Problem with render curing and cracking

[peweuk]

I have just had the front of a new extension rendered and there is a problem with cracks appearing as it cures.

The build is using a solid thermal block wall (215mm thick) and an intial scratch coat was applied to the front and sides. This was left to cure for a few days then a top coat applied to the front on Thursday.
When the renderer came back on Friday morning, the top coat on the front of the house had started to craze in places with small veins appearing (mosaic like appearance) - the porch was OK.

Yesterday, the front had dried out further and the veins were now cracks and the porch has dried out further and the veins are appearing.

The render was applied using the mix recommended by the block manufacturer (6:1 sand/cement with plasticiser for the scratch and 6:1:1 sand/cement/lime) for the top coat.

The renderer decided to not top coat the rest of the building until we find out what has caused this problem. The block manufacturer suggested using a coarser sand (the sand originally used was building sand).

The render, although cracked, does not seem to have blown off the blocks - but it has not fully cured yet.

Has anyone experienced this type of problem before, particularly on thermal block?

I'd be interested in any comments or suggestions.

 

[dazdel]

Was there any waterproofer in the scratch coat ?

 

[SUPERSLASHER]

When rendering you should always use washed plastering sand with a good waterproofer added. Building sand is for brick or block laying and not render. If the the scratch had no waterproofer in it then the top coat will pull in to fast and that is why you end up with it cracking while it cures

 

[roughcaster]

When rendering you should always use washed plastering sand with a good waterproofer added. Building sand is for brick or block laying and not render. If the the scratch had no waterproofer in it then the top coat will pull in to fast and that is why you end up with it cracking while it cures

The comment from S/Slasher, i agree with totally, and is sound advice.

Plus,
The blocks should also be given a good soaking/wetting down, just before they're scratch coated. Also, a "VERY fine LIGHT spray" from a hose, over the freshly scratch coated surface,, on and off throughout the day, will help to keep it "green", and stop it drying out too quickly, before it has a chance to cure properly..... Again,,, just enough very fine spray to "lightly" wet the scratch coat,, you don't want to wash it off.

Roughcaster.

 

[joe-90]

Thermal blocks will suck water out a rich tea biscuit. They need sealing with a good coat or two of PVA first.

 

[peweuk]

Thanks guys.

A couple of points and questions.

The blocks were wet before the scratch coat, and when the scratch coat dried it was well adhered to the blocks - no problem there. It was then left for 3-4 days before the top coat was applied.

I am not sure but I don't think any waterproofer was used in the base coat - however the top coat has been on for 3 days now and it is still drying out - the cracks appearing after the second day. So it does not appear to be drying too quick.

My questions now are:

On the front and porch where the top coat has been applied, is there a simple way to cover the cracks ( a couple of good coats of paint might do it - maybe a test area will decide this)

On the side where no top coat has been applied to the now dry base coat, should a second coat with waterproofer be used before the top coat, or what other preparation or technique should be used to ensure the top coat does not crack?