Limestone paving - Type of screed bed

[johnegrey]

Hi all, I'm planning on laying 60m2 of Natural Paving's new(ish) Cragstone Tuscan Limestone (called 'Tuscan' but actually from India...).

http://www.naturalpaving.co.uk/product/tuscan-limestone/33/14

It comes calibrated at 24mm in '600 Series' project packs, biggest format is 600x900mm. The site has been prepared with a well compacted Type 1 base. I was hoping to use an unbound screed bed for these flags as the area is large, however, I read on the main site that the size of some of these slabs makes this method questionable.

I'd welcome advice on whether I can indeed use a screed bed and if so whether I should use a bound or unbound bed, if bound would a 10:1 grit-sand to OPC be the ticket?

This new patio replaces an old Bradstone patio that I laid in 2003, I put that on an unbound screed bed with solid pointing and didn't encounter any issues.

Thanks for the help.

[r896neo]

You definately won't get anyone on here advocating a sceeded sand bed for natural stone and rightly so.

If they are very well calibrated you might get away with a screeded bed of 6:1 Grit sand : Cement

The problem with calibrated flags is that any flag which happens to be thinner, or worse just part of it thinner, than the calibrated thickness will not be milled by the machine. Therefore flags or parts of flags below 24mm will remain uncalibrated and on a screeded bed that will lead to rocking.

You will also find you have adhesion problems with a bedding mix that is dry enough to physically screed.

I, and I imagine most others on here would lay them individually on a full bed of mortar.

Others may have had good success with screeding though so wait for some others thoughts

[lutonlagerlout]

screed beds are suitable only for council greys or saxons
it will not work
LLL

[London Stone Paving]

You also need to consider John that all natural stone is subject to a minimum tolerance of +/- 2mm. You will also get a few rouge slabs which are +/- 3 or 4mm. This makes it very difficult to lay natural stone on a screed

[Tony McC]

You *can* lay calibrated natural stone on a screeded bed, but the units need to be capable of withstanding a vibrating plate compactor to settle them into the bed and accommodate any variation, of which there is always some, even with the very best suppliers.

So, you can get away with what we might call block/sett size pavers, but stone flags are rarely suitable for the plate.

You could lay flags on a screeded bed, but because of that problem with the thickness tolerances, some will sit too proud, despite being mauled down; some will sit just right; and some will need lifting to allow a scattering of additional bedding. Consequently, there's no real saving of labour, so most contractors will opt for individual beds - that way, we get it right first time. As contractors, we only get paid for laying each flag once, so we really don't want to be lifting them to adjust the bed over and over again.

To sum up: individual bedding, 10:1 mix is fine for patio use, and make it moist, rather than wet. You can use a bond bridge if adhesion is really important, but with strip-milled calibrated flags, the grooves on the underside are usually adequate to keep the paving in place on a moist bed when there'll be nowt heavier than a child's bike using the completed pavement.

[lutonlagerlout]

I know technically I may be wrong but i wont use 10:1 anywhere
on a 30m2 patio 5:1 requires 7 extra bags of cement which is £35
why risk a 4-5 grand investment over £35?
IMHO

LLL

[mickg]

we have used up to 8:1 while I was testing various mixes over the last 18 months and within a few days its hard, 2 weeks and its rock hard - this is not a scientific experiment in a lab environment just an observation as I like a lot of other people did not believe Tony Mc weak mixes would be suitable until I put it to the test and it does actually work

[lutonlagerlout]

yeah but mick I want it hard 24 hrs later
I am not disagreeing per se
just that I intend to carry on doing what has worked for me with zero failures

my next door neighbour up the allotments is a guy called Tom Togher
back in the early 90s he was lutons pre eminent CBP

anyway at the time I watched his blokes
they dug out 250mm by hand with jackhammers and picks
laid 140 of dry lean knocked up in the big diesel mixer to falls
laid grit sand to falls 50mm thick
and then 60mm blocks cut with the wizzer

fast forward a couple of years all 3 of them had a bad accident on the motorway and none of them ever worked again= apart from tom up the allotment next to me

but the massively striking thing about this story is that I see 3 of the drives he did back then 23 years on and not 1 of them has sunk anywhere
the blocks have faded to pink and lichen but they are solid

I know this goes against conventional thinking but when i asked him why the leanmix?
he said that type 1 will always rut on clay sooner or later especially when the car/van drives on the same spot every
day

maybe the initial dryness of the concrete allows water to pass through?
cheers LLL

[msh paving]

I often do a lean mix base, ready mix delivered or i collect, with 40mm sand on top. in this certain areas round hear the water table is very high , 600 down,never had a problem doing it that way. MSH

[mickg]

it is hard 24 hours later but could still crumble the morning after hence why I said a few days to cure, a 4:1 will be hard the morning after but contains too much cement content and can cause issues with bleeding through to the surface of a paving flag which is what started me off in the first place to test other mixes

we do a lean mix sub base on driveways quite often but just don't post about it as to coin a phrase "its not the correct way" or "its not to British Standard" when in reality it works, Ken gave us a lift last year on a drive we relaid that was an absolute mess due to soft ground and not having a sub base in the first place and he stood there and could not believe I was laying concrete as a sub base as the way he had been taught did not cover doing anything unconventional

like you say Luton I could drive past that driveway in 20 years time and it will still look the same and to me that's what it all about, sorting any ground problems out before you move onto the next stage of laying the block

[Tony McC]

Concrete sub-bases are all well and good as long use you can be sure the laying course will drain. They certainly have their uses on soft ground, but unless tyhere's an effective escape route for collected water, they can cause just as many problems as they solve.

10:1 for patio use is not all that far from the old 1:3:6 concrete mix we used for kerb bedding. Even at 10% cement content, the mortar will have initially hardened within 24 hours, certainly enough to withstand careful foot traffic, and, as Mick said, far fewer problems with staining which has become a bigger issue over recent years due to the public's fondness for pale flagstones.

[johnegrey]

Thanks all for your help and advice, I will go with individual bedding, cheers