel ridge beam?

[Will A]

On 04/12/2024 at 20:43, Gus Potter said:

Help ma boab ( Scottish expression for surprise / aghast)!

 

This needs investigating properly.

Agree. Send them your marked up photo as I think this will grab their attention. Also send a photo unmarked.

 

See what the NHBC and the manufacturer say first.. let them do some leg work before you get an independant SE involved.. which may not be required... but you may want a second opinion anyway.. will be cheeper for you if the donkey work is done by those who build it and underwrote it.

 

Question is.. that's the bit you can see.. what about the rest that you can't see! Sorry to say it but..

 

 

Thanks Gus, It was the manufacturer who said that the blocks shouldn't shrink 5 years after the drying out process should have completed, that said the blocks appear to have a darker shading around there edges which looks like moisture, the outer leaf is brick which we discovered has been laid frog down and unfilled, plus a large amount of the bedding joints are recessed on the cavity side and the perps are mostly tipped and not fully filled, on the outer joints it looks like some repointing was done and you can see chipping to the edges of bricks where a tool was used to removed the mortar, petrographic tests on mortar samples from the joints appears to show that a second layer of mortar was applied, samples from the edge and the middle show very high air/void content in the mortar (between 32 and 40%).

 

There's not insulation within the gable cavity, which I understand isn't required, so I'm wondering if wind driven rain is penetrating the outer leaf and soaking the inner leak of blocks, although that seems a stretch.

 

Some of the blocks sound hollow if you knock on them so I suspect that those are cracks in their width, assuming that the vertical cracks are caused by shrinkage then it seems probable that the shrinkage would be equal to the blocks width, heigh and length, all 4 roof verges have failed and rain water is leaking in, so maybe the heigh shrinkage has created a small gap to the verges, we've also had around a dozen roof tiles crack along their join channel, which I suspect is being caused by a downwards pressure, the leaking first appeared towards the end of last year and the cracking to the blocks started around Jan this year, the cracking stopped around July but has started again in the last month.

[Will A]

21 hours ago, Brickie said:

Is there a steel ridge beam?

No I don't believe there is

[Gus Potter]

13 hours ago, Will A said:

Thanks Gus, It was the manufacturer who said that the blocks shouldn't shrink 5 years

This is going to take you time to sort out.. allow for 1 - 2 years, if sooner then you have won a watch.

 

For all if you have this kind of thing get a hold of the NHBC and the Blockwork folk. Make sure you identify the blocks!

 

Each well known manufacturer has a different scratch pattern on their blocks. The scratch pattern identifies the manufacturer and the block strength.

 

I've screenshotted the scratch pattern for Themalite so you can see what I'm on about.

 

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,

13 hours ago, Will A said:

petrographic tests on mortar

 

But.. what's going on here @Will A.. are you declaring all the info you have. If you want a bit of advice then can you expain how the petrographic test came about? Who paid for that? Where are you in terms of the NHBC / Builder?

 

[Will A]

9 hours ago, Gus Potter said:

This is going to take you time to sort out.. allow for 1 - 2 years, if sooner then you have won a watch.

 

For all if you have this kind of thing get a hold of the NHBC and the Blockwork folk. Make sure you identify the blocks!

 

Each well known manufacturer has a different scratch pattern on their blocks. The scratch pattern identifies the manufacturer and the block strength.

 

I've screenshotted the scratch pattern for Themalite so you can see what I'm on about.

 

 

But.. what's going on here @Will A.. are you declaring all the info you have. If you want a bit of advice then can you expain how the petrographic test came about? Who paid for that? Where are you in terms of the NHBC / Builder?

 

 

Hi, I'm pretty sure that the blocks are from H+H and are either Aircrete or Celcon, they have a green stripe on their edge which seems to indicates that they are 2.9N/mm.

 

Around a year after we bought the house (new build 2017) we noticed that mortar joints were falling out, just 4 or 5mm deep, this turned out to be where they had repointed, we found that the mortar under this was soft and flaky, the developer engaged the BRE to carry out an inspection, they removed multiple bricks from different locations in the outer leaf and took mortar samples for petrographic testing, they also gave me samples so that I could get my own tests done, when the bricks were removed it was found that they had been laid frog down and that the perps were mostly just tipped, an inspection scope inside ethe cavity found that a lot of the bedding joints were deeply recessed and there were a limited amount of wall ties present in the areas checked, both ours and the BRE petrographic tests found that the mortar contained between 32 and 40% voids, the BRE claimed that the additional air in the mortar mix was due to the use of a dry soli mix, I did a bit of digging on this and it appears to be true, dry silo mixers are unable to control the amount of air added into the mix, although some of the newer silo mixers are now able to control air intake.

 

Obviously the more air in the mix the more voids will be present once it cures, and the more voids the weaker is it, it also appears to have an impact on bond strength due to the lime in the mix being unable to protect the shell of the voids, over time this leads to smaller voids to coalesce, the mortar manufacture doesn't use a silo when batch testing their mortar, so while batch samples might meet the compressive strength testing for M4 the actual mortar is much weaker.

 

As with most new builds the otter leaf of brickwork isn't loadbearing and only needs to carry its self weight and lateral wind force, and most are protected from severe wind when in large developments, the problem that concerns me is the cracking to the blocks which I assume is holding the outer brickwork in place to some degree, the vertical cracks are logically just unbonded joints and given that they are present in the gables there would be limited compressive force holding them, which I assume is why we don't see cracks in the blocks at the peak of the gable.    

 

[Gus Potter]

On 07/12/2024 at 07:11, Will A said:

Hi, I'm pretty sure that the blocks are from H+H and are either Aircrete or Celcon,

Hello Will.

 

I've not been about much as busy with the day job and trying to get a bit own own house done before Xmas. Excuse spelling and grammer please.. just knocking out my thoughts.

 

I can see you have put a good bit of work in trying to get to the bottom of this. Well done you!

 

 

On 07/12/2024 at 07:11, Will A said:

they are 2.9N/mm.

For me that is the benchmark. We want the mortar to be no stronger per say.

 

On 07/12/2024 at 07:11, Will A said:

the developer engaged the BRE to carry out an inspection,

Ok they seem to be taking things seriously. Insurance companies per say.. delay, deny, defend.. a pattern of behaviour which I see on a regular basis.

 

On 07/12/2024 at 07:11, Will A said:

they also gave me samples so that I could get my own tests done,

So nice of them.. as they know it will cost you lots! They are chucking enough at you, getting some testing done to see if this will head you off at the pass. Did the report give an interpretation of the results and the knock on consequences SE design wise?

 

On 07/12/2024 at 07:11, Will A said:

I did a bit of digging on this and it appears to be true, dry silo mixers are unable to control the amount of air added into the mix, although some of the newer silo mixers are now able to control air intake.

Well done you! Smart thinking.

 

On 07/12/2024 at 07:11, Will A said:

As with most new builds the otter leaf of brickwork isn't loadbearing and only needs to carry its self weight and lateral wind force,

And here is your simple way in I think. When we design masonry cavity walls we allow for the inner and outer leaf to interact. This allows us to take an overall effective wall thinkness.. both the inner and outer leaf work together.. which gives us an overall effective wall thickness. This means that both leaves have to reach the design standard. Also the wall ties are critical as is cavity width.. something to check later to make sure the ties have the correct embedment each end.

 

Now if the inner leaf is sub standard and is contributing to the effective thickness of the wall then someone is going to have to prove that the cracked inner block and mortar with extra air is still able to meet the design codes.. and that will cost them a fortune!

 

Now the questions I would be asking are:

 

OK we have a BRE report. But has an SE had a look at this to check to see how the wall was initially designed to resist the vertical and importantly the horizontal loads and if the inner leaf still is able to contribute to the effective thickness of the wall.. especially for lateral wind load.  THIS IS I THINK YOUR WAY IN at least expense / risk to you. In som way they have started the forensic process. The NHBC et al do this, get a report that is confined in scope with no interpretive element.. the knock on consequences.

 

I would go back to them and thank then for the testing. Then say can you give me a structural Engineers report supported by calculations that check both the inner and outer leaves for wind loading and slenderness under vertical and horizontal wind loading.. give the information we have on the cracking in the inner leaf and the mortar findings. They need to provide you with an interpretive report. what does the testing /  findings mean?.. and I guess at that point they may shit a brick! My gut feeling is that they are intending to appear they are doing something and racking up your percieved risk aspect. It's a game they play... gambling odds.

 

This is going to open a can of worms as the SE that has to sign off on this will want to know lots more! The main thing here is to force their hand and not spend any of you own money at this stage. For me this is a structural safety issue.. and that concentrates minds.

 

Now as a word of caution. If you instructed me to go full pelters to win as much as possible on your behalf then we could find lots of other issues. As and SE I can't sign off on the stuff that suits you and ignore other stuff I find. Depending on how things pan out you may have to make a declaration to your home insurer and you must make sure you don "blight" the property by a simple administrative error. Now say it turn out that these defects are prevelant over the whole structure? What then?

 

That's it for now. Don't act in haste here.