patio nightmare - help!!

[zark]

About half way through a large patio, slabs on 10:1 sharp sand cement semi dry mix, individually bedded (as per the excellent pavingexpert.com advice). Several attempts at each slab til I was happy, so it's taken four days so far. This morning I was checking over the work so far and was horrified to discover that most of the slabs are now rocking slightly. Aggggggggggghhhhhhhhhhhhhhh!!!!! what has happened? I tried to fix one by lifting, raking the top surface, adding a bit more bedding etc but it's pretty much impossible to get it right as the bed has now set and the couple of mm raked over top surface has no room for compacting.

The only solution I can see at the moment if lift the lot, break up and dispose of the bedding and start again, but I ***** really ***** do not want to do that.

Anyone got any ideas? Why has this happened?!!!!

someone help please. My keyboard is getting wet with the tears :-(

[Uno Hoo!]

I also have just completed laying a large patio (about 36 sq metres). I also read up on the subject before starting but decided, in view of the area and the cost, to use the 'five spot' method of slightly wet mortar on compacted sharp sand. I've had no problems with this at all and all the flags are rock steady. I know that this doesn't really help you with your problem but if you lift the rocky flags and excavate slightly, will there be enough room to 'five spot' ? If you do that I'm sure that will fix your unsteady flags.

Kev

[Rob Morley]

The earth moved for you :-)

[zark]

Not too far from the truth, I think......

As I was working this morning I spotted a low patch and thought rather than just fill in with bedding, I'll do the job right and fill the low spot with hardcore. I chucked down some broken bricks and concrete but decided I still wasnt doing the job properly enough and they were too big, so I whacked em up a fair bit with a lump hammer.

My guess is the slabs (unpointed) have jumped in the air just a little bit allowing a few grains of sand in under the corners. :-(

The rocking is very minimal on most slabs, I can't feel it underfoot but tapping with my palm makes them "click" on one corner. I don't think I can face having to move them/break up the bedding etc - I might just point them up and deal with it another year if they start breaking or become problematic. Anyone know how fast they will become a problem or will they (wishfull thinking) settle themselves back down again?

One thought - if I lifted the offending flags and put a very thin bead of silicone or similar around the edge of the slab before re-laying - or would that make them prone to snapping?

depressed. :-(

[Andy]

Did you walk on the slabs before the mix went off? The only thing I can suggest, if you haven't filled the spaces between the slabs yet, is to do that with a wet cement mix. That should stabilise them, though it's not ideal of course.

Otherwise, short of lifting them, some kind of fine-grained slurry sent down between the slabs may penetrate underneath ( note the word 'may' ). Ordinary mortar will not flow to fill small gaps at all. That would suggest either exterior ceramic tile adhesive ( fine-grained cement based-mix, probably mixed up quite sloppy ), or exterior grade PVA. All a bit desperate, I should just try the pointing option. Really, if the slabs are held firmly at the edges, the fact that they're not bedded on a 100% flat bed will not matter.

Andy.

[Rob Morley]

I'd finish laying the slabs then leave them a while before pointing - they might settle anyway with a bit of traffic or even just temperature variation.

[zark]

I am desperate! PVA might do the trick I guess, or a thin bead of silicone, or maybe some self levelling compound.

Thanks for the reassurance. I'll carry on laying the slabs. When they're all done going back over and fixing any that are really bad will probably not seem so daunting! Hopefully the jointing will help (I guess it's now best to do this with a strong mix, applied wet rather than swept in?)

What are the likely consequences of the slabs rocking slightly? (as I said this is very slight in most cases) Are they much more likely to break? Or is it just that they are annoying and the pointing will break and fall out more quickly?

[zark]

This one is 70 sq m :-(

I must admit I'm now wishing I'd used the 5 spot method, despite all that's been said against it. Or just laid them on sand without the cement. At least I'd have a chance of correcting them like that. It takes several attempts to lay each slab as they are faily uneven riven types so it's prepare bed, lay slab, lift slab, try to correct, lay slab, still not right, lift slab, etc etc etc. I guess each slab is taking average 3-4 attempts. And then I get the rocking!! I'm utterly exhausted, it's less than half finished, I've got to go to my tedious day job in the morning and I'm fast on my way to getting very drunk :-P

[Jimbo]

I hate to sound unhelpful but your hardcore isn't solid enough. yo should bite the bullet and start again. I'm afraid if you don't you'l have to accept you are going to have to live with a problem that wil get worse and worse. Sounds like bad news - I know. But you'll feel much better once you ge the "foundations" right

-- Jimbo

[Andy]

You don't say what the slabs are made of, or their thickness/dimensions. If they are average concrete 450mm slabs carrying foot traffic I can't see the breaking. All you will get is the annoying rocking reminding you of your mistake forever! My experience is that weak mixtures of cement, and 10:1 is certainly weak, will not really 'stick' to anything very much. Had you used a wet 1:5 cement, with plenty of plasticiser in it to allow it to flow a bit, and had trowelled furrows in the surface of the cement, before bedding the slabs in place, you could have had a good amount of bonding between the slabs and the cement. That not being the case, like I said, I would point the joins with a

1:5 reasonably sloppy ( but on no account watery/oozing water ) mix, with some added plasticiser ( or Squeezy if you're feeling cheap ). Doutbless the bulk of the load any slab takes is already taken by the drymix bed, but pointing the gaps should stop the annoying tipping.

I laid a lot of slate tiles on a solid bed of mortar once, but did not use any plasticiser, nor did I rake furrows in the mortar with a trowel before placing the tiles. The result was great difficulty in bedding them down at the right level, since the mortar under them would not 'flow' to allow adjustment. I finally got them as good as possible, but it is obvious from tapping them with the butt of the trowel ( you get a hollow sound ), that quite a few are not firmly in contact with all of the bed. However, the pointing seems to stabilise them from moving and I have had no problems since. If I were you, I'd go for the wet pointing approach, and not walk on the slabs for at least 3 days.

Andy.

[zark]

I don't think the sub base/grade is the problem, I've never had problems laying slabs before, several years on they havent moved.

There's no hardcore (for the bulk of it anyway), just good well compacted earth after removing 3 - 12" of topsoil to get the area flat I'm convinced that I caused the problem by breaking up bricks and ramming them in with a lump hammer nearby for fill a low spot so that I'd have a max 50mm of bedding layer. I was hitting them hard, and the vibration has caused the slabs break their weak bonding to the bedding mix and jump very slightly, allowing just a few grains of loose sand in (they were OK before I started hammering!).

If I'd have just done them how I've always done them, (i.e. slap a bit of sand down and put them on that) without doing the research I'd have been OK! (not knocking the pavingexpert site here - it is truely excellent, just wish I hadn't read it this time!)

[Hzatph]

I would just try pointing - it will bind them together. It is unlikey that they will break and they are thin, large and subject to heavy vehicles. I have seen rocking wiht the 5 spot method too.

HTH