Medium Concrete Block Shrinkage

[HHHAMSTA]

Hi All, 

 

Just need some advise please, the blockwork for my extension went up back end of last year when we had rain pretty much every day through October, so the blocks got wet.  I've now got many cracks in the mortar vertically, some step in places, some go horizontal and some split blocks vertically.  Most are hairline, a couple slightly wider about 1mm.  They're around every window, and on every wall, in some places only 2-3m apart.  The long wall of the house, 13m, has brickwork on the outside skin (blockwork inside) and doesn't have any cracks in the external brickwork but cracks on the internal blockwork.  The other walls with blockwork internally and externally have cracks in both skins.  No cracks go below damp course, none are wider at the top, they're all pretty consistent.

 

My understanding is the cracks are very likely to be from materials shrinkage which would make sense given the weather when the blockwork was completed.

 

I was hoping to wet plaster the walls and the external blockwork will be rendered.  I read thermal movement cracks shouldn't necessarily be filled as can stop the house returning to the original state which then leads to larger cracks over time, though if this is likely shrinkage (thoughts?), I don't believe I'm likely to see any further movement anyway.  Would obviously like to avoid having to scrape out the mortar and/or replacing blocks if unnecessary.

 

In summary:

* Does this sound like normal shrinkage?

* Assuming shrinkage, will plastering straight over the cracks lead to cracking of the plaster in the future?

* Would future cracking in the plaster be reduced if the cracked mortar was scraped and filled, the cracked blocks were replaced or silicone simply be run in the cracks?

* Would dot and dab be a better plaster solution or do the board joins just crack if the walls move anyway?

 

Thanks in advance for any support!

 

 

 

[Mike]

2 hours ago, HHHAMSTA said:

* Does this sound like normal shrinkage?

Sounds more typical of aerated concrete blockwork, though a medium density wall of 13m without a vertical movement joint is pushing it.

 

2 hours ago, HHHAMSTA said:

* Assuming shrinkage, will plastering straight over the cracks lead to cracking of the plaster in the future?

Chance are it will be generally good, but you may get a few.

 

2 hours ago, HHHAMSTA said:

* Would future cracking in the plaster be reduced if the cracked mortar was scraped and filled, the cracked blocks were replaced or silicone simply be run in the cracks?

Probably not. Consider retrofitting a vertical movement joint, if you want to try something that might help. For example resin-bond wall ties in every block course between inner & outer leaves either side of where you want the joint, saw through with a 350mm disk cutter (proper safety gear essential), then fill (or call in an expert). Personally I'd probably leave it.

 

2 hours ago, HHHAMSTA said:

* Would dot and dab be a better plaster solution or do the board joins just crack if the walls move anyway?

If you use dot-and-dab, make sure it's done properly and apply a parge coat first. I'd choose plaster every time.

[HHHAMSTA]

Thanks Mike, think I'll just run with it and be prepared for some making good now and again, or some well placed cupboards

[saveasteading]

On 22/08/2024 at 16:40, HHHAMSTA said:

cracks on the internal blockwork

Photo?