Will A Posted November 25 Share Posted November 25 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. Link to comment Share on other sites More sharing options...
Mike Posted November 25 Share Posted November 25 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. Link to comment Share on other sites More sharing options...
Russell griffiths Posted November 25 Share Posted November 25 I would not use aircrete blocks if you gave them away free. Link to comment Share on other sites More sharing options...
Chanmenie Posted November 25 Share Posted November 25 (edited) 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 Link to comment Share on other sites More sharing options...
saveasteading Posted November 25 Share Posted November 25 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. Link to comment Share on other sites More sharing options...
Will A Posted November 26 Author Share Posted November 26 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? Link to comment Share on other sites More sharing options...
saveasteading Posted November 26 Share Posted November 26 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? Link to comment Share on other sites More sharing options...
Chanmenie Posted November 26 Share Posted November 26 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 Link to comment Share on other sites More sharing options...
saveasteading Posted November 26 Share Posted November 26 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? Link to comment Share on other sites More sharing options...
Mike Posted November 26 Share Posted November 26 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. Link to comment Share on other sites More sharing options...
Gus Potter Posted November 26 Share Posted November 26 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! 1 Link to comment Share on other sites More sharing options...
SteamyTea Posted November 27 Share Posted November 27 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. Link to comment Share on other sites More sharing options...
Brickie Posted November 28 Share Posted November 28 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. 1 Link to comment Share on other sites More sharing options...
Will A Posted Friday at 15:17 Author Share Posted Friday at 15:17 These are the gable walls, the black lines are where the crack run 1 Link to comment Share on other sites More sharing options...
Mr Punter Posted Friday at 17:31 Share Posted Friday at 17:31 Get on to the manufacturer and see what they have to say. Link to comment Share on other sites More sharing options...
Will A Posted Friday at 18:41 Author Share Posted Friday at 18:41 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 Link to comment Share on other sites More sharing options...
Gus Potter Posted 5 hours ago Share Posted 5 hours ago 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.. 1 Link to comment Share on other sites More sharing options...
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