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 :-(

"zark"  wrote in message 
news:dgjht7$eam$1@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com...

> 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 :-(


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

In article <dgjht7$eam$1@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com>, a@b.com 
says...

> 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?


The earth moved for you  :-)

"Rob Morley"  wrote in message
news:MPG.1d975e9c520903e998984e@news.individual.net...

> In article <dgjht7$eam$1@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com>, a@b.com
> says...
> > 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?
>
> The earth moved for you  :-)


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. :-(

"Rob Morley"  wrote in message 
news:MPG.1d975e9c520903e998984e@news.individual.net...

> In article <dgjht7$eam$1@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com>, a@b.com
> says...
>> 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?


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.

In article <dgjlcm$n54$1@nwrdmz02.dmz.ncs.ea.ibs-infra.bt.com>, a@b.com 
says...
<snip>

> 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?
> 

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.

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



> 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.
>


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?





> Andy.
>
>

"Uno Hoo!"  wrote in message
news:dgjim8$ptf$1@newsg4.svr.pol.co.uk...

>

> 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.
>


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

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


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!)

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

"zark"  wrote in message 
news:dgjnbo$nfr$1@nwrdmz03.dmz.ncs.ea.ibs-infra.bt.com...

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


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.