Powdery New Render (dried to quickly in hot / direct sunshine maybe???)

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shilo

New Member
Hi,

I’m not a plasterer but I was a bricklayer in a previous life before a couple of prolapsed discs ended that. So I’ve got some knowledge of mortar but not in the spreading department.

Anyway.....I’m currently having my bungalow renovated and the external render is being renewed by a plasterer. The old dashing was hacked off back to the common concrete bricks, a 4:1 (with water proofer) mix was used for the scratch coat and that’s as far as we’ve got so far (there’s other works ongoing).

I was in work myself while most of the work was going on but I did catch some of the work at the end of the last day they were scratching and in my opinion the walls weren’t damp enough to start with and the resulting hot sunshine that was hitting the walls (it was during the hot period we had a few weeks back) has caused the mortar to dry too quickly.

There are hollow patches which obviously need to come off, but what isn’t hollow can be scored with a fingernail and pulled off and crumbled between my fingers. Surely after 2 weeks of curing I shouldn’t be able to do this, or have I got it wrong? Please tell me I’ve got it wrong otherwise I guess it all needs to come back off!

Any guidance will be appreciated and as you can guess I’m in a rush to get some help before they get on with the fining down top coat.

Thanks
 

henry

Private Member
sounds like dried out to quick what sand or were did you get it from, what mix did they use.
 

shilo

New Member
It looked like the sand I used to use when laying bricks from a local builders merchant. As far as I know it was a 4:1 mix with water proofer.
 

henry

Private Member
You will well know you cannot really lay bricks with plasterers sand and the same can be said useing a fine sand for rendering.
It's all to do with the angular shape of the sand . A good fine sharpe particals are better when mixing with cement fills the voids better. A very common fault is mixing it too wet so it spreads more easy yet we defeat the object of the binding the mix.
All adding more water will do is seperate the sand particals and cement making it a weak unstable mix .
 

DannyMac

Private Member
The sand was too fine. We always throw some river/grit sand in. Maybe 2.5 soft sand: 1 river sand: 1 cement for the scratch.

Then 3.5 soft/ 0.5 river/ 1 cement for top coat.
 

lucius

Well-Known Member
Wrong sand and unfortunately lack of experience but in all fairness we dont get loads of sun in Southern Europe spreads work around the sun and try and lay on in the shade but thats why the work long hours into the evening and early morning.
 

henry

Private Member
it's the old saying many are called few are chosen and the same today with spreads.
 

shilo

New Member
Sorry for the lack of responding...been away for 2 days.
So i'm guessing it all needs to come back off then?
 

shilo

New Member
Well....the scratch coat coming off :RpS_crying:. Any tips on the mix and preparation of the substrate. I think the bricks are concrete commons but after 40 years of curing have hardened to engineering strength. The wall also has a high degree of suction. They were previously rendered and dashed (which was hacked off) if that makes a difference. Should the joints be racked out or should the walls be sealed with something first?
 

asb

Member
Hi, I’m not a plasterer but I was a bricklayer in a previous life before a couple of prolapsed discs ended that. So I’ve got some knowledge of mortar but not in the spreading department. Anyway.....I’m currently having my bungalow renovated and the external render is being renewed by a plasterer. The old dashing was hacked off back to the common concrete bricks, a 4:1 (with water proofer) mix was used for the scratch coat and that’s as far as we’ve got so far (there’s other works ongoing). I was in work myself while most of the work was going on but I did catch some of the work at the end of the last day they were scratching and in my opinion the walls weren’t damp enough to start with and the resulting hot sunshine that was hitting the walls (it was during the hot period we had a few weeks back) has caused the mortar to dry too quickly. There are hollow patches which obviously need to come off, but what isn’t hollow can be scored with a fingernail and pulled off and crumbled between my fingers. Surely after 2 weeks of curing I shouldn’t be able to do this, or have I got it wrong? Please tell me I’ve got it wrong otherwise I guess it all needs to come back off! Any guidance will be appreciated and as you can guess I’m in a rush to get some help before they get on with the fining down top coat. Thanks[/QUBesided using wrong sand i bet labourer didnt put in enough waterproofer in scratch coat(i always use 3:1 for this coat)an top coat not enough cement in mix(too sandy)been doing this 20 odd years no trouble.good luck
 

gooner59

Private Member
OK Here are a few causes Friable Surface. Render coat or dashing aggregate easily rubbed away by hand over sizable areas. Usual causes are too weak a top coat, old age of render. Too rapid dehydration (lack of water proofer in the scratch coat) inadequate wetting down. Weather to hot during application or coat exposed to frost before curing. Miss-use of plasticiser or over mixing with too much air entrapment. i think as you have said you answer will lie within the previous probable causes , all the best
 

bubbles65

Well-Known Member
As warrior says, some kind of pre-rend or 'splatter-dash' first. Give them bricks a good drink beforehand and apply it when the wall is damp looking but not p!ss wet and you'll get a good key. I would then scratch with 4 sand 1 cement with Freeflo added, then topped with 5 sand 1 cement, also with Freeflo added :RpS_thumbup:

And leave a week between coats :RpS_thumbsup:
 
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