Sawn sandstone - Bonding between screed & slab

[Jill@Room105]

Hi,

I'm a garden designer and have spec'd a sawn 6 sides pale coloured sandstone for a patio. The slabs are 20mm (thinner than I would have liked but there we are).

The bedding layer is specified as 50mm semi-dry mortar bed (over 100mm type1) using a 1:4 mix of white cement / low iron sand. I've also asked for the back of the slabs to be washed with a white cement slurry mixed with PVA to stop absorption of water and subsequent discolouration.

The contractor is worried about the lack of adhesion between the slabs and the mortar bed (screed) - because he says that the screed will have gone off to the point of being dry before the slabs are laid and therefore because he won't be able to tamp down the slab into a bed he is thinking he'll need to use some form of adhesive between slab and screed.

I've never heard of this being done before and can't see any mention of doing this on the main website.

Do we need to worry about adding further bonding or should he be laying the slabs before the bed has gone too dry?
I'm also wondering whether I've spec'd too strong a mix for the bedding mortar @ 1:4 (rather than 1:6 or 1:8)

Any suggestions / opinions gratefully received!

Many thanks

[cookiewales]

4 to 1 washed sand and cement is fine the term you are looking for is bond bridge beding can be semi dry paint a sbr on back of slabs introduce some in the mix this will fix them. when laid at the end of day put lots of water on paving then keep of them they will be fine ronofix is the best sbr bar none cookie :;):

[Jill@Room105]

Thanks Cookie. Just found the page on bond bridge bedding on the website. Interesting to see that PVA is not recommended - I'll check out ronofix.

Cheers

[cookiewales]

Thanks Cookie. Just found the page on bond bridge bedding on the website. Interesting to see that PVA is not recommended - I'll check out ronofix.

Cheers

hi pva is for inside and wood glue :;):

[London Stone Paving]

Hi Jill

Would be very interested to know what type of sawn sandstone has been specified. More than likely it is some kind of Indian sawn sandstone. There are many different types available, some extremely hard and some extremely soft. We have extensive knowledge in sawn sandstone having supplied and installed thousands of m2. If it was one of the harder types of sawn sandstone then you will not need to add any pva to the backs of the slabs. If it was one of the softer types then you should be more concerned about the strength because a 20mm thickness on a soft sawn sandstone is not adequate. If you want to discuss anything dont hesitate too get in touch.

Steven London Stone

info@londonstone.com
www.londonstone.com

[GB_Groundworks]

i guess you didn't read the rest of the thread that said and has been said on many other threads dont use pva at all, its water soluble therefore useless outside. use SBR for a really good bond bridge.

reading what the original poster said sounds something like marshalls fairstone sawn sandstone, http://www.marshalls.co.uk/transfo....-paving

we've just been looking at it for a job.

[cookiewales]

i guess you didn't read the rest of the thread that said and has been said on many other threads dont use pva at all, its water soluble therefore useless outside. use SBR for a really good bond bridge.

reading what the original poster said sounds something like marshalls fairstone sawn sandstone, http://www.marshalls.co.uk/transfo....-paving

we've just been looking at it for a job.

a bond bridge should be used on 20mm unless you use a specfic addesive ie arduit made by ball or you will get that hollow sound when it dryes out and skrinks have seen it to many times sbr is cheaper than take up and relay :;): :;):

[Bob_A]

I'm no expert but I used the SBR Toolstation.com and even I can tell it's far better than PVA.