Lay riven buff patio slab on concrete slab. what to use?

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We decided to cover our concrete slab side walkway with some reclaimed riven buff patio slab. I know these slab usually directly on compacted soil but on my case it would go on solid 100mm concrete slab.

I guess sand and cement would be fine to lay them. I know it may sound stupid but would I need to use soft or sharp sand? what mixing ratio? would the mixing need to be pretty dry like making a screed and then sticking slab on top or mixing need to be more fluid and lay slab one by one?

Many Thanks
 
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If you are spotting, soft/building sand 3:1 not semi dry, it needs to be wetter a bit like mortar but not that wet or the slabs will sink.
Don't touch or walk on the slabs for a day or so once laid

Or a solid sand bed of concreting sand, no need for cement, and you can walk on them as you work. Depends if you have a bout 30mm of so depth for the bed.
 
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Lay it on a full bed, I find they always come loose doing the old 5 spot.
 
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I would avoid the 5 spot as we have removed those from another area and they were all loose and full of grass between them. I prefer to have those on full bed and 10mm gap staggered.

What would be the best bed to use in this case?
 
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The problem with riven slabs on a solid bed is getting the surface nice and flush and in line. If you level the bed, the slabs surface is all over the place as they are different thicknesses

Done properly, there is nothing wrong with spots and they will last for decades with no loose slabs, no movement and no weeds. You are allowed to but the mortar bed around the underside of the edges to stop the weeds.
 
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The problem with riven slabs on a solid bed is getting the surface nice and flush and in line. If you level the bed, the slabs surface is all over the place as they are different thicknesses

Done properly, there is nothing wrong with spots and they will last for decades with no loose slabs, no movement and no weeds. You are allowed to but the mortar bed around the underside of the edges to stop the weeds.

The 5 spot seems the way to go for that slab due of thickness inconsistency. Do you use any plasticiser on the mortar?

If I want lay those staggered with something like 10mm, would you leave the gaps open and then grout with sand cement the following day? How this is normally done when layed with gaps?

I know I may be making stupid questions but would try to make it right at first shot.

Thanks
 
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You don't need a plasticiser, but if it's hot it may help control the setting time. Also dampen the back of the slab and the concrete base - and brush away all debris.

5 spots and a thin border to seal the bottom of the joints, or just a wider border and a centre spot.

Always leave the joints and point up after a couple of days. Use a drier mix of sharp sand for the joints, or a brush-in product
 
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You don't need a plasticiser, but if it's hot it may help control the setting time. Also dampen the back of the slab and the concrete base - and brush away all debris.

5 spots and a thin border to seal the bottom of the joints, or just a wider border and a centre spot.

Always leave the joints and point up after a couple of days. Use a drier mix of sharp sand for the joints, or a brush-in product

Would you suggest drier mix of sharp sand/cement or brush in product? Do they both last same time? Which one is more flexible to stand cold and hot temperature?
 

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