Roughcast turning to dust.

[MrSquirrel]

Hi folks,

Just had a roughcasting job done and looking for advice.

It’s a block retaining wall split into two sections. I'll be painting it to match the house.

It was done 4 days ago and the wet dash on one half is like powder, it comes off with the slightest touch. If I tried to paint it, it would all end up in the brush!

I’m no builder, and would consider myself a pretty competent DIY’er but have never done wet-dash. It looks to me like he hasn’t used enough cement with the dash on the bad section, or too much waterproofer. Probably a combination of the two, the more I think about it.

He mixed up a new batch of dash before doing the good section, and that seems to be fine. Possibly a little weak, but certainly acceptable.

To prove a point I mixed up my own dash (4 stones, 2 sand, 1 cement properly guaged) and threw it at an unseen portion of wall. After one day to cure my test patch is tougher than the rest of the wall after 4 days!

It’s annoying as the scratch and top coats were neat and tidy. Looks like he's just made a mistake on the first batch of dash.

The sand used was nice and clean, and i washed the stones in advance for him to get rid of any fines.

The good section:

The bad section:

Close up of bad section:
You can see the tiny grains of sand if you look close, not right at all. They should all be coated in cement. From a distance it looks fine.

Rub your fingers along it and it turns to this:
Clean grains of sand with no cement coating them, no wonder it has no strength.

More:

The bolster is sitting here waiting to strip the lot off! Roughcast is supposed to be bullet proof - not like this!

Cheers,

 

[bubbles65]

Have you had a word with the bloke who done it?

 

[Olican]

nice photos...welcome along Mrsquirel

 

[MrSquirrel]

Not yet, wanted to give him the benefit of the doubt and a few opinions before i start moaning.

The suspect bit just didn't seem to be firming up like i'd expect.

I'll be getting in touch tomorrow! Need this sorted before the weather makes it impossible.

 

[lucius]

Did he use hydrated lime in the mix?

 

[johniosaif]

You must have done your nuts over this.

 

[MrSquirrel]

Did he use hydrated lime in the mix?

No lime, just OPC cement and Cementone Freeflo admix.

Nothing was getting measured out though which was ringing alarm bells. Again, gave the benefit of the doubt - many guys can get a perfect mix with their eyes closed because they've been doing it for 30 years.

I always gauge everything because that way you can't go wrong. Admix always gets measured out. 5 mins extra versus re-doing the job - it's a no brainer!

Kind of off topic, but i went on a Lime mortar course and the first thing they taught us was correct gauging. They said most guys using cement get away with badly gauged mixes because cement has so much strength. Its strength compensates for bad practise. Even the strongest lime is much weaker than OPC so you have to be very careful. They showed us how to properly work out how much binder (cement/or lime) you need to add to your sand. The only time i have ever seen this done in

You must have done your nuts over this.

Aye, you're not wrong!

 

[johniosaif]

It is a shame this has happened to your job, school boy errors

 

[MrSquirrel]

Okay, decided to tackle the job myself and it's turned out well after some trial and error. Will get some pics sorted shortly.

Have been playing around with different mixes and worked out where he went wrong. Definitley too little cement and too much plasticiser in the wet dash. If you have even a little plasticiser in the wet dash it creates far too many air bubbles in a mixer. The action of the gravel in such a wet mix froths it up very quickly.

Tried a few different mixes for the wet dash and settled on 3 gravel, 2 soft building sand, 1 lime, 1 cement. The lime makes a huge difference, a perfect sticky mix that makes a nice "whack" sound when it hits the wall.

 

[kebab king]

That job looks to nice to of wasted wet dash on.