Hi, thanks for reading, and thanks Tony for the amazing site!
Hands up, I’m a DIYer with a small amount of experience with building things, but new to paving ‘n’ patios. Most of what I know is gleaned from Tony’s excellent site.
I’m trying to get some 600x600x20 granite slabs down as a new patio and I can’t get them to stay down.
Site was dug out to about 150mm depth, type 1 whackered down and then slabs laid on a 40mm sharp sand mix of 6:1, moist but not wet. I now wonder if I should have been wetter. After the first attempt, maybe 20% came loose after a few weeks and lifted with ease. The base seemed solid enough so I relaid the loose ones on a thin layer of SBR/cement mix to try to glue them down.
Soon more came loose so I lifted the lot, pretty much by hand. I was surprised that even those I’d SBR’d seemed to lift pretty easily, leaving their layer of SBR bound to the base. Having lifted them all, I relaid them all with the SBR mix, working on the principle that SBR should stick them like “something� to a blanket. At first, I thought I’d cracked it, but something I didn’t expect has happened...
About 10 days after laying them, the kids got the paddling pool out and created some small tidal waves. Some of the water went on one side of the patio and on that side the following day the slabs almost jumped in the air. I’ve since been able to go around and literally pick the slabs on that side up with my fingers. The other side, where there were no splashes seems pretty much fine (although my confidence is rocking nearly as much as the slabs). Where the slabs have come up, the layer of SBR remained stuck to the base and almost none of it is stuck to the slabs.
Since then, it’s rained, and where rain has fallen on the exposed SBR, it’s expanded so that it’s now forming a slightly domed crusty layer above the base which cracks if you stand on it or pull on it. None of it has stuck to the slabs, and the rain seems to have unstuck it from the base. Naturally I’m expecting the remaining area of patio to pop up when it rains some more!
So, my question - where did I go so wrong with the SBR? I think my very first mistake was a mortar mix that was too dry and not using SBR as a bond bridge when I first laid them, but I don’t understand why my SBR remedy seems to be no stickier and water resistant than just a layer of regular mortar.
My SBR mixing method was to pour maybe quarter of a litre into a bucket and then add cement while mixing with a paddle mixer on a drill until it was blue and like custard. I would then pour and trowel this onto the base and lay each individual the slab on top, tapping it into place. One slab at a time.
I mixed the SBR with cement to a blue custardy paste. SBR was bought from Wickes a day or two prior to use. (KA SBR Bond Co-Polymer Bonding Compound).
As it happens, I've just been out and lifted another slab with my fingers. No tools required. This one was loose already so not a new escapee. When laid (and I assume dried) it seemed as solid as anything, but when I just lifted it, it left an almost perfectly smooth veneer of hard SBR mix stuck to the base and with barely a spec of it stuck to the underside of the slab. The only exception to the smoothness of the SBR is that it has a bump in one corner where it has risen up, I assume from expansion with water. I guess the SBR mix expansion has just popped the slab right off the surface.
The mix should have been wet, almost bricklaying mortar consistency.
The sbr should have been painted on the back of each slab, then laid on the mortar bed (full bed, not dot and dab)
The slabs are usually dressed , so where they laid the correct way up? I.e. the bond bridge painted on the correct side of the slab?
Dan the Crusher Man
01442 212315 www.crusherhire.co.uk
"a satisfied customer? we should have them stuffed!"
I'm not sure why that has failed - I'd need to see it for myself - but assuming the primer paste you made up is sound, that leaves the bedding as prime suspect.
If the bed was a genuine moist mix, there should be no problem with adhesion, but if it was a bit too much on the dry side of moist, that could be a possible cause. Any chance of photies?
Last minute thought: Is there residual primer sticking to the underside of the lifted flags?
I do think that my initial bed was too dry a mix. I subsequently laid a second space with concrete slabs (rather than granite) but used a wetter mix (still 6:1 sharp sand, full bed) and have not had an ounce of trouble from it. Wasn't sure if that was down to the mix or the concrete slabs being easier to work with. Hence the attempt to glue with the SBR slurry to remedy the granite.
All slabs are/were on a full bed - previous patio by previous occupant was dot 'n' dabbed and was terrible.
Granite slabs laid the same way up as in the retailers display, so I'm assuming they're correct. Slabs are really quite smooth on the underside and have a texture on the face side. Wouldn't expect them to be the other way up as I think they might be quite slippery when wet.
SBR slurry mix was poured onto old bed and slabs laid into/onto that rather than brushed onto underside of slabs. Old bed was thoroughly brushed of loose material on slab by slab basis before SBR slurry mix poured.
The cement is BlueCircle Mastercrete cement.
Here's a crack at some photos. All taken just now, post a rain shower so all colours and textures are wet or drying.
This first one (assuming it works) is the slab that I lifted by finger yesterday. You can see the SBR slurry layer that it was previously stuck to, and in the bottom right of the photo where the SBR has risen up off the bed and pushed the slab up.
This is the underside of said slab after it'd been pulled up:
Face side of slab:
This is a close-up of an area of my original bed, with my finger for scale. This was under the SBR slurry layer five days ago, supposedly and for about ten days apparently sticking to it:
The same bit of bed from further away and without my finger:
This is a shot of an area where I've lifted some slabs and then some of the SBR. You can see how the SBR has has risen up from the bed in places. In this shot, bottom right-ish, there is a broken off fragment of SBR that as snuck underneath the SBR layer whhile I've been pulling at it - that perhaps gives you an idea of how far it's risen. Where the SBR hasn't risen, it is stuck to the bed like glue, but where it has risen you can put your fingers under it and pull it up. It comes up in postcard sized chunks mostly. If you walk on it, much of it sinks under your weight and then springs back up. I have my very own mini-SBR trampoline.
Finally, the same spot but from a higher angle and with debris from where I've started pulling it up.
The SBR is quite brittle once up - it very definitely snaps and doesn't crumble very much.
As of this afternoon, half of the patio remains seemingly happily stuck down with the SBR slurry. But it's raining, so I'm half expecting it to levitate before the week's out.
Just so I understand correctly, you laid the entire patio on a semi dry mix and did not paint the backs of the slabs with sbr primer. You then had the problem of them lifting, so you tried to apply the primer to the bedding layer that had now dried, and then stick the slabs down to that? And this is the result?
Dan the Crusher Man
01442 212315 www.crusherhire.co.uk
"a satisfied customer? we should have them stuffed!"
The first lay had seemed relatively successful initially, a few slabs were a bit hollow so I'd read somewhere about how to re-stick them with SBR slurry. That seemed to work for those that I did, but as more of the originally laid slabs became loose, I decided to lift them all and use the SBR slurry sticking technique on them all.
"SBR slurry mix was poured onto old bed and slabs laid into/onto that rather than brushed onto underside of slabs"
Waste. Of. Time.
The *corect* way to install this type of paving is fully detailed oin the main website. If you deviate from that methodology, you shouldn't really be surprised when it goes t*ts up!
