Advice on kandla honed sandstone - Partial stained slabs not spot bedded

[Barry K]

Been planning this project for some time research my slabs size prep etc and thought I’d covered all my bases.

I’ve laid approximately 60 900 x 600 kandala honed sawn slabs, on a solid sub base of hard core to build up the level of my patio on which I’ve added 6 tonnes of mot 803.

The slabs have been prepped before laying by applying SBR diluted with water and mixed with cement and then laid on a full bed of 6/1 sand and cement.

Over the course of a week laying about 10 slabs a day all seemed week slabs looked great but then we had some shocking weather and I had to stop work and wait for a window to cut all the diagonal slabs then more rain and over this period the dreaded stains started to appear!

I’ve started to pull up a couple of these which were really dark in an attempt to let them dry from both sides to attempt to reseal before relaying.

Right now just looking for some advice as to why this has happened tried to show the small voids I discovered but didn’t think these were too bad but maybe I’m wrong any help would be greatly appreciated

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[Tony McC]

The answer is in your question, Barry!

Had the *diluted* SBR slury been effective, there is no way on this earth that you would have been able to lift those flags. Consequently, we can be fairly sure that the slurry primer you made has not worked.

That may be because you diluted the SBR, or it could be because, if I've read this right, you had all the flags primed days before actually laying them. Of course, it could be a combination of these two key factors.

An effective bond bridge slury primer usually relies on fresh, undiluted SBR mixed with fresh cement, mixed to a thick soup-like consistency or a thin paste, and applied to the underside (bed face) of the flag *immediately* before installation (or not more than 5-10 mins before installation).

So: the flags haven't properly bonded, and you now have a touch of damp, possibly some reflective stainng - it's topo soon to say whether the reflective staining is of any real consequence.

The minor voids in your bedding are of no real consequence. When we talk of a "full mortar bed" we actually look for a minimum bed contact of 80%. It's very difficult to achieve 100% suopport: there are nearly always some minor voids.

At this stage, I would suggest that the main issue os one of damp, so I'd look to re-lay those flags affected.

It may be possible to re-lay using a thing bed mortar over existing, or you may have to hack out the existing bedding - I haven't assessed the site so can't say one way or t'other. but what you MUST do is ensure you use a decent slurry primer, either an undliuted mix freshly applied as each flag is re-laid, or a proprietary brand used in much the same way. That will, I suspect, resolve most, if not all, of the issues you've identified.

[Barry K]

Thanks Tony for clarifying the full bed of 80% I hadn’t read that anywhere so outs my mind to ease in that at least!

I should of clarified that the SBR was applied to each slab just before laying too so assume as you correctly pointed out that the SBR wasn’t mixed correctly it did however say on the back of the bottle to mix it with water so frustrated that this has happened.

Also I did have to use a crowbar to lift the slabs so where pretty well stuck, but will have to see with one or two if I can save hacking up the bedding!

Could you advise what mix to use for the slurry?

Thanks for the reply too much appreciated!

[Tony McC]

Even with a crowbar, a properly-applied/constituted slurry primer would have brought the mortar bed up with the flag. The bond between primer and flag is greater than the internal strength of a typical mortar....at least it is in theory!

The problem with giving advice on how to prepare a sure-fire slurry primer based on SBR and cement is that there is so-o-o-o much variation in what gets sold as SBR. Some of the top brands are more-or-less pure SBR with litle or no added water, but the 'bargain' brands often dilute the SBR to "value engineer" the product to a particular price point.

So: if I was to say use X kg of cement to 1 lire or SBR, that assumes that it *is* 1 litre of SBR when, the actuality could well be that it's closer to 700ml of SBR and 300ml of water...or even worse!

This is why I state on the SBR/Bond Bridge pages that you have to follow the advice provided by the supplier.

When I'm supervising an installation, I always insist on a branded SBR (so: Sika, RonaFix, Cementone etc.) and add sufficient cement powder to from a thick slurry soup, one with the consistency of a bloody good gut-bursting pea'n'ham soup (without the ham, obv!). So far (touch chipboard!) that hasn't let me down.