Plaster blown

Status
Not open for further replies.

JBPlasteringmcr

New Member
Anyone have any idea why new plaster has blown from new render?
Its not my work that's blown, just been asked by someone so I thought I'd get a bit of input from you guys
 

beader

Private Member
If you mean that the skim coat has come away from the s&c backing then i would check to see if there is a sufficient key for the finish to hold onto.
 

beader

Private Member
That may be the case but has it been rubbed up with a devil float properly? .i have seen in the past walls that have been very poorly keyed and the skim coat has come away in sheets .
 

JBPlasteringmcr

New Member
She told me it has but I suppose the customers know **** all. Ill go and have a look at it over the weekend or something. Thanks mate
 

lucius

Well-Known Member
is it a gypsum based plaster ? it could be etterigite failure[/QUOTEI
I would be pretty sure its a Gypsum based plaster mate, as for etteringate failure it dosent work that way round for some reason cement on Gypsum can fail not Gypsum on cement dont ask me why i just read it somwhere.
 

flat boy skim

Private Member
Calcium sulphate and portland cement are not compatable - a whilst a failure is not guaranteed it is a real possibility. Over time ettringite crystals may appear and as they expand will cause the finish to lift and de -bond. It may well take several months to determine how bad the reaction thx fbs:RpS_thumbsup:
 

lilharbs

New Member
Without seeing it in the flesh can't really say,possibly not cured enough before skimming.no pva before skimming,not scratched up properly before skimming.someone put central heating on to early......
 

mark ross

Private Member
Calcium sulphate is added to portland cements to reduce rapid setting. My take is that the ettringite crystal formation can be lowered by air entrainment. Air voids can help relieve the stress in renders by providing a location for the delayed ettringite to form.
 

gooner59

Private Member
is it a gypsum based plaster ? it could be etterigite failure[/QUOTEI
I would be pretty sure its a Gypsum based plaster mate, as for etteringate failure it dosent work that way round for some reason cement on Gypsum can fail not Gypsum on cement dont ask me why i just read it somwhere.

The simple answer is you do get ettringite at the interface between render and plaster.

There are various reasons why this might not cause a failure. The first and most important is the level. The ettringite chrystalls are formed when moisture borne sulphate ions come into contact with calcium alluminates in the cement. The level of formation will depend on the amount of sulfate ions present. Now if you think about a wall. Firstly it is generally drier and the general direction of moisture will be from the render into the brickwork. If moisture runs the other way ettringite will form but it will also be self limiting as the water pressure will be away from the render thus forcing sulphates way from the interface. That is untill there is saturation. Secondly the weight of plaster is generally light and so gravity is unlikely to "pull" the plaster away from the render unless there ismsignificant attack which you might see with a risng dampor penetrating damp problem.Thirdly you don't exert impact and dynamic loadings to a wall.

Now let's look to the floor. The direction of moisture movement will be from the gypsum based screed toward the cement thus the concentration of sulphate ions is higher. You walk on the floor thus exerting dynamic and impact loads which will break the weakened bond at the interface. Other influences also come into play such as underfloor heating.

If you want to see an ettringite failure on a wal I shall take a pic of my kitchen wall and post it for you. The plaster is falling off the wall as I have a penetrating damp issue which I need to deal with.

The way to stop ettringite is to eliminate one of the three elements needed. The first being cement, the second being the potential contact between the cement and the third being moisture. These are dealt with by either removing the cement and using non cementitious adhesives whichis the most robust method, removing the contact by priming ideally with an epoxy primer or thirdly remove the water. This last is the hardest to do as there will always be some moisture present.
 

DannyMac

Private Member
My take is that the ettringite crystal formation can be lowered by air entrainment. Air voids can help relieve the stress in renders by providing a location for the delayed ettringite to form.

Would plasticiser alone trap enough air?
 

JBPlasteringmcr

New Member
I spoke to them and arranged to see them this weekend, the had a new dpc, walls rendered just over a metre from the floor and then the whole house skimmed. They only had it done a couple of months ago. When I go on Sunday I'll see what's what and give you guys more details. At first I did think they didn't devil it but she's told me 'there is loads of circular scratches on the render.'
 

asb

Member
Without scotty or spock to give views i would simply say they couldnt handle suction holding back on waterproofer?if after theyve skimmed to cieling an maybe not unibonde that correctly im guessing youll find plaster also coming of above the lm floated.the unibond or sealer would have run down wall an wlsovsealed again if they had.reskim the lot loads o money!!!!lol
 

donzx6

New Member
Did you ever get to the bottom of the cause JB? I've just had a room done (S & C backing with waterproofer) and the skim is delaminating all over the place.
 

essexandy

The Lake Governor
Did you ever get to the bottom of the cause JB? I've just had a room done (S & C backing with waterproofer) and the skim is delaminating all over the place.

I'll bet the S&C wasn't left for five days to cure as it should be before skimming it.
 

donzx6

New Member
Thanks chaps.

From memory the backing coat was done in two coats. One with Sika additive on a Thursday, then another coat on Friday/Monday with the skim on the Tuesday.

I kept the heating off and the doors open during the day for about another 10 days. The rads then went back on low (about 11-14 degrees) for another 10 days and then up to full temp (18) with a low boiler temp.

The worst bit for cracks is the chimney breast - its like crazy paving on there, but that is above one of the rads.

I've tapped all over the walls that have been done (fingernails) and the skim seems to be be coming away, but the S & C seems sound (i.e. not hollow).

Is it likely that I have caused this myself with the heating ( I had no choice as we have a new baby) or should it not be this bad?

For what it's worth,these are all external solid walls. I did have another wall (internal) in the same room done by another plasterer around the same time, but that was a hardwall backing coat and that seems fine.

As more cracks seem to be constantly appearing, I guess the best solution is to take the skim coat off and start again (once the cause of delamination is known)?

Many thanks in advance for your help
 
Status
Not open for further replies.
Top