Do all aerated blocks crack - picture attached

6 Jun 2007
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United Kingdom
I'm building my extension with Celcon standard 3.6N blocks both inside and out with a fully filled 100mm cavity. I'm using stainless steel ties at 450mm centers vertically and 600mm horizontally.

I've read a number of threads about settlement causing aerated blocks to crack. This hasn't been a problem for me - until now!

A crack has appeared on one of the external corners of the extension - see picture below ---

The crack extends upwards for another 3 courses and downwards for another 2 courses but only as a hairline. It stops 2 courses above the DPC and 5 courses below the top of the wall. There aren't any cracks on the inside of this corner or indeed anywhere at all in the inner leaf. There is one more hairline crack in the outer leaf about 4 meters away.

I've carefully followed the SE's spec with the mortar mix and the manufacturer's instructions with the plasticiser. The walls have been laid carefully and are plumb. The bulk of the blockwork has been in place for about 6 months.

Is it inevitable that my walls will crack because of my choice of block and won't this eventually compromise the overall strength of the building?

If this is settlement cracking (and I hope it isn't anything more serious!) will it continue for ever. The external finish will mostly be render, is it likely that it too will continue cracking for years to come?

Has the wall integrity already been compomised and should I be looking to replace the blocks shown in the picture?

Any thoughts would be very welcome. Thanks
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where is the brickwork!

can we see a photo from a distance showing more general detail as per location, proximity, height etc.

the close-up does not tell the whole story.
OK, I just nipped out and took some photos. These two are showing the corner in question.

The block shown end on in the earlier close up is the first full end on block you can see showing above the dwarf wall in the foreground currently under construction. I couldn't get a shot of the side of this wall but it's about 10m long with an expansion gap about 6M down from this corner and a small window about 4M down.

This photo is of the inside of the same corner just to give a feel for things.

Hope this helps. Thanks
it looks like a settlement reaction.

is the dwarf wall tied in and on the same size foundation as the main wall?
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The dwarf wall isn't tied in to the building. The dwarf wall foundation is as deep as the building foundation but wider. It's 1100mm wide (yes really) - 300mm wall + 200mm limestone clad either side + 200mm extra either side.

Oh, and about the bricks. I live in a small village on the Lancashire/ Cumbria border, all the houses are a mix of limestone and render - no bricks in sight.

I've never seen that before, I think I would be a little concerned.
firstly the lintol, (concrete?) shown does not seem to be on a long enough land, From memory the mimimum is 150mm for 3ft, 225mm for 6ft, more for longer.
With the steel lintol shown on the internal views there seems to be a lot of brickwork below and in the associated area, I think it is considered bad practice to mix brick and block especially with celcons, even if the bricks concerned are sand-limes.
It is easy for me to say as I am not paying for it, but I would change the outer skin to brick now as problems later will cause much worse and expensive repairs.
However, I would first get the D.S. (building inspector) in to give it all two coats of looking at. (pay him if necessary)
Looks like a classic expansion/contraction (shrinkage) crack.

Its a bit soon for settlement, the wall either side of the crack seems to be level, and its not a typical place to crack

We have had some extreme weather of late, and if this wall is getting hot/cold wet/dry then its going to expand and contract.

I've looked at a building the other week where several vertical cracks have suddenly appeared in the external face after 30-40 years - its the second one I've seen since March. In this case the resident actually heard the cracks appear when he was in the garden. I'm putting it down to extreme hot and dry weather immediately following a period of cold and damp weather.

It may be worth cutting the blocks out and replacing rather than trying to glue it. Also add some expanded metal to the bed joint.

Out of interest what mix was specified to inside and outside walls?
Were the blocks saturated when laid?
Thanks for the prompt responses.

The external concrete lintel landing is in line with the manufacturers recommendations.

The original spec was for 7N dense concrete blocks both inside and outside but after laying over 1,000 of them in the foundations my knuckles and elbow joints just couldn't take any more. The SE OK'd changing to Celcon Standard blocks providing I beefed up the areas under any padstones with 7N concrete commons. This accounts for the arrangement under the inner steel UB.

Woody, I'll take your advice and replace the cracked blocks incorporating expanded metal into the beds.

The specified mix was 1:5 OPC:sand + plasticiser (although I didn't check if this was still OK with the Celcon Standards!).

I'm lucky that I don't have any time pressure so I haven't needed to lay any saturated blocks although, in hot dry weather I do surface wet the wall and block before laying.

Is this likely to be an ongoing issue for me or should things improve after the walls are rendered/ clad in limestone?

The forces that cause cracks tend to run at right angles to the line of the crack. Thus, a vertical crack tends to be due to horizontal forces and stresses. Thermal movement and overstressing cause horizontal forces and stresses. The crack stops at the dpc level, as the blockwork can slide on this.

The wall is not overstressed, as there's not much load on it, so it's a thermal crack. As Woody says, the weather's been quite warm of late and that's the probable cause. Nothing serious, but if you're rendering directly onto the blockwork, then leaving the crack will result in this being reflected through the render; as such, would go with Woody's repair advice.
Thanks a lot Woody and Shytalkz, your thoughts are much appreciated. I will indeed follow your advice and replace the cracked blocks.


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