Limestone diy disaster.

It can be very disheartening when this happens but there's not much you can do other than take them up, dig out some of the base and relay them. I certainly wouldn't bed slabs on a mortar mix (assuming you mean a building sand mix) and your hardware guy is possibly right about the sand although I've not heard of 804 hardcore. Depending where you are in the country you should be using washed sand, grit sand, sharp sand which are probably all the same thing with some minor differences. You could even use plastering sand.

It sounds like you've been unlucky and a number of factors could be the reason. Incorrect mix - should be either 5:1 or 6:1 sand:cement (3:1 for the pointing). Your mix might have been allowed to dry somewhat during use - don't mix too much and cover the unused mix with a sheet/bag. The slabs may have been dusty underneath preventing a bond with the mix and the slabs may have been disturbed before the bond had formed - it's a chemical reaction between the sand, the cement and the slab which creates the bond.

I hope this helps and good luck.

 

Did you use a plasterzier in the mix, some thing like Feb mix, this makes the mix much stickier.

 

Have you left say a 10mm gap between slabs for pointing

If so, a good pointing mix 'may' be enough to hold slabs in place

 

How thick is your hard core layer and did you tamp it down well? What's the ground like underneath?

I'm thinking it's moved so relaying won't solve the problem. Need to work out what's moved first.

 

I think that confirms your own theory however Dr B. makes a good point and this will be a great opportunity to compact your hardcore again before you relay the slabs. Can't see the photo by the way.

 

Looking at the photos, not sure what sand has been used? Looks almost like tarmac

Referring to instruction from one manufacture and they recommend 5 parts sharp sand/or building sand to 1 cement. For limestone they refer to applying neat wet cement and PVA before laying to assist in bonding.

Laid 7 slabs here last year at house, used sharp sand and added plasterciser to the mix, makes the mix much more softer and workable.

The 1st slab was laid wihtout adding SRB neat to the back of the slabs, (used granite slabs) which aren't very pours. Nearly a year later, well 8 months, Only the first slab is slightly loose. Other ones with the SRB are rigid without any rock.

Need to be careful not to splash SRB everywhere as will stain the surface, nightmare to get ogg. I used it neat applied by brush.

 

I'm with jit on this one and the SBR

Only laid a few slabs here and there so certainly no expert but had read that sometimes the slabs (whatever material) don't always bond to the bedding layer. I also brush the backs of mine first with a dry brush to remove any loose dust, then a slurry of SBR and a handful of cement powder. They then stick lovely like **** to a bed

 

Had some paving done by a builder, it was India sandstone.

A lot of the slabs were loose and rocking afterwards, trying to stand on opposite corners for checking.

Used another person who mixed up a thin cement slurry and laid slabs on existing surface. Only a few mm thick. Bigger slabs need full support bed underneath otherwise can crack when load is on. No slab were looe afterwards.

 

I think the bedding mix is the main problem. Should be a mix of sharp sand and cement 5:1 or 6:1, not too wet, but also with SBR additive to make sure it sticks.

 

Looking at your picture, I would be tempted to lay a slightly wetter & richer mixture 4 - 1 on top of you Original mix & re-lay the slab.
Try laying one first, see how it turns out, you have nothing to loose, just a spoonful of mix...

 

When I laid a few slabs the mixture was dryish but had to add quite a bit more water than for concrete. I also wetted the slab and concrete sub-base before hand so the water wouldn't get sacked out of the mix.

 

You could even stick them down with some waterproof grab adhesive, I've done this many times with block paving on steps, it works a treat.

 

Stixall ...........

Maybe